Hooks are a new addition in React 16.8. They let you use state and other React features without writing a class.
This page answers some of the frequently asked questions about Hooks.
Starting with 16.8.0, React includes a stable implementation of React Hooks for:
Note that to enable Hooks, all React packages need to be 16.8.0 or higher. Hooks won't work if you forget to update, for example, React DOM.
React Native 0.59 and above support Hooks.
No. There are no plans to remove classes from React – we all need to keep shipping products and can't afford rewrites. We recommend trying Hooks in new code.
Hooks offer a powerful and expressive new way to reuse functionality between components. "Building Your Own Hooks" provides a glimpse of what's possible. This article by a React core team member dives deeper into the new capabilities unlocked by Hooks.
Hooks are a more direct way to use the React features you already know – such as state, lifecycle, context, and refs. They don't fundamentally change how React works, and your knowledge of components, props, and top-down data flow is just as relevant.
Hooks do have a learning curve of their own. If there's something missing in this documentation, raise an issue and we'll try to help.
When you're ready, we'd encourage you to start trying Hooks in new components you write. Make sure everyone on your team is on board with using them and familiar with this documentation. We don't recommend rewriting your existing classes to Hooks unless you planned to rewrite them anyway (e.g. to fix bugs).
You can't use Hooks inside a class component, but you can definitely mix classes and function components with Hooks in a single tree. Whether a component is a class or a function that uses Hooks is an implementation detail of that component. In the longer term, we expect Hooks to be the primary way people write React components.
Our goal is for Hooks to cover all use cases for classes as soon as possible. There are no Hook equivalents to the uncommon getSnapshotBeforeUpdate
, getDerivedStateFromError
and componentDidCatch
lifecycles yet, but we plan to add them soon.
It is an early time for Hooks, and some third-party libraries might not be compatible with Hooks at the moment.
Often, render props and higher-order components render only a single child. We think Hooks are a simpler way to serve this use case. There is still a place for both patterns (for example, a virtual scroller component might have a renderItem
prop, or a visual container component might have its own DOM structure). But in most cases, Hooks will be sufficient and can help reduce nesting in your tree.
connect()
and React Router?You can continue to use the exact same APIs as you always have; they'll continue to work.
React Redux since v7.1.0 supports Hooks API and exposes hooks like useDispatch
or useSelector
.
React Router supports hooks since v5.1.
Other libraries might support hooks in the future too.
Hooks were designed with static typing in mind. Because they're functions, they are easier to type correctly than patterns like higher-order components. The latest Flow and TypeScript React definitions include support for React Hooks.
Importantly, custom Hooks give you the power to constrain React API if you'd like to type them more strictly in some way. React gives you the primitives, but you can combine them in different ways than what we provide out of the box.
From React's point of view, a component using Hooks is just a regular component. If your testing solution doesn't rely on React internals, testing components with Hooks shouldn't be different from how you normally test components.
Note
Testing Recipes include many examples that you can copy and paste.
For example, let's say we have this counter component:
function Example() {
const [count, setCount] = useState(0);
useEffect(() => {
document.title = `You clicked ${count} times`;
});
return (

You clicked {count} times

<button onClick={() => setCount(count + 1)}>
Click me
</button>

);
}
We'll test it using React DOM. To make sure that the behavior matches what happens in the browser, we'll wrap the
code rendering and updating it into ReactTestUtils.act()
calls:
import React from 'react';
import ReactDOM from 'react-dom';
import { act } from 'react-dom/test-utils';
import Counter from './Counter';
let container;
beforeEach(() => {
container = document.createElement('div');
document.body.appendChild(container);
});
afterEach(() => {
document.body.removeChild(container);
container = null;
});
it('can render and update a counter', () => {
// Test first render and effect
act(() => {
ReactDOM.render(<Counter />, container);
});
const button = container.querySelector('button');
const label = container.querySelector('p');
expect(label.textContent).toBe('You clicked 0 times');
expect(document.title).toBe('You clicked 0 times');
// Test second render and effect
act(() => {
button.dispatchEvent(new MouseEvent('click', {bubbles: true}));
});
expect(label.textContent).toBe('You clicked 1 times');
expect(document.title).toBe('You clicked 1 times');
});
The calls to act()
will also flush the effects inside of them.
If you need to test a custom Hook, you can do so by creating a component in your test, and using your Hook from it. Then you can test the component you wrote.
To reduce the boilerplate, we recommend using React Testing Library which is designed to encourage writing tests that use your components as the end users do.
For more information, check out Testing Recipes.
We provide an ESLint plugin that enforces rules of Hooks to avoid bugs. It assumes that any function starting with
"use
" and a capital letter right after it is a Hook. We recognize this heuristic isn't perfect and
there may be some false positives, but without an ecosystem-wide convention there is just no way to make Hooks work
well – and longer names will discourage people from either adopting Hooks or following the convention.
In particular, the rule enforces that:
PascalCase
function (assumed to be a component) or another useSomething
function (assumed to be a custom Hook).There are a few more heuristics, and they might change over time as we fine-tune the rule to balance finding bugs with avoiding false positives.
constructor
: Function components don't need a constructor. You can initialize the state in the useState
call. If computing the initial state is
expensive, you can pass a function to useState
.
getDerivedStateFromProps
: Schedule an update while rendering instead.
shouldComponentUpdate
: See React.memo
below.
render
: This is the function component body itself.
componentDidMount
, componentDidUpdate
, componentWillUnmount
: The useEffect
Hook can express all combinations of these (including less common cases).
getSnapshotBeforeUpdate
, componentDidCatch
and getDerivedStateFromError
: There are no Hook equivalents for these methods yet, but they will be added soon.
Here is a small demo to get you started. To learn more, check out this article about data fetching with Hooks.
Yes! The useRef()
Hook isn't just for DOM refs. The
"ref" object is a generic container whose current
property is mutable and can hold any value, similar
to an instance property on a class.
You can write to it from inside useEffect
:
function Timer() {
const intervalRef = useRef();
useEffect(() => {
const id = setInterval(() => {
// ...
});
intervalRef.current = id;
return () => {
clearInterval(intervalRef.current);
};
});
// ...
}
If we just wanted to set an interval, we wouldn't need the ref (id
could be local to the effect), but
it's useful if we want to clear the interval from an event handler:
// ...
function handleCancelClick() {
clearInterval(intervalRef.current);
}
// ...
Conceptually, you can think of refs as similar to instance variables in a class. Unless you're doing lazy initialization, avoid setting refs during rendering – this can lead to surprising behavior. Instead, typically you want to modify refs in event handlers and effects.
If you're coming from classes, you might be tempted to always call useState()
once and put all state
into a single object. You can do it if you'd like. Here is an example of a component that follows the mouse
movement. We keep its position and size in the local state:
function Box() {
const [state, setState] = useState({ left: 0, top: 0, width: 100, height: 100 });
// ...
}
Now let's say we want to write some logic that changes left
and top
when the user moves
their mouse. Note how we have to merge these fields into the previous state object manually:
// ...
useEffect(() => {
function handleWindowMouseMove(e) {
// Spreading "...state" ensures we don't "lose" width and height
setState(state => ({ ...state, left: e.pageX, top: e.pageY }));
}
// Note: this implementation is a bit simplified
window.addEventListener('mousemove', handleWindowMouseMove);
return () => window.removeEventListener('mousemove', handleWindowMouseMove);
}, []);
// ...
This is because when we update a state variable, we replace its value. This is different from this.setState
in a class, which merges the updated fields into the object.
If you miss automatic merging, you could write a custom useLegacyState
Hook that merges object state updates. However, we recommend to split state into multiple state variables based on which values tend to change together.
For example, we could split our component state into position
and size
objects, and always replace the position
with no need for merging:
function Box() {
const [position, setPosition] = useState({ left: 0, top: 0 });
const [size, setSize] = useState({ width: 100, height: 100 });
useEffect(() => {
function handleWindowMouseMove(e) {
setPosition({ left: e.pageX, top: e.pageY });
}
// ...
Separating independent state variables also has another benefit. It makes it easy to later extract some related logic into a custom Hook, for example:
function Box() {
const position = useWindowPosition();
const [size, setSize] = useState({ width: 100, height: 100 });
// ...
}
function useWindowPosition() {
const [position, setPosition] = useState({ left: 0, top: 0 });
useEffect(() => {
// ...
}, []);
return position;
}
Note how we were able to move the useState
call for the position
state variable and the related effect into a custom Hook without changing their code. If all state was in a single object, extracting it would be more difficult.
Both putting all state in a single useState
call, and having a useState
call per each field can work. Components tend to be most readable when you find a balance between these two extremes, and group related state into a few independent state variables. If the state logic becomes complex, we recommend managing it with a reducer or a custom Hook.
This is a rare use case. If you need it, you can use a mutable ref to manually store a boolean value corresponding to whether you are on the first or a subsequent render, then check that flag in your effect. (If you find yourself doing this often, you could create a custom Hook for it.)
Currently, you can do it manually with a ref:
function Counter() {
const [count, setCount] = useState(0);
const prevCountRef = useRef();
useEffect(() => {
prevCountRef.current = count;
});
const prevCount = prevCountRef.current;
return
Now: {count}, before: {prevCount}
;
}
This might be a bit convoluted but you can extract it into a custom Hook:
function Counter() {
const [count, setCount] = useState(0);
const prevCount = usePrevious(count);
return
Now: {count}, before: {prevCount}
;
}
function usePrevious(value) {
const ref = useRef();
useEffect(() => {
ref.current = value;
});
return ref.current;
}
Note how this would work for props, state, or any other calculated value.
function Counter() {
const [count, setCount] = useState(0);
const calculation = count + 100;
const prevCalculation = usePrevious(calculation);
// ...
It's possible that in the future React will provide a usePrevious
Hook out of the box since it's a
relatively common use case.
See also the recommended pattern for derived state.
Any function inside a component, including event handlers and effects, "sees" the props and state from the render it was created in. For example, consider code like this:
function Example() {
const [count, setCount] = useState(0);
function handleAlertClick() {
setTimeout(() => {
alert('You clicked on: ' + count);
}, 3000);
}
return (

You clicked {count} times

<button onClick={() => setCount(count + 1)}>
Click me
</button>
<button onClick={handleAlertClick}>
Show alert
</button>

);
}
If you first click "Show alert" and then increment the counter, the alert will show the count
variable
at the time you clicked the "Show alert" button. This prevents bugs caused by the code assuming
props and state don't change.
If you intentionally want to read the latest state from some asynchronous callback, you could keep it in a ref, mutate it, and read from it.
Finally, another possible reason you're seeing stale props or state is if you use the "dependency array" optimization
but didn't correctly specify all the dependencies. For example, if an effect specifies []
as the second
argument but reads someProp
inside, it will keep "seeing" the initial value of someProp
.
The solution is to either remove the dependency array, or to fix it. Here's how you can deal with functions, and
here's other common strategies to run
effects less often without incorrectly skipping dependencies.
Note
We provide an
exhaustive-deps
ESLint rule as a part of theeslint-plugin-react-hooks
package. It warns when dependencies are specified incorrectly and suggests a fix.
getDerivedStateFromProps
?While you probably don't need it, in rare
cases that you do (such as implementing a <Transition>
component), you can update the state right
during rendering. React will re-run the component with updated state immediately after exiting the first render so
it wouldn't be expensive.
Here, we store the previous value of the row
prop in a state variable so that we can compare:
function ScrollView({row}) {
const [isScrollingDown, setIsScrollingDown] = useState(false);
const [prevRow, setPrevRow] = useState(null);
if (row !== prevRow) {
// Row changed since last render. Update isScrollingDown.
setIsScrollingDown(prevRow !== null && row > prevRow);
setPrevRow(row);
}
return `Scrolling down: ${isScrollingDown}`;
}
This might look strange at first, but an update during rendering is exactly what getDerivedStateFromProps
has always been like conceptually.
Both useState
and useReducer
Hooks bail out of updates if the next value is the same as the previous one. Mutating state in place and calling setState
will not cause a re-render.
Normally, you shouldn't mutate local state in React. However, as an escape hatch, you can use an incrementing counter to force a re-render even if the state has not changed:
Try to avoid this pattern if possible.
While you shouldn't need this often, you may expose some imperative methods to a parent component with the useImperativeHandle
Hook.
One rudimentary way to measure the position or size of a DOM node is to use a callback ref. React will call that callback whenever the ref gets attached to a different node. Here is a small demo:
function MeasureExample() {
const [height, setHeight] = useState(0);
const measuredRef = useCallback(node => {
if (node !== null) {
setHeight(node.getBoundingClientRect().height);
}
}, []);
return (
<>

Hello, world

The above header is {Math.round(height)}px tall

</>
);
}
We didn't choose useRef
in this example because an object ref doesn't notify us about changes
to the current ref value. Using a callback ref ensures that even if a
child component displays the measured node later (e.g. in response to a click), we still get notified about
it in the parent component and can update the measurements.
Note that we pass []
as a dependency array to useCallback
. This ensures that our ref
callback doesn't change between the re-renders, and so React won't call it unnecessarily.
In this example, the callback ref will be called only when the component mounts and unmounts, since the rendered

component stays present throughout any rerenders. If you want to be notified any time a component resizes, you may want to use ResizeObserver
or a third-party Hook built on it.
If you want, you can extract this logic into a reusable Hook:
function MeasureExample() {
const [rect, ref] = useClientRect();
return (
<>

Hello, world

{rect !== null &&

The above header is {Math.round(rect.height)}px tall

}
</>
);
}
function useClientRect() {
const [rect, setRect] = useState(null);
const ref = useCallback(node => {
if (node !== null) {
setRect(node.getBoundingClientRect());
}
}, []);
return [rect, ref];
}
const [thing, setThing] = useState()
mean?If you're not familiar with this syntax, check out the explanation in the State Hook documentation.
Yes. See conditionally firing an effect. Note that forgetting to handle updates often introduces bugs, which is why this isn't the default behavior.
Generally speaking, no.
function Example({ someProp }) {
function doSomething() {
console.log(someProp);
}
useEffect(() => {
doSomething();
}, []); // 🔴 This is not safe (it calls `doSomething` which uses `someProp`)
}
It's difficult to remember which props or state are used by functions outside of the effect. This is why usually you'll want to declare functions needed by an effect inside of it. Then it's easy to see what values from the component scope that effect depends on:
function Example({ someProp }) {
useEffect(() => {
function doSomething() {
console.log(someProp);
}
doSomething();
}, [someProp]); // ✅ OK (our effect only uses `someProp`)
}
If after that we still don't use any values from the component scope, it's safe to specify []
:
useEffect(() => {
function doSomething() {
console.log('hello');
}
doSomething();
}, []); // ✅ OK in this example because we don't use *any* values from component scope
Depending on your use case, there are a few more options described below.
Note
We provide the
exhaustive-deps
ESLint rule as a part of theeslint-plugin-react-hooks
package. It helps you find components that don't handle updates consistently.
Let's see why this matters.
If you specify a list of dependencies as the last argument to useEffect
, useLayoutEffect
, useMemo
, useCallback
, or useImperativeHandle
, it must include all values that are used inside the callback and participate in the React data flow. That includes props, state, and anything derived from them.
It is only safe to omit a function from the dependency list if nothing in it (or the functions called by it) references props, state, or values derived from them. This example has a bug:
function ProductPage({ productId }) {
const [product, setProduct] = useState(null);
async function fetchProduct() {
const response = await fetch('http://myapi/product/' + productId); // Uses productId prop
const json = await response.json();
setProduct(json);
}
useEffect(() => {
fetchProduct();
}, []); // 🔴 Invalid because `fetchProduct` uses `productId`
// ...
}
The recommended fix is to move that function inside of your effect. That makes it easy to see which props or state your effect uses, and to ensure they're all declared:
function ProductPage({ productId }) {
const [product, setProduct] = useState(null);
useEffect(() => {
// By moving this function inside the effect, we can clearly see the values it uses.
async function fetchProduct() {
const response = await fetch('http://myapi/product/' + productId);
const json = await response.json();
setProduct(json);
}
fetchProduct();
}, [productId]); // ✅ Valid because our effect only uses productId
// ...
}
This also allows you to handle out-of-order responses with a local variable inside the effect:
useEffect(() => {
let ignore = false;
async function fetchProduct() {
const response = await fetch('http://myapi/product/' + productId);
const json = await response.json();
if (!ignore) setProduct(json);
}
fetchProduct();
return () => { ignore = true };
}, [productId]);
We moved the function inside the effect so it doesn't need to be in its dependency list.
Tip
Check out this small demo and this article to learn more about data fetching with Hooks.
If for some reason you can't move a function inside an effect, there are a few more options:
useCallback
Hook. This ensures it doesn't
change on every render unless its own dependencies also change:function ProductPage({ productId }) {
// ✅ Wrap with useCallback to avoid change on every render
const fetchProduct = useCallback(() => {
// ... Does something with productId ...
}, [productId]); // ✅ All useCallback dependencies are specified
return <ProductDetails fetchProduct={fetchProduct} />;
}
function ProductDetails({ fetchProduct }) {
useEffect(() => {
fetchProduct();
}, [fetchProduct]); // ✅ All useEffect dependencies are specified
// ...
}
Note that in the above example we need to keep the function in the dependencies list. This ensures that a change in the productId
prop of ProductPage
automatically triggers a refetch in the ProductDetails
component.
Sometimes, your effect may be using state that changes too often. You might be tempted to omit that state from a list of dependencies, but that usually leads to bugs:
function Counter() {
const [count, setCount] = useState(0);
useEffect(() => {
const id = setInterval(() => {
setCount(count + 1); // This effect depends on the `count` state
}, 1000);
return () => clearInterval(id);
}, []); // 🔴 Bug: `count` is not specified as a dependency
return
{count}
;
}
The empty set of dependencies, []
, means that the effect will only run once when the component mounts,
and not on every re-render. The problem is that inside the setInterval
callback, the value of
count
does not change, because we've created a closure with the value of count
set to
0
as it was when the effect callback ran. Every second, this callback then calls setCount(0 +
1)
, so the count never goes above 1.
Specifying [count]
as a list of dependencies would fix the bug, but would cause the interval to be reset on every change. Effectively, each setInterval
would get one chance to execute before being cleared (similar to a setTimeout
.) That may not be desirable. To fix this, we can use the functional update form of setState
. It lets us specify how the state needs to change without referencing the current state:
function Counter() {
const [count, setCount] = useState(0);
useEffect(() => {
const id = setInterval(() => {
setCount(c => c + 1); // ✅ This doesn't depend on `count` variable outside
}, 1000);
return () => clearInterval(id);
}, []); // ✅ Our effect doesn't use any variables in the component scope
return
{count}
;
}
(The identity of the setCount
function is guaranteed to be stable so it's safe to omit.)
Now, the setInterval
callback executes once a second, but each time the inner call to setCount
can use an up-to-date value for count
(called c
in the callback here.)
In more complex cases (such as if one state depends on another state), try moving the state update logic outside the effect with the useReducer
Hook. This article offers an example of how you can do this. The identity of the dispatch
function from useReducer
is always stable — even if the reducer function is declared inside the component and reads its props.
As a last resort, if you want something like this
in a class, you can use a ref to hold a mutable variable. Then you can write and read to it. For example:
function Example(props) {
// Keep latest props in a ref.
const latestProps = useRef(props);
useEffect(() => {
latestProps.current = props;
});
useEffect(() => {
function tick() {
// Read latest props at any time
console.log(latestProps.current);
}
const id = setInterval(tick, 1000);
return () => clearInterval(id);
}, []); // This effect never re-runs
}
Only do this if you couldn't find a better alternative, as relying on mutation makes components less predictable. If there's a specific pattern that doesn't translate well, file an issue with a runnable example code and we can try to help.
shouldComponentUpdate
?You can wrap a function component with React.memo
to shallowly compare its props:
It's not a Hook because it doesn't compose like Hooks do. React.memo
is equivalent to
PureComponent
, but it only compares props. (You can also add a second argument to specify a custom
comparison function that takes the old and new props. If it returns true, the update is skipped.)
React.memo
doesn't compare state because there is no single state object to compare. But you can make
children pure too, or even optimize individual children
with useMemo
.
The useMemo
Hook lets you cache calculations between multiple renders by "remembering" the previous computation:
This code calls computeExpensiveValue(a, b)
. But if the dependencies [a, b]
haven't changed
since the last value, useMemo
skips calling it a second time and simply reuses the last value it
returned.
Remember that the function passed to useMemo
runs during rendering. Don't do anything there that you
wouldn't normally do while rendering. For example, side effects belong in useEffect
, not
useMemo
.
You may rely on useMemo
as a performance optimization, not as a semantic guarantee. In the future, React may choose to "forget" some previously memoized values and recalculate them on next render, e.g. to free memory for offscreen components. Write your code so that it still works without useMemo
— and then add it to optimize performance. (For rare cases when a value must never be recomputed, you can lazily initialize a ref.)
Conveniently, useMemo
also lets you skip an expensive re-render of a child:
function Parent({ a, b }) {
// Only re-rendered if `a` changes:
const child1 = useMemo(() => <Child1 a={a} />, [a]);
// Only re-rendered if `b` changes:
const child2 = useMemo(() => <Child2 b={b} />, [b]);
return (
<>
{child1}
{child2}
</>
)
}
Note that this approach won't work in a loop because Hook calls can't be placed
inside loops. But you can extract a separate component for the list item, and call useMemo
there.
useMemo
lets you memoize an expensive calculation if the
dependencies are the same. However, it only serves as a hint, and doesn't guarantee the computation won't
re-run. But sometimes you need to be sure an object is only created once.
The first common use case is when creating the initial state is expensive:
function Table(props) {
// ⚠ createRows() is called on every render
const [rows, setRows] = useState(createRows(props.count));
// ...
}
To avoid re-creating the ignored initial state, we can pass a function to useState
:
function Table(props) {
// ✅ createRows() is only called once
const [rows, setRows] = useState(() => createRows(props.count));
// ...
}
React will only call this function during the first render. See the useState
API reference.
You might also occasionally want to avoid re-creating the useRef()
initial value. For example, maybe you want to ensure some imperative class instance only gets created once:
function Image(props) {
// ⚠ IntersectionObserver is created on every render
const ref = useRef(new IntersectionObserver(onIntersect));
// ...
}
useRef
does not accept a special function overload like useState
. Instead, you can write your own function that creates and sets it lazily:
function Image(props) {
const ref = useRef(null);
// ✅ IntersectionObserver is created lazily once
function getObserver() {
if (ref.current === null) {
ref.current = new IntersectionObserver(onIntersect);
}
return ref.current;
}
// When you need it, call getObserver()
// ...
}
This avoids creating an expensive object until it's truly needed for the first time. If you use Flow or TypeScript,
you can also give getObserver()
a non-nullable type for convenience.
No. In modern browsers, the raw performance of closures compared to classes doesn't differ significantly except in extreme scenarios.
In addition, consider that the design of Hooks is more efficient in a couple ways:
Hooks avoid a lot of the overhead that classes require, like the cost of creating class instances and binding event handlers in the constructor.
Idiomatic code using Hooks doesn't need the deep component tree nesting that is prevalent in codebases that use higher-order components, render props, and context. With smaller component trees, React has less work to do.
Traditionally, performance concerns around inline functions in React have been related to how passing new callbacks on each render breaks shouldComponentUpdate
optimizations in child components. Hooks approach this problem from three sides.
The useCallback
Hook lets you keep the same callback reference between re-renders so that shouldComponentUpdate
continues to work:
// Will not change unless `a` or `b` changes
const memoizedCallback = useCallback(() => {
doSomething(a, b);
}, [a, b]);
The useMemo
Hook makes it easier to control when individual children update, reducing the need for pure components.
Finally, the useReducer
Hook reduces the need to pass callbacks deeply, as explained below.
We've found that most people don't enjoy manually passing callbacks through every level of a component tree. Even though it is more explicit, it can feel like a lot of "plumbing".
In large component trees, an alternative we recommend is to pass down a dispatch
function from useReducer
via context:
const TodosDispatch = React.createContext(null);
function TodosApp() {
// Note: `dispatch` won't change between re-renders
const [todos, dispatch] = useReducer(todosReducer);
return (
<TodosDispatch.Provider value={dispatch}>
<DeepTree todos={todos} />
</TodosDispatch.Provider>
);
}
Any child in the tree inside TodosApp
can use the dispatch
function to pass actions up to TodosApp
:
function DeepChild(props) {
// If we want to perform an action, we can get dispatch from context.
const dispatch = useContext(TodosDispatch);
function handleClick() {
dispatch({ type: 'add', text: 'hello' });
}
return (
<button onClick={handleClick}>Add todo</button>
);
}
This is both more convenient from the maintenance perspective (no need to keep forwarding callbacks), and avoids the callback problem altogether. Passing dispatch
down like this is the recommended pattern for deep updates.
Note that you can still choose whether to pass the application state down as props (more explicit) or as
context (more convenient for very deep updates). If you use context to pass down the state too, use two different
context types – the dispatch
context never changes, so components that read it don't need to rerender
unless they also need the application state.
useCallback
?Note
We recommend to pass
dispatch
down in context rather than individual callbacks in props. The approach below is only mentioned here for completeness and as an escape hatch.Also note that this pattern might cause problems in the concurrent mode. We plan to provide more ergonomic alternatives in the future, but the safest solution right now is to always invalidate the callback if some value it depends on changes.
In some rare cases you might need to memoize a callback with useCallback
but the memoization doesn't work very
well because the inner function has to be re-created too often. If the function you're memoizing is an event handler
and isn't used during rendering, you can use ref as an
instance variable, and save the last committed value into it manually:
function Form() {
const [text, updateText] = useState('');
const textRef = useRef();
useEffect(() => {
textRef.current = text; // Write it to the ref
});
const handleSubmit = useCallback(() => {
const currentText = textRef.current; // Read it from the ref
alert(currentText);
}, [textRef]); // Don't recreate handleSubmit like [text] would do
return (
<>
<input value={text} onChange={e => updateText(e.target.value)} />
<ExpensiveTree onSubmit={handleSubmit} />
</>
);
}
This is a rather convoluted pattern but it shows that you can do this escape hatch optimization if you need it. It's more bearable if you extract it to a custom Hook:
function Form() {
const [text, updateText] = useState('');
// Will be memoized even if `text` changes:
const handleSubmit = useEventCallback(() => {
alert(text);
}, [text]);
return (
<>
<input value={text} onChange={e => updateText(e.target.value)} />
<ExpensiveTree onSubmit={handleSubmit} />
</>
);
}
function useEventCallback(fn, dependencies) {
const ref = useRef(() => {
throw new Error('Cannot call an event handler while rendering.');
});
useEffect(() => {
ref.current = fn;
}, [fn, ...dependencies]);
return useCallback(() => {
const fn = ref.current;
return fn();
}, [ref]);
}
In either case, we don't recommend this pattern and only show it here for completeness. Instead, it is preferable to avoid passing callbacks deep down.
React keeps track of the currently rendering component. Thanks to the Rules of Hooks, we know that Hooks are only called from React components (or custom Hooks – which are also only called from React components).
There is an internal list of "memory cells" associated with each component. They're just JavaScript objects where we
can put some data. When you call a Hook like useState()
, it reads the current cell (or initializes it
during the first render), and then moves the pointer to the next one. This is how multiple useState()
calls each get independent local state.
Hooks synthesize ideas from several different sources:
adopt
keyword proposal
as a sugar syntax for render props.Sebastian Markbåge came up with the original design for Hooks, later refined by Andrew Clark, Sophie Alpert, Dominic Gannaway, and other members of the React team.


|
Contextualization Tool
1.0
|
This is the controller base class. More...
#include <contextualizationcontroller.h>
Public Types | |
| enum | ModelError { OkModel = 0, NoImage, ImageNotExist, NoStrings } |
| The ModelError enum. More... | |
| enum | CodeError { NoError = 0, NullPointer, StringAlreadyExists, NoImportFile, ImportFileFormat, FileNotExists, NoRemoteHost, NoValidIp, SshpassError = 254, SshError = 255, WriteFile = -1 } |
| The CodeError enum. More... | |
| enum | MatchType { ByID = 0, ByValue, ByApproximateValue } |
| The MatchType enum. More... | |
Signals | |
| void | stringsListChanged () |
| The signal is emitted when a new string is added to the model or a string is removed from the model. | |
| void | imageChanged () |
| The signal is emitted when a image is setted on the model. | |
Public Member Functions | |
| ContextualizationController (QObject *parent=Q_NULLPTR) | |
| Constructs a controller setting his member variables. More... | |
| ~ContextualizationController () | |
| Destroys the controller. | |
Protected Slots | |
| virtual void | add (QString newString, int matchType)=0 |
| Tries to add a new string into the model. More... | |
| virtual void | remove (QString stringId)=0 |
| Tries to remove the string in the model with the identifier receiven by parameter. More... | |
| virtual void | clear ()=0 |
| Removes all strings in the model. | |
| virtual void | capture (bool detectStringsOnLoad)=0 |
| Make a screen capture. More... | |
| virtual void | load (bool detectStringsOnLoad)=0 |
| Loads an image in the model. More... | |
| virtual void | detect ()=0 |
| Detects strings in the current image of the model and tries to add them into. | |
| virtual void | send ()=0 |
| Tries to send the active contextualization to a remote host. | |
| virtual void | cancel ()=0 |
| Ensures that the user wants to cancel the project and closes the application in an orderly manner. | |
| virtual bool | save ()=0 |
| Saves current project. | |
| virtual bool | saveAs ()=0 |
| Opens a dialog and saves current project in the path specied for the user. | |
| virtual void | open ()=0 |
| Opens a project saves on disk. | |
| virtual void | refresh () |
| Refreshes all components on current contextualization (DoneFpFile, Image, Strings...). | |
| virtual void | newProject ()=0 |
| Creates a new empty project. | |
| virtual void | processFiles ()=0 |
| Process VALIDATED strings in fp files and storage it in database. | |
Protected Member Functions | |
| CodeError | importProjectFromJsonFile (const QString &path) |
| Imports projet from json file. More... | |
| bool | exportToJsonFile (const QString &path) |
| Exports project to json file. More... | |
| ContextualizationController::ModelError | validateModel () |
| Checks the actual state of the model. More... | |
| QString | generateContextualization () |
| Generates a packet with the contextualization data. More... | |
| CodeError | sendContextualization (QString const &path, QString user, QString password) |
| Process that is responsible for sending the file received by parameter. More... | |
| QList< FirmwareString * > | detectStringsOnImage (QString image) |
| Extracts the strings contained in the image set in the model. More... | |
| QList< FirmwareString * > | fastDetectStringsOnImage (QString image) |
| Extracts the strings contained in the image set in the model. More... | |
| QList< FirmwareString * > | processExtractedStrings (QStringList strings) |
| Processes received strings. More... | |
| QList< FirmwareString * > | findString (const QString &text, const MatchType matchType=ByID) |
| Find the text received by parameter in fp file. More... | |
| bool | isValidState (QString &state) |
| Checks the parameter state is a valid state. More... | |
| CodeError | addString (FirmwareString *fwString) |
| Adds a new string on the model. More... | |
| int | addStrings (const QList< FirmwareString *> &strings) |
| Adds new strings on the model. More... | |
| bool | removeString (QString stringId) |
| Removes the string in the model with the id received by parameter. More... | |
| bool | removeString (int row) |
| Removes the string in the model row number received by parameter. More... | |
| bool | removeAllStrings () |
| Remove all strings in the model. | |
| bool | selectString (const QString id, bool state) |
| Select the string with the identifier received by parameter. More... | |
| void | clearImage () |
| Sets a no image in the model. | |
| virtual QString | takeCaptureArea ()=0 |
| Starts a process that allow user capture an area of the screen. More... | |
| bool | setImage (const QString &image) |
| setImage More... | |
| bool | isFwStringAlreadyExists (FirmwareString &fwString) |
| Checks that the FirmwareString is not already in the model. More... | |
| int | eraseExistStrings (QList< FirmwareString *> *strings) |
| Erases, from the string list received by parameter, strings that already are in the model. More... | |
| QString | getImageOfModel () |
| Returns the actual image of the model. More... | |
| QList< QObject * > | getTableModel () |
| Returns the model to be used by ListView, TableView or similar QML object. More... | |
| QString | getDateTime (QString format="yyyy_MM_dd_hh_mm_ss") |
| Returns a current date time with the format received by parameter. More... | |
| void | loadConfig () |
| Reads the configuration file, if it exists, and sets the class members to the values in the file. | |
| void | saveConfig () |
| Saves config in configuration file to can be recuperated in the next run of contetualization tool. | |
| virtual int | generateDoneFpFile () |
| Creates a copy of englishFp file in /tmp with only firmware strings with DONE status. More... | |
| int | filterStringsByState (QList< FirmwareString *> *list, const QString &state) |
| Filters a list of firmware strings. Remove from the list all strings that have not the same state as the one received by parameter. More... | |
| QStringList | splitImage (const QString &image, int chunkWidth, int chunkHeight, bool *someError=Q_NULLPTR) |
| Splits an image in one or more chunks depending of chunk size received by argument. More... | |
| QString | getParameterFromConfigFile (const QString parameter) |
| Returns a value of a parameter in config file. More... | |
| bool | setParameterInConfigFile (const QString parameter, const QString value) |
| Sets a parameter in configuration file. More... | |
| bool | proccessAndStorage () |
| Reads all fp language files that there was congigurated in database. More... | |
Protected Attributes | |
| ContextualizationModel * | model_ |
| Pointer to the contextualization model. | |
| QString | englishFpFile_ |
| Original file where be all firmware strings. | |
| QString | username_ |
| Username who run the app. | |
| QStringList | validStates_ |
| Valid states of firmware strings. | |
| QString | remoteHost_ |
| Host where the contextualization will be sent. | |
| bool | onlyDoneStrings_ |
| If is true, only string with state DONE will be found. | |
| bool | caseSensitive_ |
| Indicates if searches will be case sensitive or not. | |
Static Protected Attributes | |
| static const int | CHUNK_WIDTH = 300 |
| Width of each chunk when a image is splitted. | |
| static const int | CHUNK_HEIGHT = 150 |
| Height of each chunk when a image is splitted. | |
| static const QString | IMAGES_FOLDER = QDir("../storage/images").absolutePath() + '/' |
| Directory where will save project images. | |
| static const QString | PROJECTS_FOLDER = QDir("../storage/projects").absolutePath() + '/' |
| Directory where will save projects. | |
| static const QString | CONFIG_FOLDER = QDir("../config").absolutePath() + '/' |
| Directory where will save configurations. | |
| static const QString | DONE_FP_FILE = Utils::getTmpDirectory() + "/doneFpFile.fp" |
| File path where the firmware strings will be found. | |
This is the controller base class.
Definition at line 35 of file contextualizationcontroller.h.
The CodeError enum.
Contains all general error that can happen during any process of the controller.
WARNING!! Don't change the default values of enum. Binary program returns this number values.
Definition at line 60 of file contextualizationcontroller.h.
The MatchType enum.
Contains all match types by which you can search a string.
Definition at line 79 of file contextualizationcontroller.h.
The ModelError enum.
Contains all errors that can has the model.
Definition at line 46 of file contextualizationcontroller.h.
| ContextualizationController::ContextualizationController | ( | QObject * | parent = Q_NULLPTR | ) |
Constructs a controller setting his member variables.
Reads the configuration file and sets the values. If controller can't read the configuration file or hasn't a correct formart, controller sets a default values on member vaiables.
| parent |
Definition at line 20 of file contextualizationcontroller.cpp.
|
protectedpure virtualslot |
Tries to add a new string into the model.
This slot interacts with the user through the screen showing messages.
| newString | New strign to add. |
| matchType | Mode in which the string is to be searched for |
|
protected |
Adds a new string on the model.
The FirmwareString is added on the model of the tool if is not wrong or is already in the model. If the parameter fwString is null, nothing is done. Return true if FirmwareString is added successfully and false if not.
| fwString | The string to add on the model. |
Definition at line 377 of file contextualizationcontroller.cpp.
|
protected |
Adds new strings on the model.
Append all strings in the list that are not wrong or are already in the model. Returns the number of strings that have been added to the model.
| strings | Strings list to be added in the model. |
Definition at line 395 of file contextualizationcontroller.cpp.
|
protectedpure virtualslot |
Make a screen capture.
| detectStringsOnLoad | Flag to know if is neccesary detect strings on capture. |
|
protected |
Extracts the strings contained in the image set in the model.
Returns a QList of FirmwareString containing all of strings extracted converted in FirmwareString objects if are in the fp file. Each firmware string on QList is a FirmwareString Object extracted from the image.
| image | Path of image where strings will be detected. |
Definition at line 237 of file contextualizationcontroller.cpp.
|
protected |
Erases, from the string list received by parameter, strings that already are in the model.
Return the number of strings erased from the strings list.
| strings | List of strings to be processed. |
Definition at line 477 of file contextualizationcontroller.cpp.
|
protected |
Exports project to json file.
Codes the actual model to JSON format and sabe data in the file received by parameter. Returns true if project was exported succesfully, otherwise, returns false.
| path | File path where be saved data project. |
Definition at line 94 of file contextualizationcontroller.cpp.
|
protected |
Extracts the strings contained in the image set in the model.
Returns a QList of FirmwareString containing all of strings extracted converted in FirmwareString objects if are in the fp file. Each firmware string on QList is a FirmwareString Object extracted from the image. This is that #sendContextualization(QString image), this is more fast but less precise.
| image | Path of image where strings will be detected. |
Definition at line 292 of file contextualizationcontroller.cpp.
|
protected |
Filters a list of firmware strings. Remove from the list all strings that have not the same state as the one received by parameter.
Returns number of strings removed.
| list | Firmware strings list to be filtered. |
| state | State that will be filtered. |
Definition at line 598 of file contextualizationcontroller.cpp.
|
protected |
Find the text received by parameter in fp file.
There are diferrent find types. The second parameter indicates the member class of FirmwareString that the text received by parameter have to be compared. Returns a firmware string list containing all strings that be found.
| text | Text to be found. |
| matchType | Type of find to be done. |
If only have to find strings with DONE state, tries to do search in DONE_FP_FIlE, else use englishFpFile_.
Tries to use DONE_FP_FILE if is possible to do searches faster.
Definition at line 315 of file contextualizationcontroller.cpp.
|
protected |
Generates a packet with the contextualization data.
After verifying that the model contains no errors, it packages all the information in a zip file with the data to be sent. If there aren't errors, returns the path of the zip file, else returns an empty QString.
Definition at line 142 of file contextualizationcontroller.cpp.
|
protectedvirtual |
Creates a copy of englishFp file in /tmp with only firmware strings with DONE status.
If copy was created succesfully returns 0, otherwise returns the code error.
Reimplemented in WindowsConsoleController, and WindowsGuiController.
Definition at line 581 of file contextualizationcontroller.cpp.
|
protected |
Returns a current date time with the format received by parameter.
| format | Format to returns the date time. |
|
protected |
Returns the actual image of the model.
Definition at line 496 of file contextualizationcontroller.cpp.
|
protected |
Returns a value of a parameter in config file.
| parameter | Parameter name to be returned. |
Definition at line 669 of file contextualizationcontroller.cpp.
|
protected |
Returns the model to be used by ListView, TableView or similar QML object.
Definition at line 501 of file contextualizationcontroller.cpp.
|
protected |
Imports projet from json file.
Decodes JSON format of the file received by parameter and try to convert it in a ContextualizationModel object. Returns a #Error code.
| path | File path where be readed data project. |
Definition at line 66 of file contextualizationcontroller.cpp.
|
protected |
Checks that the FirmwareString is not already in the model.
If the FirmwareString has no empty id, checks that there is not any FirmwareString with the same id. If the FirmwareString id is empty, then checks that there is not any FirmwareString with the same value.
| fwString |
Definition at line 456 of file contextualizationcontroller.cpp.
|
protected |
Checks the parameter state is a valid state.
Valid states are stored in a private QStringList validstates. Return true if the state is a valid state and return false if not.
| state | QString state to check. |
Definition at line 369 of file contextualizationcontroller.cpp.
|
protectedpure virtualslot |
Loads an image in the model.
| detectStringsOnLoad | Flag to know if is neccesary detect strings on capture. |
|
protected |
Reads all fp language files that there was congigurated in database.
Process all strings and storage in database strings with state VALIDADED.
Definition at line 738 of file contextualizationcontroller.cpp.
|
protected |
Processes received strings.
Tries to find the strings value in fp file and strings that are in the file are returned converted in FirmwareString object. Returns QList of FirmwareString containing strings found in fp file..
| strings | String value to be processed. |
Definition at line 300 of file contextualizationcontroller.cpp.
|
protectedpure virtualslot |
Tries to remove the string in the model with the identifier receiven by parameter.
| stringId | Idetifier for string to remove. |
|
protected |
Removes the string in the model with the id received by parameter.
Returns true if the string was removed succesfully or false if the string is not in the model.
| row | Row number of the string to be removed. |
Definition at line 408 of file contextualizationcontroller.cpp.
|
protected |
Removes the string in the model row number received by parameter.
Returns true if the string was removed succesfully or false if the string is not in the model.
| row | Row number of the string to be removed. |
Definition at line 413 of file contextualizationcontroller.cpp.
|
protected |
Select the string with the identifier received by parameter.
Returns true if string was selected succesfully, otherwise, returns false.
| id | Identifier of strogn to select. |
| state | Selection state |
Definition at line 429 of file contextualizationcontroller.cpp.
|
protected |
Process that is responsible for sending the file received by parameter.
Using credentials received por parameters, sends by SFTP protocol the file. Return the code error returned by SFTP process or a CodeError If there were any errors during the process.
| path | File path to send. |
| user | Username credential. |
| password | Password credential. |
Definition at line 182 of file contextualizationcontroller.cpp.
|
protected |
setImage
| image | Path of image that will be set to the model. Returns true if the image is setted succesfully or returns false if the image can not setted. |
Definition at line 439 of file contextualizationcontroller.cpp.
|
protected |
Sets a parameter in configuration file.
Returns true if parameter is saved succesfully, otherwhise returns false.
| parameter | Parameter name |
| value | Parameter value |
Definition at line 704 of file contextualizationcontroller.cpp.
|
protected |
Splits an image in one or more chunks depending of chunk size received by argument.
If the width of the image is not divisible by the chunkWidth respectively, the last chunk of the row will narrower than all the others. Works the same way for height of image and chunkHeight. Chunks are saved on disk.
Returns a list with path of image chunks. Only returns chunks that have been saved on disk succesfully.
If ok is nonnull pointer, ok will be set to true if all chunks are saved on disk succesfully.
| image | Image to split. |
| chunkWidth | Width of each part of image. |
| chunkHeight | Height of each part of image. |
Definition at line 617 of file contextualizationcontroller.cpp.
|
protectedpure virtual |
Starts a process that allow user capture an area of the screen.
The user have to select the area to capture with the mouse. Return the path where the capture is stored or an empty QString if an error ocurred.
Implemented in WindowsConsoleController, LinuxConsoleController, LinuxGuiController, and WindowsGuiController.
|
protected |
Checks the actual state of the model.
Returns the error code of the actual state of the model.
Definition at line 119 of file contextualizationcontroller.cpp.


Welcome to the game! The game is simple. Repeat back the pattern by pressing buttons. RULES: 30 seconds per guess, 3 strikes! Pay attention...it gets fast!


modules.JWComorbid package¶
Subpackages¶
Submodules¶
modules.JWComorbid.JWComorbid module¶
This is a modified copy of /src/workspace.py, meant to service the loading of modules within this collection of modules.
Any required documentation can be found in /src/workspace.py.
Module contents¶
Hello! This is the JWComorbid collection of modules.
They are created to replicate a study titled “Comorbid substance use disorders with other Axis I and II mental disorders among treatment-seeking Asian Americans, Native Hawaiians/Pacific Islanders, and mixed-race people”. Here is the paper.
JWComorbid consists of 7 modules, structured as follows:
Each module falls into one of these layers:
Fetch and Preprocess Layer¶
preprocessDB.py
Runs SQL commands to work with database tables to generate a master table which has all needed information. Table will then be fetched and saved as data/intermediate/db.pickle .
Data Extraction Layer¶
getUsefulInfo.py
Retrieves useful/commonly used information such as metrics and category lists from the master table. Loads the master table from db.pickle and stores useful information in a dict called “data”. Data will then be saved in data/intermediate/data.pickle .
Results Layer¶
table1.py
Generates Table 1, which is the overview of characteristics with respect to each race.
figure1.py
Generates Figure 1, which is a bar plot of the percentage of patients of a certain race presenting with a certain mental diagnosis compared to the total number of patients in said race.
table2.py
Generates Table 2, which displays substance use disorder (SUD) diagnoses among patients aged 12 or older, by age group.
table3.py
Generates Table 3, which displays the adjusted odds ratios of substance use disorder (SUD) diagnoses among people aged 12 years or older.
table4.py
Generates Table 4, which displays the adjusted odds ratios of having a comorbid SUD diagnosis in relation to other psychiatric diagnoses among people aged 12 years or older.
Before you Begin¶
If you get this error:
Attribute Qt::AA_EnableHighDpiScaling must be set before QCoreApplication is created.
from figure1.py, don’t sweat it. I don’t think it affects the plotting of Figure 1, only limits the DPI. I’m not too sure how to fix it but it happens for some environments.
Specifications:¶
You should need to configure your own db.json for this to work.  The filter format is the same for setting and race.
Filters for SUD and Diagnoses are here and here respecitvely.
Do specify a workspace schema and final table name.
"schemaName" : "<your schema>",
"tableName" : "<your table>"
Its gonna create temp tables
<your schema>.temp1
<your schema>.temp2
<your schema>.temp3
<your schema>.temp4
So ensure that your schema does not have any of these tables already, else it will conflict.
Specifications for modules.json
¶
Make sure that the execute
statement within the modules file for each module you want activated is set to True.
For example:
"moduleName" : "preProcessDB",
"path" : "modules/JWComorbid/preProcessDB/preProcessDB.py",
"execute" : true,
"description": "",
"owner" : ""
There are a bunch like the above; one for every module in JWComorbid. Set those to “true” too.
Results¶
The tables should be printed in stdout. In addition, Figure 1 should be saved to /manuscript/figure1.png


This project includes three files. A static www/index.html
(rewritten with Routes for access from the root) file as the homepage, a static www/style.css
file for styling, and a api/date.js
serverless function that returns the date on invocation. See api/date.js
for the Date API with Node.js.
Loading date...


{% extends "../base.html" %} {% load static %} {% block title %}Basket Summary{%endblock %} {% block content %}
Please select your delivery address
{{address.full_name}}
{{address.address_line}}
{{address.address_line2}}
{{address.town_city}}
{{address.postcode}}
Phone number: {{address.phone}}


Futuristic Car Dashboard
This is a first attempt to create a realistic car dashboard
For better experience turn your sound on and put headphones (be carefull, it could be loud)
GitHub !
Not mobile friendly yet
Best experience with Chrome
Click here to continue
Made by Igor Rykhlevskyi (arfeus)
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17 °C
Credits


How to Buy Vechain – A Beginner’s Guide
April 25th, 2018 by Diego Riitano
Besides Bitcoin and Ethereum, there are many unique altcoins and tokens.
They all serve different purposes and have different functions.
In fact, well over a thousand different cryptocurrencies exist.
But with so many on the market, how can you decided which coins are worth owning?
The best way to learn about coins is by researching them, and reading guides and reviews.
In this guide, you will learn about Vechain, and how to buy Vechain on Binance. You’ll also learn how to safely store your Vechain.
How to Buy Vechain with USD
You cannot directly purchase Vechain with USD. In order to buy, you must first purchase another cryptocurrency with USD, then trade it for Vechain.
Before Buying Vechain: Purchase Bitcoin or Ethereum
Before buying Vechain, you have to buy Bitcoin or Ethereum first. To learn how to buy Bitcoin, follow our guide.
Step 1: Send to an Exchange
After buying Bitcoin, you need to send it to an exchange that trades Vechain. In this guide, we will be using Binance.
After signing into Binance, look in the upper-right corner, hover over the Funds tab, then select Balances.
Scroll down to Bitcoin, then click deposit.
Copy your Bitcoin deposit address. Be sure to use your own address, and not the one shown in the picture.
Sign into Coinbase, then select the accounts tab.
Under your Bitcoin wallet, select the send button.
Paste your Binance wallet address in the recipient box. Enter the amount of Bitcoin in dollars that you would like to send, then click continue.
You will then be prompted to confirm your transaction. You will also be shown the miner fee for your transaction. This fee varies, and does not go to Coinbase, but instead to the miners who verify blocks on Bitcoin’s blockchain. If you have 2-factor authentication enabled, you will also be asked to enter your 6-digit verification code.
After you send your Bitcoin, it may take some time to arrive. Don’t panic immediately if your Bitcoin has left Coinbase but not arrived at Binance. Depending on the Bitcoin network and websites themselves, transfers may take some time to process.
Step 2: Buy Vechain
After your Bitcoin transfers to Binance, then you can purchase your Vechain.
Back on the Balances tab, scroll down to Vechain and click Trade. In the dropdown menu, select VEN/BTC.
In the middle of the screen, select the Market tab. In this tutorial, we will be buying Vechain with a market order. There are other types of orders, but market orders are the easiest to use.
Enter the amount of Vechain you would like to buy. The price you pay per Vechain will be shown at the top right in dollars and in Bitcoin. After entering the amount you would like to buy, click Buy VEN. Binance will take a .1% fee from your transaction.
After buying, you should be able to view your Vechain in the balances tab.
Step 3: Safely Store Your Vechain!
Once you buy, your Vechain is stored on the exchange wallet. Although your Vechain has been received, it is not totally secure. Every wallet has a private key, or a sequence of numbers or letters. Each private key is unique to one wallet, and allows access to the contents of that wallet.
If you have Vechain on an exchange wallet, they will control the private keys. This leaves your tokens more vulnerable to manipulation by others. If the exchange is hacked, the hackers will use your private keys to access your Vechain.
 While these events may seem unlikely, they have occurred before multiple times.
The best way to store cryptocurrencies is usually a hardware wallet, as they combine security and accessibility. Some cryptocurrencies are unable to be stored in hardware wallets, but Vechain is hardware wallet compatible.
Vechain is an ERC20 token, a type of token based on the Ethereum blockchain. All ERC20 tokens can be stored on hardware wallets, but in order to do so, you must configure your hardware wallet with an ERC20 software wallet. One of the most popular ERC20 software wallets is MyEtherWallet (MEW).
For more information on cryptocurrency wallets, you can check out our guide.
Introduction to Vechain
Vechain is a blockchain product management platform. Vechain uses the blockchain in order to collect, manage, and share product data for better supply chain management.
While you may not realize it, every product you purchase goes through many steps before finally reaching a consumer. At a grocery store for example, each piece of produce is grown somewhere hundreds or thousands of miles away.
After being harvested, the produce then must be shipped through multiple intermediaries. It is sent to a distributor, then from there sent to your grocery store. After arriving, the produce is stocked, where it stays until you finally purchase it.
Throughout this whole process, the produce must be kept at the correct temperature and humidity to ensure it does not spoil. All these steps must be considered in supply chain management.
Vechain attempts to simplify the entire process by incorporating blockchain technology into the supply chain.
Each product on the Vechain network will receive a smart chip. This chip will store and record location data at every step along the way. Additionally, chips can be fitted with other data sensors, such as temperature sensors, to ensure items like produce are not spoiling.
Incorporating blockchain and smart chips into supply chain will save producers and consumers time and money.
Should you Buy Vechain?
If you’ve made it this far, you now know how to buy bitcoin, use it to buy Vechain, and safely store your Vechain in a wallet.
Before buying though, you should always do your research to make sure the coin you’re purchasing is sound.
Things to look for include how the coin works, use cases, partners, and the team behind the coin.
The sections below are dedicated to giving you the information you need to decide whether Vechain is a coin you’d like to purchase.
The Team
Vechain is led by CEO Sunny Lu. He served as an IT executive in Fortune 500 companies for more than 13 years and is the former CIO of Louis Vuitton China.
Jay Zhang serves as Vechain CFO, and he has been a senior manager at PwC and Deloitte for more than 14 years.
The position of COO is held by Kevin Feng, who has more than 12 years of cybersecurity experience.
Jianliang Gu is the Vechain CTO. He holds a master’s degree in Cybernetics and has more than 16 years of experience in systems development and management.
In total, the core team consists of 10 members.
Vechain is also overseen by members of the Board of Steering Committee, the governing body of the Vechain Foundation.
Additionally, Vechain is backed by Draper Dragon, Fenbushi Capital, and Breyer Capital.
Community
The Vechain community is large and active, and the team delivers news regularly. The Vechain blog posts updates quite often. The Vechain Twitter page has over 90,000 followers. Vechain’s subreddit has more than 46,000 subscribers.
Use Cases
Vechain has many valuable applications in the supply chain management ecosystem. As previously mentioned, Vechain smart chips can be added to products that must remain cold to ensure they remain unspoiled and fit for consumption. However, this is not the only use of the Vechain blockchain.
In the automotive industry, Vechain has many uses as well. Cars can be fitted with data sensors that create a digital passport of that vehicle. This can track data such as repair history, insurance, and registration.
This data is in the hands of the vehicle owner, but it can easily be shared with insurance companies in the case of an accident or other event.
Additionally, these digital passports can provide much more transparency to used car buying. A prospective buyer will be able to see the vehicle’s repair history and can decide whether or not the condition of the vehicle is fit for them.
One use case where transparency is imperative is the medical and healthcare industry. Implants and other medical devices must be held to strict standards to ensure quality and safety for the patient. The addition of Vechain smart chips will make quality assurance much easier.
Smart chips in implants can also be used to monitor biometrics. If the patient desires, they can share this data with their doctor to receive real time health monitoring.
Another major area where Vechain excels is luxury goods and fashion. CEO Sunny Lu is was formerly the CIO of Louis Vuitton China, so he has plenty of experience in this department.
Embedding a chip in luxury goods will help to prevent illegal overstock trading. When a customer plans to buy a luxury item, they can pull out their phone and scan the item.
This simple action will reveal whether an item is authentic or counterfeit. The global counterfeit goods market is 461 billion dollars, but the use of Vechain could eliminate that problem entirely.
Partnerships
Vechain already has many major partnerships.
In 2017, they partnered with Direct Imported Goods (DIG), China’s largest fine wine importer. Vechain placed their smart chips on the top of the bottle. When the bottle is opened, the chip is destroyed, which prevents counterfeiters from filling the bottle with another wine.
The chips can also be used to view the life cycle of the wine, including regulators, distributors, and retailers.
Vechain has also partnered with DNV GL. DNV GL is a $20 billion company that specializes in risk management as well as providing services to maritime, oil and gas, and energy companies. This partnership could provide Vechain with a massive list of clients.
Vechain has also joined the PwC incubator program which will give them access to additional clients. PwC, or PricewaterhouseCoopers is responsible for auditing many of the world’s largest companies, including over 80% of the Fortune Global 500.
The Chinese government selected Vechain to become the blockchain technology partner of the government of Gui’an. Gui’an New Area is a rapidly developing economic zone that has drawn Microsoft, Hyundai, Qualcomm, Alibaba, and others.
Vechain also has more than a dozen other partnerships.
Conclusion
Vechain intends to be the blockchain platform for supply chain management. This large and varied field has many sectors, such as agriculture, luxury goods, and automobiles.
Vechain already has real world usage, and boasts an impressive list of partnerships that only grows by the day. Vechain’s biggest obstacle may be other competitors, such as Waltonchain and Modum.
If Vechain can outpace its competitors, and continue to secure partnerships and real world use cases, it has the potential to become ubiquitious within the supply chain management industry.
NOTE: We are not financial advisors and none of this information should be taken as investment advice. We take no responsibility for any losses incurred through cryptocurrency trading.


| 456 | (A) | with many precyous perle and fułł proude stonys |
| 460 | (C) | wytħ many a pressious perle . & pured stones |
| 457 | (D) | wiþ meny precious perle and purid stonys |
| 456 | (E) | with many precyous peerles . & oþer ryche stones |
| (Ex) | ---- no text ---- | |
| 473 | (L) | with many preciose perle & pured stones |
| 436 | (P) | with many grete pilar & precious stones |
| 454 | (U) | Witħ many precious perles / and eke riche stones |
| (V) | ---- no text ---- |


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Julius Shulman
Julius Shulman's photo of Case Study House #22 includes people. Design by architect Pierre Koenig. Image © J. Paul Getty Trust, Julius Shulman Photography Archive
Kaufman Residence, Designed by Richard Neutra. 1947.
Chuey House, 1958


UNZIPPED A film review by James Berardinelli Copyright 1995 James Berardinelli
RATING (0 TO 10): 7.4
United States, 1995 U.S. Availability: limited release 8/95 Running Length: 1:13 MPAA Classification: R (Profanity) Theatrical Aspect Ratio: 1.66:1
Featuring: Isaac Mizrahi, Nina Santisi, Sarah Mizrahi, Cindy Crawford, Kate Moss, Christy Turlington, Naomi Campbell, Linda Evangelista, Eartha Kitt, Sandra Bernhard, and others Director: Douglas Keeve Producer: Michael Alden Cinematography: Ellen Kuras U.S. Distributor: Miramax Films
UNZIPPED is a cinematic portrait of Isaac Mizrahi, an artist whose palette is fabric. Ostensibly, the film is a documentary, but use of that term requires stretching its meaning. Many scenes appear staged, and a great deal of cutting-and-pasting has been done in the editing room. The cinema verite effect is a conceit -- genuine spontaneity is at a premium, and everyone is aware of and playing to the camera (especially would-be actresses like Cindy Crawford). Director Douglas Keeve (who was Mizrahi's lover at the time) freely admits that he "couldn't care less about the truth" but was more interested in capturing "the spirit and love in Isaac and in fashion."
Despite violating nearly every rule of "legitimate" documentary film making, however, UNZIPPED is a remarkably enjoyable piece of entertainment. While it sheds only a little light on the behind-the-scenes world of the fashion industry, it presents a fascinating, if incomplete, picture of designer Mizrahi. This man is the perfect subject for this kind of study -- he's funny, energetic, and eminently quotable. He has unusual views on just about everything, from fashion ("It's about women not wanting to look like cows") to Mary Tyler Moore ("Between her and Jackie Kennedy, they shaped this country") to style ("It's almost impossible to have style nowadays without the right dogs").
UNZIPPED also gives a glimpse into the creative process by which Mizrahi turns an idea into a dress. It's almost certainly different that anyone would imagine. The designer draws on a variety of sources for his look, including NANOOK OF THE NORTH and old Bette Davis movies, then enlists the aid of a Ouija board to help form the collection. In particular, UNZIPPED traces Mizrahi's development of his Fall 1994 line from its inception in the Spring to the final fashion show, which highlights a number of prominent models, including Cindy Crawford, Naomi Campbell, Kate Bush, and Linda Evangelista.
On the technical side, a number of interesting choices were made in filming UNZIPPED. A variety of film stocks were used: Super 8, 16 mm black-and-white, and 35 mm color. There is a purpose to this beyond simple artiness -- the stock often serves as an emotional key to the movie, and, during the climactic fashion show, color is used to show the audience's perspective, while black-and-white is for the behind-the- scenes sequences. Nevertheless, some viewers will be put off by this treatment.
While UNZIPPED isn't an expose on the fashion industry per se, Keeve has enough clips of petulant models to make the viewer realize how tame Robert Altman was with READY TO WEAR. Overall, however, this movie is far more about Mizrahi than anything else, and only when viewed from that perspective does UNZIPPED succeed. When the designer declares that "everything is frustrating except designing clothes -- that's beautiful and liberating," it fits perfectly with the image of him that UNZIPPED has constructed.
- James Berardinelli<EMAIL_ADDRESS>The review above was posted to the
rec.arts.movies.reviews newsgroup (de.rec.film.kritiken for German reviews).
The Internet Movie Database accepts no responsibility for the contents of the
review and has no editorial control. Unless stated otherwise, the copyright
belongs to the author.
Please direct comments/criticisms of the review to relevant newsgroups.
Broken URLs inthe reviews are the responsibility of the author.
The formatting of the review is likely to differ from the original due
to ASCII to HTML conversion.
Related links: index of all rec.arts.movies.reviews reviews


Uh, its a password generator. Duh.
The whole premise of this generator is based on a comic from XKCD (because XKCD rocks) and we like the idea of easy-to-remember but hard to guess passwords.
This website is run in our free time, but we try to update the word list somewhat regularly because we use this thing too.


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| Interface | Description |
|---|---|
| LeaseBackEnd |
Interface that grators of test leases implement so the test lease proxy
objects can communicate back to the grantor
|
| Class | Description |
|---|---|
| AddRenewRemoveTest |
Test creates a lease, places it in a lease renewal set, waits for
to be renewed a designated number of times and then removes it.
|
| BaseOwner |
Base class for owners that don't cause failures.
|
| BatchRenewTest |
Test creates a lease a number of leases and adds them with
diffrent desiered expiration and renew durations wait for
some time and makes sure that only the leases that should
have expired expire.
|
| ConstrainableTestLease |
Lease class use by renwal service tests when they don't want to use
LocalLease . |
| ExpiredLeaseTest |
Test that registers for renewal failure events, creates an expired
lease, adds the lease to the set and check to see if it gets a
renewal failure event.
|
| FailingOwner |
Owner implementation that throws a designated
throwable after a given number of renewals
|
| ForeverOwner |
BaseOwner for leases who have a membership expiration of Lease.FOREVER
|
| GetLeasesTest |
Test creates a number leases, places it in a lease renewal set, uses
getLeases to ensure that they are all still there, removes one, makes
sure that the other ones are still in the set and the removed one is not.
|
| LeaseBackEnd.RenewResults | |
| LeaseBackEndImpl |
Impl of the LeaseBackEnd remote interface for use by renewal service tests
|
| LeaseExpirationTest |
Test creates a number of leases places them in a renewal set and lets
the set's lease expire.
|
| LeaseOwner |
Abstract base class for objects that track the condition of a particular
lease that has been given to a Lease Renewal Service for testing
|
| NormDestroyTest | |
| NormLookupAttributeAdminTest | |
| NormLookupGroupAdminTest | |
| NormLookupLocatorAdminTest | |
| OneExpireOneNotTest |
Try to make sure that client leases associated with a set that has
expired do not affect client leases in a set that has not expired.
|
| OurAbstractLease |
Lifted from org.apache.river.lease.AbstractLease so we can have a codebase
independent from the service under test.
|
| RemoveExactlyOneTest |
Test creates a set and registers for both warning and renewail failure
events.
|
| RenewalFailureTest |
Test creates a lease a number of leases, places them all in the
renewal set and then forces a number of failure and makes sure that
the appropriate events get generated, and that the leases are removed
from the set.
|
| RenewAtOwner |
Simple owner that lets the renews the lease when asked.
|
| RenewAtTest |
Test creates a lease a number of leases and adds them with
diffrent desiered expiration and renew durations wait for
some time and makes sure that only the leases that should
have expired expire.
|
| RenewingOwner |
Simple owner that lets the lease be renewed a number of times.
|
| TestLease |
Lease class use by renwal service tests when they don't want to use
LocalLease . |
| ThreeArgFiniteOwner |
BaseOwner for leases who have a finite membership expiration
and are added using the three arg form of renewFor
|
| TwoArgFiniteOwner |
BaseOwner for leases who have a finite membership expiration
and are added using the two arg form of renewFor
|
| UseRenewalServiceLeaseRemoveTest |
Tests that an Lease Renweal Service properly grants, renews,
cancels, and/or expires leases.
|
| UseRenewalServiceLeaseTest |
Tests that an Lease Renweal Service properly grants, renews,
cancels, and/or expires leases.
|
Copyright 2007-2013, multiple authors.
Licensed under the Apache License, Version 2.0, see the NOTICE file for attributions.


Beta This is a test version of the layout of this page. Take the survey to help us improve it
Guidance
Grant funding for schools in the North West and South East to support inactive teachers who wish to return to teaching.
The application round is open until 20 February 2017 at 5pm.
Please use the attached application form to apply.
The National College for Teaching and Leadership would like to invite school partnerships to become a Returners Engagement Programme pilot school. This programme is open to the following in the North West and South East:
The programme is available to former teachers wishing to return to mathematics, physics, or a modern foreign language in the North West and South East.
If you’re a returner looking to participate in this programme please visit the Get into Teaching website.
We’re hosting a telephone meeting with partnerships interested in applying for the programme, on Tuesday 7 February at 4.30pm, to answer any questions you may have. If you’re interested in attending please contact<EMAIL_ADDRESS>.


vi
`Prev `__
V
`Next `__
--------------
**vi**: /V·I/, *not*, /vi:/, *never*, /siks/, n.
[from ‘Visual Interface’] A screen editor crufted together by Bill Joy
for an early `*BSD* <../B/BSD.html>`__ release. Became the de facto
standard Unix editor and a nearly undisputed hacker favorite outside of
MIT until the rise of `*EMACS* <../E/EMACS.html>`__ after about 1984.
Tends to frustrate new users no end, as it will neither take commands
while expecting input text nor vice versa, and the default setup on
older versions provides no indication of which mode the editor is in
(years ago, a correspondent reported that he has often heard the
editor's name pronounced /vi:l/; there is now a vi clone named vile).
Nevertheless vi (and variants such as vim and elvis) is still widely
used (about half the respondents in a 1991 Usenet poll preferred it),
and even EMACS fans often resort to it as a mail editor and for small
editing jobs (mainly because it starts up faster than the bulkier
versions of EMACS). See `*holy wars* <../H/holy-wars.html>`__.
--------------
+--------------------------+----------------------------+----------------------------------+
| `Prev `__ | `Up <../V.html>`__ | `Next `__ |
+--------------------------+----------------------------+----------------------------------+
| vgrep | `Home <../index.html>`__ | video toaster |
+--------------------------+----------------------------+----------------------------------+


Links
Statistics
URLs
Statistics
{ URLS_SHORTENED } URLs shortened.
{ TOTAL_USES } users redirected.
{ CHARACTERS } url characters saved.
{ TOTAL_CHAR_SAVED } total characters saved.
URLs
{ TABLE }


This example shows you how to create a mobile application that can connect to your RFduino.
You can browse the source code for this example at the Evothings GitHub repository.
The file index.html contains both the HTML markup and the logic of the application.
This example runs in Evothings Viewer on Android or iOS.
An iOS device or an Android device with support for Bluetooth 4.0 (which includes BLE) is required. For Android version 4.3 or later is needed.
To run this example you need a RFduino equipped with a RGB/Button shield, in order to upload the Arduino sketch an additional USB shield is needed.
There is an blog post available that explains this example in detail as well as how to get it up and running.
The following steps are just brief summary of how to get this example up and running, please refer to the blog post above for details.


JBTools: Misc Small Tools and Helper Functions for Other Code of J.
Buttlar
Collection of several tools and helper functions used across the other packages of J. Buttlar ('ncdf.tools' and 'spectral.methods').
| Version: |
<IP_ADDRESS> |
| Depends: |
R (≥ 2.14.0) |
| Imports: |
foreach, parallel, gplots, colorspace, plotrix |
| Suggests: |
doMC |
| Published: |
2015-06-02 |
| Author: |
Jannis v. Buttlar |
| Maintainer: |
Jannis v. Buttlar <jbuttlar at bgc-jena.mpg.de> |
| License: |
GPL-2 |
| NeedsCompilation: |
no |
| CRAN checks: |
JBTools results |
Downloads:
Reverse dependencies:
Linking:
Please use the canonical form
https://CRAN.R-project.org/package=JBTools
to link to this page.


We need to validate that the sender of each transaction matches
the wallet address and/or public key we are trying to send from.
This is quite tricky because every coin has a different method
to sign transactions and compute the identifying property because
some use an address, others a public key and again others a WIF.
We need to validate that the sender of each transaction matches the wallet address and/or public key we are trying to send from.
This is quite tricky because every coin has a different method to sign transactions and compute the identifying property because some use an address, others a public key and again others a WIF.


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Save


Estadítiscas
Revisa cuando has consumido, cuanto te queda o como estas contribuyendo a desperdiciar menos con MyPantry.
Con MyPantry la información sobre tus víveres esta al alcance de tu smartphone. Así puedes pasar mas tiempo haciendo lo que te gusta.
Revisa cuando has consumido, cuanto te queda o como estas contribuyendo a desperdiciar menos con MyPantry.
User generated content in real-time will have multiple touchpoints for offshoring.
Capitalize on low hanging fruit to identify a ballpark value added activity to beta test.
Nanotechnology immersion along the information highway will close the loop.
Iterative approaches to corporate strategy foster collaborative thinking to further the overall value proposition.
Organically grow the holistic world of disruptive innovation via workplace diversity and empowerment.
User generated content in real-time will have multiple touchpoints for offshoring.
Iterative approaches to corporate strategy foster collaborative thinking to further the overall value proposition.
Organically grow the holistic world view of disruptive innovation via workplace diversity.
Iterative approaches to corporate strategy foster collaborative thinking to further the overall value proposition.