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Add docs for new Context API #665

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Add docs for new Context API (R16.3)
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234 changes: 51 additions & 183 deletions content/docs/context.md
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Expand Up @@ -4,220 +4,88 @@ title: Context
permalink: docs/context.html
---

> Note:
>
> `React.PropTypes` has moved into a different package since React v15.5. Please use [the `prop-types` library instead](https://www.npmjs.com/package/prop-types) to define `contextTypes`.
>
>We provide [a codemod script](/blog/2017/04/07/react-v15.5.0.html#migrating-from-react.proptypes) to automate the conversion.

With React, it's easy to track the flow of data through your React components. When you look at a component, you can see which props are being passed, which makes your apps easy to reason about.

In some cases, you want to pass data through the component tree without having to pass the props down manually at every level.
You can do this directly in React with the powerful "context" API.

> Note:
>
> A [new, safe version of context](https://github.com/reactjs/rfcs/blob/master/text/0002-new-version-of-context.md) is under development for the upcoming 16.3 release.

Context provides a way to pass data through the component tree without having to pass props down manually at every level.

## Why Not To Use Context
Typically, data in a React application is passed top-down (parent to child) via props. But sometimes it's useful to pass values through multiple levels without adding props to every intermediate component. Examples include a language preference, or a UI theme. Many components may rely on those but you don't want to have to pass a locale prop and a theme prop through every level of the tree.
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This paragraph feels like it ends a bit prematurely. Maybe we can close by reiterating the opening line?

Wording suggestions:

In a typical React application, data is passed top-down (parent to child) via props, but this can be cumbersome for certain types of props (e.g. locale preference, UI theme) that are required by many components within an application. Context provides a way to share values like this between components without having to explicitly pass a prop through every level of the tree.


The vast majority of applications do not need to use context.
- [Motivation](#motivation)
- [API](#api)
- [React.createContext](#reactcreatecontext)
- [Provider](#provider)
- [Consumer](#consumer)
- [Examples](#examples)
- [Static Context](#static-context)
- [Dynamic Context](#dynamic-context)
- [Legacy API](#legacy-api)
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@bvaughn bvaughn Mar 21, 2018

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I like this table of contents 👍


If you want your application to be stable, don't use context. It is an experimental API and it is likely to break in future releases of React.

If you aren't familiar with state management libraries like [Redux](https://github.com/reactjs/redux) or [MobX](https://github.com/mobxjs/mobx), don't use context. For many practical applications, these libraries and their React bindings are a good choice for managing state that is relevant to many components. It is far more likely that Redux is the right solution to your problem than that context is the right solution.
## Motivation

If you're still learning React, don't use context. There is usually a better way to implement functionality just using props and state.
Context is designed to relieve the pain of passing props down through a deeply nested component tree. For example, in the code below we manually thread through a color prop in order to style the Button and Message components. Using context, we can avoid passing props through intermediate elements.
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nit: I would move the "Using context, we can avoid passing props through intermediate elements." sentence to below the coding example, so it's clear that the example isn't showing what context can do.


If you insist on using context despite these warnings, try to isolate your use of context to a small area and avoid using the context API directly when possible so that it's easier to upgrade when the API changes.
`embed:context/motivation.js`
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It still feels wrong to me to have a "motivation" section that does not show how to "fix" the problem we introduce.


## How To Use Context
## API

Suppose you have a structure like:
### `React.createContext`

```javascript
class Button extends React.Component {
render() {
return (
<button style={{background: this.props.color}}>
{this.props.children}
</button>
);
}
}

class Message extends React.Component {
render() {
return (
<div>
{this.props.text} <Button color={this.props.color}>Delete</Button>
</div>
);
}
}

class MessageList extends React.Component {
render() {
const color = "purple";
const children = this.props.messages.map((message) =>
<Message text={message.text} color={color} />
);
return <div>{children}</div>;
}
}
```js
const {Provider, Consumer} = React.createContext(defaultValue);
```

In this example, we manually thread through a `color` prop in order to style the `Button` and `Message` components appropriately. Using context, we can pass this through the tree automatically:

```javascript{6,13-15,21,28-30,40-42}
import PropTypes from 'prop-types';

class Button extends React.Component {
render() {
return (
<button style={{background: this.context.color}}>
{this.props.children}
</button>
);
}
}

Button.contextTypes = {
color: PropTypes.string
};

class Message extends React.Component {
render() {
return (
<div>
{this.props.text} <Button>Delete</Button>
</div>
);
}
}

class MessageList extends React.Component {
getChildContext() {
return {color: "purple"};
}

render() {
const children = this.props.messages.map((message) =>
<Message text={message.text} />
);
return <div>{children}</div>;
}
}

MessageList.childContextTypes = {
color: PropTypes.string
};
```
Creates a `{ Provider, Consumer }` pair.

By adding `childContextTypes` and `getChildContext` to `MessageList` (the context provider), React passes the information down automatically and any component in the subtree (in this case, `Button`) can access it by defining `contextTypes`.
Optionally accepts a default value to be passed to Consumers without a Provider ancestor.

If `contextTypes` is not defined, then `context` will be an empty object.
### `Provider`

## Parent-Child Coupling

Context can also let you build an API where parents and children communicate. For example, one library that works this way is [React Router V4](https://reacttraining.com/react-router):
```js
<Provider value={/* some value */}>
```

```javascript
import { BrowserRouter as Router, Route, Link } from 'react-router-dom';
A React component that allows Consumers to subscribe to context changes.

const BasicExample = () => (
<Router>
<div>
<ul>
<li><Link to="/">Home</Link></li>
<li><Link to="/about">About</Link></li>
<li><Link to="/topics">Topics</Link></li>
</ul>
Accepts a `value` prop to be passed to Consumers that are descendants of this Provider. One Provider can be connected to many Consumers. Providers can be nested to override values deeper within the tree.

<hr />
### `Consumer`

<Route exact path="/" component={Home} />
<Route path="/about" component={About} />
<Route path="/topics" component={Topics} />
</div>
</Router>
);
```js
<Consumer>
{ value => { /* render something based on the context value */ } }
</Consumer>
```

By passing down some information from the `Router` component, each `Link` and `Route` can communicate back to the containing `Router`.
A React component that subscribes to context changes.

Before you build components with an API similar to this, consider if there are cleaner alternatives. For example, you can pass entire React components as props if you'd like to.

## Referencing Context in Lifecycle Methods

If `contextTypes` is defined within a component, the following [lifecycle methods](/docs/react-component.html#the-component-lifecycle) will receive an additional parameter, the `context` object:

- [`constructor(props, context)`](/docs/react-component.html#constructor)
- [`componentWillReceiveProps(nextProps, nextContext)`](/docs/react-component.html#componentwillreceiveprops)
- [`shouldComponentUpdate(nextProps, nextState, nextContext)`](/docs/react-component.html#shouldcomponentupdate)
- [`componentWillUpdate(nextProps, nextState, nextContext)`](/docs/react-component.html#componentwillupdate)
Requires a [function as a child](/docs/render-props.html#using-props-other-than-render). This function receives the current context value and returns a React node, and will be called whenever the Provider's value is updated.
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I think the last sentence is a bit too long. Suggestion:

Requires a function as a child. This function receives the current context value and returns a React node. It will be called whenever the Provider's value is updated.


> Note:
>
> As of React 16, `componentDidUpdate` no longer receives `prevContext`.
>
> For more information about this pattern, see [render props](/docs/render-props.html).

## Referencing Context in Stateless Functional Components
## Examples

Stateless functional components are also able to reference `context` if `contextTypes` is defined as a property of the function. The following code shows a `Button` component written as a stateless functional component.
### Static Context

```javascript
import PropTypes from 'prop-types';

const Button = ({children}, context) =>
<button style={{background: context.color}}>
{children}
</button>;

Button.contextTypes = {color: PropTypes.string};
```
Here is an example illustrating how you might inject a "theme" using context:

## Updating Context
`embed:context/theme-example.js`

Don't do it.
### Dynamic Context

React has an API to update context, but it is fundamentally broken and you should not use it.
A more complex example with dynamic values for the theme:

The `getChildContext` function will be called when the state or props changes. In order to update data in the context, trigger a local state update with `this.setState`. This will trigger a new context and changes will be received by the children.
**theme-context.js**
`embed:context/theme-detailed-theme-context.js`

```javascript
import PropTypes from 'prop-types';
**themed-button.js**
`embed:context/theme-detailed-themed-button.js`

class MediaQuery extends React.Component {
constructor(props) {
super(props);
this.state = {type:'desktop'};
}
**app.js**
`embed:context/theme-detailed-app.js`

getChildContext() {
return {type: this.state.type};
}

componentDidMount() {
const checkMediaQuery = () => {
const type = window.matchMedia("(min-width: 1025px)").matches ? 'desktop' : 'mobile';
if (type !== this.state.type) {
this.setState({type});
}
};

window.addEventListener('resize', checkMediaQuery);
checkMediaQuery();
}

render() {
return this.props.children;
}
}

MediaQuery.childContextTypes = {
type: PropTypes.string
};
```
## Legacy API

The problem is, if a context value provided by component changes, descendants that use that value won't update if an intermediate parent returns `false` from `shouldComponentUpdate`. This is totally out of control of the components using context, so there's basically no way to reliably update the context. [This blog post](https://medium.com/@mweststrate/how-to-safely-use-react-context-b7e343eff076) has a good explanation of why this is a problem and how you might get around it.
> The legacy context API was deprecated in React 16.3 and will be removed in version 17.
>
> React previously shipped with an experimental context API. The old API will be supported in all 16.x releases, but applications using it should migrate to the new version. Read the [legacy context docs here](/docs/legacy-context.html).
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