| id | title | permalink |
|---|---|---|
context |
Context |
docs/context.html |
Context provides a way to pass data through the component tree without having to pass props down manually at every level.
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 these between components without having to explicitly pass a prop through every level of the tree.
Context is designed to share data that can be considered "global" for a tree of React components, such as the current authenticated user, theme, or preferred language. For example, in the code below we manually thread through a "theme" prop in order to style the Button component:
embed:context/motivation-problem.js
Using context, we can avoid passing props through intermediate elements:
embed:context/motivation-solution.js
Context is primarily used when some data needs to be accessible by many components at different nesting levels. Apply it sparingly because it makes component reuse more difficult.
If you only want to avoid passing some props through many levels, component composition is often a simpler solution than context.
For example, consider a Page component that passes a user and avatarSize prop several levels down so that deeply nested Link and Avatar components can read it:
<Page user={user} avatarSize={avatarSize} />
// ... which renders ...
<PageLayout user={user} avatarSize={avatarSize} />
// ... which renders ...
<NavigationBar user={user} avatarSize={avatarSize} />
// ... which renders ...
<Link href={user.permalink}>
<Avatar user={user} size={avatarSize} />
</Link>It might feel redundant to pass down the user and avatarSize props through many levels if in the end only the Avatar component really needs it. It's also annoying that whenever the Avatar component needs more props from the top, you have to add them at all the intermediate levels too.
One way to solve this issue without context is to pass down the Avatar component itself so that the intermediate components don't need to know about the user prop:
function Page(props) {
const user = props.user;
const userLink = (
<Link href={user.permalink}>
<Avatar user={user} size={props.avatarSize} />
</Link>
);
return <PageLayout userLink={userLink} />;
}
// Now, we have:
<Page user={user} />
// ... which renders ...
<PageLayout userLink={...} />
// ... which renders ...
<NavigationBar userLink={...} />
// ... which renders ...
{props.userLink}With this change, only the top-most Page component needs to know about the Link and Avatar components' use of user and avatarSize.
This inversion of control can make your code cleaner in many cases by reducing the amount of props you need to pass through your application and giving more control to the root components. However, this isn't the right choice in every case: moving more complexity higher in the tree makes those higher-level components more complicated and forces the lower-level components to be more flexible than you may want.
You're not limited to a single child for a component. You may pass multiple children, or even have multiple separate "slots" for children, as documented here:
function Page(props) {
const user = props.user;
const content = <Feed user={user} />;
const topBar = (
<NavigationBar>
<Link href={user.permalink}>
<Avatar user={user} size={props.avatarSize} />
</Link>
</NavigationBar>
);
return (
<PageLayout
topBar={topBar}
content={content}
/>
);
}This pattern is sufficient for many cases when you need to decouple a child from its immediate parents. You can take it even further with render props if the child needs to communicate with the parent before rendering.
However, sometimes the same data needs to be accessible by many components in the tree, and at different nesting levels. Context lets you "broadcast" such data, and changes to it, to all components below. Common examples where using context might be simpler than the alternatives include managing the current locale, theme, or a data cache.
const {Provider, Consumer} = React.createContext(defaultValue);Creates a { Provider, Consumer } pair. When React renders a context Consumer, it will read the current context value from the closest matching Provider above it in the tree.
The defaultValue argument is only used by a Consumer when it does not have a matching Provider above it in the tree. This can be helpful for testing components in isolation without wrapping them. Note: passing undefined as a Provider value does not cause Consumers to use defaultValue.
<Provider value={/* some value */}>A React component that allows Consumers to subscribe to context changes.
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.
<Consumer>
{value => /* render something based on the context value */}
</Consumer>A React component that subscribes to context changes.
Requires a function as a child. The function receives the current context value and returns a React node. The value argument passed to the function will be equal to the value prop of the closest Provider for this context above in the tree. If there is no Provider for this context above, the value argument will be equal to the defaultValue that was passed to createContext().
Note
For more information about the 'function as a child' pattern, see render props.
All Consumers that are descendants of a Provider will re-render whenever the Provider's value prop changes. The propagation from Provider to its descendant Consumers is not subject to the shouldComponentUpdate method, so the Consumer is updated even when an ancestor component bails out of the update.
Changes are determined by comparing the new and old values using the same algorithm as Object.is.
Note
The way changes are determined can cause some issues when passing objects as
value: see Caveats.
A more complex example with dynamic values for the theme:
theme-context.js
embed:context/theme-detailed-theme-context.js
themed-button.js
embed:context/theme-detailed-themed-button.js
app.js
embed:context/theme-detailed-app.js
It is often necessary to update the context from a component that is nested somewhere deeply in the component tree. In this case you can pass a function down through the context to allow consumers to update the context:
theme-context.js
embed:context/updating-nested-context-context.js
theme-toggler-button.js
embed:context/updating-nested-context-theme-toggler-button.js
app.js
embed:context/updating-nested-context-app.js
To keep context re-rendering fast, React needs to make each context consumer a separate node in the tree.
embed:context/multiple-contexts.js
If two or more context values are often used together, you might want to consider creating your own render prop component that provides both.
Accessing values from context in lifecycle methods is a relatively common use case. Instead of adding context to every lifecycle method, you just need to pass it as a prop, and then work with it just like you'd normally work with a prop.
embed:context/lifecycles.js
Some types of contexts are consumed by many components (e.g. theme or localization). It can be tedious to explicitly wrap each dependency with a <Context.Consumer> element. A higher-order component can help with this.
For example, a button component might consume a theme context like so:
embed:context/higher-order-component-before.js
That's alright for a few components, but what if we wanted to use the theme context in a lot of places?
We could create a higher-order component called withTheme:
embed:context/higher-order-component.js
Now any component that depends on the theme context can easily subscribe to it using the withTheme function we've created:
embed:context/higher-order-component-usage.js
One issue with the render prop API is that refs don't automatically get passed to wrapped elements. To get around this, use React.forwardRef:
fancy-button.js
embed:context/forwarding-refs-fancy-button.js
app.js
embed:context/forwarding-refs-app.js
Because context uses reference identity to determine when to re-render, there are some gotchas that could trigger unintentional renders in consumers when a provider's parent re-renders. For example, the code below will re-render all consumers every time the Provider re-renders because a new object is always created for value:
embed:context/reference-caveats-problem.js
To get around this, lift the value into the parent's state:
embed:context/reference-caveats-solution.js
Note
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. The legacy API will be removed in a future major React version. Read the legacy context docs here.