7.3 KiB
| 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.
When to Use Context
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
Note
Don't use context just to avoid passing props a few levels down. Stick to cases where the same data needs to be accessed in many components at multiple levels.
API
React.createContext
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
<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
<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.
Examples
Dynamic Context
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
Updating Context from a Nested Component
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
Consuming Multiple Contexts
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 Context in Lifecycle Methods
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
Consuming Context with a HOC
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
Forwarding Refs to Context Consumers
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
Caveats
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
Legacy API
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.