react.dev/content/blog/2018-03-27-update-on-async-rendering.md

title	author
Update on Async Rendering	[bvaughn]

For over a year, the React team has been working to implement asynchronous rendering. Last month during his talk at JSConf Iceland, Dan unveiled some of the exciting new possibilities async rendering unlocks. Now we'd like to share with you some of the lessons we've learned while working on these features, and some recipes to help prepare your components for async rendering when it launches.

One of the biggest lessons we've learned is that some of our legacy component lifecycles tend to encourage unsafe coding practices. They are:

• componentWillMount
• componentWillReceiveProps
• componentWillUpdate

These lifecycle methods have often been misunderstood and subtly misused; furthermore, we anticipate that their potential misuse may be more problematic with async rendering. Because of this, we will be adding an "UNSAFE_" prefix to these lifecycles in an upcoming release. (Here, "unsafe" refers not to security but instead conveys that code using these lifecycles will be more likely to have bugs in future versions of React, especially once async rendering is enabled.)

Gradual Migration Path

React follows semantic versioning, so this change will be gradual. Our current plan is:

• 16.3: Introduce aliases for the unsafe lifecycles, UNSAFE_componentWillMount, UNSAFE_componentWillReceiveProps, and UNSAFE_componentWillUpdate. (Both the old lifecycle names and the new aliases will work in this release.)
• A future 16.x release: Enable deprecation warning for componentWillMount, componentWillReceiveProps, and componentWillUpdate. (Both the old lifecycle names and the new aliases will work in this release, but the old names will log a DEV-mode warning.)
• 17.0: Remove componentWillMount, componentWillReceiveProps, and componentWillUpdate . (Only the new "UNSAFE_" lifecycle names will work from this point forward.)

Note that if you're a React application developer, you don't have to do anything about the legacy methods yet. The primary purpose of the upcoming version 16.3 release is to enable open source project maintainers to update their libraries in advance of any deprecation warnings. Those warnings will not be enabled until a future 16.x release.

We maintain over 50,000 React components at Facebook, and we don't plan to rewrite them all immediately. We understand that migrations take time. We will take the gradual migration path along with everyone in the React community.

If you don't have the time to migrate or test these components, we recommend running a "codemod" script that renames them automatically:

cd your_project
npx react-codemod rename-unsafe-lifecycles

Learn more about this codemod on the 16.9.0 release post.

---

Migrating from Legacy Lifecycles

If you'd like to start using the new component APIs introduced in React 16.3 (or if you're a maintainer looking to update your library in advance) here are a few examples that we hope will help you to start thinking about components a bit differently. Over time, we plan to add additional "recipes" to our documentation that show how to perform common tasks in a way that avoids the problematic lifecycles.

Before we begin, here's a quick overview of the lifecycle changes planned for version 16.3:

• We are adding the following lifecycle aliases: UNSAFE_componentWillMount, UNSAFE_componentWillReceiveProps, and UNSAFE_componentWillUpdate. (Both the old lifecycle names and the new aliases will be supported.)
• We are introducing two new lifecycles, static getDerivedStateFromProps and getSnapshotBeforeUpdate.

New lifecycle: getDerivedStateFromProps

embed:update-on-async-rendering/definition-getderivedstatefromprops.js

The new static getDerivedStateFromProps lifecycle is invoked after a component is instantiated as well as before it is re-rendered. It can return an object to update state, or null to indicate that the new props do not require any state updates.

Together with componentDidUpdate, this new lifecycle should cover all use cases for the legacy componentWillReceiveProps.

Note:

Both the older componentWillReceiveProps and the new getDerivedStateFromProps methods add significant complexity to components. This often leads to bugs. Consider simpler alternatives to derived state to make components predictable and maintainable.

New lifecycle: getSnapshotBeforeUpdate

embed:update-on-async-rendering/definition-getsnapshotbeforeupdate.js

The new getSnapshotBeforeUpdate lifecycle is called right before mutations are made (e.g. before the DOM is updated). The return value for this lifecycle will be passed as the third parameter to componentDidUpdate. (This lifecycle isn't often needed, but can be useful in cases like manually preserving scroll position during rerenders.)

Together with componentDidUpdate, this new lifecycle should cover all use cases for the legacy componentWillUpdate.

You can find their type signatures in this gist.

We'll look at examples of how both of these lifecycles can be used below.

Examples

• Initializing state
• Fetching external data
• Adding event listeners (or subscriptions)
• Updating state based on props
• Invoking external callbacks
• Side effects on props change
• Fetching external data when props change
• Reading DOM properties before an update

Note

For brevity, the examples below are written using the experimental class properties transform, but the same migration strategies apply without it.

Initializing state

This example shows a component with setState calls inside of componentWillMount: embed:update-on-async-rendering/initializing-state-before.js

The simplest refactor for this type of component is to move state initialization to the constructor or to a property initializer, like so: embed:update-on-async-rendering/initializing-state-after.js

Fetching external data

Here is an example of a component that uses componentWillMount to fetch external data: embed:update-on-async-rendering/fetching-external-data-before.js

The above code is problematic for both server rendering (where the external data won't be used) and the upcoming async rendering mode (where the request might be initiated multiple times).

The recommended upgrade path for most use cases is to move data-fetching into componentDidMount: embed:update-on-async-rendering/fetching-external-data-after.js

There is a common misconception that fetching in componentWillMount lets you avoid the first empty rendering state. In practice this was never true because React has always executed render immediately after componentWillMount. If the data is not available by the time componentWillMount fires, the first render will still show a loading state regardless of where you initiate the fetch. This is why moving the fetch to componentDidMount has no perceptible effect in the vast majority of cases.

Note

Some advanced use-cases (e.g. libraries like Relay) may want to experiment with eagerly prefetching async data. An example of how this can be done is available here.

In the longer term, the canonical way to fetch data in React components will likely be based on the “suspense” API introduced at JSConf Iceland. Both simple data fetching solutions and libraries like Apollo and Relay will be able to use it under the hood. It is significantly less verbose than either of the above solutions, but will not be finalized in time for the 16.3 release.

When supporting server rendering, it's currently necessary to provide the data synchronously – componentWillMount was often used for this purpose but the constructor can be used as a replacement. The upcoming suspense APIs will make async data fetching cleanly possible for both client and server rendering.

Adding event listeners (or subscriptions)

Here is an example of a component that subscribes to an external event dispatcher when mounting: embed:update-on-async-rendering/adding-event-listeners-before.js

Unfortunately, this can cause memory leaks for server rendering (where componentWillUnmount will never be called) and async rendering (where rendering might be interrupted before it completes, causing componentWillUnmount not to be called).

People often assume that componentWillMount and componentWillUnmount are always paired, but that is not guaranteed. Only once componentDidMount has been called does React guarantee that componentWillUnmount will later be called for clean up.

For this reason, the recommended way to add listeners/subscriptions is to use the componentDidMount lifecycle: embed:update-on-async-rendering/adding-event-listeners-after.js

Sometimes it is important to update subscriptions in response to property changes. If you're using a library like Redux or MobX, the library's container component should handle this for you. For application authors, we've created a small library, create-subscription, to help with this. We'll publish it along with React 16.3.

Rather than passing a subscribable dataSource prop as we did in the example above, we could use create-subscription to pass in the subscribed value:

embed:update-on-async-rendering/adding-event-listeners-create-subscription.js

Note

Libraries like Relay/Apollo should manage subscriptions manually with the same techniques as create-subscription uses under the hood (as referenced here) in a way that is most optimized for their library usage.

Updating state based on props

Note:

Both the older componentWillReceiveProps and the new getDerivedStateFromProps methods add significant complexity to components. This often leads to bugs. Consider simpler alternatives to derived state to make components predictable and maintainable.

Here is an example of a component that uses the legacy componentWillReceiveProps lifecycle to update state based on new props values: embed:update-on-async-rendering/updating-state-from-props-before.js

Although the above code is not problematic in itself, the componentWillReceiveProps lifecycle is often mis-used in ways that do present problems. Because of this, the method will be deprecated.

As of version 16.3, the recommended way to update state in response to props changes is with the new static getDerivedStateFromProps lifecycle. It is called when a component is created and each time it re-renders due to changes to props or state: embed:update-on-async-rendering/updating-state-from-props-after.js

You may notice in the example above that props.currentRow is mirrored in state (as state.lastRow). This enables getDerivedStateFromProps to access the previous props value in the same way as is done in componentWillReceiveProps.

You may wonder why we don't just pass previous props as a parameter to getDerivedStateFromProps. We considered this option when designing the API, but ultimately decided against it for two reasons:

• A prevProps parameter would be null the first time getDerivedStateFromProps was called (after instantiation), requiring an if-not-null check to be added any time prevProps was accessed.
• Not passing the previous props to this function is a step toward freeing up memory in future versions of React. (If React does not need to pass previous props to lifecycles, then it does not need to keep the previous props object in memory.)

Note

If you're writing a shared component, the react-lifecycles-compat polyfill enables the new getDerivedStateFromProps lifecycle to be used with older versions of React as well. Learn more about how to use it below.

Invoking external callbacks

Here is an example of a component that calls an external function when its internal state changes: embed:update-on-async-rendering/invoking-external-callbacks-before.js

Sometimes people use componentWillUpdate out of a misplaced fear that by the time componentDidUpdate fires, it is "too late" to update the state of other components. This is not the case. React ensures that any setState calls that happen during componentDidMount and componentDidUpdate are flushed before the user sees the updated UI. In general, it is better to avoid cascading updates like this, but in some cases they are necessary (for example, if you need to position a tooltip after measuring the rendered DOM element).

Either way, it is unsafe to use componentWillUpdate for this purpose in async mode, because the external callback might get called multiple times for a single update. Instead, the componentDidUpdate lifecycle should be used since it is guaranteed to be invoked only once per update: embed:update-on-async-rendering/invoking-external-callbacks-after.js

Side effects on props change

Similar to the example above, sometimes components have side effects when props change.

embed:update-on-async-rendering/side-effects-when-props-change-before.js

Like componentWillUpdate, componentWillReceiveProps might get called multiple times for a single update. For this reason it is important to avoid putting side effects in this method. Instead, componentDidUpdate should be used since it is guaranteed to be invoked only once per update:

embed:update-on-async-rendering/side-effects-when-props-change-after.js

Fetching external data when props change

Here is an example of a component that fetches external data based on props values: embed:update-on-async-rendering/updating-external-data-when-props-change-before.js

The recommended upgrade path for this component is to move data updates into componentDidUpdate. You can also use the new getDerivedStateFromProps lifecycle to clear stale data before rendering the new props: embed:update-on-async-rendering/updating-external-data-when-props-change-after.js

Note

If you're using an HTTP library that supports cancellation, like axios, then it's simple to cancel an in-progress request when unmounting. For native Promises, you can use an approach like the one shown here.

Reading DOM properties before an update

Here is an example of a component that reads a property from the DOM before an update in order to maintain scroll position within a list: embed:update-on-async-rendering/react-dom-properties-before-update-before.js

In the above example, componentWillUpdate is used to read the DOM property. However with async rendering, there may be delays between "render" phase lifecycles (like componentWillUpdate and render) and "commit" phase lifecycles (like componentDidUpdate). If the user does something like resize the window during this time, the scrollHeight value read from componentWillUpdate will be stale.

The solution to this problem is to use the new "commit" phase lifecycle, getSnapshotBeforeUpdate. This method gets called immediately before mutations are made (e.g. before the DOM is updated). It can return a value for React to pass as a parameter to componentDidUpdate, which gets called immediately after mutations.

The two lifecycles can be used together like this:

embed:update-on-async-rendering/react-dom-properties-before-update-after.js

Note

If you're writing a shared component, the react-lifecycles-compat polyfill enables the new getSnapshotBeforeUpdate lifecycle to be used with older versions of React as well. Learn more about how to use it below.

Other scenarios

While we tried to cover the most common use cases in this post, we recognize that we might have missed some of them. If you are using componentWillMount, componentWillUpdate, or componentWillReceiveProps in ways that aren't covered by this blog post, and aren't sure how to migrate off these legacy lifecycles, please file a new issue against our documentation with your code examples and as much background information as you can provide. We will update this document with new alternative patterns as they come up.

Open source project maintainers

Open source maintainers might be wondering what these changes mean for shared components. If you implement the above suggestions, what happens with components that depend on the new static getDerivedStateFromProps lifecycle? Do you also have to release a new major version and drop compatibility for React 16.2 and older?

Fortunately, you do not!

When React 16.3 is published, we'll also publish a new npm package, react-lifecycles-compat. This package polyfills components so that the new getDerivedStateFromProps and getSnapshotBeforeUpdate lifecycles will also work with older versions of React (0.14.9+).

To use this polyfill, first add it as a dependency to your library:

# Yarn
yarn add react-lifecycles-compat

# NPM
npm install react-lifecycles-compat --save

Next, update your components to use the new lifecycles (as described above).

Lastly, use the polyfill to make your component backwards compatible with older versions of React: embed:update-on-async-rendering/using-react-lifecycles-compat.js