An update on async rendering

content/blog/2018-02-07-strict-mode.md

@@ -0,0 +1,69 @@
+---
+title: Strict mode
+author: [bvaughn]
+---
+
+`StrictMode` is a tool for highlighting potential problems in an application. Like `Fragment`, `StrictMode` does not render any visible UI. It simply activates additional checks and warnings for its descendants.
+
+> Note:
+>
+> Strict mode checks are run in development mode only; _they do not impact the production build_.
+
+You can enable strict mode for any part of your application. For example:
+`embed:update-on-async-rendering/enabling-strict-mode.js`
+
+In the above example, strict mode checks will *not* be run against the `Header` and `Footer` components. However, `RouteOne` and `RouteTwo`, as well as all of their descendants, will have the checks.
+
+In version 16.3, `StrictMode` helps with:
+* [Identifying components with unsafe lifecycles](#identifying-unsafe-lifecycles)
+* [Warning about legacy string ref API usage](#warning-about-legacy-string-ref-api-usage)
+* [Detecting unexpected side effects](#detecting-unexpected-side-effects)
+
+Additional functionality will be added with future releases of React.
+
+### Identifying unsafe lifecycles
+
+As previously mentioned, certain legacy lifecycle methods are unsafe for use in async React applications. However, if your application uses third party libraries, it can be difficult to ensure that these lifecycles aren't being used. Fortunately, strict mode can help with this!
+
+When strict mode is enabled, React compiles a list of all class components using the unsafe lifecycles, and logs a warning message with information about these components, like so:
+
+![](../images/blog/strict-mode-unsafe-lifecycles-warning.png)
+
+Addressing the issues identified by strict mode _now_ will make it easier for you to take advantage of async rendering in future releases of React.
+
+### Warning about legacy string ref API usage
+
+Previously, React provided two ways for managing refs: the legacy string ref API and the callback API. Although the string ref API was the more convenient of the two, it had [several downsides](https://github.com/facebook/react/issues/1373) and so our official recomendation was to [use the callback form instead](https://reactjs.org/docs/refs-and-the-dom.html#legacy-api-string-refs).
+
+Version 16.3 adds a new option for managing refs that offers the convenience of a string ref without any of the downsides:
+`embed:16-3-release-blog-create-ref.js`
+
+> **Note:**
+>
+> Callback refs will continue to be supported in addition to the new `createRef` API.
+>
+> You don't need to replace callback refs in your components. They are slightly more flexible, so they will remain as an advanced feature.
+
+[Learn more about the new `createRef` API here.](#)
+
+### Detecting unexpected side effects
+
+As a general rule, side-effects should be avoided in certain class component methods (e.g. the `constructor`, `render`, etc). This is because React may invoke these methods more than once before committing, or it may invoke them without committing at all (because of an error or a higher priority interruption). Ignoring this rule can lead to a variety of problems, including memory leaks and invalid state. Unfortunately, it can be difficult to detect these problems as they are often non-deterministic.
+
+Strict mode can't automatically detect side effects for you, but it can help you spot them by making them a little more deterministic. This is done by intentionally double-invoking the following methods:
+
+* Class component `constructor` method
+* The `render` method
+* `setState` updater functions
+* The static `getDerivedStateFromProps` lifecycle
+
+> Note:
+>
+> This only applies to development mode. _Lifecycles will not be double-invoked in production mode._
+
+For example, consider the following code:
+`embed:update-on-async-rendering/side-effects-in-constructor.js`
+
+At first glance, this code might not seem problematic. But if `SharedApplicationState.recordEvent` is not idempotent, then instantiating this component multiple times could lead to invalid application state. This sort of subtle bug might not manifest during development, or it might do so inconsistently and so be overlooked.
+
+By intentionally double-invoking methods like the component constructor, strict mode makes patterns like this easier to spot.

content/blog/2018-02-07-update-on-async-rendering.md

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+---
+title: Update on Async Rendering
+author: [bvaughn, gaearon]
+---
+
+For the past few months, the React team has been experimenting with [asynchronous rendering](/blog/2017/09/26/react-v16.0.html#new-core-architecture), and we are very excited about the new features it enables.
+
+Along the way our research has shown that some of our legacy component lifecycles tend to encourage unsafe coding practices. They are:
+
+* `componentWillMount`
+* `componentWillReceiveProps`
+* `componentWillUpdate`
+
+Because of this, we are adding an "UNSAFE_" prefix to these lifecycles in a future release. React [follows semantic versioning](/blog/2016/02/19/new-versioning-scheme.html), so the migration path is gradual:
+
+* **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.)
+* **16.4**: Enable deprecation warning for `componentWillMount`, `componentWillReceiveProps`, and `componentWillUpdate`. (Both the old lifecycle names and the new aliases will work in this release.)
+* **17.0**: Remove `componentWillMount`, `componentWillReceiveProps`, and `componentWillUpdate` . (Only the new "UNSAFE_" lifecycle names will work in this release.)
+
+In this post, we will explore some of the potential capabilities of async rendering, and we'll outline a migration plan for components that rely on these legacy lifecycles.
+
+## What can asynchronous rendering do?
+
+#### With every new version, our goal is to improve the user experience of apps created with React.
+
+We have been fine-tuning the performance of React with every new release. However, despite what synthetic benchmarks say, we've found that the real bottleneck is generally not React itself, but the application code using it. In order to unlock the next wave of performance optimizations and new features, we need React to be smarter about when to re-render components and flush updates to the screen.
+
+We found that asynchronous rendering can help in several ways. For example:
+
+1. As users navigate within an app, newly displayed components often have asynchronous dependencies (including data, images, and code splitting). This leads to a lot of boilerplate code managing data fetching and displaying the loading states. It can also lead to a "cascade of spinners" as the data loads, causing DOM reflows and janky user experience. We'd like to make it easier for product developers to express asynchronous dependencies of components. React could keep the old UI "alive" and interactive for a certain period while the updated UI is not ready yet, and provide a declarative way to show a loading indicator if it takes more than a second.
+2. Fast updates within a short timeframe often cause jank because React processes each update individually. We'd like to automatically "combine" updates within a few hundred milliseconds when possible so that there is less re-rendering.
+3. Some updates are inherently less important than others. For example, if you're writing a live-updating search filter input like [this](https://zeit.co/blog/domains-search-web#asynchronous-rendering), it is essential that the input is updated immediately (within a few milliseconds). Re-rendering the result list can be done later, and should not block the thread or cause stutter when typing. It would be nice if React had a way to mark the latter updates as having a lower priority. (Note that even debouncing the input doesn't help because if the rendering is synchronous—like in React today—a keystroke can't interrupt the rendering if it already started. Asynchronous rendering solves this by splitting rendering into small chunks that can be paused and later restarted.)
+4. For UI elements like hidden popups and tabs, we'd like to be able to start pre-rendering their content when the browser isn't busy. This way, they can appear instantaneously in response to a later user interaction. However, we don't want to make the initial rendering slower, so it's essential to render such elements lazily ([when the browser is idle](https://developers.google.com/web/updates/2015/08/using-requestidlecallback)).
+5. For many apps, React is not the only JavaScript on the page. It often has to coordinate with other JS libraries, server-rendered widgets, and so on. Asynchronous rendering lets React better coordinate with non-React code regarding when components are inserted into the DOM so that [the user experience is smooth](https://twitter.com/acdlite/status/909926793536094209).
+
+Of course, it's possible to implement some of these features today, but it's difficult. We hope to make them effortless by building them into React itself. By replacing problematic lifecycles with safer alternatives, we also hope to make it simple to write async-safe React components.
+
+In the next section, we'll look at how to update your existing components to prepare for the upcoming lifecycle changes.
+
+## Updating class components
+
+#### If you're an application developer, **you don't have to do anything about the deprecated methods yet**. The primary purpose of this update (v16.3) is to enable open source project maintainers to update their libraries in advance of any deprecation warnings. Those warnings will be enabled with the next minor release, v16.4.
+
+However, if you'd like to start using the new component API (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's async-safe.
+
+### 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 the state-updates 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, or executed unnecessarily).
+
+The upgrade path for this is to move data-fetching into `componentDidMount`:
+`embed:update-on-async-rendering/fetching-external-data-after.js`
+
+> 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](https://gist.github.com/bvaughn/89700e525ff423a75ffb63b1b1e30a8f).
+
+### 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 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`
+
+### Updating `state` based on `props`
+
+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 has been deprecated.
+
+As of version 16.3, the recommended way to update `state` in response to `props` changes is using the new `static getDerivedStateFromProps` lifecycle:
+`embed:update-on-async-rendering/updating-state-from-props-after.js`
+
+> Note:
+>
+> The [`react-lifecycles-compat`](https://github.com/reactjs/react-lifecycles-compat) polyfill allows this new lifecycle to be used with older versions of React as well. This can be helpful if you're writing a shared component that is intended for use with multiple versions of React.
+
+### 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 unavoidable (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`
+
+## 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](https://github.com/reactjs/reactjs.org/issues/new) 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!
+
+Along with the release of 16.3, we've also released a new NPM package, [`react-lifecycles-compat`](https://github.com/reactjs/react-lifecycles-compat). This package polyfills components so that the new `getDerivedStateFromProps` lifecycle will also work with older versions of React (0.14.9+).
+
+To use this polyfill, first add it as a dependency to your library:
+
+```bash
+# Yarn
+yarn add react-lifecycles-compat
+
+# NPM
+npm install react-lifecycles-compat --save
+```
+
+Next, update your components to use the new static lifecycle, `getDerivedStateFromProps`, 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`

examples/16-3-release-blog-create-ref.js

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+class MyComponent extends React.Component {
+  // highlight-next-line
+  divRef = React.createRef();
+
+  render() {
+    // highlight-next-line
+    return <input type="text" ref={this.divRef} />;
+  }
+
+  componentDidMount() {
+    // highlight-next-line
+    this.divRef.value.focus();
+  }
+}

examples/update-on-async-rendering/adding-event-listeners-after.js

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+// After
+class ExampleComponent extends React.Component {
+  // highlight-range{1-3}
+  state = {
+    subscribedValue: this.props.dataSource.value,
+  };
+
+  // highlight-range{1-18}
+  componentDidMount() {
+    // Event listeners are only safe to add after mount,
+    // So they won't leak if mount is interrupted or errors.
+    this.props.dataSource.subscribe(
+      this._onSubscriptionChange
+    );
+
+    // External values could change between render and mount,
+    // In some cases it may be important to handle this case.
+    if (
+      this.state.subscribedValue !==
+      this.props.dataSource.value
+    ) {
+      this.setState({
+        subscribedValue: this.props.dataSource.value,
+      });
+    }
+  }
+
+  componentWillUnmount() {
+    this.props.dataSource.unsubscribe(
+      this._onSubscriptionChange
+    );
+  }
+
+  _onSubscriptionChange = subscribedValue => {
+    this.setState({subscribedValue});
+  };
+}

examples/update-on-async-rendering/adding-event-listeners-before.js

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+// Before
+class ExampleComponent extends React.Component {
+  // highlight-range{1-10}
+  componentWillMount() {
+    this.setState({
+      subscribedValue: this.props.dataSource.value,
+    });
+
+    // This is not safe; it can leak!
+    this.props.dataSource.subscribe(
+      this._onSubscriptionChange
+    );
+  }
+
+  componentWillUnmount() {
+    this.props.dataSource.unsubscribe(
+      this._onSubscriptionChange
+    );
+  }
+
+  _onSubscriptionChange = subscribedValue => {
+    this.setState({subscribedValue});
+  };
+}

examples/update-on-async-rendering/enabling-strict-mode.js

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+import React from 'react';
+
+// highlight-next-line
+const {StrictMode} = React;
+
+function ExampleApplication() {
+  return (
+    <div>
+      <Header />
+      {/* highlight-next-line */}
+      <StrictMode>
+        <>
+          <RouteOne />
+          <RouteTwo />
+        </>
+        {/* highlight-next-line */}
+      </StrictMode>
+      <Footer />
+    </div>
+  );
+}

examples/update-on-async-rendering/fetching-external-data-after.js

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+// After
+class ExampleComponent extends React.Component {
+  // highlight-next-line
+  _hasUnmounted = false;
+
+  state = {
+    externalData: null,
+  };
+
+  // highlight-range{1-7}
+  componentDidMount() {
+    asyncLoadData(this.props.someId).then(externalData => {
+      if (!this._hasUnmounted) {
+        this.setState({externalData});
+      }
+    });
+  }
+
+  // highlight-range{1-3}
+  componentWillUnmount() {
+    this._hasUnmounted = true;
+  }
+
+  render() {
+    if (this.externalData === null) {
+      // Render loading state ...
+    } else {
+      // Render real UI ...
+    }
+  }
+}

examples/update-on-async-rendering/fetching-external-data-before.js

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+// Before
+class ExampleComponent extends React.Component {
+  state = {
+    externalData: null,
+  };
+
+  // highlight-range{1-5}
+  componentWillMount() {
+    asyncLoadData(this.props.someId).then(externalData =>
+      this.setState({externalData})
+    );
+  }
+
+  render() {
+    if (this.externalData === null) {
+      // Render loading state ...
+    } else {
+      // Render real UI ...
+    }
+  }
+}

examples/update-on-async-rendering/initializing-state-after.js

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+// After
+class ExampleComponent extends React.Component {
+  // highlight-range{1-4}
+  state = {
+    currentColor: this.props.defaultColor,
+    palette: 'rgb',
+  };
+}

examples/update-on-async-rendering/initializing-state-before.js

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+// Before
+class ExampleComponent extends React.Component {
+  state = {};
+
+  // highlight-range{1-6}
+  componentWillMount() {
+    this.setState({
+      currentColor: this.props.defaultColor,
+      palette: 'rgb',
+    });
+  }
+}

examples/update-on-async-rendering/invoking-external-callbacks-after.js

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+// After
+class ExampleComponent extends React.Component {
+  // highlight-range{1-8}
+  componentDidUpdate(prevProps, prevState) {
+    if (
+      this.state.someStatefulValue !==
+      prevState.someStatefulValue
+    ) {
+      this.props.onChange(this.state.someStatefulValue);
+    }
+  }
+}