react-native/docs/Performance.md

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A compelling reason for using React Native instead of WebView-based tools is to achieve 60 FPS and a native look & feel to your apps. Where possible, we would like for React Native to do the right thing and help you to focus on your app instead of performance optimization, but there are areas where we're not quite there yet, and others where React Native (similar to writing native code directly) cannot possibly determine the best way to optimize for you and so manual intervention will be necessary.

This guide is intended to teach you some basics to help you to troubleshoot performance issues, as well as discuss common sources of problems and their suggested solutions.

What you need to know about frames

Your grandparents' generation called movies "moving pictures" for a reason: realistic motion in video is an illusion created by quickly changing static images at a consistent speed. We refer to each of these images as frames. The number of frames that is displayed each second has a direct impact on how smooth and ultimately life-like a video (or user interface) seems to be. iOS devices display 60 frames per second, which gives you and the UI system about 16.67ms to do all of the work needed to generate the static image (frame) that the user will see on the screen for that interval. If you are unable to do the work necessary to generate that frame within the allotted 16.67ms, then you will "drop a frame" and the UI will appear unresponsive.

Now to confuse the matter a little bit, open up the developer menu in your app and toggle Show FPS Monitor. You will notice that there are two different frame rates.

JavaScript frame rate

For most React Native applications, your business logic will run on the JavaScript thread. This is where your React application lives, API calls are made, touch events are processed, etc... Updates to native-backed views are batched and sent over to the native side at the end of each iteration of the event loop, before the frame deadline (if all goes well). If the JavaScript thread is unresponsive for a frame, it will be considered a dropped frame. For example, if you were to call this.setState on the root component of a complex application and it resulted in re-rendering computationally expensive component subtrees, it's conceivable that this might take 200ms and result in 12 frames being dropped. Any animations controlled by JavaScript would appear to freeze during that time. If anything takes longer than 100ms, the user will feel it.

This often happens during Navigator transitions: when you push a new route, the JavaScript thread needs to render all of the components necessary for the scene in order to send over the proper commands to the native side to create the backing views. It's common for the work being done here to take a few frames and cause jank because the transition is controlled by the JavaScript thread. Sometimes components will do additional work on componentDidMount, which might result in a second stutter in the transition.

Another example is responding to touches: if you are doing work across multiple frames on the JavaScript thread, you might notice a delay in responding to TouchableOpacity, for example. This is because the JavaScript thread is busy and cannot process the raw touch events sent over from the main thread. As a result, TouchableOpacity cannot react to the touch events and command the native view to adjust its opacity.

Main thread (aka UI thread) frame rate

Many people have noticed that performance of NavigatorIOS is better out of the box than Navigator. The reason for this is that the animations for the transitions are done entirely on the main thread, and so they are not interrupted by frame drops on the JavaScript thread. (Read about why you should probably use Navigator anyways.)

Similarly, you can happily scroll up and down through a ScrollView when the JavaScript thread is locked up because the ScrollView lives on the main thread (the scroll events are dispatched to the JS thread though, but their receipt is not necessary for the scroll to occur).

Common sources of performance problems

Development mode (dev=true)

JavaScript thread performance suffers greatly when running in dev mode. This is unavoidable: a lot more work needs to be done at runtime to provide you with good warnings and error messages, such as validating propTypes and various other assertions.

Slow navigator transitions

As mentioned above, Navigator animations are controlled by the JavaScript thread. Imagine the "push from right" scene transition: each frame, the new scene is moved from the right to left, starting offscreen (let's say at an x-offset of 320) and ultimately settling when the scene sits at an x-offset of 0. Each frame during this transition, the JavaScript thread needs to send a new x-offset to the main thread. If the JavaScript thread is locked up, it cannot do this and so no update occurs on that frame and the animation stutters.

Part of the long-term solution to this is to allow for JavaScript-based animations to be offloaded to the main thread. If we were to do the same thing as in the above example with this approach, we might calculate a list of all x-offsets for the new scene when we are starting the transition and send them to the main thread to execute in an optimized way. Now that the JavaScript thread is freed of this responsibility, it's not a big deal if it drops a few frames while rendering the scene -- you probably won't even notice because you will be too distracted by the pretty transition.

Unfortunately this solution is not yet implemented, and so in the meantime we should use the InteractionManager to selectively render the minimal amount of content necessary for the new scene as long as the animation is in progress. InteractionManager.runAfterInteractions takes a callback as its only argument, and that callback is fired when the navigator transition is complete (each animation from the Animated API also notifies the InteractionManager, but that's beyond the scope of this discussion).

Your scene component might look something like this:

class ExpensiveScene extends React.Component {
  constructor(props, context) {
    super(props, context);
    this.state = {renderPlaceholderOnly: true};
  }

  componentDidMount() {
    InteractionManager.runAfterInteractions(() => {
      this.setState({renderPlaceholderOnly: false});
    });
  }

  render() {
    if (this.state.renderPlaceholderOnly) {
      return this._renderPlaceholderView();
    }

    return (
      <View>
        <Text>Your full view goes here</Text>
      </View>
    );
  }


  _renderPlaceholderView() {
    return (
      <View>
        <Text>Loading...</Text>
      </View>
    );
  }
};

You don't need to be limited to rendering some loading indicator, you could alternatively render part of your content -- for example, when you load the Facebook app you see a placeholder news feed item with grey rectangles where text will be. If you are rendering a Map in your new scene, you might want to display a grey placeholder view or a spinner until the transition is complete as this can actually cause frames to be dropped on the main thread.

ListView initial rendering is too slow or scroll performance is bad for large lists

This is an issue that comes up frequently because iOS ships with UITableView which gives you very good performance by re-using underlying UIViews. Work is in progress to do something similar with React Native, but until then we have some tools at our disposal to help us tweak the performance to suit our needs. It may not be possible to get all the way there, but a little bit of creativity and experimentation with these options can go a long way.

initialListSize

This prop specifies how many rows we want to render on our first render pass. If we are concerned with getting something on screen as quickly as possible, we could set the initialListSize to 1, and we'll quickly see other rows fill in on subsequent frames. The number of rows per frame is determined by the pageSize.

pageSize

After the initial render where initialListSize is used, ListView looks at the pageSize to determine how many rows to render per frame. The default here is 1 -- but if your views are very small and inexpensive to render, you might want to bump this up. Tweak it and find what works for your use case.

scrollRenderAheadDistance

"How early to start rendering rows before they come on screen, in pixels."

If we had a list with 2000 items and rendered them all immediately that would be a poor use of both memory and computational resources. It would also probably cause some pretty awful jank. So the scrollRenderAhead distance allows us to specify for far beyond the current viewport we should continue to render rows.

removeClippedSubviews

"When true, offscreen child views (whose overflow value is hidden) are removed from their native backing superview when offscreen. This can improve scrolling performance on long lists. The default value is true."(The default value is false before version 0.14-rc).

This is an extremely important optimization to apply on large ListViews. On Android the overflow value is always hidden so you don't need to worry about setting it, but on iOS you need to be sure to set overflow: hidden on row containers.

My component renders too slowly and I don't need it all immediately

It's common at first to overlook ListView, but using it properly is often key to achieving solid performance. As discussed above, it provides you with a set of tools that lets you split rendering of your view across various frames and tweak that behavior to fit your specific needs. Remember that ListView can be horizontal too.

JS FPS plunges when re-rendering a view that hardly changes

If you are using a ListView, you must provide a rowHasChanged function that can reduce a lot of work by quickly determining whether or not a row needs to be re-rendered. If you are using immutable data structures, this would be as simple as a reference equality check.

Similarly, you can implement shouldComponentUpdate and indicate the exact conditions under which you would like the component to re-render. If you write pure components (where the return value of the render function is entirely dependent on props and state), you can leverage PureRenderMixin to do this for you. Once again, immutable data structures are useful to keep this fast -- if you have to do a deep comparison of a large list of objects, it may be that re-rendering your entire component would be quicker, and it would certainly require less code.

Dropping JS thread FPS because of doing a lot of work on the JavaScript thread at the same time

"Slow Navigator transitions" is the most common manifestation of this, but there are other times this can happen. Using InteractionManager can be a good approach, but if the user experience cost is too high to delay work during an animation, then you might want to consider LayoutAnimation.

The Animated api currently calculates each keyframe on-demand on the JavaScript thread, while LayoutAnimation leverages Core Animation and is unaffected by JS thread and main thread frame drops.

One case where I have used this is for animating in a modal (sliding down from top and fading in a translucent overlay) while initializing and perhaps receiving responses for several network requests, rendering the contents of the modal, and updating the view where the modal was opened from. See the Animations guide for more information about how to use LayoutAnimation.

Caveats:

• LayoutAnimation only works for fire-and-forget animations ("static" animations) -- if it must be be interruptible, you will need to use Animated.

Moving a view on the screen (scrolling, translating, rotating) drops UI thread FPS

This is especially true when you have text with a transparent background positioned on top of an image, or any other situation where alpha compositing would be required to re-draw the view on each frame. You will find that enabling shouldRasterizeIOS or renderToHardwareTextureAndroid can help with this significantly.

Be careful not to overuse this or your memory usage could go through the roof. Profile your performance and memory usage when using these props. If you don't plan to move a view anymore, turn this property off.

Animating the size of an image drops UI thread FPS

On iOS, each time you adjust the width or height of an Image component it is re-cropped and scaled from the original image. This can be very expensive, especially for large images. Instead, use the transform: [{scale}] style property to animate the size. An example of when you might do this is when you tap an image and zoom it in to full screen.

My TouchableX view isn't very responsive

Sometimes, if we do an action in the same frame that we are adjusting the opacity or highlight of a component that is responding to a touch, we won't see that effect until after the onPress function has returned. If onPress does a setState that results in a lot of work and a few frames dropped, this may occur. A solution to this is to wrap any action inside of your onPress handler in requestAnimationFrame:

handleOnPress() {
  // Always use TimerMixin with requestAnimationFrame, setTimeout and
  // setInterval
  this.requestAnimationFrame(() => {
    this.doExpensiveAction();
  });
}

Profiling

Use the built-in Profiler to get detailed information about work done in the JavaScript thread and main thread side-by-side.

For iOS, Instruments are an invaluable tool, and on Android you should learn to use systrace.