A compelling reason for using React Native instead of WebView-based tools is to achieve 60 frames per second and a native look and 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.
We try our best to deliver buttery-smooth UI performance by default, but sometimes that just isn't possible.
This guide is intended to teach you some basics to help you to [troubleshoot performance issues](docs/performance.html#profiling),
as well as discuss [common sources of problems and their suggested solutions](docs/performance.html#common-sources-of-performance-problems).
## What you need to know about frames
Your grandparents' generation called movies ["moving pictures"](https://www.youtube.com/watch?v=F1i40rnpOsA) 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 Perf Monitor`.
You will notice that there are two different frame rates.
### JS frame rate (JavaScript thread)
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](http://jankfree.org/) 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.
### UI frame rate (main thread)
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.
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, but their receipt is not necessary for the scroll to occur.
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. Always make sure to test performance in [release builds](docs/running-on-device.html#building-your-app-for-production).
You can also use this [babel plugin](https://babeljs.io/docs/plugins/transform-remove-console/) that removes all the `console.*` calls. You need to install it first with `npm i babel-plugin-transform-remove-console --save`, and then edit the `.babelrc` file under your project directory like this:
```json
{
"env": {
"production": {
"plugins": ["transform-remove-console"]
}
}
}
```
This will automatically remove all `console.*` calls in the release (production) versions of your project.
### `ListView` initial rendering is too slow or scroll performance is bad for large lists
Use the new [`FlatList`](docs/flatlist.html) or [`SectionList`](docs/sectionlist.html) component instead.
Besides simplifying the API, the new list components also have significant performance enhancements,
the main one being nearly constant memory usage for any number of rows.
### 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.
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.
One 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.
Solving this is one of the main goals behind the new [React Navigation](docs/navigation.html) library.
The views in React Navigation use native components and the [`Animated`](docs/animated.html) library to deliver 60 FPS animations that are run on the native thread.
Use the built-in profiler to get detailed information about work done in the JavaScript thread and main thread side-by-side.
Access it by selecting Perf Monitor from the Debug menu.
For iOS, Instruments is an invaluable tool, and on Android you should learn to use [`systrace`](docs/performance.html#profiling-android-ui-performance-with-systrace).
You can also use [`react-addons-perf`](https://facebook.github.io/react/docs/perf.html) to get insights into where React is spending time when rendering your components.
Another way to profile JavaScript is to use the Chrome profiler while debugging.
This won't give you accurate results as the code is running in Chrome but will give you a general idea of where bottlenecks might be.
But first, [**make sure that Development Mode is OFF!**](docs/performance.html#running-in-development-mode-dev-true) You should see `__DEV__ === false, development-level warning are OFF, performance optimizations are ON` in your application logs.
### Profiling Android UI Performance with `systrace`
Android supports 10k+ different phones and is generalized to support software rendering:
the framework architecture and need to generalize across many hardware targets unfortunately means you get less for free relative to iOS.
But sometimes, there are things you can improve -- and many times it's not native code's fault at all!
The first step for debugging this jank is to answer the fundamental question of where your time is being spent during each 16ms frame.
For that, we'll be using a standard Android profiling tool called `systrace`.
`systrace` is a standard Android marker-based profiling tool (and is installed when you install the Android platform-tools package).
Both the Android SDK and React Native framework provide standard markers that you can visualize.
#### 1. Collecting a trace
First, connect a device that exhibits the stuttering you want to investigate to your computer via USB and get it to the point right before the navigation/animation you want to profile.
Run `systrace` as follows:
```
$ <path_to_android_sdk>/platform-tools/systrace/systrace.py --time=10 -o trace.html sched gfx view -a <your_package_name>
```
A quick breakdown of this command:
-`time` is the length of time the trace will be collected in seconds
-`sched`, `gfx`, and `view` are the android SDK tags (collections of markers) we care about: `sched` gives you information about what's running on each core of your phone, `gfx` gives you graphics info such as frame boundaries, and `view` gives you information about measure, layout, and draw passes
-`-a <your_package_name>` enables app-specific markers, specifically the ones built into the React Native framework. `your_package_name` can be found in the `AndroidManifest.xml` of your app and looks like `com.example.app`
Once the trace starts collecting, perform the animation or interaction you care about. At the end of the trace, systrace will give you a link to the trace which you can open in your browser.
#### 2. Reading the trace
After opening the trace in your browser (preferably Chrome), you should see something like this:
> **HINT**:
> Use the WASD keys to strafe and zoom
If your trace .html file isn't opening correctly, check your browser console for the following:
Since `Object.observe` was deprecated in recent browsers, you may have to open the file from the Google Chrome Tracing tool. You can do so by:
- Opening tab in chrome chrome://tracing
- Selecting load
- Selecting the html file generated from the previous command.
> **Enable VSync highlighting**
>
> Check this checkbox at the top right of the screen to highlight the 16ms frame boundaries:
> You should see zebra stripes as in the screenshot above.
> If you don't, try profiling on a different device: Samsung has been known to have issues displaying vsyncs while the Nexus series is generally pretty reliable.
#### 3. Find your process
Scroll until you see (part of) the name of your package.
In this case, I was profiling `com.facebook.adsmanager`,
which shows up as `book.adsmanager` because of silly thread name limits in the kernel.
On the left side, you'll see a set of threads which correspond to the timeline rows on the right.
There are a few threads we care about for our purposes:
the UI thread (which has your package name or the name UI Thread), `mqt_js`, and `mqt_native_modules`.
If you're running on Android 5+, we also care about the Render Thread.
- **UI Thread.**
This is where standard android measure/layout/draw happens.
The thread name on the right will be your package name (in my case book.adsmanager) or UI Thread.
The events that you see on this thread should look something like this and have to do with `Choreographer`, `traversals`, and `DispatchUI`:
A smooth animation should look something like the following:
Each change in color is a frame -- remember that in order to display a frame,
all our UI work needs to be done by the end of that 16ms period.
Notice that no thread is working close to the frame boundary.
An application rendering like this is rendering at 60 FPS.
If you noticed chop, however, you might see something like this:
Notice that the JS thread is executing basically all the time, and across frame boundaries!
This app is not rendering at 60 FPS.
In this case, **the problem lies in JS**.
You might also see something like this:
In this case, the UI and render threads are the ones that have work crossing frame boundaries.
The UI that we're trying to render on each frame is requiring too much work to be done.
In this case, **the problem lies in the native views being rendered**.
At this point, you'll have some very helpful information to inform your next steps.
#### Resolving JavaScript issues
If you identified a JS problem,
look for clues in the specific JS that you're executing.
In the scenario above, we see `RCTEventEmitter` being called multiple times per frame.
Here's a zoom-in of the JS thread from the trace above:
This doesn't seem right.
Why is it being called so often?
Are they actually different events?
The answers to these questions will probably depend on your product code.
And many times, you'll want to look into [shouldComponentUpdate](https://facebook.github.io/react/docs/component-specs.html#updating-shouldcomponentupdate).
#### Resolving native UI Issues
If you identified a native UI problem, there are usually two scenarios:
2. You're constructing new UI during the animation/interaction (e.g. loading in new content during a scroll).
##### Too much GPU work
In the first scenario, you'll see a trace that has the UI thread and/or Render Thread looking like this:
Notice the long amount of time spent in `DrawFrame` that crosses frame boundaries. This is time spent waiting for the GPU to drain its command buffer from the previous frame.
To mitigate this, you should:
- investigate using `renderToHardwareTextureAndroid` for complex, static content that is being animated/transformed (e.g. the `Navigator` slide/alpha animations)
- make sure that you are **not** using `needsOffscreenAlphaCompositing`, which is disabled by default, as it greatly increases the per-frame load on the GPU in most cases.
If these don't help and you want to dig deeper into what the GPU is actually doing, you can check out [Tracer for OpenGL ES](http://developer.android.com/tools/help/gltracer.html).
##### Creating new views on the UI thread
In the second scenario, you'll see something more like this:
Notice that first the JS thread thinks for a bit, then you see some work done on the native modules thread, followed by an expensive traversal on the UI thread.
There isn't an easy way to mitigate this unless you're able to postpone creating new UI until after the interaction, or you are able to simplify the UI you're creating. The react native team is working on a infrastructure level solution for this that will allow new UI to be created and configured off the main thread, allowing the interaction to continue smoothly.
