在 Hermes 中进行性能分析

你可以使用Hermes在React Native应用中可视化JavaScript的性能。Hermes是一个小型且轻量的JavaScript引擎（你可以在这里阅读更多有关在React Native中使用它的信息）。Hermes有助于提高应用性能，并且还提供了分析其运行的JavaScript性能的方式。

在本节中，您将学习如何对在Hermes上运行的React Native应用进行分析，并如何使用Chrome DevTools上的性能选项卡可视化配置文件。

注意

Be sure to enable Hermes in your app before you get started!

Follow the instructions below to get started profiling:

1. Record a Hermes sampling profile
2. Execute command from CLI
3. Open the downloaded profile on Chrome DevTools

Record a Hermes sampling profile

To record a sampling profiler from the Dev Menu:

1. Navigate to your running Metro server terminal.
2. Press d to open the Dev Menu.
3. Select Enable Sampling Profiler.
4. Execute your JavaScript by in your app (press buttons, etc.)
5. Open the Dev Menu by pressing d again.
6. Select Disable Sampling Profiler to stop recording and save the sampling profiler.

A toast will show the location where the sampling profiler has been saved, usually in /data/user/0/com.appName/cache/*.cpuprofile

Execute command from CLI

You can use the React Native CLI to convert the Hermes tracing profile to Chrome tracing profile, and then pull it to your local machine using:

npx react-native profile-hermes [destinationDir]

Enabling source map

info

You may read about source maps on the Debugging Release Builds page.

Common errors

adb: no devices/emulators found or adb: device offline

• Why this happens The CLI cannot access the device or emulator (through adb) you are using to run the app.
• How to fix Make sure your Android device/emulator is connected and running. The command only works when it can access adb.

There is no file in the cache/ directory

• Why this happens The CLI cannot find any .cpuprofile file in your app's cache/ directory. You might have forgotten to record a profile from the device.
• How to fix Follow the instructions to enable/disable profiler from device.

Error: your_profile_name.cpuprofile is an empty file

• Why this happens The profile is empty, it might be because Hermes is not running correctly.
• How to fix Make sure your app is running on the latest version of Hermes.

Open the downloaded profile in Chrome DevTools

To open the profile in Chrome DevTools:

1. Open Chrome DevTools.
2. Select the Performance tab.
3. Right click and choose Load profile...

How does the Hermes Profile Transformer work?

The Hermes Sample Profile is of the JSON object format, while the format that Google's DevTools supports is JSON Array Format. (More information about the formats can be found on the Trace Event Format Document)

export interface HermesCPUProfile {
  traceEvents: SharedEventProperties[];
  samples: HermesSample[];
  stackFrames: {[key in string]: HermesStackFrame};
}

The Hermes profile has most of its information encoded into the samples and stackFrames properties. Each sample is a snapshot of the function call stack at that particular timestamp as each sample has a sf property which corresponds to a function call.

export interface HermesSample {
  cpu: string;
  name: string;
  ts: string;
  pid: number;
  tid: string;
  weight: string;
  /**
   * Will refer to an element in the stackFrames object of the Hermes Profile
   */
  sf: number;
  stackFrameData?: HermesStackFrame;
}

The information about a function call can be found in stackFrames which contains key-object pairs, where the key is the sf number and the corresponding object gives us all the relevant information about the function including the sf number of its parent function. This parent-child relationship can be traced upwards to find the information of all the functions running at a particular timestamp.

export interface HermesStackFrame {
  line: string;
  column: string;
  funcLine: string;
  funcColumn: string;
  name: string;
  category: string;
  /**
   * A parent function may or may not exist
   */
  parent?: number;
}

At this point, you should define a few more terms, namely:

1. Nodes: The objects corresponding to sf numbers in stackFrames
2. Active Nodes: The nodes which are currently running at a particular timestamp. A node is classified as running if its sf number is in the function call stack. This call stack can be obtained from the sf number of the sample and tracing upwards till parent sfs are available

The samples and the stackFrames in tandem can then be used to generate all the start and end events at the corresponding timestamps, wherein:

1. Start Nodes/Events: Nodes absent in the previous sample's function call stack but present in the current sample's.
2. End Nodes/Events: Nodes present in the previous sample's function call stack but absent in the current sample's.

You can now construct a flamechart of function calls as you have all the function information including its start and end timestamps.

The hermes-profile-transformer can convert any profile generated using Hermes into a format that can be directly displayed in Chrome DevTools. More information about this can be found on @react-native-community/hermes-profile-transformer