Performance testing companion for React and React Native.

---

• The problem
• This solution
• Installation and setup
  • Writing your first test
    • Writing async tests
  • Measuring test performance
  • Write performance testing script
  • CI integration
  • Optional: ESLint setup
• Assessing CI stability
• Analyzing results
• API
  • Measurements
    • measurePerformance function
    • MeasureOptions type
  • Configuration
    • Default configuration
    • configure function
    • resetToDefault function
    • Environmental variables
• Contributing
• License
• Made with ❤️ at Callstack

The problem

You want your React Native app to perform well and fast at all times. As a part of this goal, you profile the app, observe render patterns, apply memoization in the right places, etc. But it's all manual and too easy to unintentionally introduce performance regressions that would only get caught during QA or worse, by your users.

This solution

Reassure allows you to automate React Native app performance regression testing on CI or a local machine. The same way you write your integration and unit tests that automatically verify that your app is still working correctly, you can write performance tests that verify that your app still working performantly.

You can think about it as a React performance testing library. In fact, Reassure is designed to reuse as much of your React Native Testing Library tests and setup as possible.

Reassure works by measuring render characteristics – duration and count – of the testing scenario you provide and comparing that to the stable version. It repeates the scenario multiple times to reduce impact of random variations in render times caused by the runtime environment. Then it applies statistical analysis to figure out whether the code changes are statistically significant or not. As a result, it generates a human-readable report summarizing the results and displays it on the CI or as a comment to your pull request.

Installation and setup

In order to install Reassure run following command in your app folder:

Using yarn

yarn add --dev reassure

Using npm

npm install --save-dev reassure

You will also need a working Jest setup as well as one of either React Native Testing Library or React Testing Library.

Note: React Native Testing Library is fully supported, while React Testing Library in beta stage.

You can check our example projects:

• React Native (CLI)
• React Native (Expo)

Reassure will try to detect which Testing Library you have installed. In case both React Native Testing Library and React Testing Library are present it will warn you about that and give a precedence to React Native Testing Library. You can explicitly specify Testing Library to by used by using configure option:

configure({ testingLibrary: 'react-native' })
// or
configure({ testingLibrary: 'react' })

You should set it in your Jest setup file and you can override it in particular test files if needed.

Writing your first test

Now that the library is installed, you can write you first test scenario in a file with .perf-test.js/.perf-test.tsx extension:

// ComponentUnderTest.perf-test.tsx
import { measurePerformance } from 'reassure';
import { ComponentUnderTest } from './ComponentUnderTest';

test('Simple test', async () => {
  await measurePerformance(<ComponentUnderTest />);
});

This test will measure render times of ComponentUnderTest during mounting and resulting sync effects.

Note: Reassure will automatically match test filenames using Jest's --testMatch option with value "<rootDir>/**/*.perf-test.[jt]s?(x)".

Writing async tests

If your component contains any async logic or you want to test some interaction you should pass the scenario option:

import { measurePerformance } from 'reassure';
import { screen, fireEvent } from '@testing-library/react-native';
import { ComponentUnderTest } from './ComponentUnderTest';

test('Test with scenario', async () => {
  const scenario = async () => {
    fireEvent.press(screen.getByText('Go'));
    await screen.findByText('Done');
  };

  await measurePerformance(<ComponentUnderTest />, { scenario });
});

The body of the scenario function is using familiar React Native Testing Library methods.

In case of using a version of React Native Testing Library lower than v10.1.0, where screen helper is not available, the scenario function provides it as its first argument:

import { measurePerformance } from 'reassure';
import { fireEvent } from '@testing-library/react-native';

test('Test with scenario', async () => {
  const scenario = async (screen) => {
    fireEvent.press(screen.getByText('Go'));
    await screen.findByText('Done');
  };

  await measurePerformance(<ComponentUnderTest />, { scenario });
});

If your test contains any async changes, you will need to make sure that the scenario waits for these changes to settle, e.g. using findBy queries, waitFor or waitForElementToBeRemoved functions from RNTL.

For more examples look into our test example app.

Measuring test performance

In order to measure your first test performance you need to run following command in terminal:

yarn reassure

This command will run your tests multiple times using Jest, gathering render statistics, and will write them to .reassure/current.perf file. In order to check your setup, check if the output file exists after running the command for the first time.

Note: You can add .reassure/ folder to your .gitignore file to avoid accidentally committing your results.

Reassure CLI will automatically try to detect your source code branch name and commit hash when you are using Git. You can override these options, e.g. if you are using different version control system:

yarn reassure --branch [branch name] --commit-hash [commit hash]

Write performance testing script

In order to detect performance changes, you need to measure the performance of two versions of your code current (your modified code), and baseline (your reference point, e.g. main branch). In order to measure performance on two different branches you need to either switch branches in git or clone two copies of your repository.

We want to automate this task, so it can run on the CI. In order to do that you will need to create a performance testing script. You should save it in your repository, e.g. as reassure-tests.sh.

A simple version of such script, using branch changing approach is as follows:

#!/usr/bin/env bash
set -e

BASELINE_BRANCH=${BASELINE_BRANCH:="main"}

# Required for `git switch` on CI
git fetch origin

# Gather baseline perf measurements
git switch "$BASELINE_BRANCH"
yarn install --force
yarn reassure --baseline

# Gather current perf measurements & compare results
git switch --detach -
yarn install --force
yarn reassure

CI integration

As a final setup step you need to configure your CI to run the performance testing script and output the result. For presenting output at the moment we integrate with Danger JS, which supports all major CI tools.

You will need a working Danger JS setup.

Then add Reassure Danger JS plugin to your dangerfile :

import path from 'path';
import { dangerReassure } from 'reassure';

dangerReassure({
  inputFilePath: path.join(__dirname, '.reassure/output.md'),
});

You can also check our example Dangerfile.

Finally run both performance testing script & danger in your CI config:

- name: Run performance testing script
  run: ./reassure-tests.sh

- name: Run Danger.js
  run: yarn danger ci
  env:
    GITHUB_TOKEN: ${{ secrets.GITHUB_TOKEN }}

You can also check our example GitHub workflow.

Note: Your performance test will run much longer than regular integration tests. It's because we run each test scenario multiple times (by default 10), and we repeat that for two branches of your code. Hence, each test will run 20 times by default. That's unless you increase that number even higher.

Optional: ESLint setup

ESLint might require you to have at least one expect statement in each of your tests. In order to avoid this requirement for performance tests you can add following override to your .eslintrc file:

rules: {
 'jest/expect-expect': [
 'error',
    { assertFunctionNames: ['expect', 'measurePerformance'] },
  ],
}

Assessing CI stability

During performance measurements we measure React component render times with microsecond precision using React.Profiler. This means that the same code will run faster or slower depending on the machine. For this reason, baseline & current measurements need to be run on the same machine. Optimally, they should be run one after another.

Moreover, in order to achieve meaningful results your CI agent needs to have stable performance. It does not matter really if your agent is fast or slow as long as it is consistent in its performance. That's why during the performance tests the agent should not be used for any other work that might impact measuring render times.

In order to help you assess your machine stability, you can use reassure check-stability command. It runs performance measurements twice for the current code, so baseline and current measurements refer to the same code. In such case the expected changes are 0% (no change). The degree of random performance changes will reflect the stability of your machine. This command can be run both on CI and local machines.

Normally, the random changes should be below 5%. Results of 10% and more considered too high and mean that you should work on tweaking your machine stability.

Note: As a trick of last resort you can increase the run option, from the default value of 10 to 20, 50 or even 100, for all or some of your tests, based on the assumption that more test runs will even out measurement fluctuations. That will however make your tests run even longer.

You can refer to our example GitHub workflow.

Analyzing results

Looking at the example you can notice that test scenarios can be assigned to certain categories:

• Significant Changes To Render Duration shows test scenario where the change is statistically significant and should be looked into as it marks a potential performance loss/improvement
• Meaningless Changes To Render Duration shows test scenarios where the change is not stastatistically significant
• Changes To Render Count shows test scenarios where render count did change
• Added Scenarios shows test scenarios which do not exist in the baseline measurements
• Removed Scenarios shows test scenarios which do not exist in the current measurements

API

Measurements

measurePerformance function

Custom wrapper for the RNTL render function responsible for rendering the passed screen inside a React.Profiler component, measuring its performance and writing results to the output file. You can use optional options object allows customizing aspects of the testing

async function measurePerformance(ui: React.ReactElement, options?: MeasureOptions): Promise<MeasureRenderResult> {

MeasureOptions type

interface MeasureOptions {
  runs?: number;
  dropWorst?: number;
  wrapper?: (node: React.ReactElement) => JSX.Element;
  scenario?: (view?: RenderResult) => Promise<any>;
}

• runs: number of runs per series for the particular test
• dropWorst: number of worst (highest) runs dropped from a test series
• wrapper: custom JSX wrapper, such as a <Provider /> component, which the ui needs to be wrapped with
• scenario: a custom async function, which defines user interaction within the ui by utilized RNTL functions

Configuration

Default configuration

The default config which will be used by the measuring script. This configuration object can be overridden with the use of the configure function.

type Config = {
  runs?: number;
  dropWorst?: number;
  outputFile?: string;
  verbose?: boolean;
  testingLibrary?:
    | 'react-native'
    | 'react'
    | { render: (component: React.ReactElement<any>) => any; cleanup: () => any };
};

const defaultConfig: Config = {
  runs: 10,
  dropWorst: 1,
  outputFile: '.reassure/current.perf',
  verbose: false,
  testingLibrary: undefined, // Will try auto-detect first RNTL, then RTL
};

runs: number of repeated runs in a series per test (allows for higher accuracy by aggregating more data). Should be handled with care. dropWorst: number of worst dropped results from the series per test (used to remove test run outliers) outputFile: name of the file the records will be saved to verbose: make Reassure log more, e.g. for debugging purposes testingLibrary: where to look for render and cleanup functions, supported values 'react-native', 'react' or object providing custom render and cleanup functions

configure function

function configure(customConfig: Partial<Config>): void;

You can use the configure function to override the default config parameters.

resetToDefault function

resetToDefault(): void

Reset current config to the original defaultConfig object

Environmental variables

You can use available environmental variables in order to alter your test runner settings.

• TEST_RUNNER_PATH: an alternative path for your test runner. Defaults to 'node_modules/.bin/jest' or on Windows 'node_modules/jest/bin/jest'
• TEST_RUNNER_ARGS: a set of arguments fed to the runner. Defaults to '--runInBand --testMatch "<rootDir>/**/*.perf-test.[jt]s?(x)"'

Example:

TEST_RUNNER_PATH=myOwnPath/jest/bin yarn reassure

External References

• The Ultimate Guide to React Native Optimization 2023 Edition - Mentioned in "Make your app consistently fast" chapter.

Contributing

See the contributing guide to learn how to contribute to the repository and the development workflow.

License

MIT

Made with ❤️ at Callstack

Reassure is an Open Source project and will always remain free to use. The project has been developed in close partnership with Entain and was originally their in-house project. Thanks to their willingness to develop the React & React Native ecosystem, we decided to make it Open Source. If you think it's cool, please star it 🌟

Callstack is a group of React and React Native experts. If you need any help with these or just want to say hi, contact us at hello@callstack.com!

Like the project? ⚛️ Join the Callstack team who does amazing stuff for clients and drives React Native Open Source! 🔥