Mock 浏览器 API

简介

Playwright 为大多数浏览器功能提供了原生支持。但是，仍有一些实验性 API 或尚未被所有浏览器完全支持的 API。在这种情况下，Playwright 通常不提供专门的自动化 API。你可以使用 Mock（模拟）来测试应用程序在此类情况下的行为。本指南提供了一些示例。

让我们考虑一个使用 Battery API 来显示设备电池状态的 Web 应用。我们将模拟 Battery API，并检查页面是否正确显示了电池状态。

创建 Mock

由于页面可能会在加载时尽早调用 API，因此在页面开始加载之前设置好所有 Mock 非常重要。实现这一点的最简单方法是调用 page.addInitScript()。

await page.addInitScript(() => {
  const mockBattery = {
    level: 0.75,
    charging: true,
    chargingTime: 1800,
    dischargingTime: Infinity,
    addEventListener: () => { }
  };
  // Override the method to always return mock battery info.
  window.navigator.getBattery = async () => mockBattery;
});

一旦完成此操作，你就可以导航到页面并检查其 UI 状态。

// Configure mock API before each test.
test.beforeEach(async ({ page }) => {
  await page.addInitScript(() => {
    const mockBattery = {
      level: 0.90,
      charging: true,
      chargingTime: 1800, // seconds
      dischargingTime: Infinity,
      addEventListener: () => { }
    };
    // Override the method to always return mock battery info.
    window.navigator.getBattery = async () => mockBattery;
  });
});

test('show battery status', async ({ page }) => {
  await page.goto('/');
  await expect(page.locator('.battery-percentage')).toHaveText('90%');
  await expect(page.locator('.battery-status')).toHaveText('Adapter');
  await expect(page.locator('.battery-fully')).toHaveText('00:30');
});

模拟只读 API

某些 API 是只读的，因此你无法直接为 navigator 属性赋值。例如：

// Following line will have no effect.
navigator.cookieEnabled = true;

但是，如果该属性是 可配置的 (configurable)，你仍然可以使用原生 JavaScript 将其覆盖。

await page.addInitScript(() => {
  Object.defineProperty(Object.getPrototypeOf(navigator), 'cookieEnabled', { value: false });
});

验证 API 调用

有时，检查页面是否进行了所有预期的 API 调用会很有用。你可以记录所有 API 方法的调用情况，然后将其与预期结果进行比较。page.exposeFunction() 在将消息从页面传回测试代码时非常有用。

test('log battery calls', async ({ page }) => {
  const log = [];
  // Expose function for pushing messages to the Node.js script.
  await page.exposeFunction('logCall', msg => log.push(msg));
  await page.addInitScript(() => {
    const mockBattery = {
      level: 0.75,
      charging: true,
      chargingTime: 1800,
      dischargingTime: Infinity,
      // Log addEventListener calls.
      addEventListener: (name, cb) => logCall(`addEventListener:${name}`)
    };
    // Override the method to always return mock battery info.
    window.navigator.getBattery = async () => {
      logCall('getBattery');
      return mockBattery;
    };
  });

  await page.goto('/');
  await expect(page.locator('.battery-percentage')).toHaveText('75%');

  // Compare actual calls with golden.
  expect(log).toEqual([
    'getBattery',
    'addEventListener:chargingchange',
    'addEventListener:levelchange'
  ]);
});

更新 Mock

为了测试应用是否正确反映了电池状态的更新，确保模拟的电池对象触发与浏览器实现相同的事件至关重要。以下测试演示了如何实现这一点。

test('update battery status (no golden)', async ({ page }) => {
  await page.addInitScript(() => {
    // Mock class that will notify corresponding listeners when battery status changes.
    class BatteryMock {
      level = 0.10;
      charging = false;
      chargingTime = 1800;
      dischargingTime = Infinity;
      _chargingListeners = [];
      _levelListeners = [];
      addEventListener(eventName, listener) {
        if (eventName === 'chargingchange')
          this._chargingListeners.push(listener);
        if (eventName === 'levelchange')
          this._levelListeners.push(listener);
      }
      // Will be called by the test.
      _setLevel(value) {
        this.level = value;
        this._levelListeners.forEach(cb => cb());
      }
      _setCharging(value) {
        this.charging = value;
        this._chargingListeners.forEach(cb => cb());
      }
    }
    const mockBattery = new BatteryMock();
    // Override the method to always return mock battery info.
    window.navigator.getBattery = async () => mockBattery;
    // Save the mock object on window for easier access.
    window.mockBattery = mockBattery;
  });

  await page.goto('/');
  await expect(page.locator('.battery-percentage')).toHaveText('10%');

  // Update level to 27.5%
  await page.evaluate(() => window.mockBattery._setLevel(0.275));
  await expect(page.locator('.battery-percentage')).toHaveText('27.5%');
  await expect(page.locator('.battery-status')).toHaveText('Battery');

  // Emulate connected adapter
  await page.evaluate(() => window.mockBattery._setCharging(true));
  await expect(page.locator('.battery-status')).toHaveText('Adapter');
  await expect(page.locator('.battery-fully')).toHaveText('00:30');
});