HarmonyOS Next——鸿蒙监听App前后台切换的3种方式及applicationStateChange监听失效问题

1. 利用 applicationStateChange 监听前后台变化

参考文档：

• ApplicationContext.on('applicationStateChange')
• @ohos.app.ability.ApplicationStateChangeCallback (ApplicationStateChangeCallback)

该 API 目前已知问题：

1. pad 设备更新到 SP6 新版系统后，对应 API 前后台时不执行回调
2. 手机设备在最新的 135 SP6 系统，依旧是正常的

• 示例代码：

private _stateCallback?: ApplicationStateCallback
// 利用对象字面量初始化 ApplicationStateChangeCallback class
// 这是官方示例的写法，API 的设计方式比较奇怪
private _applicationStateChangeCallback: ApplicationStateChangeCallback = {
  // 换用箭头函数改变 this 指向
  onApplicationForeground: () => {
    if (this._stateCallback) {
      this._stateCallback('Foreground')
    }
  },
  onApplicationBackground: () => {
    if (this._stateCallback) {
      this._stateCallback('Background')
    }
  }
}

// 启动App前后台监听
startWatchingApplicationStateChange(callback: ApplicationStateCallback) {
  this._stateCallback = callback
  // 间接应用的应用上下文
  let applicationContext = Util.getContext().getApplicationContext()
  try {
    if (applicationContext != undefined) {
      applicationContext.on('applicationStateChange', this._applicationStateChangeCallback)
    }
  } catch (err) {
  }
}

// 停止App前后台监听
stopWatchingApplicationStateChange() {
  let applicationContext = Util.getContext().getApplicationContext()
  try {
    if (applicationContext != undefined) {
      applicationContext.off('applicationStateChange', this._applicationStateChangeCallback)
      this._stateCallback = undefined
    }
  } catch (er) {
  }
}

2. 利用 windowEvent 监听窗口生命周期变化

参考文档：

• on('windowEvent')
• WindowEventType

名称	值	说明
WINDOW_SHOWN	1	切到前台。系统能力： SystemCapability.WindowManager.WindowManager.Core。
WINDOW_ACTIVE	2	获焦状态。系统能力： SystemCapability.WindowManager.WindowManager.Core。
WINDOW_INACTIVE	3	失焦状态。系统能力： SystemCapability.WindowManager.WindowManager.Core。
WINDOW_HIDDEN	4	切到后台。系统能力： SystemCapability.WindowManager.WindowManager.Core。
WINDOW_DESTROYED11+	7	窗口销毁。系统能力： SystemCapability.Window.SessionManager。

这里可以根据主窗口的生命周期变化，间接监听 App 的前后台事件：

// 获取 App 主窗口
const windowStage = Util.getWindowStage()
windowStage.getMainWindowSync().on('windowEvent', (windowEventType: window.WindowEventType) => {
  console.log('windowEventType: ', windowEventType)
})

pad 设备切换前后台时，窗口生命周期变化的日志对应关系：

// App 切后台
windowEventType: 3
windowEventType: 4
// App 切前台
windowEventType: 1
windowEventType: 2

3. 利用 abilityLifecycle 监听应用生命周期

注册 abilityLifecycle 监听应用内生命周期，可以使用callback做异步回调，该 API 仅支持主线程调用。

• ApplicationContext.on('abilityLifecycle')
• @ohos.app.ability.abilityLifecycleCallback (AbilityLifecycleCallback)

该 API 目前已知问题：

1. pad 设备最新版本系统，前后台对应的生命周期同样无法监听到，但可以监听到窗口失焦和获焦的状态
2. 手机设备整体 API 表现正常

pad 设备切换前后台时，应用内生命周期变化的调试日志：

// App 切后台
AbilityLifecycleCallback onWindowStageInactive
// App 切前台
AbilityLifecycleCallback onWillNewWant
AbilityLifecycleCallback onNewWant
AbilityLifecycleCallback onWindowStageActive

手机设备切换前后台时，相对完整的生命周期示意日志：

// App 切后台
AbilityLifecycleCallback onWindowStageInactive
AbilityLifecycleCallback onAbilityWillBackground
AbilityLifecycleCallback onAbilityBackground
// App 切前台
AbilityLifecycleCallback onWillNewWant
AbilityLifecycleCallback onNewWant
AbilityLifecycleCallback onAbilityWillForeground
AbilityLifecycleCallback onAbilityForeground
AbilityLifecycleCallback onWindowStageActive

这里对应窗口生命周期的失焦获焦有：

1. 注册监听应用上下文的生命周期后，在windowStage失焦时触发回调： onWindowStageInactive(ability: UIAbility, windowStage: window.WindowStage): void
2. 注册监听应用上下文的生命周期后，在windowStage获焦时触发回调： onWindowStageActive(ability: UIAbility, windowStage: window.WindowStage): void
3. 这两个个生命周期和 iOS 有点类似，可以临时替代前后台监听：
   • UIApplicationWillResignActiveNotification
   • UIApplicationDidBecomeActiveNotification

• 示例代码（参考官方示例）：

export enum AppState {
  AppWindowStageActive,
  AppWindowStageInactive
}

type AppStateCallback = (state: AppState, ability?: UIAbility, windowStage?: window.WindowStage) => void

// 应用内生命周期监听回调
public startWatchingAbilityLifecycle(callback: AppStateCallback) {
  // 注意 AbilityLifecycleCallback 是 class，其中某些方法是必选实现，无法用 Partial 过滤
  // 官方示例也是用对象字面量初始化的这个类（不规范）
  // 可以考虑用继承的方式过滤不必要的实现
  let callback: AbilityLifecycleCallback = {
    onAbilityCreate(ability: UIAbility) {
    },
    onWindowStageCreate(ability, windowStage) {
    },
    // 在windowStage获焦时触发回调
    onWindowStageActive: (ability, windowStage) => {
      callback?.(AppState.WindowStageActive, ability, windowStage)
    },
    // 在windowStage失焦时触发回调
    onWindowStageInactive: (ability, windowStage) => {
      callback?.(AppState.WindowStageInactive, ability, windowStage)
    },
    onWindowStageDestroy(ability, windowStage) {
    },
    onAbilityDestroy(ability) {
    },
    onAbilityForeground: (ability) => {
    },
    onAbilityBackground: (ability) => {
    },
    onAbilityContinue(ability) {
    }
  }
  let applicationContext = Util.getContext().getApplicationContext();
  try {
    this.lifecycleId = applicationContext.on('abilityLifecycle', callback);
  } catch (err) {
  }
}

// 停止App生命周期监听 
stopWatchingAbilityLifecycle() {
  let applicationContext = Util.getContext().getApplicationContext();
  try {
    applicationContext.off('abilityLifecycle', this.lifecycleId, (error, data) => {
    });
  } catch (err) {
  }
}

• 优化后，相对符合编码规范的实现：

// 通过继承过滤方法回调
class PartialAbilityLifecycleCallback extends AbilityLifecycleCallback {
  constructor(callback: AppStateCallback) {
    super()
    this.callback = callback
  }

  callback?: AppStateCallback

  // 在windowStage获焦时触发回调
  onWindowStageActive(ability: UIAbility, windowStage: window.WindowStage) {
    super.onWindowStageActive(ability, windowStage)
    this.callback?.(AppState.WindowStageActive, ability, windowStage)
  }

  // 在windowStage失焦时触发回调
  onWindowStageInactive(ability: UIAbility, windowStage: window.WindowStage) {
    super.onWindowStageInactive(ability, windowStage)
    this.callback?.(AppState.WindowStageInactive, ability, windowStage)
  }
}

// 实现
let applicationContext = Util.getContext().getApplicationContext();
try {
  this.lifecycleId = applicationContext.on('abilityLifecycle', new PartialAbilityLifecycleCallback(callback));
} catch (err) {
}