Jetpack 之 LifeCycle

用于帮助开发者管理Activity和Fragment 的生命周期，它是LiveData和ViewModel的基础。

备注：

  • appcompat依赖androidx.fragment
  • androidx.fragment依赖ViewModel和 LiveData
  • LiveData依赖Lifecycle

问题：

activity的onStart / onStop 中，开启 / 关闭 功能组件
1.大量功能组件难以维护
2.无法保证在activity停止后（比如做一个耗时异步的操作后，再开启组建），这些组件一定会停止，从而有可能导致内存泄露

基本使用：

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override fun onCreate(savedInstanceState: Bundle?) {
    super.onCreate(savedInstanceState)
    setContentView(R.layout.activity_lifecycle)
    lifecycle.addObserver(MyObserver())
}

override fun onStart() {
    super.onStart()
    Log.d(TAG, "onStart: " + lifecycle.currentState)
}
override fun onResume() {
    super.onResume()
    Log.d(TAG, "onResume: " + lifecycle.currentState)
}
override fun onPause() {
    super.onPause()
    Log.d(TAG, "onPause: " + lifecycle.currentState)
}
override fun onStop() {
    super.onStop()
    Log.d(TAG, "onStop: " + lifecycle.currentState)
}
override fun onDestroy() {
    super.onDestroy()
    Log.d(TAG, "onDestroy: " + lifecycle.currentState)
}

class MyObserver : LifecycleObserver {

    @OnLifecycleEvent(value = Lifecycle.Event.ON_CREATE)
    fun fun1(owner: LifecycleOwner) {
        Log.d("TestLifecycleActivity", "ON_CREATE")
    }
    @OnLifecycleEvent(value = Lifecycle.Event.ON_START)
    fun fun0(owner: LifecycleOwner) {
        Log.d("TestLifecycleActivity", "ON_START")
    }
    @OnLifecycleEvent(value = Lifecycle.Event.ON_RESUME)
    fun fun2(owner: LifecycleOwner) {
        Log.d("TestLifecycleActivity", "ON_RESUME")
    }
    @OnLifecycleEvent(value = Lifecycle.Event.ON_PAUSE)
    fun fun3() {
        Log.d("TestLifecycleActivity", "ON_PAUSE")
    }
    @OnLifecycleEvent(value = Lifecycle.Event.ON_STOP)
    fun fun4() {
        Log.d("TestLifecycleActivity", "ON_STOP")
    }
    @OnLifecycleEvent(value = Lifecycle.Event.ON_DESTROY)
    fun fun5() {
        Log.d("TestLifecycleActivity", "ON_DESTROY")
    }

}

TestLifecycleActivity: onCreate: INITIALIZED
TestLifecycleActivity: ON_CREATE
TestLifecycleActivity: onStart: CREATED
TestLifecycleActivity: ON_START
TestLifecycleActivity: onResume: STARTED
TestLifecycleActivity: ON_RESUME

TestLifecycleActivity: ON_PAUSE
TestLifecycleActivity: onPause: STARTED
TestLifecycleActivity: ON_STOP
TestLifecycleActivity: onStop: CREATED
TestLifecycleActivity: ON_DESTROY
TestLifecycleActivity: onDestroy: DESTROYED




具体运用：

1.功能组件 实现 LifecycleObserver 接口
2.在@OnLifecycleEvent(value = Lifecycle.Event.ON_START)中实现开启功能，开启前还需检查当前状态：

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if(owner.getLifecycle().getCurrentState().isAtLeast(STARTED)){
	startSomething();
}

3.在@OnLifecycleEvent(value = Lifecycle.Event.ON_STOP)中实现关闭功能

  • 功能组件自动感知生命周期，组件处理自己逻辑，减轻Activity/Fragment中代码，易于管理
  • 检查当前状态，至少处于STARTED状态才会继续执行start()

以上两点解决上述问题1、2

Application生命周期 ProcessLifecycleOwner

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class MyApp : Application() {

override fun onCreate() {
    super.onCreate()
    INSTANCE = this
    ProcessLifecycleOwner.get().lifecycle.addObserver(MyAppLifeCycleObserver())
}

class MyAppLifeCycleObserver:LifecycleObserver{
    @OnLifecycleEvent(Lifecycle.Event.ON_START)
    fun onAppForeground() {
        Log.d("MyApp", "ON_START  --  onAppForeground")
    }

    @OnLifecycleEvent(Lifecycle.Event.ON_STOP)
    fun onAppBackground() {
        Log.d("MyApp", "ON_STOP  --  onAppBackground")
    }

}

}

MyApp: ON_START  --  onAppForeground
MyApp: ON_STOP  --  onAppBackground

Lifecycle.Event.ON_CREATE只会分发一次，Lifecycle.Event.ON_DESTROY不会被分发。

第一个Activity进入时，ProcessLifecycleOwner将分派Lifecycle.Event.ON_START, Lifecycle.Event.ON_RESUME。而Lifecycle.Event.ON_PAUSE, Lifecycle.Event.ON_STOP，将在最后一个Activit退出后后延迟分发。

参考链接：
https://juejin.cn/post/6893870636733890574#comment
https://juejin.cn/post/6844904111108800519