Jetpack - Lifecycle

生命周期组件Lifecycle主要成员

• LifecycleOwner 生命周期拥有者
• LifecycleRegistry 生命周期发布者,流转事件
• LifecycleObserver 生命周期事件监听者

通过生命周期成员自己时间生命周期的监听和通知

1. 生命周期持有者

/**
 * 自定义view，生命周期持有者
 */
class LifecycleSub : LinearLayout, LifecycleOwner {
    constructor(context: Context) : super(context)
    constructor(context: Context, attrs: AttributeSet?) : super(context, attrs)
    constructor(context: Context, attrs: AttributeSet?, defStyleAttr: Int) : super(
        context,
        attrs,
        defStyleAttr
    )

    private val mLifecycleRegistry = LifecycleRegistry(this)

    override fun getLifecycle(): Lifecycle = mLifecycleRegistry

    override fun onFinishInflate() {
        super.onFinishInflate()
        observer(ObserverSub())
    }

    override fun onAttachedToWindow() {
        super.onAttachedToWindow()
        mLifecycleRegistry.handleLifecycleEvent(Lifecycle.Event.ON_CREATE)
    }

    override fun onWindowVisibilityChanged(visibility: Int) {
        super.onWindowVisibilityChanged(visibility)
        if (visibility == VISIBLE) {
            mLifecycleRegistry.handleLifecycleEvent(Lifecycle.Event.ON_START)
            mLifecycleRegistry.handleLifecycleEvent(Lifecycle.Event.ON_RESUME)
        } else {
            mLifecycleRegistry.handleLifecycleEvent(Lifecycle.Event.ON_PAUSE)
            mLifecycleRegistry.handleLifecycleEvent(Lifecycle.Event.ON_STOP)
        }
    }

    override fun onDetachedFromWindow() {
        super.onDetachedFromWindow()
        mLifecycleRegistry.handleLifecycleEvent(Lifecycle.Event.ON_DESTROY)
    }

    private fun observer(observer: LifecycleEventObserver) {
        removeObserver(observer)
        lifecycle.addObserver(observer)
    }

    private fun removeObserver(observer: LifecycleEventObserver) {
        lifecycle.removeObserver(observer)
    }

}

2. 在LifecycleOwner里面注册生命周期事件的发布者

override fun getLifecycle(): Lifecycle = mLifecycleRegistry

3. 注册生命周期的观察者
   先添加观察者observer(ObserverSub())

/**
 * 生命周期观察者
 */
class ObserverSub: LifecycleEventObserver {
    override fun onStateChanged(source: LifecycleOwner, event: Lifecycle.Event) {
        Log.d("ObserverSub", "onStateChanged: ${event.targetState}")
    }
}

4. 打印的日志

2022-08-16 14:57:19.569 D/ObserverSub: onStateChanged: STARTED
2022-08-16 14:57:19.569 D/ObserverSub: onStateChanged: CREATED
2022-08-16 14:57:19.589 D/ObserverSub: onStateChanged: DESTROYED
2022-08-16 14:57:24.002 D/ObserverSub: onStateChanged: CREATED
2022-08-16 14:57:24.002 D/ObserverSub: onStateChanged: STARTED
2022-08-16 14:57:24.002 D/ObserverSub: onStateChanged: RESUMED
2022-08-16 15:01:09.753 D/ObserverSub: onStateChanged: STARTED
2022-08-16 15:01:09.753 D/ObserverSub: onStateChanged: CREATED
2022-08-16 15:01:09.771 D/ObserverSub: onStateChanged: DESTROYED

5. 用于测试的代码，在activity中添加自定义控件

上面5步就可以简单实现发布和监听，在新版本中Activity/Fragment都默认实现了LifecycleOwner接口

public class AppCompatActivity extends FragmentActivity implements AppCompatCallback

public class FragmentActivity extends ComponentActivity implements
        ActivityCompat.OnRequestPermissionsResultCallback,
        ActivityCompat.RequestPermissionsRequestCodeValidator {
    //...
//注册了LifecycleRegistry，ComponentActivity中实现了LifecycleOwner
final LifecycleRegistry mFragmentLifecycleRegistry = new LifecycleRegistry(this);
  //...
//生命周期的流转
@Override
    protected void onCreate(@Nullable Bundle savedInstanceState) {
        super.onCreate(savedInstanceState);
        mFragmentLifecycleRegistry.handleLifecycleEvent(Lifecycle.Event.ON_CREATE);
        mFragments.dispatchCreate();
    }
//...
}

在FragmentActivity中根据activity的生命周期流转Lifecycle的事件，只要向LifecycleRegistry添加观察者就会收到生命周期事件，例如：LiveData