前言

关键类介绍

Lifecycle

定义：用于存储有关组件（如 activity 或 fragment）的生命周期状态的信息，并允许其他对象观察此状态。

• 它是一个抽象类，具体实现类为LifecycleRegistry；
• 内部提供了addObserver(LifecycleObserver observer) 添加观察者、removeObserver(LifecycleObserver observer)移除观察者方法；
• 内部封装了State【状态】、Event【事件】枚举类，以及一系列事件状态处理方法；

   @MainThread@NonNullpublic abstract State getCurrentState();public enum Event {ON_CREATE,ON_START,ON_RESUME,ON_PAUSE,ON_STOP,ON_DESTROY,ON_ANY;@Nullablepublic static Event downFrom(@NonNull State state) {switch (state) {case CREATED:return ON_DESTROY;case STARTED:return ON_STOP;case RESUMED:return ON_PAUSE;default:return null;}}@Nullablepublic static Event downTo(@NonNull State state) {switch (state) {case DESTROYED:return ON_DESTROY;case CREATED:return ON_STOP;case STARTED:return ON_PAUSE;default:return null;}}@Nullablepublic static Event upFrom(@NonNull State state) {switch (state) {case INITIALIZED:return ON_CREATE;case CREATED:return ON_START;case STARTED:return ON_RESUME;default:return null;}}@Nullablepublic static Event upTo(@NonNull State state) {switch (state) {case CREATED:return ON_CREATE;case STARTED:return ON_START;case RESUMED:return ON_RESUME;default:return null;}}@NonNullpublic State getTargetState() {switch (this) {case ON_CREATE:case ON_STOP:return State.CREATED;case ON_START:case ON_PAUSE:return State.STARTED;case ON_RESUME:return State.RESUMED;case ON_DESTROY:return State.DESTROYED;case ON_ANY:break;}throw new IllegalArgumentException(this + " has no target state");}}@SuppressWarnings("WeakerAccess")public enum State {DESTROYED,INITIALIZED,CREATED,STARTED,RESUMED;//传入的state是否存在public boolean isAtLeast(@NonNull State state) {return compareTo(state) >= 0;}}

构成Activity生命周期的状态和事件，其对应关系图如下：

由上图我们可以很容易理解其中定义的方法：

	//根据传入的state获取逆序【生命周期结束方向】需要向下移动的事件public static Event downFrom(@NonNull State state) {switch (state) {case CREATED:return ON_DESTROY;case STARTED:return ON_STOP;case RESUMED:return ON_PAUSE;default:return null;}}//根据传入的state获取逆序【生命周期结束方向】触发state的事件public static Event downTo(@NonNull State state) {switch (state) {case DESTROYED:return ON_DESTROY;case CREATED:return ON_STOP;case STARTED:return ON_PAUSE;default:return null;}}//根据传入的state获取正序【生命周期上升方向】需要向下移动的事件public static Event upFrom(@NonNull State state) {switch (state) {case INITIALIZED:return ON_CREATE;case CREATED:return ON_START;case STARTED:return ON_RESUME;default:return null;}}//根据传入的state获取正序【生命周期上升方向】触发state的事件public static Event upTo(@NonNull State state) {switch (state) {case CREATED:return ON_CREATE;case STARTED:return ON_START;case RESUMED:return ON_RESUME;default:return null;}}

LifecycleObserver

生命周期观察者,用来监听Activity生命周期变化;其子类常用的有DefaultLifecycleObserver、LifecycleEventObserver;

public interface LifecycleEventObserver extends LifecycleObserver {//生命周期变化回调void onStateChanged(@NonNull LifecycleOwner source, @NonNull Lifecycle.Event event);
}

LifecycleOwner

具有生命周期的类，对应被观察者,提供获取getLifecycle()方法，ComponentActivity类实现该接口；

public interface LifecycleOwner {@NonNullLifecycle getLifecycle();
}

LifecycleRegistry

Lifecycle类的具体实现类，于ComponentActivity内部创建，其内部持有LifecycleOwner引用，通过维护mState【当前状态】、mObserverMap【存储观察者对象】等变量实现生命周期同步以及分发；

关键类UML类图关系

小结： LifecycleOwner作为被观察者，其被ComponentActivity实现，同时ComponentActivity内部持有LifecycleRegistry对象，其提供添加、移除观察者LifecycleObserver方法，而从持有注册的观察者集合，一旦Activity生命周期变化时，LifecycleOwner便会触发LifecycleRegistry对象中的方法，并由LifecycleRegistry将事件分发给注册的观察者们；

生命周期注册

生命周期分发

总结

结语