关于android:LifecycleEventObserver与DefaultLifecycleObserver的优先级

先看LifecycleEventObserver的源码文档形容

/** * Class that can receive any lifecycle change and dispatch it to the receiver. * <p> * If a class implements both this interface and * {@link androidx.lifecycle.DefaultLifecycleObserver}, then * methods of {@code DefaultLifecycleObserver} will be called first, and then followed by the call * of {@link LifecycleEventObserver#onStateChanged(LifecycleOwner, Lifecycle.Event)} * <p> * If a class implements this interface and in the same time uses {@link OnLifecycleEvent}, then * annotations will be ignored. */public interface LifecycleEventObserver extends LifecycleObserver {    /**     * Called when a state transition event happens.     *     * @param source The source of the event     * @param event The event     */    void onStateChanged(@NonNull LifecycleOwner source, @NonNull Lifecycle.Event event);}

文档很分明的阐明了，如果一个观察者同时实现了DefaultLifecycleObserver和DefaultLifecycleObserver，DefaultLifecycleObserver要先于LifecycleEventObserver的onStateChanged调用。如果也增加了OnLifecycleEvent注解。那么注解会被疏忽掉。

上面通过查看源码来证实。

从getLifecycle().addObserver(LifecycleObserver)开始。Lifecycle的实现类是LifecycleRegistry。

//LifecycleRegistry.java @Override    public void addObserver(@NonNull LifecycleObserver observer) {        State initialState = mState == DESTROYED ? DESTROYED : INITIALIZED;        ObserverWithState statefulObserver = new ObserverWithState(observer, initialState);           statefulObserver.dispatchEvent(lifecycleOwner, event);    }

在创立ObserverWithState对象时，会通过Lifecycling.lifecycleEventObserver(observer)生成LifecycleEventObserver对象。

//Lifecycling.java @NonNull    static LifecycleEventObserver lifecycleEventObserver(Object object) {        boolean isLifecycleEventObserver = object instanceof LifecycleEventObserver;        boolean isFullLifecycleObserver = object instanceof FullLifecycleObserver;        if (isLifecycleEventObserver && isFullLifecycleObserver) {            return new FullLifecycleObserverAdapter((FullLifecycleObserver) object,                    (LifecycleEventObserver) object);        }        if (isFullLifecycleObserver) {            return new FullLifecycleObserverAdapter((FullLifecycleObserver) object, null);        }        if (isLifecycleEventObserver) {            return (LifecycleEventObserver) object;        }}

能够看到如果一个观察者同时实现了DefaultLifecycleObserver和DefaultLifecycleObserver，返回的LifecycleEventObserver对象的理论类型是FullLifecycleObserverAdapter。

再看ObserverWithState.dispatchEvent(lifecycleOwner, event);

//ObserverWithState.javavoid dispatchEvent(LifecycleOwner owner, Event event) {            State newState = event.getTargetState();            mState = min(mState, newState);            mLifecycleObserver.onStateChanged(owner, event);            mState = newState;        }

这里的mLifecycleObserver就是FullLifecycleObserverAdapter类型。所以最终调用的是FullLifecycleObserverAdapter的onStateChanged。通过查看源码即可证实文章结尾的文档阐明。