LifecycleEventObserver与DefaultLifecycleObserver的优先级

先看LifecycleEventObserver的源码文档描述

/**
 * 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);
}

文档很清楚的说明了，如果一个观察者同时实现了DefaultLifecycleObserver和DefaultLifecycleObserver，DefaultLifecycleObserver要先于LifecycleEventObserver的onStateChanged调用。如果也添加了OnLifecycleEvent注解。那么注解会被忽略掉。

下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);
    }

在创建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;
        }
}

可以看到如果一个观察者同时实现了DefaultLifecycleObserver和DefaultLifecycleObserver，返回的LifecycleEventObserver对象的实际类型是FullLifecycleObserverAdapter。

再看ObserverWithState.dispatchEvent(lifecycleOwner, event);

//ObserverWithState.java
void 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开头的文档说明。