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.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结尾的文档阐明。