Android Lifecyle

Lifecycle笔记

主要记录一下自己看源码的过程

既然我们要感知Activity的生命周期，首先我们需要一个观察者：

interface IObserer{
    //可以把你需要的生命周期单独拿出来
    @OnLifecycleEvent(Lifecycle.Event.ON_CREATE)
    fun onCreate(owner:LifecycleOwner)

    @OnLifecycleEvent(Lifecycle.Event.ON_ANY)
    fun onLifecycleChange(owner:LifecycleOwner,event:Lifecycle.Event)
}

class MyObserver:IObserver{
    fun onCreate(owner:LifecycleOwner){
        //doSomeThing
    }
    fun onLifecycleChange(owner:LifecycleOwner,event:Lifecycle.Event){
        //doSomething
    }
}

观察者有了以后，就要想办法把他和生命周期关联起来

val myObserver = MyObserver()
lifecycle.addObserver(myObserver)

至此，我们已经可以在MyObserver中感知到Activity的生命周期变化

那么为什么这样就可以感知到生命周期变化呢

点开addObserver源码,发现该方法是属于Lifecycle抽象类的，那么就去找他的实现类

发现 SupportActivity中有个

 private LifecycleRegistry mLifecycleRegistry = new LifecycleRegistry(this);

那么就看LifecycleRegistry 的addObserver

 @Override
    public void addObserver(LifecycleObserver observer) {
        State initialState = mState == DESTROYED ? DESTROYED : INITIALIZED;
        // ① 
        ObserverWithState statefulObserver = new ObserverWithState(observer, initialState);
        //把myObserver和statefulObserver添加到Map里去，这里我理解为ObserverWithState是用来通知myObserver生命周期变化的
        ObserverWithState previous = mObserverMap.putIfAbsent(observer, statefulObserver);
        ......

    }

SupportActivity在onCreate的时候会添加一个ReportFragment汇报生命周期变化（类如其名）

class ReportFragment

//在onActivityCreated、onStart等生命周期方法中，均会调用dispatch
private void dispatch(Lifecycle.Event event) {
    Activity activity = getActivity();
    if (activity instanceof LifecycleRegistryOwner) {
        ((LifecycleRegistryOwner) activity).getLifecycle().handleLifecycleEvent(event);
        return;
    }
    if (activity instanceof LifecycleOwner) {
        Lifecycle lifecycle = ((LifecycleOwner) activity).getLifecycle();
        if (lifecycle instanceof LifecycleRegistry) {
        //这里就又回到LifecycleRegistry类中了
            ((LifecycleRegistry) lifecycle).handleLifecycleEvent(event);
        }
    }
}

handleLifecycleEvent最后会到这里

private void forwardPass() {
    Iterator<Entry<LifecycleObserver, ObserverWithState>> ascendingIterator =mObserverMap.iteratorWithAdditions();
    while (ascendingIterator.hasNext() && !mNewEventOccurred) {
        Entry<LifecycleObserver, ObserverWithState> entry = ascendingIterator.next();
        ObserverWithState observer = entry.getValue();
        while ((observer.mState.compareTo(mState) < 0 && !mNewEventOccurred
                && mObserverMap.contains(entry.getKey()))) {
            pushParentState(observer.mState);
            //这里进行事件分发，所以事件还是靠的ObserverWithState ，而不是myObserver（之前想岔了，总想着是myObserver来分发事件,）
            observer.dispatchEvent(mLifecycleOwner, upEvent(observer.mState));
            popParentState();
        }
    }
}

接下来进入ObserverWithState.dispatchEvent方法看下

 void dispatchEvent(LifecycleOwner owner, Event event) {
     State newState = getStateAfter(event);
     mState = min(mState, newState);
     //进行事件分发 见 ① 具体实现就是靠注解+反射来实现方法调用
     mLifecycleObserver.onStateChanged(owner, event);
     mState = newState;
 }

①
 static class ObserverWithState {
    State mState;
    GenericLifecycleObserver mLifecycleObserver;
    ObserverWithState(LifecycleObserver observer, State initialState) {
        // ② 这里会把myObserver包装成ReflectiveGenericLifecycleObserver，最终分发事件是靠这个类
        mLifecycleObserver = Lifecycling.getCallback(observer);
        mState = initialState;
    }
}

② 
ReflectiveGenericLifecycleObserver(Object wrapped) {
    //这个mWrapped 就是myObserver
    mWrapped = wrapped;
    mInfo = getInfo(mWrapped.getClass());
}

流程总结：(已感知生命周期变化为目的)

1、定义好我们自己的Observer（实现LifecycleObserver）
2、在Activity中通过lifecycle.addObserver(observer:LifecycleObserver)来关联生命周期
3、生命周期方法被调用时，ReportFragment向MyObserver汇报

lifecycle.addObserver里做的一些事情：
1、LifecycleRegistry把MyObserver和ObserverWithState对象添加到mObserverMap里以备后用
2、当生命周期发生变化是，ReportFragment通过SupportActivity的LifecycleRegistry 调用handleLifecycleEvent，最终从mObserverMap中取出ObserverWithState来分发事件
3、步骤1 中在创建ObserverWithState实例时，会在MyObserver外面包一层ReflectiveGenericLifecycleObserver，最终由ReflectiveGenericLifecycleObserver通过注解反射来调用MyObserver中由OnLifecycleEvent注解的方法