LifeCycle源码分析

LifeCycle用于帮助开发者管理Activity和Fragment 的生命周期。

介绍Lifecycle之前，首先需要了解观察者模式。

观察者模式首先需要被观察者和被观察者，被观察者里面可以添加观察者和删除观察者，以及有事件的时候通知观察的方法。而观察者则事收到事件的回调之后，即做出相应的处理。

而Lifecycle则是典型的观察者模式。

LifecycleOwner可以理解为被观察者。而它所有的处理全部在LifecycleRegistry中。

那那些又可以当作被观察者了？其实只要我们在一个类中取实现LifecycleOwner，然后在对应的生命周期去设置不同的Lifecycle.State，这样即可。

public class MyActivity extends Activity implements LifecycleOwner {
    private LifecycleRegistry mLifecycleRegistry;

    @Override
    protected void onCreate(@Nullable Bundle savedInstanceState) {
        super.onCreate(savedInstanceState);
        mLifecycleRegistry = new LifecycleRegistry(this);
        mLifecycleRegistry.setCurrentState(Lifecycle.State.CREATED);
    }


    @Override
    protected void onDestroy() {
        super.onDestroy();
        mLifecycleRegistry.setCurrentState(Lifecycle.State.DESTROYED);
    }

    @NonNull
    @Override
    public Lifecycle getLifecycle() {
        return mLifecycleRegistry;
    }
}

那同样的道理就是我们只要实现了LifecycleObserver就可以作为观察者。

public class LifecycleObserverImpl implements LifecycleObserver {

    @OnLifecycleEvent(Lifecycle.Event.ON_CREATE)
    public void onCreate(){
        Log.e("zzf","---------onCreate--------");
    }

    @OnLifecycleEvent(Lifecycle.Event.ON_START)
    public void onStart(){
        Log.e("zzf","---------onStart--------");
    }

    @OnLifecycleEvent(Lifecycle.Event.ON_RESUME)
    public void onResume(){
        Log.e("zzf","---------onResume--------");
    }

    @OnLifecycleEvent(Lifecycle.Event.ON_PAUSE)
    public void onPause(){
        Log.e("zzf","---------onPause--------");
    }

    @OnLifecycleEvent(Lifecycle.Event.ON_STOP)
    public void onStop(){
        Log.e("zzf","---------onStop--------");
    }

    @OnLifecycleEvent(Lifecycle.Event.ON_DESTROY)
    public void onDestory(){
        Log.e("zzf","---------onDestory--------");
    }
}

那么有个问题来了，我们在观察者上面对应的Lifecycle.Event，而在被观察者的是设置Lifecycle.State，那这个Event和State是怎样关联起来的？

那下面以ComponentActivity为例来讲讲整个流程。

public class ComponentActivity extends androidx.core.app.ComponentActivity implements
        LifecycleOwner {
        }
```LifecycleOwner
我们可以看到，ComponentActivity是实现了LifecycleOwner接口。

@Override
protected void onCreate(@Nullable Bundle savedInstanceState) {
super.onCreate(savedInstanceState);
mSavedStateRegistryController.performRestore(savedInstanceState);
ReportFragment.injectIfNeededIn(this);
if (mContentLayoutId != 0) {
setContentView(mContentLayoutId);
}
}

然后在oncreate中添加一个没有页面的ReportFragment，我们知道Fragment的生命周期和activity的生命周期是一致的，这让我们想到Glide的生命周期管理好像也是采用这种方法。


所以接下来我们看在ReportFragment的生命周期中做了啥。

@Override
public void onActivityCreated(Bundle savedInstanceState) {
super.onActivityCreated(savedInstanceState);
dispatchCreate(mProcessListener);
dispatch(Lifecycle.Event.ON_CREATE);
}

@Override
public void onStart() {
    super.onStart();
    dispatchStart(mProcessListener);
    dispatch(Lifecycle.Event.ON_START);
}

@Override
public void onResume() {
    super.onResume();
    dispatchResume(mProcessListener);
    dispatch(Lifecycle.Event.ON_RESUME);
}

@Override
public void onPause() {
    super.onPause();
    dispatch(Lifecycle.Event.ON_PAUSE);
}

@Override
public void onStop() {
    super.onStop();
    dispatch(Lifecycle.Event.ON_STOP);
}

@Override
public void onDestroy() {
    super.onDestroy();
    dispatch(Lifecycle.Event.ON_DESTROY);
    // just want to be sure that we won't leak reference to an activity
    mProcessListener = null;
}

在生命周期中调用dispatch方法.

private void dispatch(Lifecycle.Event event) {
Activity activity = getActivity();

    if (activity instanceof LifecycleOwner) {
        Lifecycle lifecycle = ((LifecycleOwner) activity).getLifecycle();
        if (lifecycle instanceof LifecycleRegistry) {
            ((LifecycleRegistry) lifecycle).handleLifecycleEvent(event);
        }
    }
}

继续调用LifecycleRegistry的handleLifecycleEvent()。

public void handleLifecycleEvent(@NonNull Lifecycle.Event event) {
State next = getStateAfter(event);
moveToState(next);
}

private void moveToState(State next) {
    if (mState == next) {
        return;
    }
    mState = next;
    if (mHandlingEvent || mAddingObserverCounter != 0) {
        mNewEventOccurred = true;
        // we will figure out what to do on upper level.
        return;
    }
    mHandlingEvent = true;
    sync();
    mHandlingEvent = false;
}

进入状态同步方法sync()。

private void sync() {
LifecycleOwner lifecycleOwner = mLifecycleOwner.get();
if (lifecycleOwner == null) {
throw new IllegalStateException("LifecycleOwner of this LifecycleRegistry is already"
+ "garbage collected. It is too late to change lifecycle state.");
}
while (!isSynced()) {
mNewEventOccurred = false;
// no need to check eldest for nullability, because isSynced does it for us.
if (mState.compareTo(mObserverMap.eldest().getValue().mState) < 0) {
backwardPass(lifecycleOwner);
}
Entry newest = mObserverMap.newest();
if (!mNewEventOccurred && newest != null
&& mState.compareTo(newest.getValue().mState) > 0) {
forwardPass(lifecycleOwner);
}
}
mNewEventOccurred = false;
}

mState.compareTo(mObserverMap.eldest().getValue().mState) < 0 从缓存的观察者中拿出最旧的状态，与当前状态进行比较，如果小于 0， 说明观察者的状态提前于当前状态，那么就执行一个backwardPass(lifecycleOwner) 方法，让观察者的状态回退到当前状态上

 mState.compareTo(newest.getValue().mState) > 0 这个判断正好和上面相反，说明当前的观察者状态落后于当前状态，那么就让观察者的状态追上当前状态，执行 forwardPass(lifecycleOwner)


通过分析 forwardPass 和 backwardPass 方法，我们看到其内部又调用了两个方法, downEvent 和 upEvent，我们称呼为升级事件和降级事件

private static Event downEvent(State state) {
switch (state) {
case INITIALIZED:
throw new IllegalArgumentException();
case CREATED:
return ON_DESTROY;
case STARTED:
return ON_STOP;
case RESUMED:
return ON_PAUSE;
case DESTROYED:
throw new IllegalArgumentException();
}
throw new IllegalArgumentException("Unexpected state value " + state);
}

private static Event upEvent(State state) {
    switch (state) {
        case INITIALIZED:
        case DESTROYED:
            return ON_CREATE;
        case CREATED:
            return ON_START;
        case STARTED:
            return ON_RESUME;
        case RESUMED:
            throw new IllegalArgumentException();
    }
    throw new IllegalArgumentException("Unexpected state value " + state);
}

在这个就回答了前面的哪个疑问，Event和state是怎样转换的。

在backwardPass或forwardPass方法中最终调用的是dispatchEvent方法，进而调用mLifecycleObserver.onStateChanged(owner, event)。

前面我们说了一大堆，都是对 Lifecycle 如何处理，并监听我们的生命周期的方法，也就是 Lifecycle是如何处理监听的。那么我们前面一直都没有说过，观察者那里来的？？？？？？？？？
好，我们继续观察 LifecycleRegister 中的 addObserver(LifecycleObserver) 这个也是我们自定义观察者后，需要调用的方法

@Override
public void addObserver(@NonNull LifecycleObserver observer) {
State initialState = mState == DESTROYED ? DESTROYED : INITIALIZED;
ObserverWithState statefulObserver = new ObserverWithState(observer, initialState);
ObserverWithState previous = mObserverMap.putIfAbsent(observer, statefulObserver);

if (previous != null) {
    return;
}
LifecycleOwner lifecycleOwner = mLifecycleOwner.get();
if (lifecycleOwner == null) {
    // it is null we should be destroyed. Fallback quickly
    return;
}

boolean isReentrance = mAddingObserverCounter != 0 || mHandlingEvent;
State targetState = calculateTargetState(observer);
mAddingObserverCounter++;
while ((statefulObserver.mState.compareTo(targetState) < 0
        && mObserverMap.contains(observer))) {
    pushParentState(statefulObserver.mState);
    statefulObserver.dispatchEvent(lifecycleOwner, upEvent(statefulObserver.mState));
    popParentState();
    // mState / subling may have been changed recalculate
    targetState = calculateTargetState(observer);
}

if (!isReentrance) {
    // we do sync only on the top level.
    sync();
}
mAddingObserverCounter--;

}

创建了一个 ObserverWithState ，这是一个有状态的观察者，从名字上我们就可以看出来。两个参数分别是我们传入的观察者和我们刚刚创建好的状态。这是个装饰器模式，目的是给原对象，增加一个新属性，但又不改变原数据结构的方案。
紧接着又把这个观察者和带有状态的观察者，放进了事先已经创建好的 Map 中，也就是 mObserverMap 然后返回了个东西， 阅读后发现，又和 Lifecycle 的注释对上了。

之后，计算出当前 Activity 或 Fragment 的状态，并判断，如果当前观察者的状态落后于 Activity 或 Fragment 的状态，则立即执行dispatchEvent() 方法，执行回调。