Android Lifecycle源码解析(一)

首先我们看HomeActivity中我们添加到一行代码

public class HomeActivity extends AppCompatActivity {@Overrideprotected void onCreate(Bundle savedInstanceState) {super.onCreate(savedInstanceState);setContentView(R.layout.activity_main);//heregetLifecycle().addObserver(new HomeObserver());}
}

先看下类的继承关系

public class AppCompatActivity extends FragmentActivity
public class FragmentActivity extends SupportActivity

我们看下getLifecycle()是个什么东西？点击去看发现getLifecycle()方法是定义在SupportActivity，下面是其部分源码

SupportActivity

public class SupportActivity extends Activity implements LifecycleOwner, Component {...private LifecycleRegistry mLifecycleRegistry = new LifecycleRegistry(this);public SupportActivity() {}protected void onCreate(@Nullable Bundle savedInstanceState) {super.onCreate(savedInstanceState);ReportFragment.injectIfNeededIn(this);}@CallSuperprotected void onSaveInstanceState(Bundle outState) {this.mLifecycleRegistry.markState(State.CREATED);super.onSaveInstanceState(outState);}public Lifecycle getLifecycle() {return this.mLifecycleRegistry;}...
}

可以看到getLifecycle返回的其实是个LifecycleRegistry对象；该类继承了Lifecycle抽象类；
Lifecycle类可以理解为具备获取生命周期状态的抽象类，为什么？看下面代码

public abstract class Lifecycle {@MainThreadpublic abstract void addObserver(@NonNull LifecycleObserver observer);@MainThreadpublic abstract void removeObserver(@NonNull LifecycleObserver observer);@MainThread@NonNullpublic abstract State getCurrentState();
}

Lifecycle声明了添加观察者，移除观察者，获取当前生命周期状态三个抽象方法
而LifecycleRegistry则是其子类，他是观察者和被观察者(HomeActivity)的纽带，能够管理多个观察者；HomeActivity的生命周期事件都是通过该类handleLifecycleEvent方法进行下发的，那么是谁将Activity的生命周期的事件转给LifecycleRegistry类中呢？答案是ReportFragment
这里有点像Glide框架中到设计一样使用一个空Fragment来感知Activity生命周期，避免图片加载出现内存泄漏
ReportFragment这家伙是在哪出现的？我们在SupportActivity类中的onCreate找到答案

class SupportActivity...protected void onCreate(@Nullable Bundle savedInstanceState) {super.onCreate(savedInstanceState);ReportFragment.injectIfNeededIn(this);}

ReportFragment

@RestrictTo(RestrictTo.Scope.LIBRARY_GROUP)
public class ReportFragment extends Fragment {private static final String REPORT_FRAGMENT_TAG = "android.arch.lifecycle"+ ".LifecycleDispatcher.report_fragment_tag";/****在Activity上附上一个空视图的ReportFragment**/public static void injectIfNeededIn(Activity activity) {// ProcessLifecycleOwner should always correctly work and some activities may not extend// FragmentActivity from support lib, so we use framework fragments for activitiesandroid.app.FragmentManager manager = activity.getFragmentManager();if (manager.findFragmentByTag(REPORT_FRAGMENT_TAG) == null) {manager.beginTransaction().add(new ReportFragment(), REPORT_FRAGMENT_TAG).commit();// Hopefully, we are the first to make a transaction.manager.executePendingTransactions();}}private ActivityInitializationListener mProcessListener;private void dispatchCreate(ActivityInitializationListener listener) {if (listener != null) {listener.onCreate();}}...@Overridepublic void onActivityCreated(Bundle savedInstanceState) {super.onActivityCreated(savedInstanceState);dispatchCreate(mProcessListener);dispatch(Lifecycle.Event.ON_CREATE);}@Overridepublic void onStart() {super.onStart();dispatchStart(mProcessListener);dispatch(Lifecycle.Event.ON_START);}...@Overridepublic void onDestroy() {super.onDestroy();dispatch(Lifecycle.Event.ON_DESTROY);// just want to be sure that we won't leak reference to an activitymProcessListener = null;}//可以看到Activtity的onCreate方法，以及ResportFragment中的onStart等各种生命周期方法都调用该方法，private void dispatch(Lifecycle.Event event) {Activity activity = getActivity();//v7中LifecycleRegistryOwner类不建议使用if (activity instanceof LifecycleRegistryOwner) {((LifecycleRegistryOwner) activity).getLifecycle().handleLifecycleEvent(event);return;}//因为SupportActivity实现了LifecycleOwner接口，所以会走此处分支if (activity instanceof LifecycleOwner) {Lifecycle lifecycle = ((LifecycleOwner) activity).getLifecycle();if (lifecycle instanceof LifecycleRegistry) {//关键点：生命周期事件转发给了LifecycleRegistry中了((LifecycleRegistry) lifecycle).handleLifecycleEvent(event);}}}...
}

至此我们明白了Activity的生命周期事件是如何被转发到LifecycleRegistry中了，以一张流程图总结下

ActivityReportFragmentLifecycleRegistryonCreateinjectIfNeededIn(activity)onActivityCreatedispatch(state)handleLifecycleEventActivityReportFragmentLifecycleRegistry

接下来我们看下LifecycleRegistry类是如何处理事件的

LifecycleRegistry

我理解的是LifecycleRegistry类和java中的Observable类有点像；因为Lifecycle的设计思想有部分就是使用了观察者模式，LifecycleRegistry类中可以注册或反注册多个观察者，一旦生命周期发生变更LifecycleRegistry会及时通知所有的观察者，这样就达到了观察者能够感知带生命周期组件的变化了。

/*** An implementation of {@link Lifecycle} that can handle multiple observers.* <p>* It is used by Fragments and Support Library Activities. You can also directly use it if you have* a custom LifecycleOwner.*/
public class LifecycleRegistry extends Lifecycle {private static final String LOG_TAG = "LifecycleRegistry";/*** Custom list that keeps observers and can handle removals / additions during traversal.** Invariant: at any moment of time for observer1 & observer2:* if addition_order(observer1) < addition_order(observer2), then* state(observer1) >= state(observer2),*/private FastSafeIterableMap<LifecycleObserver, ObserverWithState> mObserverMap =new FastSafeIterableMap<>();/*** Current state*/private State mState;/*** The provider that owns this Lifecycle.* Only WeakReference on LifecycleOwner is kept, so if somebody leaks Lifecycle, they won't leak* the whole Fragment / Activity. However, to leak Lifecycle object isn't great idea neither,* because it keeps strong references on all other listeners, so you'll leak all of them as* well.*///使用弱引用避免Activity/Fragment内存泄漏private final WeakReference<LifecycleOwner> mLifecycleOwner;private int mAddingObserverCounter = 0;private boolean mHandlingEvent = false;private boolean mNewEventOccurred = false;// we have to keep it for cases:// void onStart() {//     mRegistry.removeObserver(this);//     mRegistry.add(newObserver);// }// newObserver should be brought only to CREATED state during the execution of// this onStart method. our invariant with mObserverMap doesn't help, because parent observer// is no longer in the map.private ArrayList<State> mParentStates = new ArrayList<>();/*** Creates a new LifecycleRegistry for the given provider.* <p>* You should usually create this inside your LifecycleOwner class's constructor and hold* onto the same instance.** @param provider The owner LifecycleOwner*/public LifecycleRegistry(@NonNull LifecycleOwner provider) {mLifecycleOwner = new WeakReference<>(provider);mState = INITIALIZED;}/*** Moves the Lifecycle to the given state and dispatches necessary events to the observers.** @param state new state*/@SuppressWarnings("WeakerAccess")@MainThreadpublic void markState(@NonNull State state) {moveToState(state);}/*** Sets the current state and notifies the observers.* <p>* Note that if the {@code currentState} is the same state as the last call to this method,* calling this method has no effect.** @param event The event that was received*/public void handleLifecycleEvent(@NonNull Lifecycle.Event event) {State next = getStateAfter(event);moveToState(next);}private void moveToState(State next) {//1. 如果状态和当前状态一样，则不需要下发事件通知if (mState == next) {return;}//2. 更新最新生命周期事件状态mState = next;if (mHandlingEvent || mAddingObserverCounter != 0) {mNewEventOccurred = true;// we will figure out what to do on upper level.return;}//3. 表示当前正在处理事件mHandlingEvent = true;//4. 同步事件到各Observersync();//5. 同步结束，重置标志mHandlingEvent = false;}// happens only on the top of stack (never in reentrance),// so it doesn't have to take in account parentsprivate void sync() {//获取具备生命周期的组件，如果没有直接返回LifecycleOwner lifecycleOwner = mLifecycleOwner.get();if (lifecycleOwner == null) {Log.w(LOG_TAG, "LifecycleOwner is garbage collected, you shouldn't try dispatch "+ "new events from it.");return;}//如果未同步，那就同步状态直到完成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<LifecycleObserver, ObserverWithState> newest = mObserverMap.newest();if (!mNewEventOccurred && newest != null&& mState.compareTo(newest.getValue().mState) > 0) {forwardPass(lifecycleOwner);}}mNewEventOccurred = false;}//判断最新的和最旧的观察者状态是否一致，如果一致说明同步状态成功；如果无观察者默认已同步private boolean isSynced() {if (mObserverMap.size() == 0) {return true;}State eldestObserverState = mObserverMap.eldest().getValue().mState;State newestObserverState = mObserverMap.newest().getValue().mState;return eldestObserverState == newestObserverState && mState == newestObserverState;}private void forwardPass(LifecycleOwner lifecycleOwner) {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);//可以看到最终事件是在这里分发给了observerobserver.dispatchEvent(lifecycleOwner, upEvent(observer.mState));popParentState();}}}private void backwardPass(LifecycleOwner lifecycleOwner) {Iterator<Entry<LifecycleObserver, ObserverWithState>> descendingIterator =mObserverMap.descendingIterator();while (descendingIterator.hasNext() && !mNewEventOccurred) {Entry<LifecycleObserver, ObserverWithState> entry = descendingIterator.next();ObserverWithState observer = entry.getValue();while ((observer.mState.compareTo(mState) > 0 && !mNewEventOccurred&& mObserverMap.contains(entry.getKey()))) {Event event = downEvent(observer.mState);pushParentState(getStateAfter(event));observer.dispatchEvent(lifecycleOwner, event);popParentState();}}}...
}

通过上面代码我们知道handleLifecycleEvent事件最后传递给observer的dispatchEvent方法中了；那么这个observer是什么？
首先看下它的类型是ObserverWithState，它是在什么时候被创建的呢？答案是addObserver方法

public class LifecycleRegistry extends Lifecycle {private static final String LOG_TAG = "LifecycleRegistry";private FastSafeIterableMap<LifecycleObserver, ObserverWithState> mObserverMap =new FastSafeIterableMap<>();/*** Current state*/private State mState;//使用弱引用避免Activity/Fragment内存泄漏private final WeakReference<LifecycleOwner> mLifecycleOwner;...private ArrayList<State> mParentStates = new ArrayList<>();...@Overridepublic void addObserver(@NonNull LifecycleObserver observer) {State initialState = mState == DESTROYED ? DESTROYED : INITIALIZED;//创建一个带状态的Observer，可以理解为是observer的包装器ObserverWithState statefulObserver = new ObserverWithState(observer, initialState);//将observer以及带状态的observer保存到map中ObserverWithState previous = mObserverMap.putIfAbsent(observer, statefulObserver);//如果observer存在则无需添加，直接返回if (previous != null) {return;}//生命周期组件校验LifecycleOwner lifecycleOwner = mLifecycleOwner.get();if (lifecycleOwner == null) {// it is null we should be destroyed. Fallback quicklyreturn;}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 recalculatetargetState = calculateTargetState(observer);}if (!isReentrance) {// we do sync only on the top level.sync();}mAddingObserverCounter--;}//带状态的Observerstatic class ObserverWithState {State mState;GenericLifecycleObserver mLifecycleObserver;ObserverWithState(LifecycleObserver observer, State initialState) {//此处划重点，感兴趣可以进去仔细研究mLifecycleObserver = Lifecycling.getCallback(observer);mState = initialState;}void dispatchEvent(LifecycleOwner owner, Event event) {State newState = getStateAfter(event);mState = min(mState, newState);mLifecycleObserver.onStateChanged(owner, event);mState = newState;}}
}

可以看到在ObserverWithState的dispatchEvent方法最终将事件委托给mLifecycleObserver，
运行demo可以看到这个mLifecycleObserver类型其实是ReflectiveGenericLifecycleObserver类型；其实也好蛮理解，因为我们的观察者的方法标注了注解，而要想调用它，其实此处是通过反射来查找观察者的带生命周期注解的方法，一旦状态改变时，通过反射直接调用就行；
因为反射代价较大，所以不太建议此种写法，建议通过注解处理器，动态生成辅助类来提高性能，这个后面再说

class ReflectiveGenericLifecycleObserver implements GenericLifecycleObserver {private final Object mWrapped;private final CallbackInfo mInfo;ReflectiveGenericLifecycleObserver(Object wrapped) {mWrapped = wrapped;mInfo = ClassesInfoCache.sInstance.getInfo(mWrapped.getClass());}@Overridepublic void onStateChanged(LifecycleOwner source, Event event) {mInfo.invokeCallbacks(source, event, mWrapped);}
}

GenericLifecycleObserver的实现类有五种，感兴趣可以看看

辅助类代码

public class HomeObserver_LifecycleAdapter implements GeneratedAdapter {final HomeObserver mReceiver;HomeObserver_LifecycleAdapter(HomeObserver receiver) {this.mReceiver = receiver;}@Overridepublic void callMethods(LifecycleOwner owner, Lifecycle.Event event, boolean onAny,MethodCallsLogger logger) {boolean hasLogger = logger != null;if (onAny) {return;}if (event == Lifecycle.Event.ON_CREATE) {if (!hasLogger || logger.approveCall("onCreate", 1)) {mReceiver.onCreate();}return;}if (event == Lifecycle.Event.ON_RESUME) {if (!hasLogger || logger.approveCall("onResume", 1)) {mReceiver.onResume();}return;}if (event == Lifecycle.Event.ON_DESTROY) {if (!hasLogger || logger.approveCall("onDestroy", 1)) {mReceiver.onDestroy();}return;}}
}

给大家的问题

ReflectiveGenericLifecycleObserver使用了反射机制类调用方法，为了减少反射带来的性能问题，它做了哪些优化？
我们写的HomeObserver中onCreate方法的参数有哪些限制？为什么
使用推荐写法时Lifecycling.getCallback(observer);返回的类型是什么？为什么？
使用推荐写法时到底有没使用到反射？