對于LifeCycle的簡單使用可以看上一篇文章：LiveData+ViewModel+Repository搭建MVVM

這篇文章主要是為了閱讀源碼，盡量深入閱讀，能力還是一般，見諒：

LifeCycle的作用

解決onCreate等生命周期方法，由于各種原因，后期越來越臃腫的問題。

關鍵類闡述

LifecycleRegistryOwner/LifecycleOwner

在Activity等組件生命周期發生變化的時候，發出相應的Event給LifecycleRegistry。

LifecycleRegistry

控制state的轉換、接受分發Event事件。

LifecycleObserver

通過注解綁定Event和自定義的函數，實現對生命周期的監聽并處理。

Event

@SuppressWarnings("WeakerAccess") public enum Event {/*** Constant for onCreate event of the {@link LifecycleOwner}.*/ON_CREATE,/*** Constant for onStart event of the {@link LifecycleOwner}.*/ON_START,/*** Constant for onResume event of the {@link LifecycleOwner}.*/ON_RESUME,/*** Constant for onPause event of the {@link LifecycleOwner}.*/ON_PAUSE,/*** Constant for onStop event of the {@link LifecycleOwner}.*/ON_STOP,/*** Constant for onDestroy event of the {@link LifecycleOwner}.*/ON_DESTROY,/*** An {@link Event Event} constant that can be used to match all events.*/ON_ANY } 復制代碼

源碼分析

依賴庫版本：implementation "android.arch.lifecycle:extensions:1.1.1"

項目地址：github.com/fengqingxiu…

入口

getLifecycle().addObserver(new ActivityLifecycleObserver(getApplicationContext())); 復制代碼

進入addObserver

@Override public void addObserver(@NonNull LifecycleObserver observer) { //這里的LifecycleObserver就是我們自定義的ActivityLifecycleObserverState initialState = mState == DESTROYED ? DESTROYED : INITIALIZED;//把帶著狀態的observer封裝成ObserverWithStateObserverWithState 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 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--; } 復制代碼

進入ObserverWithState

static class ObserverWithState {State mState;GenericLifecycleObserver mLifecycleObserver;ObserverWithState(LifecycleObserver observer, State initialState) {//通過不同的observer返回不同的mLifecycleObservermLifecycleObserver = 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;} } 復制代碼

進入getCallback

@NonNull static GenericLifecycleObserver getCallback(Object object) {if (object instanceof FullLifecycleObserver) {return new FullLifecycleObserverAdapter((FullLifecycleObserver) object);}if (object instanceof GenericLifecycleObserver) {return (GenericLifecycleObserver) object;}final Class<?> klass = object.getClass();int type = getObserverConstructorType(klass);if (type == GENERATED_CALLBACK) {List<Constructor<? extends GeneratedAdapter>> constructors =sClassToAdapters.get(klass);if (constructors.size() == 1) {GeneratedAdapter generatedAdapter = createGeneratedAdapter(constructors.get(0), object);return new SingleGeneratedAdapterObserver(generatedAdapter);}GeneratedAdapter[] adapters = new GeneratedAdapter[constructors.size()];for (int i = 0; i < constructors.size(); i++) {adapters[i] = createGeneratedAdapter(constructors.get(i), object);}return new CompositeGeneratedAdaptersObserver(adapters);}return new ReflectiveGenericLifecycleObserver(object); //走到了這里 } 復制代碼

進入ReflectiveGenericLifecycleObserver

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); //這里：通過反射實現事件分發} } 復制代碼

那么是在哪里調用onStateChanged的呢？下面我們繼續分析：

LifecycleDispatcher

When initialized, it hooks into the Activity callback of the Application and observes Activities. It is responsible to hook in child-fragments to activities and fragments to report their lifecycle events. Another responsibility of this class is to mark as stopped all lifecycle providers related to an activity as soon it is not safe to run a fragment transaction in this activity.

第一次冷啟動App的時候會通過ActivityThread創建，并且之后會在Activity創建的時候，添加ReportFragment去實現生命周期事件的分發，下面看下源碼：

@SuppressWarnings("WeakerAccess") @VisibleForTesting static class DispatcherActivityCallback extends EmptyActivityLifecycleCallbacks {//省略...@Overridepublic void onActivityCreated(Activity activity, Bundle savedInstanceState) {if (activity instanceof FragmentActivity) {((FragmentActivity) activity).getSupportFragmentManager().registerFragmentLifecycleCallbacks(mFragmentCallback, true);}ReportFragment.injectIfNeededIn(activity); //這里添加了ReportFragment}//省略... } 復制代碼

進入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";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();}}static ReportFragment get(Activity activity) {return (ReportFragment) activity.getFragmentManager().findFragmentByTag(REPORT_FRAGMENT_TAG);}private ActivityInitializationListener mProcessListener;private void dispatchCreate(ActivityInitializationListener listener) {if (listener != null) {listener.onCreate();}}private void dispatchStart(ActivityInitializationListener listener) {if (listener != null) {listener.onStart();}}private void dispatchResume(ActivityInitializationListener listener) {if (listener != null) {listener.onResume();}}@Overridepublic void onActivityCreated(Bundle savedInstanceState) {super.onActivityCreated(savedInstanceState);dispatchCreate(mProcessListener);dispatch(Lifecycle.Event.ON_CREATE); //這里分發事件，我們自定義的觀察者定義的注解其實就是Lifecycle.Event.ON_CREATE這一類}@Overridepublic void onStart() {super.onStart();dispatchStart(mProcessListener);dispatch(Lifecycle.Event.ON_START);}@Overridepublic void onResume() {super.onResume();dispatchResume(mProcessListener);dispatch(Lifecycle.Event.ON_RESUME);}@Overridepublic void onPause() {super.onPause();dispatch(Lifecycle.Event.ON_PAUSE);}@Overridepublic void onStop() {super.onStop();dispatch(Lifecycle.Event.ON_STOP);}@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;}private void dispatch(Lifecycle.Event event) {Activity activity = getActivity();if (activity instanceof LifecycleRegistryOwner) {((LifecycleRegistryOwner) activity).getLifecycle().handleLifecycleEvent(event); //這里將事件分發交接給了LifecycleRegistryOwnerreturn;}if (activity instanceof LifecycleOwner) {Lifecycle lifecycle = ((LifecycleOwner) activity).getLifecycle();if (lifecycle instanceof LifecycleRegistry) {((LifecycleRegistry) lifecycle).handleLifecycleEvent(event); //這里將事件分發交接給了LifecycleRegistry}}}void setProcessListener(ActivityInitializationListener processListener) {mProcessListener = processListener;}interface ActivityInitializationListener {void onCreate();void onStart();void onResume();} } 復制代碼

進入handleLifecycleEvent

/*** 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); //獲取下一個statemoveToState(next); } 復制代碼

這里借用官方的一張圖(LifeCycle-States)：

大概意思就是可以通過不同的Event知道不同的State。例如：當你的Event是ON_RESUME的時候就代表他當前的State是STARTED，所以Next State就是RESUMED，接著分析getStateAfter：

//對照著表其實很簡單，events是ON_CREATE和ON_STOP的下一個狀態指向都是CREATED，所以返回CREATED，其他的類似。 static State getStateAfter(Event event) {switch (event) {case ON_CREATE:case ON_STOP:return CREATED;case ON_START:case ON_PAUSE:return STARTED;case ON_RESUME:return RESUMED;case ON_DESTROY:return DESTROYED;case ON_ANY:break;}throw new IllegalArgumentException("Unexpected event value " + event); } 復制代碼

所以getStateAfter獲取的是下一個State，也就是下一個生命周期。

進入moveToState

private void moveToState(State next) {//判斷當前State是不是等于我們上面獲取的if (mState == next) {return;}//不一樣則重新賦值mState = next;if (mHandlingEvent || mAddingObserverCounter != 0) {mNewEventOccurred = true;// we will figure out what to do on upper level.return;}mHandlingEvent = true;sync(); //同步mHandlingEvent = false; }// happens only on the top of stack (never in reentrance), // so it doesn't have to take in account parents private void sync() {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;}//當前的State和最早的State或最新的State進行比較，mObserverMap這個集合存放的是ObserverWithState，而ObserverWithState有我們的狀態。 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); //繼承Activity走這里}Entry<LifecycleObserver, ObserverWithState> newest = mObserverMap.newest();if (!mNewEventOccurred && newest != null&& mState.compareTo(newest.getValue().mState) > 0) {forwardPass(lifecycleOwner); //繼承FramgentActivity走這里，因為Activity的子類SupportActivity實現了LifecycleOwner，而且SupportActivity是FragmentActivity的父類}}mNewEventOccurred = false; }private void forwardPass(LifecycleOwner lifecycleOwner) {Iterator<Entry<LifecycleObserver, ObserverWithState>> ascendingIterator =mObserverMap.iteratorWithAdditions();//第一個while循壞遍歷我們存儲觀察者的集合while (ascendingIterator.hasNext() && !mNewEventOccurred) {Entry<LifecycleObserver, ObserverWithState> entry = ascendingIterator.next();ObserverWithState observer = entry.getValue();//第二個是要處理各個狀態經過的eventwhile ((observer.mState.compareTo(mState) < 0 && !mNewEventOccurred&& mObserverMap.contains(entry.getKey()))) {pushParentState(observer.mState);//upEvent返回所要經歷的Event，例如：當前是STARTED, 那么他的經過的Event就是ON_RESUMEobserver.dispatchEvent(lifecycleOwner, upEvent(observer.mState)); //在這里分發事件popParentState();}} }private void backwardPass(LifecycleOwner lifecycleOwner) {Iterator<Entry<LifecycleObserver, ObserverWithState>> descendingIterator =mObserverMap.descendingIterator();//第一個while循壞遍歷我們存儲觀察者的集合while (descendingIterator.hasNext() && !mNewEventOccurred) {Entry<LifecycleObserver, ObserverWithState> entry = descendingIterator.next();ObserverWithState observer = entry.getValue();//第二個是要處理各個狀態經過的eventwhile ((observer.mState.compareTo(mState) > 0 && !mNewEventOccurred&& mObserverMap.contains(entry.getKey()))) {//downEvent返回下一個EventEvent event = downEvent(observer.mState);pushParentState(getStateAfter(event));//upEvent返回所要經歷的Event，例如：當前是STARTED, 那么他的經過的Event就是ON_RESUMEobserver.dispatchEvent(lifecycleOwner, event); //在這里分發事件popParentState();}} } 復制代碼

進入dispatchEvent

static class ObserverWithState { //是不是很熟悉，又回到了這個類State mState;GenericLifecycleObserver mLifecycleObserver;ObserverWithState(LifecycleObserver observer, State initialState) {mLifecycleObserver = Lifecycling.getCallback(observer);mState = initialState;}void dispatchEvent(LifecycleOwner owner, Event event) {State newState = getStateAfter(event);mState = min(mState, newState);mLifecycleObserver.onStateChanged(owner, event); //最后在這里調用事件分發，并通過反射實現事件分發mState = newState;} } 復制代碼

附上一個簡單的流程圖(圖片來源于參考文章)

參考文章

• Android新組件Lifecycle詳解

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