首先確定如下幾個類并說明每個類的職責。?

接口LifecycleOwner:生命周期持有者，代碼如下：

其中google提供了接口LifecycleRegistryOwner繼承了LifecycleOwner

具有生命周期的Activity、Fragment應該實現LifecycleRegistry并重寫getLifecycle()方法。

抽象類Lifecycle主要具有添加觀察者addObserver()、移除觀察者removeObserver()功能

抽象類LifecycleRegister繼承Lifecycle，重寫了addObserver()、removeObserver()，除此之外實現將Lifecycle.Event和State之間的轉換，并且狀態發生變化時，通知觀察者。

一般使用Activity、Fragment繼承LifecycleRegistryOwner重寫getLifecyle()，方法示例如下：

需要針對生命周期做出變化的類實現LifecycleObserver也就是觀察者，并通過LifecycleRegistry的addObserver()方法將觀察者添加到LifecycleRegistry中，其中紅框部分稍后分析，這里僅以Activity為例來分析Lifecycle的運作過程。啟動Activity調用棧如下所示：

可以看到Activity啟動時，在ActivityThread類中installProvider方法中調用了ContentProvider的attchInfo方法，進而調用了LifecycleRuntimeTrojanProvider類的onCreate()方法，該方法如下所示

跟進LifecycleDispatcher

private static final String REPORT_FRAGMENT_TAG = "android.arch.lifecycle"+ ".LifecycleDispatcher.report_fragment_tag";private static AtomicBoolean sInitialized = new AtomicBoolean(false);static void init(Context context) {if (sInitialized.getAndSet(true)) {return;}((Application) context.getApplicationContext()).registerActivityLifecycleCallbacks(new DispatcherActivityCallback()); }復制代碼

init方法只是將DispatherActivityCallback添加到Application存儲的map中去，繼續查看DispatcherActivityCallBack類

static class DispatcherActivityCallback extends EmptyActivityLifecycleCallbacks {private final FragmentCallback mFragmentCallback;DispatcherActivityCallback() {mFragmentCallback = new FragmentCallback();}@Overridepublic void onActivityCreated(Activity activity, Bundle savedInstanceState) {if (activity instanceof FragmentActivity) {((FragmentActivity) activity).getSupportFragmentManager().registerFragmentLifecycleCallbacks(mFragmentCallback, true);}ReportFragment.injectIfNeededIn(activity);}@Overridepublic void onActivityStopped(Activity activity) {if (activity instanceof FragmentActivity) {markState((FragmentActivity) activity, CREATED);}}@Overridepublic void onActivitySaveInstanceState(Activity activity, Bundle outState) {if (activity instanceof FragmentActivity) {markState((FragmentActivity) activity, CREATED);}} }復制代碼

經過斷點試驗發現當ThreadActivity執行performCreate方法時會調用DispatcherActivityCallback的onActivityCreated方法，該方法中會調用ReportFragment.injectIfNeedIn(activity),其中該activity即為實現了LifecycleRegistryOwner的對象，繼續更近ReportFragment代碼如下：

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();} }復制代碼

從代碼可以看出相當于在activity中添加了一個沒有任何內容的Fragment，該Fragment用來監聽activity生命周期的變化并通知觀察者，利用Fragment應該是是因為Fragment的onCreate()、onStart()、onResume()方法均在其所在的Activity的相應的onCreate()、onStart()、onResume()方法之后執行，但是Fragment的onPause()、onStop()、onDestroy()均在所在Activit相應的onPause()、onStop()、onDestroy()之前執行，這一點兒很重要！

當執行完畢Activity中的onCreate方法后，會執行ReportFragment方法中的onAcitivityCreate方法

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

其中dispatch方法如下所示

private void dispatch(Lifecycle.Event event) {if (getActivity() instanceof LifecycleRegistryOwner) {((LifecycleRegistryOwner) getActivity()).getLifecycle().handleLifecycleEvent(event);} }復制代碼

這里個getActivity就是實現了第三張圖中實現LifecycleRegistryOwner的Activity，此處也應該注意被監聽生命周期變化的Activity應該繼承的是LifecycleRegisterOwner類，而不應該是LifecycleOwner。繼續跟進handleLifecycleEvent方法

public void handleLifecycleEvent(Lifecycle.Event event) {if (mLastEvent == event) {return;}mLastEvent = event;mState = getStateAfter(event);for (Map.Entry<LifecycleObserver, ObserverWithState> entry : mObserverSet) {entry.getValue().sync();} }復制代碼

只要mLastEvent!=event就會執行for循環，其中mObserverSet中的元素是在addObserver的時候添加的，如下所示

@Override public void addObserver(LifecycleObserver observer) {ObserverWithState observerWithState = new ObserverWithState(observer);mObserverSet.putIfAbsent(observer, observerWithState);observerWithState.sync(); }復制代碼

其中ObserverWithState對象對observer進行了封裝，

ObserverWithState(LifecycleObserver observer) {mCallback = Lifecycling.getCallback(observer); }復制代碼

ObserverWithState構造函數中調用了Lifecycleing.getCallback(observer)查看該方法（這里不再貼代碼）只是根據observer所在包名和類名通過反射構建出了一個“className”+"_LifecycleAdapter的類并調用該類的構造函數將observer作為參數傳遞給該類的實例，例如自己的demo中名為CustomLifecycleObser_LifeAdapter，該類在app/build/generated/source/apt/debug下，查看該類如下

public class CustomLifecycleObserver_LifecycleAdapter implements GenericLifecycleObserver {final CustomLifecycleObserver mReceiver;CustomLifecycleObserver_LifecycleAdapter(CustomLifecycleObserver receiver) {this.mReceiver = receiver;}@Overridepublic void onStateChanged(LifecycleOwner owner, Lifecycle.Event event) {if (event == Lifecycle.Event.ON_CREATE) {mReceiver.onCreate(owner);}if (event == Lifecycle.Event.ON_START) {mReceiver.onStart(owner);}if (event == Lifecycle.Event.ON_RESUME) {mReceiver.onResume(owner);}if (event == Lifecycle.Event.ON_STOP) {mReceiver.onStop(owner);}if (event == Lifecycle.Event.ON_PAUSE) {mReceiver.onPause(owner);}if (event == Lifecycle.Event.ON_DESTROY) {mReceiver.onDestroy(owner);}mReceiver.onAny(owner,event);}public Object getReceiver() {return mReceiver;} }復制代碼

注解處理器apt生成該類時會生成一個onStateChanged方法，并根據event調用observer中相應的方法，這些方法是observer中被onLifecycleEvent注解的方法，那onStateChanged方法在哪里被調用呢，例如上面的Activity的onCreate方法執行后會調用onActivityCreated方法，然后調用handleLifecycleEvent方法，該方法遍歷mObserverSet后調用sync方法，該方法如下所示

void sync() {if (mState == DESTROYED && mObserverCurrentState == INITIALIZED) {mObserverCurrentState = DESTROYED;}while (mObserverCurrentState != mState) {Event event = mObserverCurrentState.isAtLeast(mState)? downEvent(mObserverCurrentState) : upEvent(mObserverCurrentState);mObserverCurrentState = getStateAfter(event);mCallback.onStateChanged(mLifecycleOwner, event);} }復制代碼

當mState不等于mObserverCurrentState的時候就會進行兩種狀態的比較，其中返回的Event其實就是mObserverCurrentState過渡到mState時所需的Event詳見頭圖，例如mObserverCurrentState是INITIALIZED，而mState是CREATED的時候，這個時候然需要經過ON_CREATE事件后，mObserverCurrentState才可以過度到mState的狀態，進而更新mObserverState對應的狀態為mState，然后調用“className”+"_LifecycleAdapter的onStateChange方法，進而調用到觀察者中相應的方法。

再說紅色方框中為啥要調用markState方法，因為Activity的onCreate方法要比Fragment的早，如果不手動調用的話mObserverCurrentState和mState都是INITIALIZED，進而不會執行

mCallBack.onStateChanged方法。

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