Android中的Lifecycle有什么用
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Lifecycle的简单介绍
Lifecycle的介绍，我们还是拿Google的官方文档作为参考吧。
Lifecycle主要解决的是业务和Activity/Fragment生命周期相关的问题。例如：我们在onResume()/onStart()中请求定位，在onPause()/onStop()中停止定位。那么我们一般的做法：
class MyLocationListener { public MyLocationListener(Context context, Callback callback) { // ... } void start() { // connect to system location service } void stop() { // disconnect from system location service }}class MyActivity extends AppCompatActivity { private MyLocationListener myLocationListener; @Override public void onCreate(...) { myLocationListener = new MyLocationListener(this, (location) -> { // update UI }); } @Override public void onStart() { super.onStart(); myLocationListener.start(); // manage other components that need to respond // to the activity lifecycle } @Override public void onStop() { super.onStop(); myLocationListener.stop(); // manage other components that need to respond // to the activity lifecycle }}上面的代码虽然看起来还可以，但在真实的应用程序中，可能会有很多的方法都需要根据当前Activity/Fragment的生命周期来进行不同的操作。因此其生命周期方法中可能会被放置大量代码，例如onStart()和 onStop()中，这使得它们难以维护。因此Lifecycle应运而生！
3. Lifecycle的使用
Lifecycle已经发布了release版，所以其中的一些默认支持已经包含到了support-v7:26.1.0以及更高。
添加依赖：
implementation "android.arch.lifecycle:extensions:1.1.1"annotationProcessor "android.arch.lifecycle:compiler:1.1.1"
使用方式1：实现LifecycleObserver接口，使用@OnLifecycleEvent注解，通过编译时注解生成代码：
public class MyLifecycleObserver implements LifecycleObserver { private static final String TAG = MyLifecycleObserver.class.getSimpleName(); @OnLifecycleEvent(Lifecycle.Event.ON_CREATE) public void start(LifecycleOwner lifecycleOwner) { Lifecycle.State currentState = lifecycleOwner.getLifecycle().getCurrentState(); Log.d(TAG, "start: " + currentState); } @OnLifecycleEvent(Lifecycle.Event.ON_PAUSE) public void stop(LifecycleOwner lifecycleOwner) { Log.d(TAG, "stop: " + lifecycleOwner.getLifecycle().getCurrentState()); }}
APT生成的代码
使用方式2：实现GenericLifecycleObserver接口，实现其onStateChanged方法：
public class MyLifecycleObserver implements GenericLifecycleObserver { private static final String TAG = MyLifecycleObserver.class.getSimpleName(); @Override public void onStateChanged(LifecycleOwner source, Lifecycle.Event event) { Log.d(TAG, event.name()); }}创建完成后，我们需要将其添加：
public class MainActivity extends AppCompatActivity { @Override protected void onCreate(Bundle savedInstanceState) { super.onCreate(savedInstanceState); setContentView(R.layout.activity_main); getLifecycle().addObserver(new MyLifecycleObserver()); }}结果图：
方法1
方法2
4. 源码分析
1.Lifecycle的获取
根据源码追踪我们可以看到Lifecycle是在SupportActivity中：
private LifecycleRegistry mLifecycleRegistry = new LifecycleRegistry(this);@Overridepublic Lifecycle getLifecycle() { return mLifecycleRegistry;}这里还有个挺重要的代码：
SupportActivity.java
protected void onCreate(@Nullable Bundle savedInstanceState) { super.onCreate(savedInstanceState); ReportFragment.injectIfNeededIn(this);}ReportFragment.java
public static void injectIfNeededIn(Activity activity) { android.app.FragmentManager manager = activity.getFragmentManager(); if (manager.findFragmentByTag(REPORT_FRAGMENT_TAG) == null) { manager.beginTransaction().add(new ReportFragment(), REPORT_FRAGMENT_TAG).commit(); manager.executePendingTransactions(); }}这里在Activity中添加了一个Fragment，至于作用，留到后面在讲。
2.addObserver
addObserver方法的话，还是简单来看吧：
LifecycleRegistry.java
@Overridepublic void addObserver(@NonNull LifecycleObserver observer) { State initialState = mState == DESTROYED ? DESTROYED : INITIALIZED; ObserverWithState statefulObserver = new ObserverWithState(observer, initialState); ObserverWithState previous = mObserverMap.putIfAbsent(observer, statefulObserver); ...... // 对状态的修正，这里会对已经过去的状态进行分发。也就是说如果添加observer时状态已经改变，那么也会被通知到！}// 有状态的Observerstatic class ObserverWithState { 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); mState = newState; }}我们将实现的接口通过装饰者（我认为是这样）模式转成ObserverWithState对象，并将该对象添加到mObserverMap中。
在ObserverWithState的构造方法中，有对我们传入的LifecycleObserver进行包装：
Lifecycling.java
@NonNullstatic GenericLifecycleObserver getCallback(Object object) { // 这里应该算第三种实现方式，然而FullLifecycleObserver不是public，所以不能使用 if (object instanceof FullLifecycleObserver) { return new FullLifecycleObserverAdapter((FullLifecycleObserver) object); } // 我们的第一种方式 if (object instanceof GenericLifecycleObserver) { return (GenericLifecycleObserver) object; } // 通过注解生成的 final Class klass = object.getClass(); // 将生成的MyLifecycleObserver_LifecycleAdapter放到Map中 int type = getObserverConstructorType(klass); if (type == GENERATED_CALLBACK) { List> 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);} 根据我们传入的对象进行解析，最终返回结果为：GenericLifecycleObserver或者GenericLifecycleObserver的实现类。附获取生成MyLifecycleObserver_LifecycleAdapter代码：
// 通过名称获取public static String getAdapterName(String className) { return className.replace(".", "_") + "_LifecycleAdapter";}最终通过Class.forName方式获得Class。
3.事件分发
Lifecycle的事件分发在ObserverWithState类中：
// 事件分发void dispatchEvent(LifecycleOwner owner, Event event) { State newState = getStateAfter(event); mState = min(mState, newState); mLifecycleObserver.onStateChanged(owner, event); mState = newState;}看下哪里调用了dispatchEvent方法：
调用
我们看这个方法，其中一个在addObserver中调用，另外的两个都会通过sync()方法调用：
// happens only on the top of stack (never in reentrance),// so it doesn't have to take in account parentsprivate 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; } // 最新的状态和当前状态不一致，则需要进行状态修改 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;}// 是否状态同步的private boolean isSynced() { if (mObserverMap.size() == 0) { return true; } // 最新的和最后添加的Observer状态一致，并且当前的状态和最新状态一致，则已经同步了 State eldestObserverState = mObserverMap.eldest().getValue().mState; State newestObserverState = mObserverMap.newest().getValue().mState; return eldestObserverState == newestObserverState && mState == newestObserverState;} 这里的同步方法用于同步当前Observer的状态，如果最新的和最老的Observer的状态不一致或者当前的状态和最新的状态不一致时，那么需要进行状态同步。同步包括了向前同步和向后同步。
调用sync()方法一共就两处，一处在addObserver方法，另一处是moveToState方法，而调用moveToState方法也有两处：
@SuppressWarnings("WeakerAccess")@MainThreadpublic void markState(@NonNull State state) { moveToState(state);}public void handleLifecycleEvent(@NonNull Lifecycle.Event event) { State next = getStateAfter(event); moveToState(next);}handleLifecycleEvent方法有很多处调用：
handleLifecycleEvent调用
看图的话，就可以知道Fragment在不同的生命周期调用了handleLifecycleEvent方法，随便看一个吧：
ON_CREATE
还记得上面说的ReportFragment吗？这里也出现了，我们看下ReportFragment到底做什么的：
// 调用Activity的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) { ((LifecycleRegistry) lifecycle).handleLifecycleEvent(event); } }}// 生命周期@Overridepublic 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;}可以看到，ReportFragment只是作为一个中间层，通过它来分发各种事件！
由于篇幅原因（已经啰嗦了很多了），这里就不写LifecycleDispatcher和ProcessLifecycleOwner了，这两个通过自定义的内容提供者ProcessLifecycleOwnerInitializer进行初始化，并且通过registerActivityLifecycleCallbacks和registerFragmentLifecycleCallbacks注册统一的观察回调。
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