在上一篇文章中，我们学习了如何去使用Lifecycle； 当然之会使用是不够的，还需要了解它的原理，这是成为优秀工程师必备的；这篇文章就来学习Lifecycle的基本原理
Lifecycle使用两个枚举来跟踪其关联组件的生命周期状态，这两个枚举分别是Event和State；State指的是Lifecycle的生命周期所处的状态；Event代表Lifecycle生命周期对应的事件，这些事件会映射到Activity和Fragment中的回调事件中
Android 9.0的Lifecycle的源码如下所示
public abstract class Lifecycle { @MainThread public abstract void addObserver(@NonNull LifecycleObserver observer); @MainThread public abstract void removeObserver(@NonNull LifecycleObserver observer); @MainThread @NonNull public abstract State getCurrentState(); @SuppressWarnings("WeakerAccess") public enum Event { ON_CREATE, ON_START, ON_RESUME, ON_PAUSE, ON_STOP, ON_DESTROY, ON_ANY } @SuppressWarnings("WeakerAccess") public enum State { DESTROYED, INITIALIZED, CREATED, STARTED, RESUMED; public boolean isAtLeast(@NonNull State state) { return compareTo(state) >= 0; } }}Lifecycle是一个抽象类； 其内部不仅包括了添加和移除观察者的方法，还包括了此前说到的Event和State枚举。可以看到Event中的事件和Activity的生命周期几乎是对应的，除了ON_ANY，它可用于匹配所有事件
在Android Support Library 26.1.0 及其之后的版本，Activity和Fragment已经默认实现了LifecycleOwner接口，LifecycleOwner可以理解为被观察者，那么Lifecycle是如何观察Activity和Fragment的生命周期的呢？
在上一篇文章举的例子中，MainActivity继承了AppCompatActivity，而AppCompatActivity继承了FragmentActivity。在Android 8.0时，FragmentActivity继承自SupportActivity，而在Android 9.0，FragmentActivity继承自ComponentActivity 。SupportActivity和ComponentActivity的代码区别不大，这里以ComponentActivity举例，如下所示
@RestrictTo(LIBRARY_GROUP)public class ComponentActivity extends Activity implements LifecycleOwner { private SimpleArrayMap, ExtraData> mExtraDataMap = new SimpleArrayMap<>(); private LifecycleRegistry mLifecycleRegistry = new LifecycleRegistry(this);//1 @RestrictTo(LIBRARY_GROUP) public void putExtraData(ExtraData extraData) { mExtraDataMap.put(extraData.getClass(), extraData); } @Override @SuppressWarnings("RestrictedApi") protected void onCreate(@Nullable Bundle savedInstanceState) { super.onCreate(savedInstanceState); ReportFragment.injectIfNeededIn(this);//2 } @CallSuper @Override protected void onSaveInstanceState(Bundle outState) { mLifecycleRegistry.markState(Lifecycle.State.CREATED);//3 super.onSaveInstanceState(outState); } @RestrictTo(LIBRARY_GROUP) public T getExtraData(Class extraDataClass) { return (T) mExtraDataMap.get(extraDataClass); } @Override public Lifecycle getLifecycle() { return mLifecycleRegistry;//4 } @RestrictTo(LIBRARY_GROUP) public static class ExtraData { }} 注释1处创建了LifecycleRegistry，它是Lifecycle的实现类；注释4处实现了LifecycleOwner接口定义的getLifecycle方法，返回了LifecycleRegistry。在注释3处，将Lifecycle的State设置为CREATED；
正常来说应该在ComponentActivity的各个生命周期方法中改变Lifecycle的State，显然在ComponentActivity中没有做这些，而是将这个任务交给了ReportFragment，注释2处的将ComponentActivity注入到ReportFragment中
@RestrictTo(RestrictTo.Scope.LIBRARY_GROUP)public class ReportFragment extends Fragment { private static final String REPORT_FRAGMENT_TAG = "androidx.lifecycle" + ".LifecycleDispatcher.report_fragment_tag"; 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(); } } static ReportFragment get(Activity activity) { return (ReportFragment) activity.getFragmentManager().findFragmentByTag( REPORT_FRAGMENT_TAG); } ... @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);//1 } @Override public void onResume() { super.onResume(); dispatchResume(mProcessListener); dispatch(Lifecycle.Event.ON_RESUME); } ... private void dispatch(Lifecycle.Event event) { Activity activity = getActivity(); if (activity instanceof LifecycleRegistryOwner) {//2 ((LifecycleRegistryOwner) activity).getLifecycle().handleLifecycleEvent(event); return; } if (activity instanceof LifecycleOwner) {//3 Lifecycle lifecycle = ((LifecycleOwner) activity).getLifecycle(); if (lifecycle instanceof LifecycleRegistry) { ((LifecycleRegistry) lifecycle).handleLifecycleEvent(event); } } } ...}ReportFragment的onStart方法中会调用注释1处的dispatch方法； 在dispatch方法的注释2处，判断Activity是否实现了LifecycleRegistryOwner接口，LifecycleRegistryOwner继承了LifecycleOwner接口
这两个接口不同的是： LifecycleRegistryOwner定义的getLifecycle方法返回的是LifecycleRegistry类型，而LifecycleOwner定义的getLifecycle方法返回的是Lifecycle类型。注释3处如果Activity实现了LifecycleOwner接口，会调用LifecycleRegistry的handleLifecycleEvent方法
public void handleLifecycleEvent(@NonNull Lifecycle.Event event) { State next = getStateAfter(event); moveToState(next); }getStateAfter方法会获取“即将的事件” ：当前事件执行后，即将会处于什么事件，代码如下所示
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); }这个和文章开头给出的State与Event关系的时序图对照看会比较好理解； 比如当前执行了ON_CREATE事件或者ON_STOP事件，那么状态就会处于CREATED；回到handleLifecycleEvent方法，其内部还会调用moveToState方法
private void moveToState(State next) { if (mState == next) { return; } mState = next; if (mHandlingEvent || mAddingObserverCounter != 0) { mNewEventOccurred = true; return; } mHandlingEvent = true; sync(); mHandlingEvent = false; }如果当前所处的状态和即将要处于的状态一样就不做任何操作，sync方法如下所示
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; } while (!isSynced()) { mNewEventOccurred = false; 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; } sync方法中会根据当前状态和mObserverMap中的eldest和newest的状态做对比 ，判断当前状态是向前还是向后； 比如由STARTED到RESUMED是状态向前，反过来就是状态向后，这个不要和Activity的生命周期搞混；向前还是向后的代码大同小异，这里以向后为例
private void forwardPass(LifecycleOwner lifecycleOwner) { Iterator> ascendingIterator = mObserverMap.iteratorWithAdditions(); while (ascendingIterator.hasNext() && !mNewEventOccurred) { Entry entry = ascendingIterator.next(); ObserverWithState observer = entry.getValue();//1 while ((observer.mState.compareTo(mState) < 0 && !mNewEventOccurred && mObserverMap.contains(entry.getKey()))) { pushParentState(observer.mState); observer.dispatchEvent(lifecycleOwner, upEvent(observer.mState));//2 popParentState(); } } } 注释1处的用于获取ObserverWithState，后面会在提到它。 注释2处的upEvent方法会得到当前状态的向前状态
ObserverWithState的dispatchEvent方法如下所示
static class ObserverWithState { State mState; GenericLifecycleObserver mLifecycleObserver; ObserverWithState(LifecycleObserver observer, State initialState) { mLifecycleObserver = Lifecycling.getCallback(observer);//1 mState = initialState; } void dispatchEvent(LifecycleOwner owner, Event event) { State newState = getStateAfter(event); mState = min(mState, newState); mLifecycleObserver.onStateChanged(owner, event); mState = newState; } }从名称就可以看出来，它内部包括了State和GenericLifecycleObserver，GenericLifecycleObserver是一个接口，它继承了LifecycleObserver接口； ReflectiveGenericLifecycleObserver和CompositeGeneratedAdaptersObserver是GenericLifecycleObserver的实现类，这里主要查看ReflectiveGenericLifecycleObserver的onStateChanged方法是如何实现的
class ReflectiveGenericLifecycleObserver implements GenericLifecycleObserver { private final Object mWrapped; private final CallbackInfo mInfo; ReflectiveGenericLifecycleObserver(Object wrapped) { mWrapped = wrapped; mInfo = ClassesInfoCache.sInstance.getInfo(mWrapped.getClass()); } @Override public void onStateChanged(LifecycleOwner source, Event event) { mInfo.invokeCallbacks(source, event, mWrapped);//1 }}注释1处会调用CallbackInfo的invokeCallbacks方法，在讲这个方法前，需要先了解CallbackInfo是怎么创建的，是由createInfo方法创建的，如下所示
private CallbackInfo createInfo(Class klass, @Nullable Method[] declaredMethods) { Class superclass = klass.getSuperclass(); Map handlerToEvent = new HashMap<>(); ... Method[] methods = declaredMethods != null ? declaredMethods : getDeclaredMethods(klass); boolean hasLifecycleMethods = false; for (Method method : methods) { OnLifecycleEvent annotation = method.getAnnotation(OnLifecycleEvent.class);//1 if (annotation == null) { continue; } hasLifecycleMethods = true; Class<?>[] params = method.getParameterTypes(); int callType = CALL_TYPE_NO_ARG; if (params.length > 0) { callType = CALL_TYPE_PROVIDER; if (!params[0].isAssignableFrom(LifecycleOwner.class)) { throw new IllegalArgumentException( "invalid parameter type. Must be one and instanceof LifecycleOwner"); } } Lifecycle.Event event = annotation.value();//2 ... MethodReference methodReference = new MethodReference(callType, method);//3 verifyAndPutHandler(handlerToEvent, methodReference, event, klass);//4 } CallbackInfo info = new CallbackInfo(handlerToEvent);//5 mCallbackMap.put(klass, info); mHasLifecycleMethods.put(klass, hasLifecycleMethods); return info; } 关键点在注释1处； 不断的遍历各个方法，获取方法上的名为OnLifecycleEvent的注解，这个注解正是实现LifecycleObserver接口时用到的。
注释2处获取该注解的值； 也就是在@OnLifecycleEvent中定义的事件
注释3处新建了一个MethodReference； 其内部包括了使用了该注解的方法
注释4处的verifyAndPutHandler方法用于将MethodReference和对应的Event存在类型为Map
注释5处新建CallbackInfo，并将handlerToEvent传进去
接着回头看CallbackInfo的invokeCallbacks方法，代码如下所示
static class CallbackInfo { final Map> mEventToHandlers; final Map mHandlerToEvent; CallbackInfo(Map handlerToEvent) { mHandlerToEvent = handlerToEvent; mEventToHandlers = new HashMap<>(); for (Map.Entry entry : handlerToEvent.entrySet()) {//1 Lifecycle.Event event = entry.getValue(); List methodReferences = mEventToHandlers.get(event); if (methodReferences == null) { methodReferences = new ArrayList<>(); mEventToHandlers.put(event, methodReferences); } methodReferences.add(entry.getKey()); } } @SuppressWarnings("ConstantConditions") void invokeCallbacks(LifecycleOwner source, Lifecycle.Event event, Object target) { invokeMethodsForEvent(mEventToHandlers.get(event), source, event, target);//2 invokeMethodsForEvent(mEventToHandlers.get(Lifecycle.Event.ON_ANY), source, event, target); } private static void invokeMethodsForEvent(List handlers, LifecycleOwner source, Lifecycle.Event event, Object mWrapped) { if (handlers != null) { for (int i = handlers.size() - 1; i >= 0; i--) { handlers.get(i).invokeCallback(source, event, mWrapped);//1 } } } 注释1处的循环的意义在于将handlerToEvent进行数据类型转换，转化为一个HashMap，key的值为事件，value的值为MethodReference。注释2处的invokeMethodsForEvent方法会传入mEventToHandlers.get(event)，也就是事件对应的MethodReference的集合。invokeMethodsForEvent方法中会遍历MethodReference的集合，调用MethodReference的invokeCallback方法
@SuppressWarnings("WeakerAccess") static class MethodReference { final int mCallType; final Method mMethod; MethodReference(int callType, Method method) { mCallType = callType; mMethod = method; mMethod.setAccessible(true); } void invokeCallback(LifecycleOwner source, Lifecycle.Event event, Object target) { try { switch (mCallType) { case CALL_TYPE_NO_ARG: mMethod.invoke(target); break; case CALL_TYPE_PROVIDER: mMethod.invoke(target, source); break; case CALL_TYPE_PROVIDER_WITH_EVENT: mMethod.invoke(target, source, event); break; } } catch (InvocationTargetException e) { throw new RuntimeException("Failed to call observer method", e.getCause()); } catch (IllegalAccessException e) { throw new RuntimeException(e); } } ... }MethodReference类中有两个变量，一个是callType，它代表调用方法的类型，另一个是Method； 它代表方法，不管是哪种callType都会通过invoke对方法进行反射。 简单来说，实现LifecycleObserver接口的类中，注解修饰的方法和事件会被保存起来，通过反射对事件的对应方法进行调用
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最后我想说：
对于程序员来说，要学习的知识内容、技术有太多太多，要想不被环境淘汰就只有不断提升自己，从来都是我们去适应环境，而不是环境来适应我们
技术是无止境的，你需要对自己提交的每一行代码、使用的每一个工具负责，不断挖掘其底层原理，才能使自己的技术升华到更高的层面
Android 架构师之路还很漫长，与君共勉
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