case ON_ANY:
throw new IllegalArgumentException(“ON_ANY must not been send by anybody”);
}
if (mLifecycleEventObserver != null) {
mLifecycleEventObserver.onStateChanged(source, event);
}
}
}

Activity 的 LifecycleRegistry 如何感知生命周期

先来看下 Activity 的基类 ComponentActivity

// androidx.core.app.ComponentActivity
@RestrictTo(LIBRARY_GROUP_PREFIX)
public class ComponentActivity extends Activity implements
LifecycleOwner,
KeyEventDispatcher.Component {
…
private LifecycleRegistry mLifecycleRegistry = new LifecycleRegistry(this);
…
@SuppressLint(“RestrictedApi”)
@Override
protected void onCreate(@Nullable Bundle savedInstanceState) {
super.onCreate(savedInstanceState);
ReportFragment.injectIfNeededIn(this);
}

@CallSuper
@Override
protected void onSaveInstanceState(@NonNull Bundle outState) {
mLifecycleRegistry.markState(Lifecycle.State.CREATED);
super.onSaveInstanceState(outState);
}
…
@NonNull
@Override
public Lifecycle getLifecycle() {
return mLifecycleRegistry;
}
}

再来看下 ReportFragment

// ReportFragment.java
public class ReportFragment extends android.app.Fragment {
private static final String REPORT_FRAGMENT_TAG = “androidx.lifecycle”

• “.LifecycleDispatcher.report_fragment_tag”;

public static void injectIfNeededIn(Activity activity) {
if (Build.VERSION.SDK_INT >= 29) {
// 在API 29+上，我们可以直接注册正确的生命周期回调
LifecycleCallbacks.registerIn(activity);
}
// 在API 29之前，并且为了保持与较旧版本的ProcessLifecycleOwner的兼容性（当生命周期运行时已经更新的时候可能不会更新
// 和需要支持未从support lib的FragmentActivity继承的activities），请使用framework fragment来获取正确的生命周期事件
android.app.FragmentManager manager = activity.getFragmentManager();
if (manager.findFragmentByTag(REPORT_FRAGMENT_TAG) == null) {
manager.beginTransaction().add(new ReportFragment(), REPORT_FRAGMENT_TAG).commit();
manager.executePendingTransactions();
}
}

@SuppressWarnings(“deprecation”)
static void dispatch(@NonNull Activity activity, @NonNull Lifecycle.Event event) {
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);
}
}
}

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();
}
}

@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);
}

@Override
public void onResume() {
super.onResume();
dispatchResume(mProcessListener);
dispatch(Lifecycle.Event.ON_RESUME);
}

@Override
public void onPause() {
super.onPause();
dispatch(Lifecycle.Event.ON_PAUSE);
}

@Override
public void onStop() {
super.onStop();
dispatch(Lifecycle.Event.ON_STOP);
}

@Override
public void onDestroy() {
super.onDestroy();
dispatch(Lifecycle.Event.ON_DESTROY);
// 只是想确保我们不会泄漏对activity的引用
mProcessListener = null;
}

private void dispatch(@NonNull Lifecycle.Event eve
nt) {
if (Build.VERSION.SDK_INT < 29) {
// 仅在API 29之前的API上从ReportFragment调度事件。在API 29+上，
// 这由ReportFragment.injectIfNeededIn中添加的ActivityLifecycleCallbacks处理
dispatch(getActivity(), event);
}
}

void setProcessListener(ActivityInitializationListener processListener) {
mProcessListener = processListener;
}

interface ActivityInitializationListener {
void onCreate();

void onStart();

void onResume();
}

// 此类不是内联的，仅因为我们需要为其添加一个proguard规则（b / 142778206）除此之外，registerIn方法还可以避免类验证失败，
// 因为registerActivityLifecycleCallbacks仅从api 29开始可用。
@RequiresApi(29)
static class LifecycleCallbacks implements Application.ActivityLifecycleCallbacks {

static void registerIn(Activity activity) {
activity.registerActivityLifecycleCallbacks(new LifecycleCallbacks());
}

@Override
public void onActivityCreated(@NonNull Activity activity,
@Nullable Bundle bundle) {
}

@Override
public void onActivityPostCreated(@NonNull Activity activity,
@Nullable Bundle savedInstanceState) {
dispatch(activity, Lifecycle.Event.ON_CREATE);
}

@Override
public void onActivityStarted(@NonNull Activity activity) {
}

@Override
public void onActivityPostStarted(@NonNull Activity activity) {
dispatch(activity, Lifecycle.Event.ON_START);
}

@Override
public void onActivityResumed(@NonNull Activity activity) {
}

@Override
public void onActivityPostResumed(@NonNull Activity activity) {
dispatch(activity, Lifecycle.Event.ON_RESUME);
}

@Override
public void onActivityPrePaused(@NonNull Activity activity) {
dispatch(activity, Lifecycle.Event.ON_PAUSE);
}

@Override
public void onActivityPaused(@NonNull Activity activity) {
}

@Override
public void onActivityPreStopped(@NonNull Activity activity) {
dispatch(activity, Lifecycle.Event.ON_STOP);
}

@Override
public void onActivityStopped(@NonNull Activity activity) {
}

@Override
public void onActivitySaveInstanceState(@NonNull Activity activity,
@NonNull Bundle bundle) {
}

@Override
public void onActivityPreDestroyed(@NonNull Activity activity) {
dispatch(activity, Lifecycle.Event.ON_DESTROY);
}

@Override
public void onActivityDestroyed(@NonNull Activity activity) {
}
}
}

LifecycleRegistry 如何将事件分发到 LifecycleObserver

// LifecycleRegistry.java
// 设置当前状态并通知观察者。
// 请注意，如果{@code currentState}与该方法的最后一次调用的状态相同，则调用此方法无效。
public void handleLifecycleEvent(@NonNull Lifecycle.Event event) {
enforceMainThreadIfNeeded(“handleLifecycleEvent”);
moveToState(event.getTargetState());
}

private void moveToState(State next) {
if (mState == next) {
return;
}
mState = next;
if (mHandlingEvent || mAddingObserverCounter != 0) {
mNewEventOccurred = true;
// we will figure out what to do on upper level.
return;
}
mHandlingEvent = true;
sync();
mHandlingEvent = false;
}

// 仅发生在堆栈的顶部（永远不可再进入）
// 因此不必考虑父类
private void sync() {
LifecycleOwner lifecycleOwner = mLifecycleOwner.get();
if (lifecycleOwner == null) {
throw new IllegalStateException(“LifecycleOwner of this LifecycleRegistry is already”

• “garbage collected. It is too late to change lifecycle state.”);
  }
  while (!isSynced()) {
  mNewEventOccurred = false;
  // 无需检查最老的是否可空, 因为在 isSynced 已经验证了
  if (mState.compareTo(mObserverMap.eldest().getValue().mState) < 0) {
  backwardPass(lifecycleOwner);
  }
  Map.Entry<LifecycleObserver, ObserverWithState> newest = mObserverMap.newest();
  if (!mNewEventOccurred && newest != null
  && mState.compareTo(newest.getValue().mState) > 0) {
  forwardPass(lifecycleOwner);
  }
  }
  mNewEventOccurred = false;
  }

private void forwardPass(LifecycleOwner lifecycleOwner) {
Iterator<Map.Entry<LifecycleObserver, ObserverWithState>> ascendingIterator =
mObserverMap.iteratorWithAdditions();
while (ascendingIterator.hasNext() && !mNewEventOccurred) {
Map.Entry<LifecycleObserver, ObserverWithState> entry = ascendingIterator.next();
ObserverWithState observer = entry.getValue();
while ((observer.mState.compareTo(mState) < 0 && !mNewEventOccurred
&& mObserverMap.contains(entry.getKey()))) {
pushParentState(observer.mState);
final Event event = Event.upFrom(observer.mState);
if (event == null) {
throw new IllegalStateException("no event up from " + observer.mState);
}
// 进行事件分发
observer.dispatchEvent(lifecycleOwner, event);
popParentState();
}
}
}

static class ObserverWithState {
State mState;
LifecycleEventObserver mLifecycleObserver;

ObserverWithState(LifecycleObserver observer, State initialState) {
mLifecycleObserver = Lifecycling.lifecycleEventObserver(observer);
mState = initialState;
}

void dispatchEvent(LifecycleOwner owner, Event event) {
State newState = event.getTargetState();
mState = min(mState, newState);
// 回调 LifecycleEventObserver 的 onStateChanged
mLifecycleObserver.onStateChanged(owner, event);
mState = newState;
}
}

@Override
public void addObserver(@NonNull LifecycleObserver observer) {
enforceMainThreadIfNeeded(“addObserver”);
State initialState = mState == DESTROYED ? DESTROYED : INITIALIZED;
// 创建 ObserverWithState 的位置
ObserverWithState statefulObserver = new ObserverWithState(observer, initialState);
ObserverWithState previous = mObserverMap.putIfAbsent(observer, statefulObserver);

// 不重复添加，直接返回
if (previous != null) {
return;
}
//如果 LifecycleOwner 对象已经被回收了，直接返回
LifecycleOwner lifecycleOwner = mLifecycleOwner.get();
if (lifecycleOwner == null) {
// it is null we should be destroyed. Fallback quickly
return;
}

boolean isReentrance = mAddingObserverCounter != 0 || mHandlingEvent;
State targetState = calculateTargetState(observer);
mAddingObserverCounter++;
// State值是否到目标状态 && observer 未移除
while ((statefulObserver.mState.compareTo(targetState) < 0
&& mObserverMap.contains(observer))) {
pushParentState(statefulObserver.mState);
final Event event = Event.upFrom(statefulObserver.mState);
if (event == null) {
throw new IllegalStateException("no event up from " + statefulObserver.mState);
}
statefulObserver.dispatchEvent(lifecycleOwner, event);
popParentState();
// mState / subling may have been changed recalculate
targetState = calculateTargetState(observer);
}

if (!isReentrance) {
// we do sync only on the top level.
sync();
}
mAddingObserverCounter–;
}

Lifecycling

// Lifecycling.java
@NonNull
static LifecycleEventObserver lifecycleEventObserver(Object object) {
boolean isLifecycleEventObserver = object instanceof LifecycleEventObserver;
boolean isFullLifecycleObserver = object instanceof FullLifecycleObserver;
if (isLifecycleEventObserver && isFullLifecycleObserver) {
return new FullLifecycleObserverAdapter((FullLifecycleObserver) object,
(LifecycleEventObserver) object);
}
if (isFullLifecycleObserver) {
return new FullLifecycleObserverAdapter((FullLifecycleObserver) object, null);
}

if (isLifecycleEventObserver) {
return (LifecycleEventObserver) object;
}

final Class<?> klass = object.getClass();
// 往里跳 跳到 resolveObserverCallbackType(Class<?>) - ClassesInfoCache.sInstance.hasLifecycleMethods(Class<?>)，跳完看下面
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);
}
// 通过反射来调用各回调函数
return new ReflectiveGenericLifecycleObserver(object);
}

通过反射获取注解信息

// ReflectiveGenericLifecycleObserver.java
class ReflectiveGenericLifecycleObserver implements LifecycleEventObserver {
private final Object mWrapped;
private final CallbackInfo mInfo;

ReflectiveGenericLifecycleObserver(Object wrapped) {
mWrapped = wrapped;
// 因为反射的代价是比较大的，所以对 ReflectiveGenericLifecycleObserver 类要调用的各种方法的相关信息进行了缓存。
mInfo = ClassesInfoCache.sInstance.getInfo(mWrapped.getClass());
}

@Override
public void onStateChanged(@NonNull LifecycleOwner source, @NonNull Event event) {
mInfo.invokeCallbacks(source, event, mWrapped);
}
}

// ClassesInfoCache.java
// 判断该 class 对象是否包含带有 OnLifecycleEvent 注解的方法
boolean hasLifecycleMethods(Class<?> klass) {
Boolean hasLifecycleMethods = mHasLifecycleMethods.get(klass);
// 如果本地有缓存的话则直接返回缓存值
if (hasLifecycleMethods != null) {
return hasLifecycleMethods;
}

//获取 klass 包含的所有方法
Method[] methods = getDeclaredMethods(klass);
for (Method method : methods) {
// 获取方法注解
OnLifecycleEvent annotation = method.getAnnotation(OnLifecycleEvent.class);
if (annotation != null) {
// Optimization for reflection, we know that this method is called
// when there is no generated adapter. But there are methods with @OnLifecycleEvent
// so we know that will use ReflectiveGenericLifecycleObserver,
// so we createInfo in advance.
// CreateInfo always initialize mHasLifecycleMethods for a class, so we don’t do it
// here.
createInfo(klass, methods);
return true;
}
}
mHasLifecycleMethods.put(klass, false);
return false;
otation != null) {
// Optimization for reflection, we know that this method is called
// when there is no generated adapter. But there are methods with @OnLifecycleEvent
// so we know that will use ReflectiveGenericLifecycleObserver,
// so we createInfo in advance.
// CreateInfo always initialize mHasLifecycleMethods for a class, so we don’t do it
// here.
createInfo(klass, methods);
return true;
}
}
mHasLifecycleMethods.put(klass, false);
return false;