public interface LifecycleOwner {
/**
* Returns the Lifecycle of the provider.
*
* @return The lifecycle of the provider.
*/
@NonNull
Lifecycle getLifecycle();
}LifecycleOwner 是一个接口，定义了 getLifeCycle 方法。ComponentActivity 和 Fragment 都实现了该接口。
private final LifecycleRegistry mLifecycleRegistry = new LifecycleRegistry(this);
@NonNull
@Override
public Lifecycle getLifecycle() {
return mLifecycleRegistry;
}可以看到，在 ComponentActivity 的 getLifecycle 中直接返回了 mLifecycleRegistry 对象，它是 LifecycleRegistry 类型的对象。
public LifecycleRegistry(@NonNull LifecycleOwner provider) {
this(provider, true);
}
private LifecycleRegistry(@NonNull LifecycleOwner provider, boolean enforceMainThread) {
mLifecycleOwner = new WeakReference<>(provider);
mState = INITIALIZED;
mEnforceMainThread = enforceMainThread;
}在 LifecycleRegistry 的构造函数中，将传入的 LifecycleOwner 放入弱引用中保存。那么 ComponentActivity 的生命周期又是如何与 LifecycleRegistry 关联起来的呢？
@Override
protected void onCreate(@Nullable Bundle savedInstanceState) {
super.onCreate(savedInstanceState);
...
ReportFragment.injectIfNeededIn(this);
...
}在 ComponentActivity 的 onCreate 方法中，有一行重要代码 ReportFragment.injectIfNeededIn(this);，正是在 ReportFragment 中实现了 ComponentActivity 的生命周期分发到 LifecycleRegistry 中。接下来看看 injectIfNeededIn 方法。
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) {
// On API 29+, we can register for the correct Lifecycle callbacks directly
LifecycleCallbacks.registerIn(activity);
}
// Prior to API 29 and to maintain compatibility with older versions of
// ProcessLifecycleOwner (which may not be updated when lifecycle-runtime is updated and
// need to support activities that don't extend from FragmentActivity from support lib),
// use a framework fragment to get the correct timing of Lifecycle events
android.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();
}
}
}对于 API 29 及以上，会执行 LifecycleCallbacks 的 registerIn 方法，然后将 ReportFragment 添加到 Activity 中。LifecycleCallbacks 实现了 Application.ActivityLifecycleCallbacks 接口，在 Activity 生命周期回调方法中会执行 ReportFragment 的 dispatch 方法。
@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) {
}
}而在 ReportFragment 的各个生命周期方法中也会调用 dispatch 方法，最终执行的方法与上述一致。
@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);
// just want to be sure that we won't leak reference to an activity
mProcessListener = null;
}
private void dispatch(@NonNull Lifecycle.Event event) {
if (Build.VERSION.SDK_INT < 29) {
// Only dispatch events from ReportFragment on API levels prior
// to API 29. On API 29+, this is handled by the ActivityLifecycleCallbacks
// added in ReportFragment.injectIfNeededIn
dispatch(getActivity(), event);
}
}那么我们看看 dispatch(@NonNull Activity activity, @NonNull Lifecycle.Event event) 方法的实现：
@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);
}
}
}其中 LifecycleRegistryOwner 是一个继承自 LifecycleOwner 的接口，所以，最终都会执行 LifecycleRegistry 的 handleLifecyleEvent 方法。
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;
}
private boolean isSynced() {
if (mObserverMap.size() == 0) {
return true;
}
State eldestObserverState = mObserverMap.eldest().getValue().mState;
State newestObserverState = mObserverMap.newest().getValue().mState;
return eldestObserverState == newestObserverState && mState == newestObserverState;
}
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;
// no need to check eldest for nullability, because isSynced does it for us.
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);
}
observer.dispatchEvent(lifecycleOwner, event);
popParentState();
}
}
}
private void backwardPass(LifecycleOwner lifecycleOwner) {
Iterator<Map.Entry<LifecycleObserver, ObserverWithState>> descendingIterator =
mObserverMap.descendingIterator();
while (descendingIterator.hasNext() && !mNewEventOccurred) {
Map.Entry<LifecycleObserver, ObserverWithState> entry = descendingIterator.next();
ObserverWithState observer = entry.getValue();
while ((observer.mState.compareTo(mState) > 0 && !mNewEventOccurred
&& mObserverMap.contains(entry.getKey()))) {
Event event = Event.downFrom(observer.mState);
if (event == null) {
throw new IllegalStateException("no event down from " + observer.mState);
}
pushParentState(event.getTargetState());
observer.dispatchEvent(lifecycleOwner, event);
popParentState();
}
}
}
static class ObserverWithState {
State mState;
LifecycleEventObserver mLifecycleObserver;
...
void dispatchEvent(LifecycleOwner owner, Event event) {
...
mLifecycleObserver.onStateChanged(owner, event);
mState = newState;
}
}在 handleLifecycleEvent 中会执行 moveToState 方法，并传入当前生命周期状态，在 moveToState 方法中会将当前生命周期状态赋值为新的状态，接着会执行 sync 方法，在该方法中会执行遍历比较添加的 LifecycleObserver 的状态和当前状态，最终会执行 ObserverWithState 的 dispatchEvent 方法，此方法中会回调 LifecycleObserver 的 onStateChanged 方法。
LifecycleRegistry mLifecycleRegistry;
@Override
@NonNull
public Lifecycle getLifecycle() {
return mLifecycleRegistry;
}
public Fragment() {
initLifecycle();
}
private void initLifecycle() {
mLifecycleRegistry = new LifecycleRegistry(this);
mSavedStateRegistryController = SavedStateRegistryController.create(this);
if (Build.VERSION.SDK_INT >= 19) {
mLifecycleRegistry.addObserver(new LifecycleEventObserver() {
@Override
public void onStateChanged(@NonNull LifecycleOwner source,
@NonNull Lifecycle.Event event) {
if (event == Lifecycle.Event.ON_STOP) {
if (mView != null) {
mView.cancelPendingInputEvents();
}
}
}
});
}
}
void performCreate(Bundle savedInstanceState) {
...
mLifecycleRegistry.handleLifecycleEvent(Lifecycle.Event.ON_CREATE);
}
void performStart() {
...
mChildFragmentManager.dispatchStart();
}
void performResume() {
...
mLifecycleRegistry.handleLifecycleEvent(Lifecycle.Event.ON_RESUME);
if (mView != null) {
mViewLifecycleOwner.handleLifecycleEvent(Lifecycle.Event.ON_RESUME);
}
mChildFragmentManager.dispatchResume();
...
}
void performPause() {
...
if (mView != null) {
mViewLifecycleOwner.handleLifecycleEvent(Lifecycle.Event.ON_PAUSE);
}
mLifecycleRegistry.handleLifecycleEvent(Lifecycle.Event.ON_PAUSE);
...
}
void performStop() {
...
if (mView != null) {
mViewLifecycleOwner.handleLifecycleEvent(Lifecycle.Event.ON_STOP);
}
mLifecycleRegistry.handleLifecycleEvent(Lifecycle.Event.ON_STOP);
...
}
void performDestroyView() {
...
if (mView != null) {
mViewLifecycleOwner.handleLifecycleEvent(Lifecycle.Event.ON_DESTROY);
}
...
}
void performDestroy() {
...
mLifecycleRegistry.handleLifecycleEvent(Lifecycle.Event.ON_DESTROY);
...
}在 Fragment 的构造方法中，会调用 initLifecycle 方法初始化 mLifecycleRegistry ，在 getLifecycle 中返回。在 Fragment 的各个生命周期方法中同样会调用 LifecycleRegistry 的 handleLifecycleEvent 方法进行生命周期事件分发。
public interface LifecycleEventObserver extends LifecycleObserver {
/**
* Called when a state transition event happens.
*
* @param source The source of the event
* @param event The event
*/
void onStateChanged(@NonNull LifecycleOwner source, @NonNull Lifecycle.Event event);
}LifecycleObserver 是一个空接口。LifecycleEventObserver 继承了它，并定义了 onStateChanged 方法，该方法在生命周期变化时会被回调。LifecycleRegistry 的 addObserver 方法：
private FastSafeIterableMap<LifecycleObserver, ObserverWithState> mObserverMap =
new FastSafeIterableMap<>();
@Override
public void addObserver(@NonNull LifecycleObserver observer) {
enforceMainThreadIfNeeded("addObserver");
State initialState = mState == DESTROYED ? DESTROYED : INITIALIZED;
ObserverWithState statefulObserver = new ObserverWithState(observer, initialState);
ObserverWithState previous = mObserverMap.putIfAbsent(observer, statefulObserver);
...
}在该方法中，首先创建了一个 ObserverWithState 类型的 statefulObserver 。然后执行了 mObserverMap 的 putIfAbsent 方法，该方法会取出当前 key 对应的 Entry，如果 Entry 不为空则返回它对应的 value，否则将新的键值对存入并返回 null。