Lifecycle-Aware是Google官方的生命周期感知组件。同时，在Google推出的Android Jetpack组件中，也将Lifecycle-Aware放在了Architecture类别之中。我们可以利用 android.arch.lifecycle 包提供的类来控制数据、监听器等的 lifecycle。同时，LiveData和ViewModel的lifecycle也依赖于Lifecycle框架，所以我们有必要去了解一下Lifecycle。
Google官方Lifecycle组件跟踪状态图
一、Lifecycle组件初始化
/app/build/intermediates/manifests/full/debug/AndroidManifest.xml，是我们应用在构建完成之后完整生成的AndroidManifest.xml文件。其中，我们可以找到Lifecycle-Aware组件在AndroidManifest的定义。
<provider
android:name="android.arch.lifecycle.ProcessLifecycleOwnerInitializer"
android:authorities="com.boohee.one.lifecycle-trojan"
android:exported="false"
android:multiprocess="true" />
ProcessLifecycleOwnerInitializer是ContentProvider的子类，利用其onCreate()生命周期方法，处理Lifecycle组件初始化。因此，这是一种隐式初始化的方式。
@Override
public boolean onCreate() {
LifecycleDispatcher.init(getContext());
ProcessLifecycleOwner.init(getContext());
return true;
}
二、Lifecycle生命周期事件感知
继续关注ProcessLifecycleOwnerInitializer的onCreate()方法，其调用了LifecycleDispatcher的init()方法进行相应初始化。
static void init(Context context) {
if (sInitialized.getAndSet(true)) {
return;
}
((Application) context.getApplicationContext())
.registerActivityLifecycleCallbacks(new DispatcherActivityCallback());
}
@SuppressWarnings("WeakerAccess")
@VisibleForTesting
static class DispatcherActivityCallback extends EmptyActivityLifecycleCallbacks {
private final FragmentCallback mFragmentCallback;
DispatcherActivityCallback() {
mFragmentCallback = new FragmentCallback();
}
@Override
public void onActivityCreated(Activity activity, Bundle savedInstanceState) {
if (activity instanceof FragmentActivity) {
((FragmentActivity) activity).getSupportFragmentManager()
.registerFragmentLifecycleCallbacks(mFragmentCallback, true);
}
ReportFragment.injectIfNeededIn(activity);
}
@Override
public void onActivityStopped(Activity activity) {
if (activity instanceof FragmentActivity) {
markState((FragmentActivity) activity, CREATED);
}
}
@Override
public void onActivitySaveInstanceState(Activity activity, Bundle outState) {
if (activity instanceof FragmentActivity) {
markState((FragmentActivity) activity, CREATED);
}
}
}
init()方法其实就是在Application中注册了一个ActivityLifecycleCallbacks监听（即DispatcherActivityCallback），用来监听每个Activity的生命周期变化。
我们再看下DispatcherActivityCallback的onActivityCreated()回调方法，如果Activity是FragmentActivity，则注册一个FragmentLifecycleCallbacks（即FragmentCallback）用来监听Activity中Fragment的生命周期。
static class FragmentCallback extends FragmentManager.FragmentLifecycleCallbacks {
@Override
public void onFragmentCreated(FragmentManager fm, Fragment f, Bundle savedInstanceState) {
dispatchIfLifecycleOwner(f, ON_CREATE);
if (!(f instanceof LifecycleRegistryOwner)) {
return;
}
if (f.getChildFragmentManager().findFragmentByTag(REPORT_FRAGMENT_TAG) == null) {
f.getChildFragmentManager().beginTransaction().add(new DestructionReportFragment(),
REPORT_FRAGMENT_TAG).commit();
}
}
@Override
public void onFragmentStarted(FragmentManager fm, Fragment f) {
dispatchIfLifecycleOwner(f, ON_START);
}
@Override
public void onFragmentResumed(FragmentManager fm, Fragment f) {
dispatchIfLifecycleOwner(f, ON_RESUME);
}
}
我们看到FragmentCallback的onFragmentCreated()回调方法中，当Fragment如果实现了LifecycleRegistryOwner，就会为其添加一个DestructionReportFragment，用来监听Fragment的生命周期。
之后，会在Activity中添加一个ReportFragment，用来监听Activity生命周期。
// ReportFragment
@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(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);
}
}
}
如此，利用一个附着在Activity上无UI的Fragment，通过相应的生命周期回调方法，实质上是对Activity生命周期的监听。
至此，我们知道了Lifecycle组件通过ActivityLifecycleCallbacks、FragmentLifecycleCallbacks及 无UI的Fragment来感知Activity/Fragment的生命周期变化并产生相应的事件。
三、Lifecycle生命周期事件分发
在上面的分析中，我们只是重点关注了Lifecycle如何感知生命周期，也就是如何监听Activity/Fragment的生命周期。在相应的回调方法当中，其实都包含了很多生命周期事件分发相关的逻辑。通过相关的源码分析，我们可以发现最终都会调用到Activity/Fragment的getLifecycle()方法获取Lifecycle对象，之后再调用Lifecycle对象的handleLifecycleEvent()方法进行事件分发。26.1.0及更高版本Support支持库中的Activity和Fragment已经实现了LifecycleOwner接口。
public interface LifecycleOwner {
/**
* Returns the Lifecycle of the provider
* @return The lifecycle of the provider.
*/
@NonNull
Lifecycle getLifecycle();
}
public class Fragment implements implements ComponentCallbacks, OnCreateContextMenuListener, LifecycleOwner {
... ...
LifecycleRegistry mLifecycleRegistry = new LifecycleRegistry(this);
@Override
public Lifecycle getLifecycle() {
return mLifecycleRegistry;
}
... ...
}
public class FragmentActivity extends BaseFragmentActivityApi16 implements
ActivityCompat.OnRequestPermissionsResultCallback,
ActivityCompat.RequestPermissionsRequestCodeValidator {
... ...
// FragmentActivity最终继承自SupportActivity
@Override
public Lifecycle getLifecycle() {
return super.getLifecycle();
}
... ...
}
public class SupportActivity extends Activity implements LifecycleOwner {
... ...
private LifecycleRegistry mLifecycleRegistry = new LifecycleRegistry(this);
@Override
public Lifecycle getLifecycle() {
return mLifecycleRegistry;
}
... ...
}
有上述源码可以，其实getLifecycle() 方法最终返回的都是LifecycleRegistry对象，事件分发最后也都是调用LifecycleRegistry的handleLifecycleEvent()方法。
public void handleLifecycleEvent(@NonNull Lifecycle.Event event) {
State next = getStateAfter(event);
moveToState(next);
}
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 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;
// no need to check eldest for nullability, because isSynced does it for us.
if (mState.compareTo(mObserverMap.eldest().getValue().mState) < 0) {
backwardPass(lifecycleOwner);
}
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<Entry<LifecycleObserver, ObserverWithState>> ascendingIterator =
mObserverMap.iteratorWithAdditions();
while (ascendingIterator.hasNext() && !mNewEventOccurred) {
Entry<LifecycleObserver, ObserverWithState> entry = ascendingIterator.next();
ObserverWithState observer = entry.getValue();
while ((observer.mState.compareTo(mState) < 0 && !mNewEventOccurred
&& mObserverMap.contains(entry.getKey()))) {
pushParentState(observer.mState);
observer.dispatchEvent(lifecycleOwner, upEvent(observer.mState));
popParentState();
}
}
}
private void backwardPass(LifecycleOwner lifecycleOwner) {
Iterator<Entry<LifecycleObserver, ObserverWithState>> descendingIterator =
mObserverMap.descendingIterator();
while (descendingIterator.hasNext() && !mNewEventOccurred) {
Entry<LifecycleObserver, ObserverWithState> entry = descendingIterator.next();
ObserverWithState observer = entry.getValue();
while ((observer.mState.compareTo(mState) > 0 && !mNewEventOccurred
&& mObserverMap.contains(entry.getKey()))) {
Event event = downEvent(observer.mState);
pushParentState(getStateAfter(event));
observer.dispatchEvent(lifecycleOwner, event);
popParentState();
}
}
}
这些只是分发的逻辑，那么在事件分发之前，肯定要有事件的注册，这就得看一下addObserver()方法。
@Override
public void addObserver(@NonNull LifecycleObserver observer) {
State initialState = mState == DESTROYED ? DESTROYED : INITIALIZED;
ObserverWithState statefulObserver = new ObserverWithState(observer, initialState);
ObserverWithState previous = mObserverMap.putIfAbsent(observer, statefulObserver);
if (previous != null) {
return;
}
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++;
while ((statefulObserver.mState.compareTo(targetState) < 0
&& mObserverMap.contains(observer))) {
pushParentState(statefulObserver.mState);
statefulObserver.dispatchEvent(lifecycleOwner, upEvent(statefulObserver.mState));
popParentState();
// mState / subling may have been changed recalculate
targetState = calculateTargetState(observer);
}
if (!isReentrance) {
// we do sync only on the top level.
sync();
}
mAddingObserverCounter--;
}
static 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;
}
}
至此，我们可以知道，当lifecycle发生变化时，handleLifecycleEvent
会通过 getStateAfter()方法获取当前应处的状态并修改mState值，紧接着遍历所有 ObserverWithState并调用他们的sync方法来同步且通知LifecycleObserver状态发生变化。
总结
Lifecycle生命周期事件感知由LifecycleDispatcher(ActivityLifecycleCallbacks, FragmentLifecycleCallbacks, Fragment)完成，并通过 LifecycleRegistry传递给 LifecycleObserver。
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