Lifecycle-Aware是Google官方的生命周期感知組件。同時，在Google推出的Android Jetpack組件中，也將Lifecycle-Aware放在了Architecture類別之中。我們可以利用 android.arch.lifecycle 包提供的類來控制數(shù)據(jù)、監(jiān)聽器等的 lifecycle。同時，LiveData和ViewModel的lifecycle也依賴于Lifecycle框架，所以我們有必要去了解一下Lifecycle。
一、Lifecycle組件初始化
/app/build/intermediates/manifests/full/debug/AndroidManifest.xml，是我們應用在構建完成之后完整生成的AndroidManifest.xml文件。其中，我們可以找到Lifecycle-Aware組件在AndroidManifest的定義。
<provider
android:name="android.arch.lifecycle.ProcessLifecycleOwnerInitializer"
android:authorities="com.boohee.one.lifecycle-trojan"
android:exported="false"
android:multiprocess="true" />
ProcessLifecycleOwnerInitializer是ContentProvider的子類，利用其onCreate()生命周期方法，處理Lifecycle組件初始化。因此，這是一種隱式初始化的方式。
@Override
public boolean onCreate() {
LifecycleDispatcher.init(getContext());
ProcessLifecycleOwner.init(getContext());
return true;
}
二、Lifecycle生命周期事件感知
繼續(xù)關注ProcessLifecycleOwnerInitializer的onCreate()方法，其調(diào)用了LifecycleDispatcher的init()方法進行相應初始化。
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()方法其實就是在Application中注冊了一個ActivityLifecycleCallbacks監(jiān)聽（即DispatcherActivityCallback），用來監(jiān)聽每個Activity的生命周期變化。
我們再看下DispatcherActivityCallback的onActivityCreated()回調(diào)方法，如果Activity是FragmentActivity，則注冊一個FragmentLifecycleCallbacks（即FragmentCallback）用來監(jiān)聽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);
}
}
我們看到FragmentCallback的onFragmentCreated()回調(diào)方法中，當Fragment如果實現(xiàn)了LifecycleRegistryOwner，就會為其添加一個DestructionReportFragment，用來監(jiān)聽Fragment的生命周期。
之后，會在Activity中添加一個ReportFragment，用來監(jiān)聽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，利用一個附著在Activity上無UI的Fragment，通過相應的生命周期回調(diào)方法，實質(zhì)上是對Activity生命周期的監(jiān)聽。
至此，我們知道了Lifecycle組件通過ActivityLifecycleCallbacks、FragmentLifecycleCallbacks及 無UI的Fragment來感知Activity/Fragment的生命周期變化并產(chǎn)生相應的事件。
三、Lifecycle生命周期事件分發(fā)
在上面的分析中，我們只是重點關注了Lifecycle如何感知生命周期，也就是如何監(jiān)聽Activity/Fragment的生命周期。在相應的回調(diào)方法當中，其實都包含了很多生命周期事件分發(fā)相關的邏輯。通過相關的源碼分析，我們可以發(fā)現(xiàn)最終都會調(diào)用到Activity/Fragment的getLifecycle()方法獲取Lifecycle對象，之后再調(diào)用Lifecycle對象的handleLifecycleEvent()方法進行事件分發(fā)。26.1.0及更高版本Support支持庫中的Activity和Fragment已經(jīng)實現(xiàn)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最終繼承自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;
}
... ...
}
有上述源碼可以，其實getLifecycle() 方法最終返回的都是LifecycleRegistry對象，事件分發(fā)最后也都是調(diào)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();
}
}
}
這些只是分發(fā)的邏輯，那么在事件分發(fā)之前，肯定要有事件的注冊，這就得看一下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，我們可以知道，當lifecycle發(fā)生變化時，handleLifecycleEvent
會通過 getStateAfter()方法獲取當前應處的狀態(tài)并修改mState值，緊接著遍歷所有 ObserverWithState并調(diào)用他們的sync方法來同步且通知LifecycleObserver狀態(tài)發(fā)生變化。
總結
Lifecycle生命周期事件感知由LifecycleDispatcher(ActivityLifecycleCallbacks, FragmentLifecycleCallbacks, Fragment)完成，并通過 LifecycleRegistry傳遞給 LifecycleObserver。
