Demo
Activity中
getLifecycle().addObserver(new LifecycleObserver2());
其他需要声明周期的对象中
public class LifecycleObserver2 implements LifecycleObserver {
@OnLifecycleEvent(Lifecycle.Event.ON_RESUME)
public void onResume(){
L.e("onResume");
}
@OnLifecycleEvent(Lifecycle.Event.ON_DESTROY)
public void onDestroy(){
L.e("onDestroy");
}
}
原理
以前看Glide时，第一次知道建立不可见碎片Fragment用来管理生命周期这种方式。现在Lifecycle也是通过这种方式。知道这个，其实大致过程都能猜出来吧，Fragment绑定到Activity上，然后分发生命周期到订阅的对象。
向上找Activity的父类，找到ComponentActivity，在onCreate()中有个ReportFragment.injectIfNeededIn(this);
public class ComponentActivity extends androidx.core.app.ComponentActivity implements
LifecycleOwner,
ViewModelStoreOwner,
SavedStateRegistryOwner,
OnBackPressedDispatcherOwner {
@Override
protected void onCreate(@Nullable Bundle savedInstanceState) {
super.onCreate(savedInstanceState);
mSavedStateRegistryController.performRestore(savedInstanceState);
ReportFragment.injectIfNeededIn(this);
if (mContentLayoutId != 0) {
setContentView(mContentLayoutId);
}
}
injectIfNeededIn()中，创建FragmentManager，然后new一个ReportFragment，add到Activity中。
@RestrictTo(RestrictTo.Scope.LIBRARY_GROUP_PREFIX)
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);
}
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);
// 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);
}
}
}
void setProcessListener(ActivityInitializationListener processListener) {
mProcessListener = processListener;
}
interface ActivityInitializationListener {
void onCreate();
void onStart();
void onResume();
}
}
碎片的生命周期中调用了dispatch(Lifecycle.Event.ON_CREATE)
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);
}
}
}
ComponentActivity实现了LifecycleOwner接口，所以看第二个if。
public void handleLifecycleEvent(@NonNull Lifecycle.Event event) {
State next = getStateAfter(event);
moveToState(next);
}
找到这次生命周期切换事件后的下一状态，并修改状态
事件event和状态state就是
public enum Event {
ON_CREATE,
ON_START,
ON_RESUME,
ON_PAUSE,
ON_STOP,
ON_DESTROY,
ON_ANY
}
public enum State {
DESTROYED,
INITIALIZED,
CREATED,
STARTED,
RESUMED;
public boolean isAtLeast(@NonNull State state) {
return compareTo(state) >= 0;
}
}
moveToState()会调用sync()
private void sync() {
//标记1
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;
if (mState.compareTo(mObserverMap.eldest().getValue().mState) < 0) {
backwardPass(lifecycleOwner);
}
Entry newest = mObserverMap.newest();
if (!mNewEventOccurred && newest != null
&& mState.compareTo(newest.getValue().mState) > 0) {
//标记2
forwardPass(lifecycleOwner);
}
}
mNewEventOccurred = false;
}
- 标记1处，这个LifecycleOwner对象，向前寻找可以得知是ComponentActivity对象
- 标记2处的forwardPass和上面的backwardPass类似,点进去一个
private void forwardPass(LifecycleOwner lifecycleOwner) {
Iterator> ascendingIterator =
mObserverMap.iteratorWithAdditions();
while (ascendingIterator.hasNext() && !mNewEventOccurred) {
Entry entry = ascendingIterator.next();
//标记5
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();
}
}
}
static class ObserverWithState {
State mState;
LifecycleEventObserver mLifecycleObserver;
ObserverWithState(LifecycleObserver observer, State initialState) {
//标记3
mLifecycleObserver = Lifecycling.lifecycleEventObserver(observer);
mState = initialState;
}
void dispatchEvent(LifecycleOwner owner, Event event) {
State newState = getStateAfter(event);
mState = min(mState, newState);
//标记4
mLifecycleObserver.onStateChanged(owner, event);
mState = newState;
}
}
- 标记3处lifecycleEventObserver()方法找进去，最终找到处理实现LifecycleObserver接口对象的注解方法，时间最终就会回调到这些方法
boolean hasLifecycleMethods(Class klass) {
Boolean hasLifecycleMethods = mHasLifecycleMethods.get(klass);
if (hasLifecycleMethods != null) {
return hasLifecycleMethods;
}
Method[] methods = getDeclaredMethods(klass);
for (Method method : methods) {
OnLifecycleEvent annotation = method.getAnnotation(OnLifecycleEvent.class);
if (annotation != null) {
createInfo(klass, methods);
return true;
}
}
mHasLifecycleMethods.put(klass, false);
return false;
}
- 标记4处onStateChanged()方法是接口方法，找进去后最终是通过反射调用了实现LifecycleObserver接口对象的方法
CallbackInfo mInfo;
mInfo.invokeCallbacks(source, event, mWrapped);
void invokeCallback(LifecycleOwner source, Lifecycle.Event event, Object target) {
//noinspection TryWithIdenticalCatches
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);
}
}
- 标记5处，ObserverWithState类初始化时会走标记3处理注解方法，mObserverMap保存的是LifecycleObserver和ObserverWithState对象。找到put的地方就是LifecycleRegistry的addObserver()，正是使用是在Activity中调用的方法
@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);
...
调用addObserver()的getLifecycle()，返回的就是LifecycleRegistry对象
@NonNull
@Override
public Lifecycle getLifecycle() {
return mLifecycleRegistry;
}