Lifecycle是什么
Lifecycle是Google官方提供的方便管理生命周期事件的方式，可以更方便的监听生命周期变化事件，它以注解的方式作用在方法上，当生命周期方法调用时，它也会被跟随调用。
Lifecycle是一个抽象类，它持有关于组件（如Activity 或 Fragment）生命周期状态的信息，并且允许其他对象观察此状态。
从Lifecycle类作切入点读代码：
public abstract class Lifecycle {
AtomicReference<Object> mInternalScopeRef = new AtomicReference<>();
@MainThread
public abstract void addObserver(@NonNull LifecycleObserver observer);
@MainThread
public abstract void removeObserver(@NonNull LifecycleObserver observer);
@MainThread
@NonNull
public abstract State getCurrentState();
@SuppressWarnings("WeakerAccess")
public enum Event {
ON_CREATE,
ON_START,
ON_RESUME,
ON_PAUSE,
ON_STOP,
ON_DESTROY,
ON_ANY
}
@SuppressWarnings("WeakerAccess")
public enum State {
DESTROYED,
INITIALIZED,
CREATED,
STARTED,
RESUMED;
}
}
分析：Lifecycle使用两个枚举来跟踪其关联组件的生命周期状态，这两个枚举分别是Event和State。Event中的事件和Activity的生命周期几乎是对应的，除了ON_ANY，它可用于匹配所有事件。还有添加和删除观察者的方法。
State：当前生命周期所处状态
Event：当前生命周期改变对应的事件
Lifecycle如何观察到Activity和Fragment的生命周期
Activity的继承关系：
AppCompatActivity继承了FragmentActivity，FragmentActivity继承了ComponentActivity，ComponentActivity最终去实现了LifecycleOwner接口。
LifecycleOwner接口代码：只有一个getLifecycle()抽象方法。
@SuppressWarnings({"WeakerAccess", "unused"})
public interface LifecycleOwner {
@NonNull
Lifecycle getLifecycle();
}
ComponentActivity类关于实现lifecycle的代码：
public class ComponentActivity extends androidx.core.app.ComponentActivity implements LifecycleOwner {
private final LifecycleRegistry mLifecycleRegistry = new LifecycleRegistry(this);
@Override
protected void onCreate(@Nullable Bundle savedInstanceState) {
super.onCreate(savedInstanceState);
mSavedStateRegistryController.performRestore(savedInstanceState);
ReportFragment.injectIfNeededIn(this); //注释处
if (mContentLayoutId != 0) {
setContentView(mContentLayoutId);
}
}
@Override
public Lifecycle getLifecycle() {
return mLifecycleRegistry;
}
}
正常来说应该在ComponentActivity的各个生命周期方法中改变Lifecycle的State，显然在ComponentActivity中没有做这些，注释处表明是通过injectIfNeededIn()这个方法将这个任务交给了ReportFragment。
ReportFragment类部分实现：
@RestrictTo(RestrictTo.Scope.LIBRARY_GROUP_PREFIX)
public class ReportFragment extends Fragment {
@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);
}
}
}
}
同时，ComponentActivity中获取LifecycleOwner，返回LifecycleRegistry对象，LifecycleRegistry继承了Lifecycle，让LifecycleRegistry去实现Lifecycle的功能。
LifecycleRegistry类部分代码实现：
public class LifecycleRegistry extends Lifecycle {
private State mState;
private final WeakReference<LifecycleOwner> mLifecycleOwner;
public LifecycleRegistry(@NonNull LifecycleOwner provider) {
mLifecycleOwner = new WeakReference<>(provider);
mState = INITIALIZED;
}
@Override
public void addObserver(@NonNull LifecycleObserver observer) {
...
}
@Override
public void removeObserver(@NonNull LifecycleObserver observer) {
...
}
static State getStateAfter(Event event) {
switch (event) {
case ON_CREATE:
case ON_STOP:
return CREATED;
case ON_START:
case ON_PAUSE:
return STARTED;
case ON_RESUME:
return RESUMED;
case ON_DESTROY:
return DESTROYED;
case ON_ANY:
break;
}
throw new IllegalArgumentException("Unexpected event value " + event);
}
private static Event downEvent(State state) {
switch (state) {
case INITIALIZED:
throw new IllegalArgumentException();
case CREATED:
return ON_DESTROY;
case STARTED:
return ON_STOP;
case RESUMED:
return ON_PAUSE;
case DESTROYED:
throw new IllegalArgumentException();
}
throw new IllegalArgumentException("Unexpected state value " + state);
}
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 = getStateAfter(event);
mState = min(mState, newState);
mLifecycleObserver.onStateChanged(owner, event);
mState = newState;
}
}
}
分析这个核心过程：
当ReportFragment生命周期发生改变时，调用dispatch()方法并将当前生命周期对应的事件Event分发，交给当前所属的实现了lifecycle或LifecycleOwner的activity处理，通过activity.getLifecycle()获取到内部持有的LifecycleRegistry，最终是调用了LifecycleRegistry的handleLifecycleEvent(event)方法处理。
public void handleLifecycleEvent(@NonNull Lifecycle.Event event) {
State next = getStateAfter(event);
moveToState(next);
}
getStateAfter方法会获取“即将的事件” ：当前事件执行后，即将会处于什么事件，代码如下：
static State getStateAfter(Event event) {
switch (event) {
case ON_CREATE:
case ON_STOP:
return CREATED;
case ON_START:
case ON_PAUSE:
return STARTED;
case ON_RESUME:
return RESUMED;
case ON_DESTROY:
return DESTROYED;
case ON_ANY:
break;
}
throw new IllegalArgumentException("Unexpected event value " + event);
}
然后其内部还会调用moveToState方法，改变当前生命周期为下一个生命周期。而如果当前所处的状态和即将要处于的状态一样就不做任何操作。
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;
}
ObserverWithState类代码：
static class ObserverWithState {
State mState;
GenericLifecycleObserver mLifecycleObserver;
ObserverWithState(LifecycleObserver observer, State initialState) {
mLifecycleObserver = Lifecycling.getCallback(observer);//1
mState = initialState;
}
void dispatchEvent(LifecycleOwner owner, Event event) {
State newState = getStateAfter(event);
mState = min(mState, newState);
mLifecycleObserver.onStateChanged(owner, event);
mState = newState;
}
}
从名称就可以看出来，它内部包括了State和GenericLifecycleObserver，GenericLifecycleObserver是一个接口，它继承了LifecycleObserver接口。
ReflectiveGenericLifecycleObserver和CompositeGeneratedAdaptersObserver是GenericLifecycleObserver的实现类，这里主要查看ReflectiveGenericLifecycleObserver的onStateChanged方法是如何实现的。
/**
* An internal implementation of {@link LifecycleObserver} that relies on reflection.
*/
class ReflectiveGenericLifecycleObserver implements LifecycleEventObserver {
private final Object mWrapped;
private final CallbackInfo mInfo;
ReflectiveGenericLifecycleObserver(Object wrapped) {
mWrapped = wrapped;
mInfo = ClassesInfoCache.sInstance.getInfo(mWrapped.getClass());
}
@Override
public void onStateChanged(LifecycleOwner source, Event event) {
mInfo.invokeCallbacks(source, event, mWrapped); //注释处
}
}
注释处会调用CallbackInfo的invokeCallbacks方法。
CallbackInfo类代码：
static class CallbackInfo {
final Map<Lifecycle.Event, List<MethodReference>> mEventToHandlers;
final Map<MethodReference, Lifecycle.Event> mHandlerToEvent;
CallbackInfo(Map<MethodReference, Lifecycle.Event> handlerToEvent) {
mHandlerToEvent = handlerToEvent;
mEventToHandlers = new HashMap<>();
for (Map.Entry<MethodReference, Lifecycle.Event> entry : handlerToEvent.entrySet()) { //注释1
Lifecycle.Event event = entry.getValue();
List<MethodReference> methodReferences = mEventToHandlers.get(event);
if (methodReferences == null) {
methodReferences = new ArrayList<>();
mEventToHandlers.put(event, methodReferences);
}
methodReferences.add(entry.getKey());
}
}
@SuppressWarnings("ConstantConditions")
void invokeCallbacks(LifecycleOwner source, Lifecycle.Event event, Object target) {
invokeMethodsForEvent(mEventToHandlers.get(event), source, event, target);
invokeMethodsForEvent(mEventToHandlers.get(Lifecycle.Event.ON_ANY), source, event,
target); //注释2
}
private static void invokeMethodsForEvent(List<MethodReference> handlers,
LifecycleOwner source, Lifecycle.Event event, Object mWrapped) {
if (handlers != null) {
for (int i = handlers.size() - 1; i >= 0; i--) {
handlers.get(i).invokeCallback(source, event, mWrapped);
}
}
}
}
注释1处的循环的意义在于将handlerToEvent进行数据类型转换，转化为一个HashMap，key的值为事件，value的值为MethodReference。注释2处的invokeMethodsForEvent方法会传入mEventToHandlers.get(event)，也就是事件对应的MethodReference的集合。invokeMethodsForEvent方法中会遍历MethodReference的集合，调用MethodReference的invokeCallback方法。
MethodReference类代码：
@SuppressWarnings("WeakerAccess")
static class MethodReference {
final int mCallType;
final Method mMethod;
MethodReference(int callType, Method method) {
mCallType = callType;
mMethod = method;
mMethod.setAccessible(true);
}
void invokeCallback(LifecycleOwner source, Lifecycle.Event event, Object target) {
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);
}
}
...
}
MethodReference类中有两个变量，一个是callType，它代表调用方法的类型，另一个是Method，它代表方法，不管是哪种callType都会通过invoke对方法进行反射。
简单来说，实现LifecycleObserver接口的类中，注解修饰的方法和事件会被保存起来，通过反射对事件的对应方法进行调用。
总结如上过程中调用链的时序图：
lifecycle家族关联类及集成关系图：
网友评论