Lifecycle是用来观察和监听Activity、Fragment生命周期的组件。
一、Lifecycle的基本使用
Lifecycle有两个核心类，一个是LifecycleOwner接口，该接口有一个getLifecycle()方法，实现这个接口的类就可以作为一个被观察者，AppCompatActivity和Fragment就实现了这个接口，所以它们的 生命周期可以被观察和监听。另一个是LifecycleObserver接口，实现了这个接口可以作为一个观察者。
1、自定义一个观察者类
class MyLifecycleObserver : LifecycleObserver {
//监听onCreate
@OnLifecycleEvent(Lifecycle.Event.ON_START)
fun onStart(){
Log.w("bestchangge", "LifecycleObserver_onStart")
}
@OnLifecycleEvent(Lifecycle.Event.ON_RESUME)
fun onResume() {
Log.w("bestchangge", "LifecycleObserver_onResume")
}
//...
}
2、创建一个AppCompatActivity作为被观察者
class MainActivity : AppCompatActivity() {
override fun onCreate(savedInstanceState: Bundle?) {
super.onCreate(savedInstanceState)
setContentView(R.layout.activity_main)
//将观察者和被观察者关联起来
lifecycle.addObserver(MyLifecycleObserver())
}
override fun onStart() {
super.onStart()
Log.w("bestchangge","onStart...")
}
override fun onResume() {
super.onResume()
Log.w("bestchangge","onResume...")
}
}
3、打开MainActivity观察生命周期方法日志
2021-02-24 15:29:50.297 10988-10988/com.sun.jetpack W/bestchangge: onStart...
2021-02-24 15:29:50.297 10988-10988/com.sun.jetpack W/bestchangge: LifecycleObserver_onStart
2021-02-24 15:29:50.299 10988-10988/com.sun.jetpack W/bestchangge: onResume...
2021-02-24 15:29:50.300 10988-10988/com.sun.jetpack W/bestchangge: LifecycleObserver_onResume
可以看出，当Activity中的生命周期方法回调时，都会回调LifecycleObserver中对应的监听方法。
二、Lifecycle的原理
从使用的地方addObserver入手
//将观察者和被观察者关联起来
lifecycle.addObserver(MyLifecycleObserver())
//LifecycleRegistry.java
@Override
public void addObserver(@NonNull LifecycleObserver observer) {
State initialState = mState == DESTROYED ? DESTROYED : INITIALIZED;
//关键代码1 new ObserverWithState(observer, initialState)
ObserverWithState statefulObserver = new ObserverWithState(observer, initialState);
ObserverWithState previous = mObserverMap.putIfAbsent(observer, statefulObserver);
//...
}
//最终追溯到ClassesInfoCache 会通过反射将带有@OnLifecycleEvent注解的方法信息缓存起来
//ClassesInfoCache.java
private CallbackInfo createInfo(Class klass, @Nullable Method[] declaredMethods) {
//...
Method[] methods = declaredMethods != null ? declaredMethods : getDeclaredMethods(klass);
boolean hasLifecycleMethods = false;
for (Method method : methods) {
OnLifecycleEvent annotation = method.getAnnotation(OnLifecycleEvent.class);
if (annotation == null) {
continue;
}
//...
Lifecycle.Event event = annotation.value();
//...
verifyAndPutHandler(handlerToEvent, methodReference, event, klass);
}
CallbackInfo info = new CallbackInfo(handlerToEvent);
mCallbackMap.put(klass, info);
mHasLifecycleMethods.put(klass, hasLifecycleMethods);
return info;
}
至于什么时候触发生命周期回调，我们回到ComponentActivity的onCreate方法中。
//ComponentActivity.java
@Override
protected void onCreate(@Nullable Bundle savedInstanceState) {
super.onCreate(savedInstanceState);
//...
//通过一个没有UI的ReportFragment绑定到Activity上来实现触发
ReportFragment.injectIfNeededIn(this);
//...
}
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();
}
}
在ReportFragment中的生命周期方法中，会去分发生命周期事件，以onResume为例：
@Override
public void onResume() {
super.onResume();
dispatchResume(mProcessListener);
dispatch(Lifecycle.Event.ON_RESUME);
}
private void dispatch(Lifecycle.Event event) {
Activity activity = getActivity();
//...
if (activity instanceof LifecycleOwner) {
Lifecycle lifecycle = ((LifecycleOwner) activity).getLifecycle();
if (lifecycle instanceof LifecycleRegistry) {
//关键代码handleLifecycleEvent
((LifecycleRegistry) lifecycle).handleLifecycleEvent(event);
}
}
}
//设置当前状态并通知观察者
public void handleLifecycleEvent(@NonNull Lifecycle.Event event) {
State next = getStateAfter(event);
moveToState(next);
}
//getStateAfter方法，会根据生命周期事件获取当前生命周期状态
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 void moveToState(State next) {
//...
sync();
//...
}
private void sync() {
LifecycleOwner lifecycleOwner = mLifecycleOwner.get();
//...
while (!isSynced()) {
//...
//关键代码forwardPass：sync中会调用backwardPass或forwardPass去同步生命周期并将当前生命周期 Event事件回调给观察者，这里以forwardPass为例
forwardPass(lifecycleOwner);
//...
}
}
}
private void forwardPass(LifecycleOwner lifecycleOwner) {
Iterator> ascendingIterator =
mObserverMap.iteratorWithAdditions();
while (ascendingIterator.hasNext() && !mNewEventOccurred) {
Entry entry = ascendingIterator.next();
//ObserverWithState在addObserver时记录了反射方法
ObserverWithState observer = entry.getValue();
while ((observer.mState.compareTo(mState) < 0 && !mNewEventOccurred
&& mObserverMap.contains(entry.getKey()))) {
pushParentState(observer.mState);
//分发事件
observer.dispatchEvent(lifecycleOwner, upEvent(observer.mState));
popParentState();
}
}
}
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);
//关键代码 找到实现类ReflectiveGenericLifecycleObserver
mLifecycleObserver.onStateChanged(owner, event);
mState = newState;
}
}
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) {
//根据Activity生命周期状态反射对应的方法，这样就完成了整个流程
mInfo.invokeCallbacks(source, event, mWrapped);
}
}
一、小结
lifecycle.png