介绍
Lifecycle的诞生使我们监听组件生命周期不需要那么麻烦了，只需要注册一个监听器就行了，这种AOP的编程使得程序完全解耦，易于维护。
使用
实现LifecycleObserver接口，在需要监听Activity生命周期的方法上添加注解。
public class Presenter implements LifecycleObserver {
@OnLifecycleEvent(Lifecycle.Event.ON_CREATE)
public void onCreate(){
Log.i("jawe", "onCreate: Presenter");
}
@OnLifecycleEvent(Lifecycle.Event.ON_START)
public void onStart(){
Log.i("jawe", "onStart: Presenter");
}
@OnLifecycleEvent(Lifecycle.Event.ON_RESUME)
public void onResume(){
Log.i("jawe", "onResume: Presenter");
}
@OnLifecycleEvent(Lifecycle.Event.ON_PAUSE)
public void onPause(){
Log.i("jawe", "onPause: Presenter");
}
@OnLifecycleEvent(Lifecycle.Event.ON_STOP)
public void onStop(){
Log.i("jawe", "onStop: Presenter");
}
@OnLifecycleEvent(Lifecycle.Event.ON_DESTROY)
public void onDestroy(){
Log.i("jawe", "onDestroy: Presenter");
}
}
在Activity中进行注册监听器
public class MainActivity extends AppCompatActivity {
@Override
protected void onCreate(Bundle savedInstanceState) {
super.onCreate(savedInstanceState);
setContentView(R.layout.activity_main);
Presenter presenter = new Presenter();
getLifecycle().addObserver(presenter);
}
}
分析原理
.首先看一下LifecycleObserver的源码
/**
* Marks a class as a LifecycleObserver. It does not have any methods, instead, relies on
* {@link OnLifecycleEvent} annotated methods.
* <p>
* @see Lifecycle Lifecycle - for samples and usage patterns.
*/
public interface LifecycleObserver {
}
这个接口很简单，里边没有任何方法。但是请大家看接口的注释描述：标记一个类是LifecycleObserver，接口内没有任何方法，它的实现类需要使用OnLifecycleEvent 注解方法。
getLifecycle()的实现
private final LifecycleRegistry mLifecycleRegistry = new LifecycleRegistry(this);
...
@NonNull
@Override
public Lifecycle getLifecycle() {
return mLifecycleRegistry;
}
LifecycleRegistry的构造
/**
* Current state
*/
private State mState;
...
private final WeakReference<LifecycleOwner> mLifecycleOwner;
...
public LifecycleRegistry(@NonNull LifecycleOwner provider) {
mLifecycleOwner = new WeakReference<>(provider);
mState = INITIALIZED;
}
初始化的时候mState是INITIALIZED状态。
LifecycleRegistry#addObserver
@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);//1
mAddingObserverCounter++;
while ((statefulObserver.mState.compareTo(targetState) < 0//2
&& 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--;
}
mObserverMap是一个对hashMap的封装，列表内的元素是Entry<LifecycleObserver, ObserverWithState>
注释1处的targetState是INITIALIZED，注释2处的compareTo=0，所以直接到了sync()方法
private void sync() {
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;
// 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;
}
sync方法根据链表中的头节点（eldest()）和尾节点（newest ）的状态进行比较，决定向前还是向后传递。但是这时候while的循环条件是true，所以不执行任何操作。我们看一下isSynced的源码
private boolean isSynced() {
if (mObserverMap.size() == 0) {
return true;
}
State eldestObserverState = mObserverMap.eldest().getValue().mState;
State newestObserverState = mObserverMap.newest().getValue().mState;
return eldestObserverState == newestObserverState && mState == newestObserverState;
}
初始化的mObserverMap 头节点和尾节点指向的是同一个entrystate是(INITIALIZED)。所以这里都是true。
那么Lifecycle是如何做到监听组件生命周期，并且通知观察者的呢？我们继续向下分析。
我们看一下ComponentActivity的onCreate方法
@Override
protected void onCreate(@Nullable Bundle savedInstanceState) {
...
ReportFragment.injectIfNeededIn(this);
...
}
看到这里明白了吧，系统是在Activity上添加了一个ReportFragment监听Activity的生命周期，这一点和Glide是一样的。
public static void injectIfNeededIn(Activity activity) {
// ProcessLifecycleOwner should always correctly work and some activities may not extend
// FragmentActivity from support lib, so we use framework fragments for activities
android.app.FragmentManager manager = activity.getFragmentManager();
if (manager.findFragmentByTag(REPORT_FRAGMENT_TAG) == null) {
manager.beginTransaction().add(new ReportFragment(), REPORT_FRAGMENT_TAG).commit();
// Hopefully, we are the first to make a transaction.
manager.executePendingTransactions();
}
}
然后就在所有的生命周期的方法内分发事件
@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) {//1
Lifecycle lifecycle = ((LifecycleOwner) activity).getLifecycle();
if (lifecycle instanceof LifecycleRegistry) {
((LifecycleRegistry) lifecycle).handleLifecycleEvent(event);
}
}
}
ComponentActivity实现的是LifecycleOwner接口，所以执行注释1处的条件。
我们看一下LifecycleRegistry的handleLifecycleEvent,
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;
}
我们以create事件ON_CREATE分析，getStateAfter返回的是CREATED状态，在moveToState有执行sync方法了。
private void sync() {
...
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) {//1
backwardPass(lifecycleOwner);
}
Entry<LifecycleObserver, ObserverWithState> newest = mObserverMap.newest();
if (!mNewEventOccurred && newest != null
&& mState.compareTo(newest.getValue().mState) > 0) {//2
forwardPass(lifecycleOwner);
}
}
mNewEventOccurred = false;
}
这次的isSynced()方法内mState是CREATED，newestObserverState是INITIALIZED，所以进入while循环。目前eldest和newest的mState都是INITIALIZED，所以注释2的条件是成立的。
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()))) {//1
pushParentState(observer.mState);
observer.dispatchEvent(lifecycleOwner, upEvent(observer.mState));//2
popParentState();
}
}
}
主要的是注释2处
static class ObserverWithState {
State mState;
LifecycleEventObserver mLifecycleObserver;
ObserverWithState(LifecycleObserver observer, State initialState) {
mLifecycleObserver = Lifecycling.lifecycleEventObserver(observer);//1
mState = initialState;
}
void dispatchEvent(LifecycleOwner owner, Event event) {
State newState = getStateAfter(event);
mState = min(mState, newState);
mLifecycleObserver.onStateChanged(owner, event);
mState = newState;//2
}
}
我们先看注释2会把最新的状态赋值给map内的observer，这样下次在forwardPass的while循环条件就不成立了。
注释1处会把我们的LifecycleObserver包装成LifecycleEventObserver。所以调用的是LifecycleEventObserver的onStateChanged方法。
public interface LifecycleEventObserver extends LifecycleObserver {
/**
* Called when a state transition event happens.
*
* @param source The source of the event
* @param event The event
*/
void onStateChanged(@NonNull LifecycleOwner source, @NonNull Lifecycle.Event event);
}
至此整个定义分发过程就都一目了然了。
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