问题思考
- 为啥lifecycle能感知生命周期呢
- 新增的生命周期观察者是如何监听生命周期的
状态和事件图解
lifecycle重点是它的事件和状态的理解，根据上面的图理解下状态和事件的一个流程
1. lifecycle 如何绑定activity 的生命周期
ComponentActivity实现LifecycleOwner接口，并且oncreate()创建ReportFragment，之后在这个fragment监听生命周期
ComponentActivity.java
protected void onCreate(@Nullable Bundle savedInstanceState) {
super.onCreate(savedInstanceState);
mSavedStateRegistryController.performRestore(savedInstanceState);
ReportFragment.injectIfNeededIn(this);
if (mContentLayoutId != 0) {
setContentView(mContentLayoutId);
}
}
ReportFragment.java
public static void injectIfNeededIn(Activity activity) {
if (Build.VERSION.SDK_INT >= 29) {
// On API 29+, we can register for the correct Lifecycle callbacks directly
activity.registerActivityLifecycleCallbacks(
new LifecycleCallbacks());
}
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();
}
}
// 多的不写了
@Override
public void onStart() {
super.onStart();
dispatchStart(mProcessListener);
dispatch(Lifecycle.Event.ON_START); // 为啥能感知生命周期就在这体现
}
@Override
public void onResume() {
super.onResume();
dispatchResume(mProcessListener);
dispatch(Lifecycle.Event.ON_RESUME);
}
private void dispatch(@NonNull Lifecycle.Event event) {
if (Build.VERSION.SDK_INT < 29) {
dispatch(getActivity(), event);
}
}
// 最后还是调用了handleLifecycleEvent，下面详细讲解这个
static void dispatch(@NonNull Activity activity, @NonNull Lifecycle.Event event) {
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);
}
}
}
是不是有点像glide生命周期的管理，一样都是创建一个空白的fragment从而监听生命周期，接着往下分析handleLifecycleEvent干了什么。
2. 在监听到生命周期后如何通知观察者LifecycleObserver
接着讲解handleLifecycleEvent。我们一开始就讲了，ComponentActivity实现LifecycleOwner接口，接口下面函数getLifecycle()对应具体返回就是LifecycleRegistry。本质就是LifecycleRegistry管理我们的针对生命周期的观察者，接着通过LifecycleRegistry同步所有观察者的状态+事件
//ComponentActivity.java
private final LifecycleRegistry mLifecycleRegistry = new LifecycleRegistry(this);
@Override
public Lifecycle getLifecycle() {
return mLifecycleRegistry;
}
LifecycleRegistry.java
// 处理lifecycleEvent
public void handleLifecycleEvent(@NonNull Lifecycle.Event event) {
State next = getStateAfter(event); // 获取下个状态
moveToState(next); // 移动到下个状态
}
// 通过事件获取到状态
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);
}
// 移动到下个状态
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(); // 这才是重点
mHandlingEvent = false;
}
// 状态同步
private void sync() {
LifecycleOwner lifecycleOwner = mLifecycleOwner.get();
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) {
// 向后，例如created -> started
backwardPass(lifecycleOwner);
}
Entry<LifecycleObserver, ObserverWithState> newest = mObserverMap.newest();
if (!mNewEventOccurred && newest != null
&& mState.compareTo(newest.getValue().mState) > 0) {
// 向前 onresumed -> onStarted
forwardPass(lifecycleOwner);
}
}
mNewEventOccurred = false;
}
// 重点关注mObserverMap，这是就是我们的观察者Observer集合
private void forwardPass(LifecycleOwner lifecycleOwner) {
Iterator<Entry<LifecycleObserver, ObserverWithState>> ascendingIterator =
mObserverMap.iteratorWithAdditions();
while (ascendingIterator.hasNext() && !mNewEventOccurred) {
Entry<LifecycleObserver, ObserverWithState> entry = ascendingIterator.next();
// 观察者的包装类
ObserverWithState observer = entry.getValue();
while ((observer.mState.compareTo(mState) < 0 && !mNewEventOccurred
&& mObserverMap.contains(entry.getKey()))) {
pushParentState(observer.mState);
// 重要代码，观察者事件分发
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();
}
}
}
上面可以看出，我们来到了我们观察者的包装类，并且调用了它的dispatchEvent函数，那包装类有啥？我们又是在哪生成这个包装类的呢？
// 先看包装类具体内容
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); // 这个就是状态的监听了，我们重点观察内容，怎么监听生命周期
mState = newState;
}
}
// 看下我们的主线
lifecycle.addObserver(MyLocationListener())// 代码调用
// LifecycleRegitstry.java
// 重点看到我们的包装类
@Override
public void addObserver(@NonNull LifecycleObserver observer) {
State initialState = mState == DESTROYED ? DESTROYED : INITIALIZED;
ObserverWithState statefulObserver = new ObserverWithState(observer, initialState); // 观察者和状态一起包装起来
ObserverWithState previous = mObserverMap.putIfAbsent(observer, statefulObserver); // mObserverMap 进行缓存
......
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));// 第一次添加也是触发生命周期
popParentState();
targetState = calculateTargetState(observer);
}
if (!isReentrance) {
// we do sync only on the top level.
sync(); // 又回到我们之前说的代码同步
}
mAddingObserverCounter--;
}
从我们的支线流程开始看，lifecycle.addObserver内部是会把我们的观察者LifecycleObserver和状态封装ObserverWithState，封装类内部dispatchEvent的方法去调用LifecycleObserver.onStateChanged。然后我们回到我们的原点空fragment如何管理生命周期，可以看到onstart\onresume等内部调用了dispatch->handleLifecycleEvent->backwardPass/forwardPass->ObserverWithState.dispatchEvent.。也就形成了闭环了
