Android Lifecycle
Lifecycle是一个类，用于存储有关组件（如 Activity 或 Fragment）的生命周期状态的信息，并允许其他对象观察此状态。
Lifecycle 使用两种主要枚举跟踪其关联组件的生命周期状态：
事件
从框架和 Lifecycle 类分派的生命周期事件。这些事件映射到 Activity 和 Fragment 中的回调事件。
状态
由 Lifecycle 对象跟踪的组件的当前状态。
如下图：
Lifecycle是如何跟踪Activity的生命周期的呢？Activity刚打开，此时Lifecycle State处于INITIALIZED , Lifecycle不分派事件，Activity生命周期达到onCreate时，Lifecycle State转变为CREATED , 分派一个ON_CREATE事件，Activity生命周期到达onStart() ,Lifecycle State变为STARTED ,分派ON_START事件，依此图类推，需要注意：STATE没有PAUSED 或者 STOPED状态，为什么没有？Lifecycle服务于MVVM架构，STATE的状态应该尽可能精简 ，可以结合Activity切换时的生命周期去理解
Lifecycle.java
public abstract class Lifecycle {
....
....
// 事件，对应Activity，或者Fragment的生命周期,ON_ANY是一个补齐字段，一般无须特别关注
public enum Event {
ON_CREATE,
ON_START,
ON_RESUME,
ON_PAUSE,
ON_STOP,
ON_DESTROY,
ON_ANY
}
//状态,对应Activity或者Fragment不同生命周期，表示当前处于生命状态
//可以看到onPause()和onStop()分别对应到了STARTED 和 CREATED状态
public enum State {
DESTROYED,
INITIALIZED,
CREATED,
STARTED,
RESUMED;
public boolean isAtLeast(@NonNull State state) {
return compareTo(state) >= 0;
}
}
}
简单用法
/ MainActiviy.kt
class MainActivity : AppCompatActivity() {
override fun onCreate(savedInstanceState: Bundle?) {
super.onCreate(savedInstanceState)
setContentView(R.layout.activity_main)
lifecycle.addObserver(Observer())
}
}
// MainActObserver.kt
class Observer : LifecycleObserver {
@OnLifecycleEvent(Lifecycle.Event.ON_CREATE)
fun onCreate() {
// on create
Log.e("test","onCreate");
}
@OnLifecycleEvent(Lifecycle.Event.ON_RESUME)
fun onResume() {
// onResume
Log.e("test","onResume");
}
Log会打印出 onCreate 和 onResume ，我们可以在Observer中感知到Activity的生命周期了
源码分析
Lifecycle的源码并不复杂，下来从看一下LifecycleObserver是如何收到Activity的生命周期回调的
// ComponentActivity
@NonNull
@Override
public Lifecycle getLifecycle() {
return mLifecycleRegistry;
}
lifecycle.addObserver(Observer())实质是mLifecycleRegistry.addObserver(Observer())
// ComponentActivity
private final LifecycleRegistry mLifecycleRegistry = new LifecycleRegistry(this);
LiefcycleRegistery.addObserver()
@Override
public void addObserver(@NonNull LifecycleObserver observer) {
State initialState = mState == DESTROYED ? DESTROYED : INITIALIZED;// 拿到初始状态
ObserverWithState statefulObserver = new ObserverWithState(observer, initialState);//包装类ObserverWithState
// 把observer当做key，添加过包装过的ObserverWithState对象，如果之前已经注册过，那么返回老的包装对象；
ObserverWithState previous = mObserverMap.putIfAbsent(observer, statefulObserver);// 将ObserverWithState放到mObserverMap里
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);// 计算observer添加时处于的State
mAddingObserverCounter++;
while ((statefulObserver.mState.compareTo(targetState) < 0
&& mObserverMap.contains(observer))) {
pushParentState(statefulObserver.mState);
statefulObserver.dispatchEvent(lifecycleOwner, upEvent(statefulObserver.mState));// 分发生命周期事件;
popParentState();
// mState / subling may have been changed recalculate
targetState = calculateTargetState(observer);
}
if (!isReentrance) {
// we do sync only on the top level.
// 同步生命周期
sync();
}
mAddingObserverCounter--;
}
/*mParentStates的用处在于嵌套添加，比如有A和B两个Obserser,在A的onStart回调里面执行removeObserver(A), addObserver(B),执行dispatchEvent 的时候,当mObserverMap里面只有一个ObserverA，首先会移除ObserverA,然后添加B，B在添加的时候需要给自己状态设置为A所在的状态，但是mObserverMap里面已 经找不到A了,B的状态就需要从mParentStates里面同步，所以每次dispatchEvent都需要考虑这种极端情况;*/
private State calculateTargetState(LifecycleObserver observer) {
Entry<LifecycleObserver, ObserverWithState> previous = mObserverMap.ceil(observer);
State siblingState = previous != null ? previous.getValue().mState : null;
State parentState = !mParentStates.isEmpty() ? mParentStates.get(mParentStates.size() - 1)
: null;
return min(min(mState, siblingState), parentState);
}
上面是一个典型的观测者模式的代码，observer被包装成一个ObserverWithState来作为真正的关注者，sync();是生命周期同步过程，里面通知所有观测者生命周期的分发
// happens only on the top of stack (never in reentrance),
// so it doesn't have to take in account parents
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()) { // 是否已经同步过生命周期，false时执行while
mNewEventOccurred = false;
// no need to check eldest for nullability, because isSynced does it for us.
// mObserverMap实质是一个链表，每一个element是一个键值对的entry，eldest()返回的是最先添加到表中的元素
//当前的生命周期mState和最先添加的Observer的state比对， < 0 ,表示需要回退Observer的生命周期事件
// 比如当前Activity处于onCreate,但是Observer里面记录的是onDestory，那么需要给Observer同步一个onCreate事件;
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) {
// 同理，表示需要向前同步Observer的生命周期，比如当前Activity处于RESUMED，但是Observer是ON_CREATE，那么需要触发ON_START
forwardPass(lifecycleOwner);
}
}
mNewEventOccurred = false;
}
// 遍历mObserverMap,发送订阅事件,注意这里事件的顺序是down事件
//比如当前mState是RESUMED,observer的状态是CREATED,那么observer需要从CREATED变为RESUMED,需要经历的生命周期onStart，onResume
//这个时候就需要发送两次事件.第一个while是遍历observer，第二个while是遍历需要同步的生命周期
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()))) {
pushParentState(observer.mState);
observer.dispatchEvent(lifecycleOwner, upEvent(observer.mState));// 分发生命周期事件;
popParentState();
}
}
}
// 遍历mObserverMap,发送订阅事件,注意这里事件的顺序是down事件
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);// 分发生命周期事件;
popParentState();
}
}
}
ObserverWithState.java
static class ObserverWithState {
State mState;
LifecycleEventObserver mLifecycleObserver;
ObserverWithState(LifecycleObserver observer, State initialState) {
// observer被包装成mLifecycleObserver
mLifecycleObserver = Lifecycling.lifecycleEventObserver(observer);
mState = initialState;
}
//分发生命周期方法
void dispatchEvent(LifecycleOwner owner, Event event) {
State newState = getStateAfter(event);
mState = min(mState, newState);
// 实际调用mLifecycleObserver.onStateChanged()方法;
mLifecycleObserver.onStateChanged(owner, event);
mState = newState;
}
}
LifecycleEventObserver.java
...
...
// 生成生命周期回调的适配器,不在更多描述
@NonNull
static LifecycleEventObserver lifecycleEventObserver(Object object) {
boolean isLifecycleEventObserver = object instanceof LifecycleEventObserver;
boolean isFullLifecycleObserver = object instanceof FullLifecycleObserver;
if (isLifecycleEventObserver && isFullLifecycleObserver) {
return new FullLifecycleObserverAdapter((FullLifecycleObserver) object,
(LifecycleEventObserver) object);
}
if (isFullLifecycleObserver) {
return new FullLifecycleObserverAdapter((FullLifecycleObserver) object, null);
}
if (isLifecycleEventObserver) {
return (LifecycleEventObserver) object;
}
final Class<?> klass = object.getClass();
int type = getObserverConstructorType(klass);
if (type == GENERATED_CALLBACK) {
List<Constructor<? extends GeneratedAdapter>> constructors =
sClassToAdapters.get(klass);
if (constructors.size() == 1) {
GeneratedAdapter generatedAdapter = createGeneratedAdapter(
constructors.get(0), object);
return new SingleGeneratedAdapterObserver(generatedAdapter);
}
GeneratedAdapter[] adapters = new GeneratedAdapter[constructors.size()];
for (int i = 0; i < constructors.size(); i++) {
adapters[i] = createGeneratedAdapter(constructors.get(i), object);
}
return new CompositeGeneratedAdaptersObserver(adapters);
}
return new ReflectiveGenericLifecycleObserver(object);
}
上述代码描述的是Lifecycle执行生命周期同步的具体步骤，但是Lifecycle是如何感知到Activity或者Fragment生命周期变化呢？
我们使用的AppCompatActivity是ComponentActivity.java的子类
//ComponentActivity onCrate里面注入了一个ReportFragemnt，联想一下Glide
@SuppressLint("RestrictedApi")
@Override
protected void onCreate(@Nullable Bundle savedInstanceState) {
super.onCreate(savedInstanceState);
ReportFragment.injectIfNeededIn(this);
}
ReportFragment.java
public class ReportFragment extends Fragment {
private static final String REPORT_FRAGMENT_TAG = "androidx.lifecycle"
+ ".LifecycleDispatcher.report_fragment_tag";
// 将ReportFragment添加到Activity中作为Activity生命周期的监听;
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();
}
}
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);//发出Lifecycle.Event.ON_CREATE
}
@Override
public void onStart() {
super.onStart();
dispatchStart(mProcessListener);
dispatch(Lifecycle.Event.ON_START);//发出(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;
}
// 分发生命周期事件
private void dispatch(Lifecycle.Event event) {
Activity activity = getActivity();
if (activity instanceof LifecycleRegistryOwner) {
((LifecycleRegistryOwner) activity).getLifecycle().handleLifecycleEvent(event);
return;
}
// Activity实现了LifecycleOwner接口，getLifecycle()返回mLifecycleRegistry，所以实际调用的是mLifecycleRegistry的
//handleLifecycleEvent(event)，从而Activity的生命周期被Lifecycle感知到
// Fragment里面与Activity不同，Fragment是在生命周期里面主动调用LifecycleRegistry的handleLifecycleEvent(event)
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();
}
}
public void handleLifecycleEvent(@NonNull Lifecycle.Event event) {
State next = getStateAfter(event);// 根据事件找到对应State的状态
moveToState(next);// 同步状态
}
// 与上面的图对应
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) {
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();// 开始同步state
mHandlingEvent = false;
}
补充
常用Lifecycle的LifecycleObserver
-
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); } -
interface FullLifecycleObserver extends LifecycleObserver { void onCreate(LifecycleOwner owner); void onStart(LifecycleOwner owner); void onResume(LifecycleOwner owner); void onPause(LifecycleOwner owner); void onStop(LifecycleOwner owner); void onDestroy(LifecycleOwner owner); } -
// MainActObserver.kt class Observer : LifecycleObserver { @OnLifecycleEvent(Lifecycle.Event.ON_CREATE) // 这种Observer是通过获取方法的注解来调用生命周期方法,方法的名称可以随意命名 fun onCreate() { // on create Log.e("test","onCreate"); } @OnLifecycleEvent(Lifecycle.Event.ON_RESUME) fun onResume() { // onResume Log.e("test","onResume"); }
