相信大家平时经常用到Lifecycle，对它怎么使用应该已经相当熟悉了吧，所以今天省略这一块内容。
想个问题，它解决了什么痛点？
在真实的应用中，最终会有太多管理界面和其他组件的调用，以响应生命周期的当前状态。管理多个组件会在生命周期方法（如 onStart() 和 onStop()）中放置大量的代码，这使得它们难以维护。同时也无法保证组件会在 Activity/Fragment停止后不执行启动。
那Lifecycle是怎么解决这些问题的呢，我们直接进入分析源码正题吧。
Lifecycle类
分析之前先看下Lifecycle内部做了什么事情？
public abstract class Lifecycle {
//添加 LifecycleObserver，当 LifecycleOwner 更改状态时将收到通知。
@MainThread
public abstract void addObserver(@NonNull LifecycleObserver observer);
//从观察者列表中删除给定的观察者。
@MainThread
public abstract void removeObserver(@NonNull LifecycleObserver observer);
//返回生命周期的当前状态
public abstract State getCurrentState();
//生命周期事件，对应Activity生命周期方法
public enum Event {
ON_CREATE,
ON_START,
ON_RESUME,
ON_PAUSE,
ON_STOP,
ON_DESTROY,
//可以响应任意一个事件
ON_ANY
}
//生命周期状态. （Event是进入这种状态的事件）
public enum State {
DESTROYED,
INITIALIZED,
CREATED,
STARTED,
RESUMED;
//判断至少是某一状态
public boolean isAtLeast(@NonNull State state) {
return compareTo(state) >= 0;
}
}
可以看到提供了两种枚举来关联组件的生命周期状态：
Event：生命周期事件，对应Activity/Fragment生命周期方法。State：生命周期状态，而Event是指你进入一种状态的事件。
好了，我们现在正式开始来分析了。
如何进行生命中周期分发
public class ComponentActivity extends androidx.core.app.ComponentActivity implements
LifecycleOwner,
ViewModelStoreOwner,
SavedStateRegistryOwner,
OnBackPressedDispatcherOwner {
.....
private final LifecycleRegistry mLifecycleRegistry = new LifecycleRegistry(this);
....
@NonNull
@Override
public Lifecycle getLifecycle() {
return mLifecycleRegistry;
}
@Override
protected void onCreate(@Nullable Bundle savedInstanceState) {
super.onCreate(savedInstanceState);
mSavedStateRegistryController.performRestore(savedInstanceState);
//标注
ReportFragment.injectIfNeededIn(this);
if (mContentLayoutId != 0) {
setContentView(mContentLayoutId);
}
}
....
}
在Android Support Library 26.1.0 及其之后的版本，Activity和Fragment已经默认实现了LifecycleOwner接口，LifecycleOwner可以理解为被观察者。这里看到getLifeCycle()得到了LifecycleRegistry，这里的LifecycleRegistry是LifeCyle的具体实现。并在onCreate()中创建了ReportFragment来作为生命周期的观察，是不是感觉豁然开朗，原来是ReportFragment在帮我们事件分发。
public class ReportFragment extends Fragment {
public static void injectIfNeededIn(Activity activity) {
if (Build.VERSION.SDK_INT >= 29) {
//在API 29及以上，可以直接注册回调 获取生命周期
activity.registerActivityLifecycleCallbacks(
new LifecycleCallbacks());
}
android.app.FragmentManager manager = activity.getFragmentManager();
//API29以前，使用fragment 获取生命周期
if (manager.findFragmentByTag(REPORT_FRAGMENT_TAG) == null) {
manager.beginTransaction().add(new ReportFragment(), REPORT_FRAGMENT_TAG).commit();
manager.executePendingTransactions();
}
}
...
@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);
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) {
//使用LifecycleRegistry的handleLifecycleEvent方法处理事件
((LifecycleRegistry) lifecycle).handleLifecycleEvent(event);
}
}
}
}
这里injectIfNeededIn给activity添加了ReportFragment，并且是没有布局的。
注意到fragment在这里的作用是对生命周期进行分发，里面都走了dispatch()方法，而它内部真正实现的是LifecycleRegistry的handleLifecycleEvent方法处理事件。
LifecycleRegistry事件分发
### LifecycleRegistry
private FastSafeIterableMap<LifecycleObserver, ObserverWithState> mObserverMap =
public void handleLifecycleEvent(@NonNull Lifecycle.Event event) {
//根据event生命周期方法的相应的状态
State next = getStateAfter(event);
//移动到下面的状态
moveToState(next);
}
private void moveToState(State next) {
//状态不一致则不处理
if (mState == next) {
return;
}
//设置新状态
mState = next;
if (mHandlingEvent || mAddingObserverCounter != 0) {
mNewEventOccurred = true;
// we will figure out what to do on upper level.
return;
}
mHandlingEvent = true;
//同步生命周期分发
sync();
mHandlingEvent = false;
}
private void sync() {
LifecycleOwner lifecycleOwner = mLifecycleOwner.get();
.....
//所有观察者都同步完了
while (!isSynced()) {
mNewEventOccurred = false;
// mObserverMap储存的观察者们
if (mState.compareTo(mObserverMap.eldest().getValue().mState) < 0) {
//这里调用observer.dispatchEvent(lifecycleOwner, event); 进行分发，也就是调用ObserverWithState里的方法
//mState比最老观察者状态小
backwardPass(lifecycleOwner);
}
Entry<LifecycleObserver, ObserverWithState> newest = mObserverMap.newest();
if (!mNewEventOccurred && newest != null
&& mState.compareTo(newest.getValue().mState) > 0) {
//mState比最新观察者状态大
forwardPass(lifecycleOwner);
}
}
mNewEventOccurred = false;
}
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);
}
处理分发时，首先要获得event所对应的状态，，然后再调用movetoState移动到新状态，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,最后把生命周期状态同步到所有的观察者。
sync方法中会根据当前状态和mObserverMap中的eldest和newest的状态做对比，判断当前状态是向前还是向后，backwardPass和forwardPass基本差不多，就拿forwardPass来看。
### LifecycleRegistry
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);
//调用了observer.dispatchEvent对事件处理
observer.dispatchEvent(lifecycleOwner, upEvent(observer.mState));
popParentState();
}
}
}
看到了调用了observer.dispatchEvent对事件处理，不在这里分析，先看完下面在分析。
所有的观察者都存放在mObserverMap，我们再来看下是怎么存放进去的？
回到LifecycleRegistry代码，当我们调用getLifecycle().addObserver(myLocationListener)内部实现：
public class LifecycleRegistry extends Lifecycle {
private FastSafeIterableMap<LifecycleObserver, ObserverWithState> mObserverMap =
new FastSafeIterableMap<>();
....
@Override
public void addObserver(@NonNull LifecycleObserver observer) {
State initialState = mState == DESTROYED ? DESTROYED : INITIALIZED;
//都会被包装成对应的Observer
ObserverWithState statefulObserver = new ObserverWithState(observer, initialState);
//将观察者放入map中，后续作为分发
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);
mAddingObserverCounter++;
//通过while循环，把新的观察者的状态 连续地 同步到最新状态mState。
while ((statefulObserver.mState.compareTo(targetState) < 0
&& mObserverMap.contains(observer))) {
pushParentState(statefulObserver.mState);
//继续深入
statefulObserver.dispatchEvent(lifecycleOwner, upEvent(statefulObserver.mState));
popParentState();
targetState = calculateTargetState(observer);
}
if (!isReentrance) {
sync();
}
mAddingObserverCounter--;
}
}
主要是将观察者加入到mObserverMap，然后通过循环找到最新的观察者并把状态同步过去，可以把之前的事件一个个都分发过去，具有粘性。
接着来看下ObserverWithState，如何让加了对应注解的方法执行的?
沿着statefulObserve.dispatchEvent()方法继续往下看。
### LifecycleRegister/ObserverWithState
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);
//防止重复
mState = min(mState, newState);
//事件通知观察者
mLifecycleObserver.onStateChanged(owner, event);
mState = newState;
}
}
ObserverWithState是LifecycleRegister的内部类，在这里mState的作用是新事件时机重新赋值，防止重复通知。
在Lifecycling.lifecycleEventObserver(observer)方法中会返回LifecycleEventObserver对象。
public interface LifecycleEventObserver extends LifecycleObserver {
//当状态转换事件发生时调用
void onStateChanged(@NonNull LifecycleOwner source, @NonNull Lifecycle.Event event);
}
再来看下Lifechcling做了什么操作：
//Lifecycling.java
@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();
//这份方法通过klass获取注解，进行记录
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);
}
//getObserverConstructorType(klass)，最终调用的方法
boolean hasLifecycleMethods = ClassesInfoCache.sInstance.hasLifecycleMethods(klass);
返回到ObserverWithState中，获取到观察者后状态发生变化会调用mLifecycleObserver.onStateChanged(owner, event);其中有三个FullLifecycleObserverAdapter、 CompositeGeneratedAdaptersObserver、ReflectiveGenericLifecycleObserver。而我们关注的是ComponentActivity，所以LifecycleEventObserver的实现类是ReflectiveGenericLifecycleObserver。
LifecycleObserver方法执行
### ReflectiveGenericLifecycleObserver
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(@NonNull LifecycleOwner source, @NonNull Event event) {
mInfo.invokeCallbacks(source, event, mWrapped);
}
}
这里看到调用了CallbackInfo的invokeCallbacks来进行分发。我们先要了解下CallbackInfo是怎么产生的呢，那就要从ClassesInfoCache内部看了。
### ClassInfoCache
CallbackInfo getInfo(Class<?> klass) {
CallbackInfo existing = mCallbackMap.get(klass);
if (existing != null) {
return existing;
}
existing = createInfo(klass, null);
return existing;
}
private CallbackInfo createInfo(Class<?> klass, @Nullable Method[] declaredMethods) {
Class<?> superclass = klass.getSuperclass();
Map<MethodReference, Lifecycle.Event> handlerToEvent = new HashMap<>();
if (superclass != null) {
CallbackInfo superInfo = getInfo(superclass);
if (superInfo != null) {
handlerToEvent.putAll(superInfo.mHandlerToEvent);
}
}
Class<?>[] interfaces = klass.getInterfaces();
//不断的遍历各个方法，获取方法上的名为OnLifecycleEvent的注解，这个注解正是实现LifecycleObserver接口时用到的
for (Class<?> intrfc : interfaces) {
for (Map.Entry<MethodReference, Lifecycle.Event> entry : getInfo(
intrfc).mHandlerToEvent.entrySet()) {
verifyAndPutHandler(handlerToEvent, entry.getKey(), entry.getValue(), klass);
}
}
Method[] methods = declaredMethods != null ? declaredMethods : getDeclaredMethods(klass);
boolean hasLifecycleMethods = false;
for (Method method : methods) {
OnLifecycleEvent annotation = method.getAnnotation(OnLifecycleEvent.class);
if (annotation == null) {
continue;
}
hasLifecycleMethods = true;
Class<?>[] params = method.getParameterTypes();
int callType = CALL_TYPE_NO_ARG;
if (params.length > 0) {
callType = CALL_TYPE_PROVIDER;
if (!params[0].isAssignableFrom(LifecycleOwner.class)) {
throw new IllegalArgumentException(
"invalid parameter type. Must be one and instanceof LifecycleOwner");
}
}
Lifecycle.Event event = annotation.value();
if (params.length > 1) {
callType = CALL_TYPE_PROVIDER_WITH_EVENT;
if (!params[1].isAssignableFrom(Lifecycle.Event.class)) {
throw new IllegalArgumentException(
"invalid parameter type. second arg must be an event");
}
if (event != Lifecycle.Event.ON_ANY) {
throw new IllegalArgumentException(
"Second arg is supported only for ON_ANY value");
}
}
if (params.length > 2) {
throw new IllegalArgumentException("cannot have more than 2 params");
}
//新建了一个MethodReference，其内部包括了使用了该注解的方法
//这个method就是最后要调用invoke的方法
MethodReference methodReference = new MethodReference(callType, method);
//用于将MethodReference和对应的Event存在类型为Map<MethodReference, Lifecycle.Event> 的handlerToEvent中
verifyAndPutHandler(handlerToEvent, methodReference, event, klass);
}
//并将handlerToEvent传进去
CallbackInfo info = new CallbackInfo(handlerToEvent);
mCallbackMap.put(klass, info);
mHasLifecycleMethods.put(klass, hasLifecycleMethods);
return info;
}
了解了之后，再回过来看下invokeCallbacks方法：
### ClassesInfoCache/CallbackInfo
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<>();
//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。
for (Map.Entry<MethodReference, Lifecycle.Event> entry : handlerToEvent.entrySet()) {
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) {
//这里会传入mEventToHandlers.get(event)，也就是事件对应的MethodReference的集合。
invokeMethodsForEvent(mEventToHandlers.get(event), source, event, target);
invokeMethodsForEvent(mEventToHandlers.get(Lifecycle.Event.ON_ANY), source, event,
target);
}
private static void invokeMethodsForEvent(List<MethodReference> handlers,
LifecycleOwner source, Lifecycle.Event event, Object mWrapped) {
if (handlers != null) {
//会遍历MethodReference的集合，调用MethodReference的invokeCallback方法。
for (int i = handlers.size() - 1; i >= 0; i--) {
handlers.get(i).invokeCallback(source, event, mWrapped);
}
}
}
}
主要做了执行对应event的方法，最后会遍历MethodReference的集合，调用MethodReference的invokeCallback方法。
static final 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) {
//noinspection TryWithIdenticalCatches
try {
switch (mCallType) {
case CALL_TYPE_NO_ARG:
//没有参数的
mMethod.invoke(target);
break;
case CALL_TYPE_PROVIDER:
//一个参数的：LifecycleOwner
mMethod.invoke(target, source);
break;
case CALL_TYPE_PROVIDER_WITH_EVENT:
两个参数的：LifecycleOwner，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);
}
}
......
}
在MethodReference根据callType的类型通过invoke对方法进行反射。
最后用一张图来概括下今天所看的内容：
图片来自–Android Jetpack架构组件（三）一文带你了解Lifecycle（原理篇)
参考
Android Jetpack架构组件（三）一文带你了解Lifecycle（原理篇)