这是今天在使用Lifecycle 时因为好奇，所以看了一下源码， 初次阅读，可能理解的不够深，讲的不够好请大家踊跃批评谢谢。
初级使用简单介绍
Lifecycle 是google为我们提供的一个专门用来监听Activity或者Fragment生命周期的一个组件。它是通过观察生命周期状态来回调LifecycleObserver中的方法的。
先看一个简单的小例子
Activity
protected void onCreate(Bundle savedInstanceState) {
super.onCreate(savedInstanceState);
setContentView(R.layout.activity_main);
getLifecycle().addObserver(new PageLifecycleObserver());
}
public class PageLifecycleObserver implements LifecycleObserver {
@OnLifecycleEvent(Lifecycle.Event.ON_START)
public void onStart(){
Log.d("Tag","start");
}
}
就像上面这样，Activity的onStart方法就被监听。
加入我们想在Activity活动的时候，注册一个LocationListener来获取位置信息，然后在onPause的时候，移除监听器，那我们可以通过Activity的生命周期事件来实现。
源码阅读
追根溯源看看为什么会这样呢 ？
getLifecycle().addObserver(new PageLifecycleObserver()); 这段话其实做了两件是一个是拿到LifecycleRegistry 然后把PageLifecycleObserver监听添加进去。
private final LifecycleRegistry mFragmentLifecycleRegistry = new LifecycleRegistry(this);
private final SavedStateRegistryController mSavedStateRegistryController =
SavedStateRegistryController.create(this);
public LifecycleRegistry(@NonNull LifecycleOwner provider) {
mLifecycleOwner = new WeakReference<>(provider);
mState = INITIALIZED;
}
可以看出LifecycleRegistry创建的时候就把Activity引用传进来了。并且把mState 变成了INITIALIZED。 就如最前面我贴的那个图一样。
LifecycleRegistry.java
@Override
public void addObserver(@NonNull LifecycleObserver observer) {
State initialState = mState == DESTROYED ? DESTROYED : INITIALIZED;
//状态和observer 封装在一起
ObserverWithState statefulObserver = new ObserverWithState(observer, initialState);
//previous null
ObserverWithState previous = mObserverMap.putIfAbsent(observer, statefulObserver);
if (previous != null) {
return;
}
//Activity 引用
LifecycleOwner lifecycleOwner = mLifecycleOwner.get();
if (lifecycleOwner == null) {
// it is null we should be destroyed. Fallback quickly
return;
}
boolean isReentrance = mAddingObserverCounter != 0 || mHandlingEvent;
//计算下一个state CREATE
State targetState = calculateTargetState(observer);
mAddingObserverCounter++;
while ((statefulObserver.mState.compareTo(targetState) < 0
&& mObserverMap.contains(observer))) {
pushParentState(statefulObserver.mState);
//最终执行这个方法
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--;
}
static class ObserverWithState {
State mState;
LifecycleEventObserver mLifecycleObserver;
ObserverWithState(LifecycleObserver observer, State initialState) {
//重点在这个代码
mLifecycleObserver = Lifecycling.lifecycleEventObserver(observer);
mState = initialState;
}
void dispatchEvent(LifecycleOwner owner, Event event) {
State newState = getStateAfter(event);
mState = min(mState, newState);
mLifecycleObserver.onStateChanged(owner, event);
mState = newState;
}
}
这个就是程序的调用逻辑。看看是如何驱动这个逻辑走起来的呢？
@NonNull
static LifecycleEventObserver lifecycleEventObserver(Object object) {
// 1 、由于 PageLifecycleObserver implements LifecycleObserver 所以下面全部是false
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;
}
// 2、第二部判断
final Class<?> klass = object.getClass();
int type = getObserverConstructorType(klass);
// false
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);
}
int type = getObserverConstructorType(klass);
看到代码可以发现
Event标记的方法为什么会被调用
最后创建了一个ReflectiveGenericLifecycleObserver
class ReflectiveGenericLifecycleObserver implements LifecycleEventObserver {
private final Object mWrapped;
private final CallbackInfo mInfo;
ReflectiveGenericLifecycleObserver(Object wrapped) {
mWrapped = wrapped;
//核心的代码在这里
mInfo = ClassesInfoCache.sInstance.getInfo(mWrapped.getClass());
}
@Override
public void onStateChanged(LifecycleOwner source, Event event) {
mInfo.invokeCallbacks(source, event, mWrapped);
}
}
看到这里等于又封装了一层object 其实还是最后的那个PageLifecycleObserver
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();
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");
}
MethodReference methodReference = new MethodReference(callType, method);
// event 和 method只不过这里又包了一层
verifyAndPutHandler(handlerToEvent, methodReference, event, klass);
}
//最后遍历完以后 放到CallbackInfo 返回。
CallbackInfo info = new CallbackInfo(handlerToEvent);
mCallbackMap.put(klass, info);
mHasLifecycleMethods.put(klass, hasLifecycleMethods);
return info;
}
非常长的一段代码， 其实猜可以猜到反射拿到所有的OnLifecycleEvent标注的方法。在注释中我们也看到， 最后所有的method 和Event 放在了CallbackInfo中。
回到之前的那个对象ReflectiveGenericLifecycleObserver中。
public void onStateChanged(LifecycleOwner source, Event event) {
mInfo.invokeCallbacks(source, event, mWrapped);
}
//看名字也可以知道 inthod 对应的event方法
void invokeCallbacks(LifecycleOwner source, Lifecycle.Event event, Object target) {
invokeMethodsForEvent(mEventToHandlers.get(event), source, event, target);
invokeMethodsForEvent(mEventToHandlers.get(Lifecycle.Event.ON_ANY), source, event,
target);
}
至此这里为什么Event方法被调用。 其实就是使用的反射拿到注解然后method和Event绑定起来， 看起来和EventBus 很像。
生命周期方法为什么会和Event方法相关联
FragmentActivity 生命周期方法 你会看到很多这样的代码
mFragmentLifecycleRegistry.handleLifecycleEvent(Lifecycle.Event.ON_START);
看名字猜可以看到 处理Event
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;
}
// 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()) {
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
backwardPass(lifecycleOwner);
}
Entry<LifecycleObserver, ObserverWithState> newest = mObserverMap.newest();
if (!mNewEventOccurred && newest != null
&& mState.compareTo(newest.getValue().mState) > 0) {
//向前
forwardPass(lifecycleOwner);
}
}
mNewEventOccurred = false;
}
//向后
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();
}
}
}
上面这段代码以后你们可能会经常看到，它其实就是在计算 State 的大小，判断是向前或者向后，然后调用observer的dispatchEvent方法。其中的observer 从一个mObserverMap中取到的，它是一个ObserverWithState statefulObserver = new ObserverWithState(observer, initialState);
而这个类就是跟反射和注解的那个ReflectiveGenericLifecycleObserver 封装一次，调用它的dispatchEvent。
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) {
mInfo.invokeCallbacks(source, event, mWrapped);
}
}
最后回到了CallbackInfo invokeCallbacks 的那两个invoke方法。
