1.Lifecycle的获取
简单的获取Lifecycle
getLifecycle().addObserver(new SampleObserver();
点进去发现getLifecycle是在ComponentActivity中并且实现了LifecycleOwner接口
public class ComponentActivity extends androidx.core.app.ComponentActivity implements
LifecycleOwner{
//1
private final LifecycleRegistry mLifecycleRegistry = new LifecycleRegistry(this);
@Override
protected void onCreate(@Nullable Bundle savedInstanceState) {
super.onCreate(savedInstanceState);
···
//2
ReportFragment.injectIfNeededIn(this);
···
}
@NonNull
@Override
public Lifecycle getLifecycle() {
return mLifecycleRegistry;
}
}
在1 处构建了LifecycleRegistry对象 这个是负责分发组件状态的
ComponentActivity中有用的就这几行代码那么Lifecycle是如何获取Activity的状态并且分发的呢？
重点在2处Lifecycle使用了和Glide相同的做法，使用Fragment来分发状态这样的好处就是可以复用，即ReportFragment
ReportFragment是怎么做的呢 点进去看
public class ReportFragment extends Fragment {
private static final String REPORT_FRAGMENT_TAG = "androidx.lifecycle"
+ ".LifecycleDispatcher.report_fragment_tag";
//1 把Fragment加入到Activity中
public static void injectIfNeededIn(Activity activity) {
android.app.FragmentManager manager = activity.getFragmentManager();
if (manager.findFragmentByTag(REPORT_FRAGMENT_TAG) == null) {
//加入了一个空的Fragment
manager.beginTransaction().add(new ReportFragment(), REPORT_FRAGMENT_TAG).commit();
// Hopefully, we are the first to make a transaction.
manager.executePendingTransactions();
}
}
@Override
public void onActivityCreated(Bundle savedInstanceState) {
super.onActivityCreated(savedInstanceState);
//2 分发状态
dispatch(Lifecycle.Event.ON_CREATE);
}
@Override
public void onStart() {
super.onStart();
dispatchStart(mProcessListener);
dispatch(Lifecycle.Event.ON_START);
}
···
@Override
public void onDestroy() {
super.onDestroy();
dispatch(Lifecycle.Event.ON_DESTROY);
···
}
private void dispatch(Lifecycle.Event event) {
Activity activity = getActivity();
//已经过时忽略
if (activity instanceof LifecycleRegistryOwner) {
((LifecycleRegistryOwner) activity).getLifecycle().handleLifecycleEvent(event);
return;
}
if (activity instanceof LifecycleOwner) {
Lifecycle lifecycle = ((LifecycleOwner) activity).getLifecycle();
if (lifecycle instanceof LifecycleRegistry) {
//分发生命周期
((LifecycleRegistry) lifecycle).handleLifecycleEvent(event);
}
}
}
}
1在ComponentActivity中调用了ReportFragment#injectIfNeededIn方法把Fragment加入到了Activity中 用Fragment来感知Activity的生命周期
2在Fragment生命周期发生变化时调用dispatch方法来分发生命周期在里面调用了LifecycleRegistry的handleLifecycleEvent方法
现在如何感知Activity的生命周期已经清楚了，那么最终是怎么分发到LifecycleObserve的呢？
2.Lifecycle分发Observe
在上面我们可以看到最后实际是调用了LifecycleRegistry#handleLifecycleEvent方法点进去
public void handleLifecycleEvent(@NonNull Lifecycle.Event event) {
// 这里根据Event状态来获取State状态
State next = getStateAfter(event);
//分发状态
moveToState(next);
}
private void moveToState(State next) {
···
mState = next;
//如果正在分发状态或者没有注册Observer则终止分发
if (mHandlingEvent || mAddingObserverCounter != 0) {
···
}
mHandlingEvent = true;
//重点
sync();
mHandlingEvent = false;
}
private boolean isSynced() {
if (mObserverMap.size() == 0) {
return true;
}
//1 这里再次判断是否在分发状态 并且Observer的状态是否和要分发的状态一致
State eldestObserverState = mObserverMap.eldest().getValue().mState;
State newestObserverState = mObserverMap.newest().getValue().mState;
return eldestObserverState == newestObserverState && mState == newestObserverState;
}
private void sync() {
LifecycleOwner lifecycleOwner = mLifecycleOwner.get();
···
while (!isSynced()) {
···
// 获取最新的Observer的状态 当前状态在Observer状态前面
if (mState.compareTo(mObserverMap.eldest().getValue().mState) < 0) {
backwardPass(lifecycleOwner);
}
Entry<LifecycleObserver, ObserverWithState> newest = mObserverMap.newest();
//最新的Observer状态在Observer状态后面
if (!mNewEventOccurred && newest != null
&& mState.compareTo(newest.getValue().mState) > 0) {
forwardPass(lifecycleOwner);
}
}
···
}
private void forwardPass(LifecycleOwner lifecycleOwner) {
Iterator<Entry<LifecycleObserver, ObserverWithState>> ascendingIterator =
mObserverMap.iteratorWithAdditions();
while (ascendingIterator.hasNext() && !mNewEventOccurred) {
Entry<LifecycleObserver, ObserverWithState> entry = ascendingIterator.next();
//3
ObserverWithState observer = entry.getValue();
while ((observer.mState.compareTo(mState) < 0 && !mNewEventOccurred
&& mObserverMap.contains(entry.getKey()))) {
pushParentState(observer.mState);
// 2 获取到每个observer分发状态
observer.dispatchEvent(lifecycleOwner, upEvent(observer.mState));
popParentState();
}
}
}
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;
}
···
}
1调用handleLifecycleEvent方法后会根据当前的Event事件来获取到最新的State然后根据最新的State和Observer的State做比较来进行状态的分发
FastSafeIterableMap SafeIterableMap 是jetpack新加的一个链表结构模仿HashMap 适用于小量的Map结构
支持在遍历中增删元素是线程不安全的源码分析
2这里会根据最新的State来获取最新的Event进行分发
3这里从map集合里面拿到的是一个ObserverWithState，那么ObserverWithState是什么呢？
3. ObserverWithState
ObserverWithState是LifecycleRegistry里面的一个静态内部类
static class ObserverWithState {
//State 状态
State mState;
//#1
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);
mState = newState;
}
}
ObserverWithState 包含两个参数State LifecycleEventObserver
LifecycleEventObserver 是继承自LifecycleObserver的接口声明了onStateChanged()函数
在调用dispatchEvent的时候间接调用了LifecycleEventObserver的onStateChanged函数
那么我们自己LifecycleObserver在哪呢 ObserverWithState 是在哪里实例化的呢
通过追踪代码我们发现
在LifecycleRegistry#addObserver中实例化了ObserverWithState对象
@Override
public void addObserver(@NonNull LifecycleObserver observer) {
State iniObserverWithStatetialState = mState == DESTROYED ? DESTROYED : INITIALIZED;
//构造ObserverWithState
ObserverWithState statefulObserver = new ObserverWithState(observer, initialState);
//这里查看jetpack新加的两个链表
ObserverWithState previous = mObserverMap.putIfAbsent(observer, statefulObserver);
if (previous != null) {
return;
}
···
···
mAddingObserverCounter--;
}
在ObserverWithState的构造方法中调用了
mLifecycleObserver = Lifecycling.lifecycleEventObserver(observer);
追踪代码
@NonNull
static LifecycleEventObserver lifecycleEventObserver(Object object) {
boolean isLifecycleEventObserver = object instanceof LifecycleEventObserver;
//包安全的类 不用关注
boolean isFullLifecycleObserver = object instanceof FullLifecycleObserver;
···
// 如果Observer 实现的是LifecycleEventObserver 则不会处理注解信息直接返回
if (isLifecycleEventObserver) {
return (LifecycleEventObserver) object;
}
final Class<?> klass = object.getClass();
//获取类型 是否包含注解处理器
int type = getObserverConstructorType(klass);
if (type == GENERATED_CALLBACK) {
//这里是包含注解处理器 返回SingleGeneratedAdapterObserver 或者CompositeGeneratedAdaptersObserver
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);
}
//通过反射调用方法
return new ReflectiveGenericLifecycleObserver(object);
}
// 这里会通过Observer的“类名__LifecycleAdapter” 去包里寻找可以找到说面有注解处理器
@Nullable
private static Constructor<? extends GeneratedAdapter> generatedConstructor(Class<?> klass) {
try {
Package aPackage = klass.getPackage();
String name = klass.getCanonicalName();
final String fullPackage = aPackage != null ? aPackage.getName() : "";
final String adapterName = getAdapterName(fullPackage.isEmpty() ? name :
name.substring(fullPackage.length() + 1));
@SuppressWarnings("unchecked") final Class<? extends GeneratedAdapter> aClass =
(Class<? extends GeneratedAdapter>) Class.forName(
fullPackage.isEmpty() ? adapterName : fullPackage + "." + adapterName);
Constructor<? extends GeneratedAdapter> constructor =
aClass.getDeclaredConstructor(klass);
if (!constructor.isAccessible()) {
constructor.setAccessible(true);
}
return constructor;
} catch (ClassNotFoundException e) {
return null;
} catch (NoSuchMethodException e) {
// this should not happen
throw new RuntimeException(e);
}
}
流程大致如下
- 判断该Observer是否是LifecycleEventObserver 是的话返回本身
- 判断是否包含注解处理器 查找是否包含“类名__LifecycleAdapter”的类 包含并且有OnLifecycleEvent注解则返回SingleGeneratedAdapterObserver/CompositeGeneratedAdaptersObserver
- 如果以上提交都不满足就通过反射调用回调方法
查看SingleGeneratedAdapterObserver
class SingleGeneratedAdapterObserver implements LifecycleEventObserver {
private final GeneratedAdapter mGeneratedAdapter;
SingleGeneratedAdapterObserver(GeneratedAdapter generatedAdapter) {
mGeneratedAdapter = generatedAdapter;
}
@Override
public void onStateChanged(LifecycleOwner source, Lifecycle.Event event) {
mGeneratedAdapter.callMethods(source, event, false, null);
mGeneratedAdapter.callMethods(source, event, true, null);
}
}
这里就脉络就清楚了通过ObserverWithState#dispatchEvent方法最后调用的实际是SingleGeneratedAdapterObserver里面的onStateChanged方法。在SingleGeneratedAdapterObserver里面调用了Adapter的callMethods方法 关于注解处理器生成的adapter请自行分析
通过反射调用回调函数
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);
}
}
追踪代码查看 ClassesInfoCache.sInstance.getInfo(mWrapped.getClass());
// 先从map里拿 拿不到通过createInfo函数扫描类里面的方法
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<>();
··· 获取父类里面的method
··· 获取接口里面的method
Method[] methods = declaredMethods != null ? declaredMethods : getDeclaredMethods(klass);
boolean hasLifecycleMethods = false;
for (Method method : methods) {
//获取当前方法是否有OnLifecycleEvent注解
OnLifecycleEvent annotation = method.getAnnotation(OnLifecycleEvent.class);
if (annotation == null) {
continue;
}
hasLifecycleMethods = true;
Class<?>[] params = method.getParameterTypes();
//没有参数
int callType = CALL_TYPE_NO_ARG;
//有参数的情况下第一个必须是LifecycleOwner
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();
//有两个参数的情况下第二个参数类型必须是Lifecycle.Event.ANY
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");
}
}
//如果大于两个参数 直接抛出异常
if (params.length > 2) {
throw new IllegalArgumentException("cannot have more than 2 params");
}
MethodReference methodReference = new MethodReference(callType, method);
verifyAndPutHandler(handlerToEvent, methodReference, event, klass);
}
CallbackInfo info = new CallbackInfo(handlerToEvent);
mCallbackMap.put(klass, info);
mHasLifecycleMethods.put(klass, hasLifecycleMethods);
return info;
}
总结
- Lifecycle是在Activity里面放置一个空的Fragment来监听生命周期变化
- 当addObserver的时候最后实际传入的是一个包装好的ObserverWithState对象 然后调用onStateChanged方法来分发状态
- 使用处理器来提高性能 避免反射造成的性能消耗
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