Lifecycle是Android Architecture Components的成员，是一个生命周期感知组件，能够感知Activity、Fragment等组件的生命周期变化，并将变化通知到已注册的观察者。正确的使用有助于更好地组织代码，减少内存泄漏，增强稳定。下面分析他的实现原理，看看到底只怎么感知生命周期的。
一、基本使用
1、引入依赖库
dependencies {
def lifecycle_version = "2.5.0-alpha01"
def arch_version = "2.1.0"
implementation "androidx.lifecycle:lifecycle-viewmodel:$lifecycle_version"
implementation "androidx.lifecycle:lifecycle-livedata:$lifecycle_version"
implementation "androidx.lifecycle:lifecycle-runtime:$lifecycle_version"
//非java8使用
annotationProcessor "androidx.lifecycle:lifecycle-compiler:$lifecycle_version"
//java8使用
implementation "androidx.lifecycle:lifecycle-common-java8:$lifecycle_version"
}2、自定义一个类实现LifecycleObserver，在类中监听生命周期回调
public class BaseJsApi implements LifecycleObserver {
@OnLifecycleEvent(Lifecycle.Event.ON_CREATE)
void create() {
onCreate();
}
@OnLifecycleEvent(Lifecycle.Event.ON_DESTROY)
void destroy() {
onDestroy();
}
@OnLifecycleEvent(Lifecycle.Event.ON_RESUME)
void resume() {
onResume();
}
@OnLifecycleEvent(Lifecycle.Event.ON_PAUSE)
void pause() {
onPause();
}
……………………………………
}3、在Activity或者fragment中注册。
getLifecycle().addObserver(getJsBridge());通过以上两步就可以感知当前Activity的生命周期了，非常的简便实用。
二、LifecycleObserver接口和LifecycleOwner接口
public interface LifecycleObserver {
}
public interface LifecycleOwner {
@NonNull
Lifecycle getLifecycle();
}LifecycleObserver是一个空接口，LifecycleOwner提供getLifecycle返回Lifecycle，该接口由ComponentActivity实现，提供了getLifecycle的实现。
三、getLifecycle()
getLifecycle方法实际上是调用的ComponentActivity的getLifecycle方法，通过查看，Activity确实也是继承的ComponentActivity，该方法返回mLifecycleRegistry对象，该对象继承自Lifecycle，具体实现了添加观察者，移出观察者的方法，还定义了一个生命周期的枚举。
public class LifecycleRegistry extends Lifecycle {
}
public abstract class Lifecycle {
@MainThread
public abstract void addObserver(@NonNull LifecycleObserver observer);
@MainThread
public abstract void removeObserver(@NonNull LifecycleObserver observer);
@MainThread
@NonNull
public abstract State getCurrentState();
public enum Event {
ON_CREATE,
ON_START,
ON_RESUME,
ON_PAUSE,
ON_STOP,
ON_DESTROY,
ON_ANY;
…………………………………………
}
………………………………
}也就可以看出getLifecycle().addObserver实际上就是执行LifecycleRegistry的addObserver的方法添加观察者，做一个小总结如下：
- 观察者实现LifecycleObserver，实现需要的生命周期。
- 在Activity、Fragment中添通过getLifecycle().addObserver加观察者。
- 实际上调用的是ComponentActivity的getLifecycle方法返回mLifecycleRegistry。
- 调用LifecycleRegistry的addObserver加观察者。
三、添加观察者的流程
详情看注释：
@Override
public void addObserver(@NonNull LifecycleObserver observer) {
//检查是否在主线程
enforceMainThreadIfNeeded("addObserver");
//设置状态值
State initialState = mState == DESTROYED ? DESTROYED : INITIALIZED;
//将观察者和初始化的值封装到ObserverWithState中
ObserverWithState statefulObserver = new ObserverWithState(observer, initialState);
//以观察者为key，statefulObserver为value放到hashMap集合中
ObserverWithState previous = mObserverMap.putIfAbsent(observer, statefulObserver);
//不为null表示之前已经放进去了，否则没放进去，放进去了就添加结束
if (previous != null) {
return;
}
//获取被观察者对象，即Activity，如果被销毁了，则结束
LifecycleOwner lifecycleOwner = mLifecycleOwner.get();
if (lifecycleOwner == null) {
// it is null we should be destroyed. Fallback quickly
return;
}
//mAddingObserverCounter 表示正在添加的观察者的数量，
//开始添加新的observe之前，对该变量进行++操作，添加完成后，进行--操作。
//mHandlingEvent表示是否正在处理
boolean isReentrance = mAddingObserverCounter != 0 || mHandlingEvent;
//计算状态，进行while循环
State targetState = calculateTargetState(observer);
mAddingObserverCounter++;
//新添加的LifecycleObserver ，会不断更新自己的生命周期到指定的targetState
while ((statefulObserver.mState.compareTo(targetState) < 0
&& mObserverMap.contains(observer))) {
//添加状态到list，计算当前状态，用于计算状态calculateTargetState
pushParentState(statefulObserver.mState);
//
final Event event = Event.upFrom(statefulObserver.mState);
if (event == null) {
throw new IllegalStateException("no event up from " + statefulObserver.mState);
}
//生命周期分发：分发到mLifecycleObserver里面
statefulObserver.dispatchEvent(lifecycleOwner, event);
//移除状态到list
popParentState();
// mState / subling may have been changed recalculate
//重新计算状态
targetState = calculateTargetState(observer);
}
//没有事件正在处理，或者是添加了observe,就需要重新同步一次状态,下面详细介绍
if (!isReentrance) {
// we do sync only on the top level.
sync();
}
mAddingObserverCounter--;
}
//计算状态规则
private State calculateTargetState(LifecycleObserver observer) {
//在map中获取到当前observer的前一个observer
Map.Entry<LifecycleObserver, ObserverWithState> previous = mObserverMap.ceil(observer);
//前一个observer的state
State siblingState = previous != null ? previous.getValue().mState : null;
//如果是嵌套添加observer，获取父级observer的状态
State parentState = !mParentStates.isEmpty() ? mParentStates.get(mParentStates.size() - 1)
: null;
//获取最小状态，DESTROYED < INITIALIZED < CREATED < STARTED < RESUMED
return min(min(mState, siblingState), parentState);
}四、绑定生命周期
上面介绍了addObserver的流程，在ComponentActivity的onCreate方法中：
ReportFragment.injectIfNeededIn(this);
ReportFragment是一个Fragment，它负责分派生命周期的事件，injectIfNeededIn()就是在当前的Activity里添加一个ReportFragment，和glide的绑定生命周期比较类似。
public static void injectIfNeededIn(Activity activity) {
//当API>=29注册registerIn绑定生命周期
if (Build.VERSION.SDK_INT >= 29) {
// On API 29+, we can register for the correct Lifecycle callbacks directly
LifecycleCallbacks.registerIn(activity);
}
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();
}
}当API>=29是通过Application.ActivityLifecycleCallbacks的方式注册生命周期， 以onStart为例，其他onResume、onPause等类似：
@Override
public void onStart() {
super.onStart();
dispatchStart(mProcessListener);
dispatch(Lifecycle.Event.ON_START);
}重点看看dispatch：
private void dispatch(@NonNull Lifecycle.Event event) {
//当API<29分发dispatch绑定生命周期
if (Build.VERSION.SDK_INT < 29) {
// Only dispatch events from ReportFragment on API levels prior
// to API 29. On API 29+, this is handled by the ActivityLifecycleCallbacks
// added in ReportFragment.injectIfNeededIn
dispatch(getActivity(), event);
}
}不管是pplication.ActivityLifecycleCallbacks还是当前fragment的生命周期方式都会执行如下dispatch：
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);
}
}
}调用LifecycleRegistry的handleLifecycleEvent将生命周期分发出去，LifecycleRegistry上面已经介绍过了addObserve逻辑了，代码比较长，只显示关键代码：
handleLifecycleEvent -> moveToState -> sync
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.");
}
//没有同步过，则同步
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) {
//如果ObserverWithState的state小于当前state，那么就调用forwardPass方法，
backwardPass(lifecycleOwner);
}
Map.Entry<LifecycleObserver, ObserverWithState> newest = mObserverMap.newest();
if (!mNewEventOccurred && newest != null
&& mState.compareTo(newest.getValue().mState) > 0) {
//如果大于当前state，那么就调用backwardPass方法。
forwardPass(lifecycleOwner);
}
}
mNewEventOccurred = false;
}backwardPass和forwardPass的代码都会调用：
observer.dispatchEvent(lifecycleOwner, event)
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 = event.getTargetState();
mState = min(mState, newState);
mLifecycleObserver.onStateChanged(owner, event);
mState = newState;
}
}调用了Lifecycling.getCallback方法来获得mLifecycleObserver的实例，通过这个实例调用onStateChanged：
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;
}
//通过反射，找到Lifecycle 自动生成的类
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);
最终使用生成的Adapter，创建SingleGeneratedAdapterObserver
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);
}class SingleGeneratedAdapterObserver implements LifecycleEventObserver {
private final GeneratedAdapter mGeneratedAdapter;
SingleGeneratedAdapterObserver(GeneratedAdapter generatedAdapter) {
mGeneratedAdapter = generatedAdapter;
}
@Override
public void onStateChanged(@NonNull LifecycleOwner source, @NonNull Lifecycle.Event event) {
mGeneratedAdapter.callMethods(source, event, false, null);
mGeneratedAdapter.callMethods(source, event, true, null);
}
}到这里会根据依赖中添加的annotationProcessor "androidx.lifecycle:lifecycle-compiler:2.2.0"根据不同生命周期注解生成不同的类，整个过程还是比较复杂的。整体过程如下图：