[移动开发]Lifecycle 源码分析

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前言

Lifecycle 是 Jetpack 的一员同时也是 LiveData 和 ViewModel 的基础，在 Lifecycle 之前依赖生命周期的操作都是通过重写父类的对应方法来实现的，通过 Lifecycle 可以把这部分操作定义在别的地方，也就是解耦。之前只是看过大概原理，今天来细读一下源码

主要成员

• LifecycleOwner 是一个接口用于被观察的组件实现，唯一一个方法用于返回一个 Lifecycle 对象，实现它的对象表示具有生命周期
• Lifecycle 是一个抽象类，三个抽象方法分别用于添加、移除观察者和获取组件（具有生命周期的对象比如 Activity、Fragment）当前状态，具体实现在 LifecycleRegistry 里，同时定义了两个枚举类 State（代表具有生命周期的对象的状态）、Event（代表具有生命周期的对象发出的事件）
• LifecycleRegistry Lifecycle 的具体实现，整个 Lifecycle 框架的核心
• LifecycleObserver 生命周期事件的观察者，是一个空实现，标记接口
• LifecycleEventObserver LifecycleObserver 的子接口，定义了一个方法可以接收所有的生命周期事件
• FullLifecycleObserver LifecycleObserver 的子接口，定义了接收所有生命周期事件的方法
• DefaultLifecycleObserver FullLifecycleObserver 的子接口，仅在编译平台是 Java 8 时可用，利用了 Java8 的接口方法空实现的特性，是推荐的使用方式
• OnLifecycleEvent 注解，可通过此注解标记对象方法接收指定的生命周期事件，是不推荐的使用方式

在 Activity 中的实现

// androidx.core.app.ComponentActivity
public class ComponentActivity extends Activity implements
        LifecycleOwner,
        KeyEventDispatcher.Component {
    
    private LifecycleRegistry mLifecycleRegistry = new LifecycleRegistry(this);

    @Override
    protected void onCreate(@Nullable Bundle savedInstanceState) {
        super.onCreate(savedInstanceState);
        ReportFragment.injectIfNeededIn(this);
    }

    @CallSuper
    @Override
    protected void onSaveInstanceState(@NonNull Bundle outState) {
        mLifecycleRegistry.markState(Lifecycle.State.CREATED);
        super.onSaveInstanceState(outState);
    }

    @NonNull
    @Override
    public Lifecycle getLifecycle() {
        return mLifecycleRegistry;
    }

}

主要逻辑在 ReportFragment 里

// androidx.lifecycle.ReportFragment#injectIfNeededIn
    public static void injectIfNeededIn(Activity activity) {
        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();
        }
    }

injectIfNeededIn 方法里区分了版本，如果是 29 及以上版本调用 LifecycleCallbacks.registerIn 否则添加 ReportFragment 用于感知宿主 Activity 的生命周期变化，这里与 Glide 的处理一样。先看一下高版本的实现

// androidx.lifecycle.ReportFragment.LifecycleCallbacks
    static class LifecycleCallbacks implements Application.ActivityLifecycleCallbacks {

        static void registerIn(Activity activity) {
            activity.registerActivityLifecycleCallbacks(new LifecycleCallbacks());
        }

        @Override
        public void onActivityPostCreated(@NonNull Activity activity,
                @Nullable Bundle savedInstanceState) {
            dispatch(activity, Lifecycle.Event.ON_CREATE);
        }

        @Override
        public void onActivityPostStarted(@NonNull Activity activity) {
            dispatch(activity, Lifecycle.Event.ON_START);
        }

        @Override
        public void onActivityPostResumed(@NonNull Activity activity) {
            dispatch(activity, Lifecycle.Event.ON_RESUME);
        }

        @Override
        public void onActivityPrePaused(@NonNull Activity activity) {
            dispatch(activity, Lifecycle.Event.ON_PAUSE);
        }

        @Override
        public void onActivityPreStopped(@NonNull Activity activity) {
            dispatch(activity, Lifecycle.Event.ON_STOP);
        }

        @Override
        public void onActivityPreDestroyed(@NonNull Activity activity) {
            dispatch(activity, Lifecycle.Event.ON_DESTROY);
        }

    }

LifecycleCallbacks 实现了 Application.ActivityLifecycleCallbacks 接口 Activity 通过 registerActivityLifecycleCallbacks 方法注册接口并且在 onCreate onStart onResume 生命周期方法后和 onPause onStop onDestroy 方法前调用 dispatch 去通知所有观察者事件变化。再看一下低版本的实现

// androidx.lifecycle.ReportFragment
public class ReportFragment extends android.app.Fragment {

    @Override
    public void onActivityCreated(Bundle savedInstanceState) {
        super.onActivityCreated(savedInstanceState);
        dispatch(Lifecycle.Event.ON_CREATE);
    }

    @Override
    public void onStart() {
        super.onStart();
        dispatch(Lifecycle.Event.ON_START);
    }

    @Override
    public void onResume() {
        super.onResume();
        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);
    }

    private void dispatch(@NonNull Lifecycle.Event event) {
        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);
        }
    }
    
}

Fragment 的 onActivityCreated、onStart、onResume 等方法是后于 Activity 的相应方法的，Fragment 的 onPause、onStop、onDestroy 等方法是先于 Activity 的相应方法的，所以与低版本一样保证了先后顺序，并且在 dispatch 方法里判断了版本只有小于 29 才继续执行，因为上面不管是高低版本都会向宿主 Activity 添加 ReportFragment 避免高版本重复发送事件。总结一下就是在大于等于 29 的版本通过 android.app.Activity#registerActivityLifecycleCallbacks 方法感知生命周期，低版本通过向宿主 Activity 中添加 ReportFragment 感知生命周期

    static void dispatch(@NonNull Activity activity, @NonNull Lifecycle.Event event) {
    	// 忽略，LifecycleRegistryOwner 已经废弃了
        if (activity instanceof LifecycleRegistryOwner) {
            ((LifecycleRegistryOwner) activity).getLifecycle().handleLifecycleEvent(event);
            return;
        }

        if (activity instanceof LifecycleOwner) {
            Lifecycle lifecycle = ((LifecycleOwner) activity).getLifecycle();
            if (lifecycle instanceof LifecycleRegistry) {
            	// Activity 中的 lifecycle 就是 LifecycleRegistry 所以会走到这里
                ((LifecycleRegistry) lifecycle).handleLifecycleEvent(event);
            }
        }
    }

可以想到接下来肯定是遍历所有的观察者把事件转发出去，所以在 handleLifecycleEvent 之前先看一下添加观察者的逻辑

    @Override
    public void addObserver(@NonNull LifecycleObserver observer) {
    	// 必须在主线程调用
        enforceMainThreadIfNeeded("addObserver");
        // 初始状态
        State initialState = mState == DESTROYED ? DESTROYED : INITIALIZED;
        // 将 LifecycleObserver 和初始状态一起封装成 ObserverWithState 待会再来说
        ObserverWithState statefulObserver = new ObserverWithState(observer, initialState);
        // androidx.arch.core.internal.FastSafeIterableMap 是一个有序的 Map 内部维护了一个链表来保证顺序
        // putIfAbsent 的逻辑是如果有老的值则返回老的值否则将新值加入并返回 null
        ObserverWithState previous = mObserverMap.putIfAbsent(observer, statefulObserver);
		// 如果有老的值说明重复添加直接返回
        if (previous != null) {
            return;
        }
        // mLifecycleOwner 是构造方法中传入创建的弱引用对象
        // 如果 Activity 已经被回收了直接返回
        LifecycleOwner lifecycleOwner = mLifecycleOwner.get();
        if (lifecycleOwner == null) {
            // it is null we should be destroyed. Fallback quickly
            return;
        }

		// 如果 isReentrance 为 true 意味着存在事件嵌套
        // mAddingObserverCounter != 0 表示在观察者的回调事件中又通过 addObserver 添加了一个观察者
    	// mHandlingEvent 为 true 表示此时正在向观察者同步生命周期事件
        boolean isReentrance = mAddingObserverCounter != 0 || mHandlingEvent;
        // 计算观察者应该处于的生命周期状态，待会再来说
        State targetState = calculateTargetState(observer);
        // 记录正在添加观察者
        mAddingObserverCounter++;
        // 如果观察者所处的状态小于（定义的枚举所以会调用 Enum 的 compareTo 比较每个枚举值的 ordinal）上面计算的状态就回调事件给观察者并且是从观察者所处的状态到当前组件所处的状态一个事件一个事件的回调（比如在 RESUMED 状态添加的观察者也会接收到 CREATED STARTED 事件即粘性）
        while ((statefulObserver.mState.compareTo(targetState) < 0
                && mObserverMap.contains(observer))) {
            // 在回调生命周期事件前记录观察者当前状态    
            pushParentState(statefulObserver.mState);
            // 得到生命周期状态后对应的生命周期事件
            final Event event = Event.upFrom(statefulObserver.mState);
            if (event == null) {
                throw new IllegalStateException("no event up from " + statefulObserver.mState);
            }
            // 把生命周期事件回调给观察者
            statefulObserver.dispatchEvent(lifecycleOwner, event);
            // 在回调生命周期事件后移除记录的观察者状态    
            popParentState();
            // 重新计算
            targetState = calculateTargetState(observer);
        }

        if (!isReentrance) {
            // 如果不存在事件嵌套则执行同步
            sync();
        }
        // 执行完后减 1
        mAddingObserverCounter--;
    }

方法做的事情先做了一些校验然后依次回调给观察者生命周期事件，即使生命周期事件已经发生了才添加观察者也会回调事件即粘性。下面看一下 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 = event.getTargetState();
            mState = min(mState, newState);
            mLifecycleObserver.onStateChanged(owner, event);
            mState = newState;
        }
    }

ObserverWithState 有两个作用：一、观察者可以是注解也可以是接口实现 ObserverWithState 统一了它们的接口方便调用，二、ObserverWithState 内部也保存了生命周期事件用于记录这个观察者已经回调到的状态，防止重复调用 ObserverWithState 在构造方法中调用了 androidx.lifecycle.Lifecycling#lifecycleEventObserver 对观察者解析，最后把不同方式的观察者都解析成 LifecycleEventObserver 方便回调事件

    static LifecycleEventObserver lifecycleEventObserver(Object object) {
    	// 观察者实现了 LifecycleEventObserver 接口
        boolean isLifecycleEventObserver = object instanceof LifecycleEventObserver;
        // 观察者实现了 FullLifecycleObserver 接口
        boolean isFullLifecycleObserver = object instanceof FullLifecycleObserver;
        if (isLifecycleEventObserver && isFullLifecycleObserver) {
        	// 如果两个条件都满足就创建一个 FullLifecycleObserverAdapter 对象实现了 LifecycleEventObserver 接口
            return new FullLifecycleObserverAdapter((FullLifecycleObserver) object,
                    (LifecycleEventObserver) object);
        }
        // 同上
        if (isFullLifecycleObserver) {
            return new FullLifecycleObserverAdapter((FullLifecycleObserver) object, null);
        }

        if (isLifecycleEventObserver) {
        	// 如果就是 LifecycleEventObserver 直接返回
            return (LifecycleEventObserver) object;
        }

        final Class<?> klass = object.getClass();
        // 观察者的类型 GENERATED_CALLBACK 为使用代码生成 REFLECTIVE_CALLBACK 为使用反射调用
        int type = getObserverConstructorType(klass);
        if (type == GENERATED_CALLBACK) {
        	// 生成的 GeneratedAdapter 子类的构造器
            List<Constructor<? extends GeneratedAdapter>> constructors =
                    sClassToAdapters.get(klass);
            if (constructors.size() == 1) {
                GeneratedAdapter generatedAdapter = createGeneratedAdapter(
                        constructors.get(0), object);
                // 如果构造器的个数为 1 则创建 SingleGeneratedAdapterObserver 对象 
                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);
            }
            // 如果有多个构造器则创建 CompositeGeneratedAdaptersObserver 对象
            return new CompositeGeneratedAdaptersObserver(adapters);
        }
        // 通过反射调用
        return new ReflectiveGenericLifecycleObserver(object);
    }

只有引入 lifecycle-compiler 时才会生成 GeneratedAdapter 的子类，如果没有引入则会通过反射调用

    private static int resolveObserverCallbackType(Class<?> klass) {
        // 匿名内部类采用反射
        if (klass.getCanonicalName() == null) {
            return REFLECTIVE_CALLBACK;
        }
		// 如果生成了 GeneratedAdapter 的子类
        Constructor<? extends GeneratedAdapter> constructor = generatedConstructor(klass);
        if (constructor != null) {
            sClassToAdapters.put(klass, Collections
                    .<Constructor<? extends GeneratedAdapter>>singletonList(constructor));
            return GENERATED_CALLBACK;
        }
		
		// 如果有标记了 OnLifecycleEvent 注解的方法
        boolean hasLifecycleMethods = ClassesInfoCache.sInstance.hasLifecycleMethods(klass);
        if (hasLifecycleMethods) {
            return REFLECTIVE_CALLBACK;
        }
		// 如果没有找到生成类，也没有方法被 OnLifecycleEvent 注解
		// 需要去看其父类和接口，或许它们被注解，因此这里又递归调用
        Class<?> superclass = klass.getSuperclass();
        List<Constructor<? extends GeneratedAdapter>> adapterConstructors = null;
        // 如果父类是 LifecycleObserver 的子类
        if (isLifecycleParent(superclass)) {
            if (getObserverConstructorType(superclass) == REFLECTIVE_CALLBACK) {
                return REFLECTIVE_CALLBACK;
            }
            adapterConstructors = new ArrayList<>(sClassToAdapters.get(superclass));
        }
		// 如果实现的接口是 LifecycleObserver 的子类
        for (Class<?> intrface : klass.getInterfaces()) {
            if (!isLifecycleParent(intrface)) {
                continue;
            }
            if (getObserverConstructorType(intrface) == REFLECTIVE_CALLBACK) {
                return REFLECTIVE_CALLBACK;
            }
            if (adapterConstructors == null) {
                adapterConstructors = new ArrayList<>();
            }
            adapterConstructors.addAll(sClassToAdapters.get(intrface));
        }
        if (adapterConstructors != null) {
            sClassToAdapters.put(klass, adapterConstructors);
            return GENERATED_CALLBACK;
        }

        return REFLECTIVE_CALLBACK;
    }

再来看一下是怎么解析标记了 OnLifecycleEvent 注解的方法的

	// 判断指定的 class 对象是否包含使用了 OnLifecycleEvent 进行注解的方法
    boolean hasLifecycleMethods(Class<?> klass) {
    	// 因为使用了反射效率低所以这里使用了缓存
        Boolean hasLifecycleMethods = mHasLifecycleMethods.get(klass);
        if (hasLifecycleMethods != null) {
            return hasLifecycleMethods;
        }
		// 拿到自身定义的所有方法
        Method[] methods = getDeclaredMethods(klass);
        for (Method method : methods) {
            OnLifecycleEvent annotation = method.getAnnotation(OnLifecycleEvent.class);
            // 如果标记了 OnLifecycleEvent 注解
            if (annotation != null) {
                createInfo(klass, methods);
                return true;
            }
        }
        mHasLifecycleMethods.put(klass, false);
        return false;
    }

遍历所有方法看是否标记了 OnLifecycleEvent 注解，主要逻辑都在 createInfo 里

    private CallbackInfo createInfo(Class<?> klass, @Nullable Method[] declaredMethods) {
        Class<?> superclass = klass.getSuperclass();
        Map<MethodReference, Lifecycle.Event> handlerToEvent = new HashMap<>();
        if (superclass != null) {
        	// 解析父类的方法注解信息
            CallbackInfo superInfo = getInfo(superclass);
            if (superInfo != null) {
                handlerToEvent.putAll(superInfo.mHandlerToEvent);
            }
        }
		// 解析所有接口的方法注解信息
        Class<?>[] interfaces = klass.getInterfaces();
        for (Class<?> intrfc : interfaces) {
            for (Map.Entry<MethodReference, Lifecycle.Event> entry : getInfo(
                    intrfc).mHandlerToEvent.entrySet()) {
                // 校验重复定义
                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;
            // 标记了 OnLifecycleEvent 注解的方法参数类型
            Class<?>[] params = method.getParameterTypes();
            // CALL_TYPE_NO_ARG 方法没有参数
            // CALL_TYPE_PROVIDER 方法有一个参数
            // CALL_TYPE_PROVIDER_WITH_EVENT 方法有两个参数
            int callType = CALL_TYPE_NO_ARG;
            if (params.length > 0) {
                callType = CALL_TYPE_PROVIDER;
                // 如果只有一个参数类型必须是 LifecycleOwner 或者其子类
                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;
                // 如果有两个参数类型第二个参数必须是 Lifecycle.Event 类型
                if (!params[1].isAssignableFrom(Lifecycle.Event.class)) {
                    throw new IllegalArgumentException(
                            "invalid parameter type. second arg must be an event");
                }
                // 并且注解的值必须是 ON_ANY
                if (event != Lifecycle.Event.ON_ANY) {
                    throw new IllegalArgumentException(
                            "Second arg is supported only for ON_ANY value");
                }
            }
            // 如果参数超过 2 个则抛异常
            if (params.length > 2) {
                throw new IllegalArgumentException("cannot have more than 2 params");
            }
            // 封装方法信息（包含参数个数和方法）
            MethodReference methodReference = new MethodReference(callType, method);	// 校验
            verifyAndPutHandler(handlerToEvent, methodReference, event, klass);
        }
        // 封装 CallbackInfo  
        CallbackInfo info = new CallbackInfo(handlerToEvent);
        // 缓存起来
        mCallbackMap.put(klass, info);
        mHasLifecycleMethods.put(klass, hasLifecycleMethods);
        return info;
    }

解析所有方法判断是否有 OnLifecycleEvent 注解并且对方法的参数进行校验最后缓存起来，可以看到不管是以什么方式使用最后都会返回 LifecycleEventObserver 的子类实例最后再通过 androidx.lifecycle.LifecycleRegistry.ObserverWithState#dispatchEvent 调用实例的 onStateChanged 方法把事件回调出去。接下来再回去看 handleLifecycleEvent 方法

    public void handleLifecycleEvent(@NonNull Lifecycle.Event event) {
    	// 检查线程必须在主线程调用
        enforceMainThreadIfNeeded("handleLifecycleEvent");
        moveToState(event.getTargetState());
    }

    private void moveToState(State next) {
        if (mState == next) {
            return;
        }
        mState = next;
       	// 如果有事件嵌套就把 mNewEventOccurred 置为 true 
        // 在同步过程中如果发现了此值为 true 就会停止同步
        if (mHandlingEvent || mAddingObserverCounter != 0) {
            mNewEventOccurred = true;
            // we will figure out what to do on upper level.
            return;
        }
        // 标记正在同步
        mHandlingEvent = true;
        sync();
        // 同步结束
        mHandlingEvent = false;
    }

    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.");
        }
        // 如果没有同步完成
        while (!isSynced()) {
            mNewEventOccurred = false;
            if (mState.compareTo(mObserverMap.eldest().getValue().mState) < 0) {
                backwardPass(lifecycleOwner);
            }
            Map.Entry<LifecycleObserver, ObserverWithState> newest = mObserverMap.newest();
            if (!mNewEventOccurred && newest != null
                    && mState.compareTo(newest.getValue().mState) > 0) {
                forwardPass(lifecycleOwner);
            }
        }
        mNewEventOccurred = false;
    }

isSynced() 内部通过判断最新和最老的订阅者的状态是否都等于当前状态 mState 来判断是否已经同步完毕的，如果没有就通过订阅者状态和当前状态的大小比较来通过 backwardPass 或者 forwardPass 来更新订阅者状态
sync() 里这个判断状态的方式就是依赖了订阅队列的状态有序性：判断订阅队列存在状态大于当前状态的订阅者不需要检查整个队列，只需要跟订阅队列中状态最大的也就是最先订阅的订阅者比较；同样的判断订阅队列存在状态小于于当前状态的订阅者也只需要跟最新订阅的订阅者比较
backwardPass 或者 forwardPass 用于更新状态，其中 backwardPass 的作用是将订阅队列中所有状态大于当前状态的订阅者同步到当前状态。而 forwardPass 就是将订阅队列中的所有状态小于当前状态的订阅者同步到当前状态

状态往 INITIALIZED->RESUMED 方向变化时调用 forwardPass 方法，状态往 RESUMED->DESTROYED 方向变化时调用 backwardPass 方法。下面看下 backwardPass

    private void backwardPass(LifecycleOwner lifecycleOwner) {
    	// 从最老到最新的订阅者的迭代器
        Iterator<Map.Entry<LifecycleObserver, ObserverWithState>> descendingIterator =
                mObserverMap.descendingIterator();
        while (descendingIterator.hasNext() && !mNewEventOccurred) {
            Map.Entry<LifecycleObserver, ObserverWithState> entry = descendingIterator.next();
            ObserverWithState observer = entry.getValue();
            while ((observer.mState.compareTo(mState) > 0 && !mNewEventOccurred
                    && mObserverMap.contains(entry.getKey()))) {
                Event event = Event.downFrom(observer.mState);
                if (event == null) {
                    throw new IllegalStateException("no event down from " + observer.mState);
                }
                pushParentState(event.getTargetState());
                // 通过观察者把事件回调出去
                observer.dispatchEvent(lifecycleOwner, event);
                popParentState();
            }
        }
    }

方法中会判断观察者的状态依次回调事件，在回调之前会记录当前观察者的状态回调之后再移除记录是为了处理事件嵌套的情况，保证回调事件的顺序和添加观察者的顺序一致

        void dispatchEvent(LifecycleOwner owner, Event event) {
            State newState = event.getTargetState();
            mState = min(mState, newState);
            mLifecycleObserver.onStateChanged(owner, event);
            mState = newState;
        }

最后就会通过添加观察者时创建的 ObserverWithState 对象回调事件，而 ObserverWithState 是通过在构造方法中根据观察者的方式解析出的 LifecycleEventObserver 回调出去的，下面依次看一下

class FullLifecycleObserverAdapter implements LifecycleEventObserver {

    private final FullLifecycleObserver mFullLifecycleObserver;
    private final LifecycleEventObserver mLifecycleEventObserver;

    FullLifecycleObserverAdapter(FullLifecycleObserver fullLifecycleObserver,
            LifecycleEventObserver lifecycleEventObserver) {
        mFullLifecycleObserver = fullLifecycleObserver;
        mLifecycleEventObserver = lifecycleEventObserver;
    }

    @Override
    public void onStateChanged(@NonNull LifecycleOwner source, @NonNull Lifecycle.Event event) {
        switch (event) {
            case ON_CREATE:
                mFullLifecycleObserver.onCreate(source);
                break;
            case ON_START:
                mFullLifecycleObserver.onStart(source);
                break;
            case ON_RESUME:
                mFullLifecycleObserver.onResume(source);
                break;
            case ON_PAUSE:
                mFullLifecycleObserver.onPause(source);
                break;
            case ON_STOP:
                mFullLifecycleObserver.onStop(source);
                break;
            case ON_DESTROY:
                mFullLifecycleObserver.onDestroy(source);
                break;
            case ON_ANY:
                throw new IllegalArgumentException("ON_ANY must not been send by anybody");
        }
        if (mLifecycleEventObserver != null) {
            mLifecycleEventObserver.onStateChanged(source, event);
        }
    }
}

FullLifecycleObserverAdapter 在 onStateChanged 中把事件回调出去最终调用了观察者的对应方法

class SingleGeneratedAdapterObserver implements LifecycleEventObserver {

    private final GeneratedAdapter mGeneratedAdapter;

    SingleGeneratedAdapterObserver(GeneratedAdapter generatedAdapter) {
        mGeneratedAdapter = generatedAdapter;
    }

    @Override
    public void onStateChanged(@NonNull LifecycleOwner source, @NonNull Lifecycle.Event event) {
    	// 调用生成代码的 callMethods 方法
        mGeneratedAdapter.callMethods(source, event, false, null);
        mGeneratedAdapter.callMethods(source, event, true, null);
    }
}

public class AnnotationLifecycleTest_LifecycleAdapter implements GeneratedAdapter {
  final AnnotationLifecycleTest mReceiver;

  AnnotationLifecycleTest_LifecycleAdapter(AnnotationLifecycleTest receiver) {
    this.mReceiver = receiver;
  }

  @Override
  public void callMethods(LifecycleOwner owner, Lifecycle.Event event, boolean onAny,
      MethodCallsLogger logger) {
    boolean hasLogger = logger != null;
    if (onAny) {
      if (!hasLogger || logger.approveCall("onAny", 4)) {
      	// 最终调用了定义的方法
        mReceiver.onAny(owner,event);
      }
      return;
    }
  }
}

CompositeGeneratedAdaptersObserver 与 SingleGeneratedAdapterObserver 一致不重复贴代码了，最后看一下 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) {
    	// 调用处
        mInfo.invokeCallbacks(source, event, mWrapped);
    }
}

        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);
        }

        private static void invokeMethodsForEvent(List<MethodReference> handlers,
                LifecycleOwner source, Lifecycle.Event event, Object mWrapped) {
            if (handlers != null) {
                for (int i = handlers.size() - 1; i >= 0; i--) {
                	// androidx.lifecycle.ClassesInfoCache.MethodReference#invokeCallback
                    handlers.get(i).invokeCallback(source, event, mWrapped);
                }
            }
        }

        void invokeCallback(LifecycleOwner source, Lifecycle.Event event, Object target) {
            //noinspection TryWithIdenticalCatches
            try {
                switch (mCallType) {
                    case CALL_TYPE_NO_ARG:
                        mMethod.invoke(target);
                        break;
                    case CALL_TYPE_PROVIDER:
                        mMethod.invoke(target, source);
                        break;
                    case CALL_TYPE_PROVIDER_WITH_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);
            }
        }

最后在封装的方法信息中调用了 invokeCallback 方法根据解析出的参数个数回调观察者的方法，到这里 Activity 的生命周期所有流程就都结束了，再来看一下 Fragment 是怎么实现的

在 Fragment 中的实现

Fragment 本身实现了 LifecycleOwner 接口提供的 Lifecycle 实现也是 LifecycleRegistry

public class Fragment implements LifecycleOwner {

    LifecycleRegistry mLifecycleRegistry;

    public Fragment() {
        initLifecycle();
    }

    private void initLifecycle() {
        mLifecycleRegistry = new LifecycleRegistry(this);
    }

    public Lifecycle getLifecycle() {
        return mLifecycleRegistry;
    }

    void performCreate(Bundle savedInstanceState) {
        onCreate(savedInstanceState);
        mLifecycleRegistry.handleLifecycleEvent(Lifecycle.Event.ON_CREATE);
    }

    void performStart() {
        onStart();
        mLifecycleRegistry.handleLifecycleEvent(Lifecycle.Event.ON_START);
    }

    void performResume() {
        onResume();
        mLifecycleRegistry.handleLifecycleEvent(Lifecycle.Event.ON_RESUME);
    }

    void performPause() {
        mLifecycleRegistry.handleLifecycleEvent(Lifecycle.Event.ON_PAUSE);
        onPause();
    }

    void performStop() {
        mLifecycleRegistry.handleLifecycleEvent(Lifecycle.Event.ON_STOP);
        onStop();
    }

    void performDestroy() {
        mLifecycleRegistry.handleLifecycleEvent(Lifecycle.Event.ON_DESTROY);
        onDestroy();
    }
    
}

Fragment 本身提供的 Lifecycle 也与 Activity 一致都是 ON_CREATE ON_START ON_RESUME 事件在相应的生命周期方法后回调 ON_PAUSE ON_STOP ON_DESTROY 事件在相应的生命周期方法前回调。另外由于 Fragment 本身的生命周期与 Fragment 中 mView 的生命周期并不一致 Fragment 额外又提供了一个持有的 View 的 FragmentViewLifecycleOwner

class FragmentViewLifecycleOwner implements HasDefaultViewModelProviderFactory,
        SavedStateRegistryOwner, ViewModelStoreOwner {
    private final Fragment mFragment;
    private final ViewModelStore mViewModelStore;

    private ViewModelProvider.Factory mDefaultFactory;

    private LifecycleRegistry mLifecycleRegistry = null;
    private SavedStateRegistryController mSavedStateRegistryController = null;

    FragmentViewLifecycleOwner(@NonNull Fragment fragment, @NonNull ViewModelStore viewModelStore) {
        mFragment = fragment;
        mViewModelStore = viewModelStore;
    }

    @NonNull
    @Override
    public ViewModelStore getViewModelStore() {
        initialize();
        return mViewModelStore;
    }

    void initialize() {
        if (mLifecycleRegistry == null) {
            mLifecycleRegistry = new LifecycleRegistry(this);
            mSavedStateRegistryController = SavedStateRegistryController.create(this);
        }
    }

    boolean isInitialized() {
        return mLifecycleRegistry != null;
    }

    @NonNull
    @Override
    public Lifecycle getLifecycle() {
        initialize();
        return mLifecycleRegistry;
    }

    void setCurrentState(@NonNull Lifecycle.State state) {
        mLifecycleRegistry.setCurrentState(state);
    }

    void handleLifecycleEvent(@NonNull Lifecycle.Event event) {
        mLifecycleRegistry.handleLifecycleEvent(event);
    }

}

FragmentViewLifecycleOwner 也实现了 LifecycleOwner 接口向外提供的 Lifecycle 实现还是 LifecycleRegistry 它们俩除了 ON_CREATE ON_DESTROY 两个事件回调时机不同其他事件都是相同的

    final void restoreViewState(Bundle savedInstanceState) {
        onViewStateRestored(savedInstanceState);
        if (mView != null) {
            mViewLifecycleOwner.handleLifecycleEvent(Lifecycle.Event.ON_CREATE);
        }
    }

    void performDestroyView() {
        if (mView != null && mViewLifecycleOwner.getLifecycle().getCurrentState()
                        .isAtLeast(Lifecycle.State.CREATED)) {
            mViewLifecycleOwner.handleLifecycleEvent(Lifecycle.Event.ON_DESTROY);
        }
        mState = CREATED;
        mCalled = false;
        onDestroyView();
    }

Fragment View 提供的 Lifecycle 的 ON_CREATE 事件在 onViewStateRestored 之后执行 ON_DESTROY 事件在 onDestroyView 之前执行。官网有一张图描述了它们的区别

另外在 fragment 版本的更新说明中有这样一段描述

添加了新的 Lint 检查，以确保您在从 onCreateView()、onViewCreated() 或 onActivityCreated() 观察 LiveData 时使用 getViewLifecycleOwner()

原因是在 Fragment 中订阅 LiveData 时在 LiveData 中实现的 Lifecycle 观察者会在 Lifecycle 组件处于 DESTROYED 状态时移除 Fragment 对 LiveData 的订阅，当在 Fragment 中 View 销毁重建而 Fragment 没有重建的一些场景可以避免重复订阅，因为 Fragment 在 onDestroy() 前才处于 DESTROYED 而 Fragment 中的 View 在 onDestroyView 前就处于 DESTROYED 通过上面的图也可以看出来

在 Service 中的实现

public class LifecycleService extends Service implements LifecycleOwner {

    private final ServiceLifecycleDispatcher mDispatcher = new ServiceLifecycleDispatcher(this);

    @CallSuper
    @Override
    public void onCreate() {
        mDispatcher.onServicePreSuperOnCreate();
        super.onCreate();
    }

    @CallSuper
    @Nullable
    @Override
    public IBinder onBind(@NonNull Intent intent) {
        mDispatcher.onServicePreSuperOnBind();
        return null;
    }

    @SuppressWarnings("deprecation")
    @CallSuper
    @Override
    public void onStart(@Nullable Intent intent, int startId) {
        mDispatcher.onServicePreSuperOnStart();
        super.onStart(intent, startId);
    }

    // this method is added only to annotate it with @CallSuper.
    // In usual service super.onStartCommand is no-op, but in LifecycleService
    // it results in mDispatcher.onServicePreSuperOnStart() call, because
    // super.onStartCommand calls onStart().
    @CallSuper
    @Override
    public int onStartCommand(@Nullable Intent intent, int flags, int startId) {
        return super.onStartCommand(intent, flags, startId);
    }

    @CallSuper
    @Override
    public void onDestroy() {
        mDispatcher.onServicePreSuperOnDestroy();
        super.onDestroy();
    }

    @Override
    @NonNull
    public Lifecycle getLifecycle() {
        return mDispatcher.getLifecycle();
    }
}

ServiceLifecycleDispatcher 承担了所有的工作

public class ServiceLifecycleDispatcher {
    private final LifecycleRegistry mRegistry;
    private final Handler mHandler;
    private DispatchRunnable mLastDispatchRunnable;

    public ServiceLifecycleDispatcher(@NonNull LifecycleOwner provider) {
    	// 在构造方法中创建 LifecycleRegistry 和 Handler 对象
        mRegistry = new LifecycleRegistry(provider);
        mHandler = new Handler();
    }

    private void postDispatchRunnable(Lifecycle.Event event) {
    	// 如果在发送事件的时候发现上一个事件还没有发送则发送上一个事件
    	// 目的是为了保证事件的有序性，内部做了判断不会重复执行
        if (mLastDispatchRunnable != null) {
            mLastDispatchRunnable.run();
        }
        // 把事件封装成了一个 Runnable 对象 
        mLastDispatchRunnable = new DispatchRunnable(mRegistry, event);
        // 将消息发送到 MessageQueue 链表头部可以尽快执行
        mHandler.postAtFrontOfQueue(mLastDispatchRunnable);
    }

    public void onServicePreSuperOnCreate() {
        postDispatchRunnable(Lifecycle.Event.ON_CREATE);
    }

	// 发送的 ON_START 事件，对应 bindService 的方式
    public void onServicePreSuperOnBind() {
        postDispatchRunnable(Lifecycle.Event.ON_START);
    }

	// 发送的 ON_START 事件，对应 startService 的方式
    public void onServicePreSuperOnStart() {
        postDispatchRunnable(Lifecycle.Event.ON_START);
    }

    public void onServicePreSuperOnDestroy() {
        postDispatchRunnable(Lifecycle.Event.ON_STOP);
        postDispatchRunnable(Lifecycle.Event.ON_DESTROY);
    }

    @NonNull
    public Lifecycle getLifecycle() {
        return mRegistry;
    }

    static class DispatchRunnable implements Runnable {
        private final LifecycleRegistry mRegistry;
        final Lifecycle.Event mEvent;
        private boolean mWasExecuted = false;

        DispatchRunnable(@NonNull LifecycleRegistry registry, Lifecycle.Event event) {
            mRegistry = registry;
            mEvent = event;
        }

        @Override
        public void run() {
        	// 避免通过消息循环和直接 run 的方式重复执行
            if (!mWasExecuted) {
                mRegistry.handleLifecycleEvent(mEvent);
                mWasExecuted = true;
            }
        }
    }
}

Service 的 Lifecycle 比较简单并且没有 ON_PAUSE ON_RESUME 事件

在 Process 中的实现

// androidx.lifecycle.ProcessLifecycleOwnerInitializer
public class ProcessLifecycleOwnerInitializer extends ContentProvider {
    @Override
    public boolean onCreate() {
        LifecycleDispatcher.init(getContext());
        ProcessLifecycleOwner.init(getContext());
        return true;
    }
}

// androidx.lifecycle.LifecycleDispatcher
class LifecycleDispatcher {

    private static AtomicBoolean sInitialized = new AtomicBoolean(false);

    static void init(Context context) {
        if (sInitialized.getAndSet(true)) {
            return;
        }
        ((Application) context.getApplicationContext())
                .registerActivityLifecycleCallbacks(new DispatcherActivityCallback());
    }

    // EmptyActivityLifecycleCallbacks 是 Application.ActivityLifecycleCallbacks 接口的所有方法空实现
    static class DispatcherActivityCallback extends EmptyActivityLifecycleCallbacks {

        @Override
        public void onActivityCreated(Activity activity, Bundle savedInstanceState) {
        	// 注入 ReportFragment 内部会通过 manager.findFragmentByTag(REPORT_FRAGMENT_TAG) 做判断会重复调用不会重复注入
            ReportFragment.injectIfNeededIn(activity);
        }

        @Override
        public void onActivityStopped(Activity activity) {
        }

        @Override
        public void onActivitySaveInstanceState(Activity activity, Bundle outState) {
        }
    }

    private LifecycleDispatcher() {
    }
}

ContentProvider 中只做了两件事，第一是调用 LifecycleDispatcher 的 init 方法注入 ReportFragment 监听宿主 Activity 的生命周期，第二是对 ProcessLifecycleOwner 进行初始化

// androidx.lifecycle.ProcessLifecycleOwner#attach
void attach(Context context) {
    mHandler = new Handler();
    // 直接发送一个 ON_CREATE 事件
    mRegistry.handleLifecycleEvent(Lifecycle.Event.ON_CREATE);
    Application app = (Application) context.getApplicationContext();
    app.registerActivityLifecycleCallbacks(new EmptyActivityLifecycleCallbacks() {
        @Override
        public void onActivityPreCreated(@NonNull Activity activity,
                @Nullable Bundle savedInstanceState) {
            activity.registerActivityLifecycleCallbacks(new EmptyActivityLifecycleCallbacks() {
                @Override
                public void onActivityPostStarted(@NonNull Activity activity) {
                    activityStarted();
                }

                @Override
                public void onActivityPostResumed(@NonNull Activity activity) {
                    activityResumed();
                }
            });
        }

        @Override
        public void onActivityCreated(Activity activity, Bundle savedInstanceState) {
        	// 低于 API 29 的版本使用 ReportFragment 的回调方法
        	// 大于等于 API 29 的版本使用 Activity 注册的 ActivityLifecycleCallbacks 的 pre/post 相关方法，这些方法是在 API 29 才添加的
            if (Build.VERSION.SDK_INT < 29) {
                ReportFragment.get(activity).setProcessListener(mInitializationListener);
            }
        }

        @Override
        public void onActivityPaused(Activity activity) {
            activityPaused();
        }

        @Override
        public void onActivityStopped(Activity activity) {
            activityStopped();
        }
    });
}

跟 Activity 一样同样区分版本使用不同的回调方法 ReportFragment 中的代码就不再贴了，文章太长了，先看一下 ProcessLifecycleOwner 中定义的几个变量

// 当有 Activity 走到 Started 状态时，则 mStartedCounter 加一
// 当有 Activity 走到 Stopped 状态时，则 mStartedCounter 减一
private int mStartedCounter = 0;

// 当有 Activity 走到 Resumed 状态时，则 mResumedCounter 加一
// 当有 Activity 走到 Paused 状态时，则 mResumedCounter 减一
private int mResumedCounter = 0;

// 当发布了 ON_RESUME 事件时，值变为 false
// 当发布了 ON_PAUSE 事件时，值变为 true
private boolean mPauseSent = true;

// 当发布了 ON_START 事件时，值变为 false
// 当发布了 ON_STOP 事件时，值变为 true
private boolean mStopSent = true;

再看一下 ProcessLifecycleOwner 中对其他几个事件的处理

private Runnable mDelayedPauseRunnable = new Runnable() {
    @Override
    public void run() {
        dispatchPauseIfNeeded();
        dispatchStopIfNeeded();
    }
};

// 当创建第一个页面时回调此事件，如果只有一个页面并且页面销毁重建并不会回调 ON_START 事件因为延时处理不会触发 ON_PAUSE ON_STOP 事件 mStopSent 为 false
void activityStarted() {
    mStartedCounter++;
    if (mStartedCounter == 1 && mStopSent) {
        mRegistry.handleLifecycleEvent(Lifecycle.Event.ON_START);
        mStopSent = false;
    }
}

void activityResumed() {
    mResumedCounter++;
    if (mResumedCounter == 1) {
        if (mPauseSent) {
        	// 如果发送了 ON_PAUSE 事件说明是从后台回到前台发送 ON_RESUME 事件
            mRegistry.handleLifecycleEvent(Lifecycle.Event.ON_RESUME);
            mPauseSent = false;
        } else {
        	// 如果是页面重建移除 mDelayedPauseRunnable 不必延迟检查了因为不用发送 ON_PAUSE 事件
            mHandler.removeCallbacks(mDelayedPauseRunnable);
        }
    }
}

void activityPaused() {
    mResumedCounter--;
    if (mResumedCounter == 0) {
    	// 因为要处理页面重建的情况所以发送延时消息处理
        mHandler.postDelayed(mDelayedPauseRunnable, TIMEOUT_MS);
    }
}

void activityStopped() {
    mStartedCounter--;
    // 因为要处理页面重建的情况所以发送延时消息处理
    dispatchStopIfNeeded();
}

void dispatchPauseIfNeeded() {
	// mResumedCounter 等于 0 说明确实进入了后台则发送 ON_PAUSE 事件
    if (mResumedCounter == 0) {
        mPauseSent = true;
        mRegistry.handleLifecycleEvent(Lifecycle.Event.ON_PAUSE);
    }
}

void dispatchStopIfNeeded() {
	// 如果不是页面重建并且已经发送了 ON_PAUSE 事件则发送 ON_STOP 事件
    if (mStartedCounter == 0 && mPauseSent) {
        mRegistry.handleLifecycleEvent(Lifecycle.Event.ON_STOP);
        mStopSent = true;
    }
}

ProcessLifecycleOwner 整体也比较简单

ViewTreeLifecycleOwner

ViewTreeLifecycleOwner 是 lifecycle-runtime-ktx 中提供的 View 的一个扩展方法，可以方便的获取所属 Fragment 或者 Activity 的 LifecycleOwner

val lifecycleOwner = binding.textView.findViewTreeLifecycleOwner()

public fun View.findViewTreeLifecycleOwner(): LifecycleOwner? = ViewTreeLifecycleOwner.get(this)

使用和源码都比较简单直接调用了 ViewTreeLifecycleOwner 的 get 方法

public class ViewTreeLifecycleOwner {
    private ViewTreeLifecycleOwner() {
        // No instances
    }

	// 调用 setTag 把 lifecycleOwner 存起来
    public static void set(@NonNull View view, @Nullable LifecycleOwner lifecycleOwner) {
        view.setTag(R.id.view_tree_lifecycle_owner, lifecycleOwner);
    }

    // 通过不断循环遍历 parent 获取 LifecycleOwner 
    public static LifecycleOwner get(@NonNull View view) {
        LifecycleOwner found = (LifecycleOwner) view.getTag(R.id.view_tree_lifecycle_owner);
        if (found != null) return found;
        ViewParent parent = view.getParent();
        while (found == null && parent instanceof View) {
            final View parentView = (View) parent;
            found = (LifecycleOwner) parentView.getTag(R.id.view_tree_lifecycle_owner);
            parent = parentView.getParent();
        }
        return found;
    }
}

在 androidx.appcompat.app.AppCompatActivity、androidx.activity.ComponentActivity 的 setContentView 方法以及 androidx.fragment.app.Fragment#performCreateView （onCreateView 之后调用）方法和 androidx.fragment.app.DialogFragment 的 onStart 方法中都调用了 set 方法，所以在这些组件的子类中可以使用 ViewTreeLifecycleOwner 获取 LifecycleOwner

参考与感谢

从源码看 Jetpack（1）- Lifecycle 源码详解
从源码看 Jetpack（2）- Lifecycle 衍生物源码详解
Android架构组件（2）LifecycleRegistry 源码分析
Jetpack MVVM七宗罪 之一 拿Fragment当LifecycleOwner
为什么Fragment中要使用viewLifecycleOwner代替this