概述

Lifecycle 是用来管理感知生命周期的Jetpack基础组件，主要是为了方便需要依赖生命周期来搭建的业务逻辑。其本质原理是观察者模式，即把生命周期组件作为被观察的对象，通过注册观察者，构建状态或者事件变化通知，从而达到感知生命周期的。

生命周期感知型组件可执行操作来响应另一个组件（如 Activity 和 Fragment）的生命周期状态的变化。这些组件有助于您编写出更有条理且往往更精简的代码，此类代码更易于维护。

Lifecycle 主要构成

三个重要角色

通过Lifecycle-common的源码，归结Lifecycle的三个重要角色

• LifeCycleOwner 生命周期拥有者，即Activity与Fragment（也可自定义，但局限性大）
• LifeCycleObserver 生命周期观察者，可以是任何类，常见的有mvp的p，自定义View等
• Lifecycle 是一个抽象类，其内部不仅包括了添加和移除观察者的方法，还包括了 Event 和 State 枚举。可以看到 Event 中的事件和 Activity 的生命周期几乎是对应的，除了 ON_ANY，它可用于匹配所有事件。

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();

    @SuppressWarnings("WeakerAccess")
    public enum Event {
        ON_CREATE,
        ON_START,
        ON_RESUME,
        ON_PAUSE,
        ON_STOP,
        ON_DESTROY,
        ON_ANY;
    }

    /**
     * Lifecycle states. You can consider the states as the nodes in a graph and
     * {@link Event}s as the edges between these nodes.
     * 见下图1
     */
    @SuppressWarnings("WeakerAccess")
    public enum State {
        DESTROYED,
        INITIALIZED,
        CREATED,
        STARTED,
        RESUMED;
        public boolean isAtLeast(@NonNull State state) {
            return compareTo(state) >= 0;
        }
    }
}

state与event事件关系

state与event事件关系如下图，您可以将状态看作图中的节点，将事件看作这些节点之间的边。

基本使用方法

• 引入Lifecycle-lib

基础组件只需要通过appcompat, 无需再单独引用某一个lib

implementation 'androidx.appcompat:appcompat:1.3.0'

• 以activity生命周期感知

以activity生命周期的感知举例使用（lifecycle-runtime）

public class MainActivity extends AppCompatActivity {

    @Override
    protected void onCreate(Bundle savedInstanceState) {
        super.onCreate(savedInstanceState);
        setContentView(R.layout.activity_main);
        getLifecycle().addObserver(new MainLifecycleObserver(getLifecycle()));
    }

    public class MainLifecycleObserver implements LifecycleObserver {
        private static final String TAG = "MainLifecycleObserver";
        Lifecycle lifecycle;

        MainLifecycleObserver(Lifecycle lifecycle) {
            this.lifecycle = lifecycle;
        }

        @OnLifecycleEvent(Lifecycle.Event.ON_CREATE)
        public void onActivityCreate() {
            Log.d(TAG, "onActivityCreate");
        }
        ...     
        @OnLifecycleEvent(Lifecycle.Event.ON_DESTROY)
        public void onActivityDestroy() {
            Log.d(TAG, "onActivityDestroy");
        }

        @OnLifecycleEvent(Lifecycle.Event.ON_ANY)
        public void onActivityAny() {
            Log.d(TAG, "onActivityAny : " + lifecycle.getCurrentState().name());
        }
    }
}

lifecycle-runtime 源码分析

在上面的例子中，我们注册的 MainLifecycleObserver 注册的监听者如何保存、增删的？到底是如何监听到activity生命周期变化的？事件又是如何分发的？接下来按照事件分发的流程逐步分析。

ReportFragment

Activity 生命周期的监听，是通过 ReportFragment向外传递的。

ComponentActivity | oncreate

在 androidx.activity.ComponentActivity 的oncreate方法中, 会调用ReportFragment.injectIfNeededIn

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

injectIfNeededIn

接入，在injectIfNeededIn（） 方法中分为两种情况来处理

• SDK_INT >= 29 此情况会向 Activity 注册一个 LifecycleCallbacks ，以此来直接获得各个生命周期事件的回调通知。但也会继续执行第二种情况。
• SDK_INT < 29 此情况会通过向 Activity 添加一个无 UI 界面的 Fragment（即 ReportFragment），间接获得 Activity 的各个生命周期事件的回调通知

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);
    }
    // Prior to API 29 and to maintain compatibility with older versions of
    // ProcessLifecycleOwner (which may not be updated when lifecycle-runtime is updated and
    // need to support activities that don't extend from FragmentActivity from support lib),
    // use a framework fragment to get the correct timing of Lifecycle events
    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();
    }
}

SDK_INT >= 29

直接通过注册 activity.registerActivityLifecycleCallbacks

@RequiresApi(29)
static class LifecycleCallbacks implements Application.ActivityLifecycleCallbacks {

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

    @Override
    public void onActivityCreated(@NonNull Activity activity,
            @Nullable Bundle bundle) {
    }

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

SDK_INT < 29

向activity添加了一个fragment后，ReportFragment 本身的生命周期函数和所在的 Activity 是相关联的，进而实现间接监听activity生命周期，这里列出fragment部分生命周期方法

@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);
}

此处会继续对版本判断，防止重复分发事件

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);
    }
}

dispatch

两种情况中，最终都会调到 dispatch方法

@SuppressWarnings("deprecation")
static void dispatch(@NonNull Activity activity, @NonNull Lifecycle.Event event) {

    if (activity instanceof LifecycleRegistryOwner) { // 废弃了
        ((LifecycleRegistryOwner) activity).getLifecycle().handleLifecycleEvent(event);
        return;
    }

    if (activity instanceof LifecycleOwner) {
        Lifecycle lifecycle = ((LifecycleOwner) activity).getLifecycle();
        if (lifecycle instanceof LifecycleRegistry) {
            ((LifecycleRegistry) lifecycle).handleLifecycleEvent(event);
        }
    }
}

在这个方法中最终通过 LifecycleRegistry.handleLifecycleEvent 来将事件传递出去，从而使得外部得到各个生命周期事件的通知。

LifecycleRegistry

拿到事件后，再来看下 LifecycleRegistry 是如何将 Event 值转发给 LifecycleObserver 的

可以看到在 androidx.activity.ComponentActivity、androidx.fragment.app.Fragment 中都定义了 LifecycleRegistry

// 在ComponentActivity
private final LifecycleRegistry mLifecycleRegistry = new LifecycleRegistry(this);

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

//在 Fragment
LifecycleRegistry mLifecycleRegistry;
...
private void initLifecycle() {
    mLifecycleRegistry = new LifecycleRegistry(this);
    ....
}
...
@Override
@NonNull
public Lifecycle getLifecycle() {
    return mLifecycleRegistry;
}
...

handleLifecycleEvent

public void handleLifecycleEvent(@NonNull Lifecycle.Event event) {
    enforceMainThreadIfNeeded("handleLifecycleEvent");
    moveToState(event.getTargetState());
}

moveToState

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;
}

sync

private void sync() {
    LifecycleOwner lifecycleOwner = mLifecycleOwner.get();
    if (lifecycleOwner == null) {  // 注释1
        throw new IllegalStateException("LifecycleOwner of this LifecycleRegistry is already"
                + "garbage collected. It is too late to change lifecycle state.");
    }
    while (!isSynced()) {  // 注释2
        mNewEventOccurred = false;
        // no need to check eldest for nullability, because isSynced does it for us.      
        // 注释 3
        if (mState.compareTo(mObserverMap.eldest().getValue().mState) < 0) {
            backwardPass(lifecycleOwner);
        }
        Map.Entry<LifecycleObserver, ObserverWithState> newest = mObserverMap.newest();
        // 注释4
        if (!mNewEventOccurred && newest != null
                && mState.compareTo(newest.getValue().mState) > 0) {
            forwardPass(lifecycleOwner);
        }
    }
    mNewEventOccurred = false;
}

针对这个同步到各个观察者重要方法，我们逐个拆解

• 注释1 通过弱引用，判断当前的lifecycleowner是否已经被回收，避免无效同步
• 注释2 循环条件，是否已经同步完成，具体判断：当前的状态mState、第一个观察者状态eldestObserverState、最后一个观察者状态newestObserverState，三者否是相等，相等表明同步完成

private boolean isSynced() {
    if (mObserverMap.size() == 0) { //无观察者了
        return true;
    }
    State eldestObserverState = mObserverMap.eldest().getValue().mState;
    State newestObserverState = mObserverMap.newest().getValue().mState;
    return eldestObserverState == newestObserverState && mState == newestObserverState;
}

• 如果这三个状态值不相等进入循环，就需要判断当前状态是向前还是向后，比如由 STARTED 到 RESUMED 是状态向前，反过来就是状态向后
• 注释3 表明当前状态是在第一个观察者前面，此时状态是向后，故走到 backwardPass
• 注释4 表明当前状态在最后一个观察者后面，此时状态是向前，故走到 forwardPass

接下里我们选择 forwardPass 分析，另一个类似，不在赘析

forwardPass

其实到这里，既然决定了同步状态方向，那就要轮询通知所有的观察者了吧？

private void forwardPass(LifecycleOwner lifecycleOwner) {
    Iterator<Map.Entry<LifecycleObserver, ObserverWithState>> ascendingIterator =
            mObserverMap.iteratorWithAdditions();
    while (ascendingIterator.hasNext() && !mNewEventOccurred) {
        // 注释1
        Map.Entry<LifecycleObserver, ObserverWithState> entry = ascendingIterator.next();
        ObserverWithState observer = entry.getValue();
        // 注释2
        while ((observer.mState.compareTo(mState) < 0 && !mNewEventOccurred
                && mObserverMap.contains(entry.getKey()))) {
            // 注释3
            pushParentState(observer.mState);
            // 注释4
            final Event event = Event.upFrom(observer.mState);
            if (event == null) {
                throw new IllegalStateException("no event up from " + observer.mState);
            }
            // 注释5
            observer.dispatchEvent(lifecycleOwner, event);
            popParentState();
        }
    }
}

• 注释1 进入ascendingIterator循环遍历，取出entry的value,即每一个观察者 ObserverWithState
• 注释2 由于我们选取的是向前同步，此处的判定条件有三个
  • 此处的小于0 ，表明观察者状态位于当前状态之前 ，所以需要向前同步
  • 无新的事件发生
  • 再次判断map中依然含有此观察者，防止被移除
• 注释3 将 observer 已经遍历到的当前的状态值 mState 保存下来
• 注释4 获取当前观察者状态向前同步后，对应的事件
• 注释5 确定事件后，此处真正的去分发给观察者，即ObserverWithState

最终会调用到 ObserverWithState.dispatchEvent

ObserverWithState

这是LifecycleRegistry 一个内部类，会对注册进来的观察者会重新进行一次包装，成为 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;
    }
}

dispatchEvent

这里主要做了两件事

• 更新观察者的当前的状态为最新的状态，即我们要同步的状态
• 调用 LifecycleObserver.onStateChanged 传递事件。这个LifecycleObserver 是在构造方法中通过 Lifecycling.``*lifecycleEventObserver*来统一包装后的接口对象

其实到这里整个事件分发的流程已经完成了，状态也同步了，也分发到每个观察者了，但可以注意到这里并没有直接调用开头示例中的那些注解方法？接下来我们继续分析 Lifecycling的包装流程

Lifecycling

如果在 LifecycleRegistry 中直接对外部传入的 Observer 来进行类型判断、接口回调、反射调用等一系列操作的话，那势必会使得 LifecycleRegistry 整个类非常的臃肿，所以 Lifecycling 的作用就是来将这一系列的逻辑给封装起来，仅仅开放一个 onStateChanged 方法即可让 LifecycleRegistry 完成整个事件分发，从而使得整个流程会更加清晰明了且职责分明。

lifecycleEventObserver

@NonNull
static LifecycleEventObserver lifecycleEventObserver(Object object) {
    boolean isLifecycleEventObserver = object instanceof LifecycleEventObserver;
    boolean isFullLifecycleObserver = object instanceof FullLifecycleObserver;
    //注释1
    if (isLifecycleEventObserver && isFullLifecycleObserver) {
        return new FullLifecycleObserverAdapter((FullLifecycleObserver) object,
                (LifecycleEventObserver) object);
    }
    // 注释2
    if (isFullLifecycleObserver) {
        return new FullLifecycleObserverAdapter((FullLifecycleObserver) object, null);
    }

    if (isLifecycleEventObserver) {
        return (LifecycleEventObserver) object;
    }
    // 注释3
    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);
            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);
}

这里分为三种情况

• 注释1 自定义观察者：实现两个接口LifecycleEventObserver、FullLifecycleObserver
• 注释2 自定义观察者：只实现接口FullLifecycleObserver或者 LifecycleEventObserver
• 注释3 以注解的方式，此时就要通过反射来进行回调注解的生命周期方法

这里三种case后续的逻辑不在赘述，主要是采用适配器的设计模式，对观察者做onstatechange调用适配。

总结

这里总结一下整个事件分发的调用顺序

LifeCycle时序图.png

更多应用场景

你也可以单独使用其他lib,这些都是官方提供的具体的应用实现场景

• 如开头示例使用，解耦监听与activity/fragment组件的绑定
• 引入lifecycle-process， 使用ProcessLifecycleOwner快速监听应用前后台，监听应用生命周期
• 引入lifecycle-service， 解耦监听 Service 组件
• 可以使用lifecycle-viewmodel,lifecycle-livedata 等

https://developer.android.com/jetpack/androidx/releases/lifecycle

【参考文档】

https://developer.android.com/topic/libraries/architecture/lifecycle

https://developer.android.com/jetpack/androidx/releases/lifecycle