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Lifecycle 源码分析

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Lifecycle 源码解析

SafeIterableMap - 本质上是一个双链表伪装成的 Map，支持在遍历时候的修改(删除)操作

FastSafeIterableMap - 继承自 SafeIterableMap ，使用 HashMap 存取数据，速度更快，但同时占用内存也更大

Lifecycle 枚举常量：

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enum Event {
    // LifecycleOwner 的 onCreate 事件
    ON_CREATE,
    // LifecycleOwner 的 onStart 事件
    ON_START,
    // LifecycleOwner 的 onResume 事件
    ON_RESUME,
    // LifecycleOwner 的 onPause 事件
    ON_PAUSE,
    // LifecycleOwner 的 onStop 事件
    ON_STOP,
    // LifecycleOwner 的 onDestroy 事件
    ON_DESTROY,
    // ANY事件 能匹配以上所有事件
    ON_ANY
}
``` 

```java
// Lifecycle 对象跟踪的组件（即 LifecycleOwner ）当前的状态
enum State {
    // LifecycleOwner 的 DESTROYED 状态，到达这个事件后，当前生命周期不会再发送其他事件。
    // 对于 Activity 来说，Activity#onDestroy() 调用之后，就是 DESTROYED 状态
    DESTROYED,
    // LifecycleOwner 的初始状态。对于 Activity 来说，在 Activity 构造之后，在 Activity#onCreate() 之前的状态就是 INITIALIZED
    INITIALIZED,
    // LifecycleOwner CREATED 状态，对于 Activity 来说，在以下两种情况下会到达此状态:
    // 1. 在 Activity#onCreate() 调用之后
    // 2. 在 Activity#onStop() 调用之前
    CRETED,
    // LifecycleOwner STARTED 状态。对于 Activity 来说，在以下两种情况下回到达此状态:
    // 1. 在 Activity#onStart() 调用之后
    // 2. 在 Activity#onPause() 调用之前
    STARTED,
    // LifecycleOwner RESUME 状态。对于 Activity 来说，在 Activity#onResume() 调用之后就是 RESUMED 状态
    RESUMED;

    // 判断当前状态是否至少是指定状态
    // RESUMED > STARTED > CREATED > INITIALIZED > DESTROYED
    public boolean isAtLeast(@NonNull State state) {
        return compare(state) >= 0;
    }
}

Event 是一种事件，是一种过程，好比是一条线段，State 是一个状态，好比是一个点。线段两端是点，两点之间是线段。
所以对于 Activity 来说，其生命周期状态和事件关系如下图:

Activity生命周期状态与事件关系图

Lifecycle 抽象类:

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// 生命周期对象
// 事件 Event#ON_CREATE, Event#ON_START, Event#ON_RESUME 会在 LifecycleOwner 对应的方法返回之后发送
// 事件 Event#ON_PAUSE, Event#ON_STOP, Event#ON_DESTROY 会在 LifecycleOwner 调用之前被发送
// 比如: Event#ON_START 会在 Activity#onStart() 方法返回的时候发送； Event#ON_STOP 会在 Activity#onStop() 方法调用前发送
// 这样就保证了，观察者在观察的事件周期一定在 LifecycleOwner 生命周期内
abstract class Lifecycle {

    // 将指定 LifecycleObserver 添加到观察者列表中去
    // 当 LifecyclerOwner 发生变化的时候，会通知 LifecycleObserver
    // 注意: 参数 LifecycleObserver 的状态会带到当前 LifecycleOwner 的状态
    // 举个栗子：如果 LifecyleOwner 状态是 State#STARTED，当添加 LifecycleObserver 
    // 之后，会接收到事件: Event#ON_CREATE 和 EVENT#ON_START
    public abstract void addObserver(LifecycleObserver observer);

    // 将指定 LifecycleObserver 从观察者列表中移除
    public abstract void removeObserver(LifecycleObserver observer);

    // 获取当前生命周期状态
    public abstract State getCurrentState();
}

LifecycleOwner 接口代表拥有生命周期的组件，例如 androidx.fragment.app.FragmentActivity，androidx.fragment.app.Fragment 都实现了该接口。当然也可以自己定义拥有生命周期的组件来作为 LifecyleOwner。

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interface LifecycleOwner {
    Lifecycle getLifecycle();
}

Lifecycle 是一个抽象类，其方法诸如 addObserver 等定义了方法规范，其实际实现类是 LifecycleRegistry。 该类可以处理多个观察者。

添加 Observer: 

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class LifecycleRegistry extends Lifecycle {

    // FastSafeIterableMap - 支持在遍历过程中增删元素的双向链表
    // 这个 mObserverMap 元素中，恒成立的是：如果 addtion_order(observer1) < addtion_order(observer2)
    // 则 state(observer1) >= state(observer2)
    // 即如果把所有 observer 组成的队列的 State 看成是一个数列，那么这个数列是非递增数列
    private FastSafeIterableMap<LifecycleObserver, ObserverWithState> mObserverMap =
            new FastSafeIterableMap<>();

    @Override
    public void addObserver(@NonNull LifecycleObserver observer) {
        State initialState = mState == DESTROYED ? DESTROYED : INITIALIZED;
        // 1. 将传入的 LifecyleObserver 对象包装成 ObserverWithState 对象
        ObserverWithState statefulObserver = new ObserverWithState(observer, initialState);
        // 2. 如果当前 observer 没有被添加，则将其添加到队列中；如果已添加则 return
        ObserverWithState previous = mObserverMap.putIfAbsent(observer, statefulObserver);

        if (previous != null) {
            return;
        }

        LifecycleOwner lifecycleOwner = mLifecycleOwner.get();
        if (lifecycleOwner == null) {
            // it is null we should be destroyed. Fallback quickly
            return;
        }

        boolean isReentrance = mAddingObserverCounter != 0 || mHandlingEvent;
        State targetState = calculateTargetState(observer);
        mAddingObserverCounter++;
        // 3. 将参数 LifecycleObserver 的状态带到当前 LifecycleOwner 的状态
        while ((statefulObserver.mState.compareTo(targetState) < 0
                && mObserverMap.contains(observer))) {
            pushParentState(statefulObserver.mState);
            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.
            // 4. 同步整个观察者队列中的状态
            sync();
        }
        mAddingObserverCounter--;
    }
}

注释1，ObserverWithState 是 LifecycleRegistry 的静态内部类

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static class ObserverWithState {
    State mState;
    LifecycleEventObserver mLifecycleObserver;

    ObserverWithState(LifecycleObserver observer, State initialState) {
        // 5. 通过 Lifecycling#lifecycleEventObserver 方法将 LifecycleObserver 转换成 LifecycleEventObserver
        mLifecycleObserver = Lifecycling.lifecycleEventObserver(observer);
        mState = initialState;
    }
}

// LifecyeleEventObserver 继承自 LifecycleObserver，能接收任何生命周期的变化。
public interface LifecyeleEventObserver extends LifecycleObserver {
    void onStateChanged(LifecycleOwner source, Lifecycle.Event event);
}

注释5：Lifecycling#lifecycleEventObserver 方法

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static LifecycleEventObserver lifecycleEventObserver(Object object) {
    boolean isLifecycleEventObserver = object instanceof LifecycleEventObserver;
    boolean isFullLifecycleObserver = object instanceof FullLifecycleObserver;

    // 如果参数 object 是 LifecycleEventObserver 也是 FullLifecycleObserver，则返回 FullLifecycleObserverAdapter
    if (isLifecycleEventObserver && isFullLifecycleObserver) {
        return new FullLifecycleObserverAdapter((FullLifecycleObserver) object,
                (LifecycleEventObserver) object);
    }

    // 如果 object 是 FullLifecycleObserver, 则返回 FullLifecycleObserverAdapter
    if (isFullLifecycleObserver) {
        return new FullLifecycleObserverAdapter((FullLifecycleObserver) object, null);
    }

    // 如果 object 是 LifecycleEventObserver，则返回 LifecycleEventObserver
    if (isLifecycleEventObserver) {
        return (LifecycleEventObserver) object;
    }

    final Class<?> klass = object.getClass();
    // 判断 Observer 构造器类型，有两个返回值 GENERATED_CALLBACK(注解生成器类型) 和 REFLECTIVE_CALLBACK (反射类型)
    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);
    }
    // 基于反射调用的 LifecycleEventObserver
    return new ReflectiveGenericLifecycleObserver(object);
}

发送 Event 事件：
继承关系:
AppCompatActivity -> FragmentActivity -> androidx.fragment.app.ComponentActivity

androidx.fragment.app.ComponentActivity#onCreate()

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protected void onCreate(@Nullable Bundle savedInstanceState) {
    super.onCreate(savedInstanceState);
    mSavedStateRegistryController.performRestore(savedInstanceState);
    // 6. 在必要情况下，注入一个 ReportFragment
    ReportFragment.injectIfNeededIn(this);
    if (mContentLayoutId != 0) {
        setContentView(mContentLayoutId);
    }
}

ReportFragment#injectIfNeededIn(this)

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public class ReportFragment extends android.app.Fragment {
    public static void injectIfNeededIn(Activity activity) {
        if (Build.VERSION.SDK_INT >= 29) {
            // API 29+，即 Android 10 及以上，通过 activity 注册 ActivityLifecycleCallbacks 来实现对 Activity 生命周期的监听
            activity.registerActivityLifecycleCallbacks(
                    new LifecycleCallbacks());
        }

        // 在当前 Activity 中生成一个空的 ReportFragment, 通过 ReportFragment 的生命周期事件来对 Lifecycle.Event 事件进行分发
        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();
        }
    }

    // API 29+ 
    static class LifecycleCallbacks implements Application.ActivityLifecycleCallbacks {

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

        //... 其他方法实现省略
    }

    static void dispatch(@NonNull Activity activity, @NonNull Lifecycle.Event event) {
        // 兼容之前 android.support.v7.app.AppCompatActivity
        if (activity instanceof LifecycleRegistryOwner) {
            ((LifecycleRegistryOwner) activity).getLifecycle().handleLifecycleEvent(event);
            return;
        }

        if (activity instanceof LifecycleOwner) {
            Lifecycle lifecycle = ((LifecycleOwner) activity).getLifecycle();
            if (lifecycle instanceof LifecycleRegistry) {
                // 7. 调用 LifecycleRegistry#handleLifecycleEvent() 对 Lifecycle.Event 事件进行分发
                ((LifecycleRegistry) lifecycle).handleLifecycleEvent(event);
            }
        }
    }

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

    private void dispatch(@NonNull Lifecycle.Event event) {
        if (Build.VERSION.SDK_INT < 29) {
            // API 29+ 已经通过 registerActivityLifecycleCallbacks 的方式实现了对 Activity 生命周期的监听
            dispatch(getActivity(), event);
        }
    }
}

LifecycleRegistry#handleLifecycleEvent() 对 Lifecyle.Event 事件进行分发:

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public void handleLifecycleEvent(@NonNull Lifecycle.Event event) {
    State next = getStateAfter(event);
    // 将当前 LifecycleOwner 中的 Observer 列表中的状态移至指定状态
    moveToState(next);
}

// 事件(Event) 的触发，就会引起 状态(State) 的变化
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;
    }
    throw new IllegalArgumentException("Unexpected event value " + event);
}

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;
}

LifecycleRegistry#sync() 同步 Observer 队列的状态

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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.");
    }
    // 当队列的状态没有同步
    while (!isSynced()) {
        // 则需要进行同步操作
        mNewEventOccurred = false;
        
        // 如果目标状态 小于 队首 Observer 的状态，则向后同步
        if (mState.compareTo(mObserverMap.eldest().getValue().mState) < 0) {
            backwardPass(lifecycleOwner);
        }

        // 如果目标状态 大于 队尾 Observer 的状态，则向前同步
        Entry<LifecycleObserver, ObserverWithState> newest = mObserverMap.newest();
        if (!mNewEventOccurred && newest != null
                && mState.compareTo(newest.getValue().mState) > 0) {
            forwardPass(lifecycleOwner);
        }

        // 举个例子: 如果队列状态是 [4, 4, 3, 2, 1]，目标状态是 3，经过 backwardPass()，状态队列变为 [3, 3, 3, 2, 1]，
        // 经过 forwardPass() 状态队列会变成 [3, 3, 3, 3, 3]，此时同步完成，isSynced() 为 true，跳出循环
    }
    mNewEventOccurred = false;
}

// 如果队尾 和 队首的状态与目标状态保持一致，则说明已经同步完成；否则同步未完成。
private boolean isSynced() {
    if (mObserverMap.size() == 0) {
        return true;
    }
    State eldestObserverState = mObserverMap.eldest().getValue().mState;
    State newestObserverState = mObserverMap.newest().getValue().mState;
    return eldestObserverState == newestObserverState && mState == newestObserverState;
}

private void backwardPass(LifecycleOwner lifecycleOwner) {
    // 从后往前遍历的 Iterator
    Iterator<Entry<LifecycleObserver, ObserverWithState>> descendingIterator =
            mObserverMap.descendingIterator();
    while (descendingIterator.hasNext() && !mNewEventOccurred) {
        Entry<LifecycleObserver, ObserverWithState> entry = descendingIterator.next();
        ObserverWithState observer = entry.getValue();
        // 当 observer 当前 state 值大于目标 state，且没有新事件触发，则触发循环
        while ((observer.mState.compareTo(mState) > 0 && !mNewEventOccurred
                && mObserverMap.contains(entry.getKey()))) {
            // 通过 downEvent() 将 observer 当前的 状态 向下转换成对应的 Event
            Event event = downEvent(observer.mState);
            pushParentState(getStateAfter(event));
            // 分发 Event 事件
            observer.dispatchEvent(lifecycleOwner, event);
            popParentState();
        }
    }
}

private void forwardPass(LifecycleOwner lifecycleOwner) {
    // 从前往后遍历
    Iterator<Entry<LifecycleObserver, ObserverWithState>> ascendingIterator =
            mObserverMap.iteratorWithAdditions();
    while (ascendingIterator.hasNext() && !mNewEventOccurred) {
        Entry<LifecycleObserver, ObserverWithState> entry = ascendingIterator.next();
        ObserverWithState observer = entry.getValue();
        while ((observer.mState.compareTo(mState) < 0 && !mNewEventOccurred
                && mObserverMap.contains(entry.getKey()))) {
            pushParentState(observer.mState);
            // 通过 upEvent() 将 observer 当前的 状态 向上转换成对应的 Event
            // 并发送该 Event 事件
            observer.dispatchEvent(lifecycleOwner, upEvent(observer.mState));
            popParentState();
        }
    }
}

ObserverWithState#dispatchEvent :

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static class ObserverWithState {
    void dispatchEvent(LifecycleOwner owner, Event event) {
        // 获取事件发生之后对应的状态
        State newState = getStateAfter(event);
        mState = min(mState, newState);
        // 发送指定事件
        mLifecycleObserver.onStateChanged(owner, event);
        mState = newState;
    }
}

这里的 mLifecycleObserver 即我们之前分析的 Lifecycling.lifecycleEventObserver() 返回的 Observer