深入了解架构组件之 Lifecycle

官方文档连接（需翻墙）

一、Lifecycle 简介

Google 最新推荐的 Lifecycle 架构是什么呢，简单来说，就是可以让你自己的类拥有像 activity 或 fragment 一样生命周期的功能。
为什么要这样呢？直接利用 activity 或 fragment 不行吗？当然可以，但Lifecycle肯定是解决了某些痛点的。
比如：我们现在要实现定位回调的功能，通常我们这样做：

class MyLocationListener {
    public MyLocationListener(Context context, Callback callback) {
        // ...
    }

    void start() {
        // connect to system location service
    }

    void stop() {
        // disconnect from system location service
    }
}

class MyActivity extends AppCompatActivity {
    private MyLocationListener myLocationListener;

    @Override
    public void onCreate(...) {
        myLocationListener = new MyLocationListener(this, (location) -> {
            // update UI
        });
    }

    @Override
    public void onStart() {
        super.onStart();
        myLocationListener.start();
        // manage other components that need to respond
        // to the activity lifecycle
    }

    @Override
    public void onStop() {
        super.onStop();
        myLocationListener.stop();
        // manage other components that need to respond
        // to the activity lifecycle
    }
}

看起来似乎是很好，但在实际开发过程中还是要考虑很多东西的。随着业务的复杂 生命周期方法会变的臃肿难以维护。而且某些耗时的异步操作，也很难保证它会规规矩矩的按生命周期顺序执行。
所以，今天的主角就出现了。继承Lifecycle 的组件将生命周期脱离出 activity 而转到自己身上完美的解决了以上问题。

二、Lifecycle 使用

1、添加观察者

这里有两种方式：java7的方式和 java8，java7为通过注解的方式，而 java8不需要注解。官方推荐是 java8，理由是随着 android java8的普及要逐渐废弃注解的方式。
java7：

public class MyObserver implements LifecycleObserver {
    @OnLifecycleEvent(Lifecycle.Event.ON_RESUME)
    public void connectListener() {
        ...
    }

    @OnLifecycleEvent(Lifecycle.Event.ON_PAUSE)
    public void disconnectListener() {
        ...
    }
}

myLifecycleOwner.getLifecycle().addObserver(new MyObserver());

直接实现LifecycleObserver，然后通过注解即可生命周期实现回调。
java8：

class TestObserver implements DefaultLifecycleObserver {
     @Override
     public void onCreate(LifecycleOwner owner) {
         // your code
     }
 }

实现的是DefaultLifecycleObserver（LifecycleObserver的子类），然后重写其生命周期方法即可。
最后同过LifecycleOwner，添加观察。什么是LifecycleOwner往下看

2、LifecycleOwner

LifecycleOwner是只有一个方法getLifecycle()的接口，是让拥有生命周期的东西实现比如（activity）用来获取Lifecycle。（大白话）
在Android Support Library 26.1.0 及其之后已经activity 和 fragment 已经默认实现了LifecycleOwner。
所以在 activity 里我们可以直接：

getLifecycle().addObserver(new MyObserver());

ok 这样我们的MyObserver就会感知 activity 的生命周期了。

三、原理分析

1、Lifecycle 如何感知 activity 或 fragment 生命周期

既然activity 和 fragment 已经实现了 Lifecycle 架构。就先来看看如何感知的。
首先找到 activity 实现LifecycleOwner的地方SupportActivity：

public class SupportActivity extends Activity implements LifecycleOwner {
    /**
     * Storage for {@link ExtraData} instances.
     *
     * <p>Note that these objects are not retained across configuration changes</p>
     */
    private SimpleArrayMap<Class<? extends ExtraData>, ExtraData> mExtraDataMap =
            new SimpleArrayMap<>();

    private LifecycleRegistry mLifecycleRegistry = new LifecycleRegistry(this);

    /**
     * Store an instance of {@link ExtraData} for later retrieval by class name
     * via {@link #getExtraData}.
     *
     * <p>Note that these objects are not retained across configuration changes</p>
     *
     * @see #getExtraData
     * @hide
     */
    @RestrictTo(LIBRARY_GROUP)
    public void putExtraData(ExtraData extraData) {
        mExtraDataMap.put(extraData.getClass(), extraData);
    }

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

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

    /**
     * Retrieves a previously set {@link ExtraData} by class name.
     *
     * @see #putExtraData
     * @hide
     */
    @RestrictTo(LIBRARY_GROUP)
    public <T extends ExtraData> T getExtraData(Class<T> extraDataClass) {
        return (T) mExtraDataMap.get(extraDataClass);
    }

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

    /**
     * @hide
     */
    @RestrictTo(LIBRARY_GROUP)
    public static class ExtraData {
    }
}

很简单，出现了一个LifecycleRegistry，是 Lifecycle 的一个实现类。通过markState方法在onSaveInstanceState把 Lifecycle 状态标记为Lifecycle.State.CREATED。
然后，就没有然后了，我原想着 会在 Activity 生命周期方法里改变Lifecycle的各种状态，然后一路寻找到笑容逐渐缺失，也没找到。卡了好大一阵后 我忽然发现 onCreate 方法里有个ReportFragment，点进去看了下才发现了新世界。

public class ReportFragment extends Fragment {
    public static void injectIfNeededIn(Activity activity) {
        // ProcessLifecycleOwner should always correctly work and some activities may not extend
        // FragmentActivity from support lib, so we use framework fragments for activities
        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();
        }
    }
    ……
    @Override
    public void onActivityCreated(Bundle savedInstanceState) {
        super.onActivityCreated(savedInstanceState);
        dispatchCreate(mProcessListener);
        dispatch(Lifecycle.Event.ON_CREATE);
    }

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

    @Override
    public void onResume() {
        super.onResume();
        dispatchResume(mProcessListener);
        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);
        // just want to be sure that we won't leak reference to an activity
        mProcessListener = null;
    }

    private void dispatch(Lifecycle.Event event) {
        Activity activity = getActivity();
        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);
            }
        }
    }
}

原来 Google 是利用 fragment 的特性，绑定了一个 fragment 然后在其生命周期方法中调用了
LifecycleRegistry的handleLifecycleEvent，此方法便是通知观察者的地方。

另外需注意 Lifecycle.State.CREATED状态，在 FragmentActivity 中 也有递归的调用：

    @Override
    protected void onSaveInstanceState(Bundle outState) {
        super.onSaveInstanceState(outState);
        markState(getSupportFragmentManager(), Lifecycle.State.CREATED);
         ……
    }
    private static void markState(FragmentManager manager, Lifecycle.State state) {
        Collection<Fragment> fragments = manager.getFragments();
        for (Fragment fragment : fragments) {
            if (fragment == null) {
                continue;
            }
            fragment.mLifecycleRegistry.markState(state);
            markState(fragment.getChildFragmentManager(), state);
        }
    }

Lifecycle 会在 自身构造方法中把自己标记为INITIALIZED

    public LifecycleRegistry(@NonNull LifecycleOwner provider) {
        mLifecycleOwner = new WeakReference<>(provider);
        mState = INITIALIZED;
    }

2、如何通知观察者

先看addObserver

     private FastSafeIterableMap<LifecycleObserver, ObserverWithState> mObserverMap =
            new FastSafeIterableMap<>();

    public void addObserver(@NonNull LifecycleObserver observer) {
        State initialState = mState == DESTROYED ? DESTROYED : INITIALIZED;
        ObserverWithState statefulObserver = new ObserverWithState(observer, initialState);
        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++;
        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.
            sync();
        }
        mAddingObserverCounter--;
    }

感觉跟 livedata 那一套差不多，把observer维护到ObserverWithState然后装到 map 里。

然后通过handleLifecycleEvent方法最终遍历map 通知 observer。

    public void handleLifecycleEvent(@NonNull Lifecycle.Event event) {
        State next = getStateAfter(event);
        moveToState(next);
    }
    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;
    }

    private void sync() {
        LifecycleOwner lifecycleOwner = mLifecycleOwner.get();
        if (lifecycleOwner == null) {
            Log.w(LOG_TAG, "LifecycleOwner is garbage collected, you shouldn't try dispatch "
                    + "new events from it.");
            return;
        }
        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) {
                backwardPass(lifecycleOwner);
            }
            Entry<LifecycleObserver, ObserverWithState> newest = mObserverMap.newest();
            if (!mNewEventOccurred && newest != null
                    && mState.compareTo(newest.getValue().mState) > 0) {
                forwardPass(lifecycleOwner);
            }
        }
        mNewEventOccurred = false;
    }

调用了forwardPass或backwardPass随便看一个：

    private void forwardPass(LifecycleOwner lifecycleOwner) {
        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);
                observer.dispatchEvent(lifecycleOwner, upEvent(observer.mState));
                popParentState();
            }
        }
    }

循环所有保存的观察者，然后调用ObserverWithState的dispatchEvent

    static class ObserverWithState {
        State mState;
        GenericLifecycleObserver mLifecycleObserver;

        ObserverWithState(LifecycleObserver observer, State initialState) {
            mLifecycleObserver = Lifecycling.getCallback(observer);
            mState = initialState;
        }

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

还有比较重要的一点，ObserverWithState并不是简单的保存 observer 而是通过Lifecycling.getCallback(observer):

   static GenericLifecycleObserver getCallback(Object object) {
        if (object instanceof FullLifecycleObserver) {
            return new FullLifecycleObserverAdapter((FullLifecycleObserver) object);
        }

        if (object instanceof GenericLifecycleObserver) {
            return (GenericLifecycleObserver) object;
        }

        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);
    }

从而做到了，区分不同 observer 类型（比如前面提到的 java7与java8不同）。

四、总结

SupportActivity 实现了LifecycleOwner并通过 ReportFragment生命周期实现Lifecycle 生命周期的感知。

还是有点费解的地方，后来发现还有一种通过 ContentProvider （具体是这个ProcessLifecycleOwnerInitializer）然后用一个LifecycleDispatcher也同样实现了一套ReportFragment那些逻辑。初步猜测是为了不继承 SupportActivity 的 Activity 使用？还不太确定。

转自：https://www.jianshu.com/p/b6fff29ab498