Android Architecture Components 之Lifecycle-Aware 的源码及trick

用户1127566

发布于 2018-06-04 18:03:02

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发布于 2018-06-04 18:03:02

文章被收录于专栏：Android 开发学习Android 开发学习

一背景

google的Architecture Components Components(lz 简写AAC)出来好久了，但一直没时间阅读源码，趁最近空挡，阅读了AAC的源码，分享下阅读的理解。

二 AAC是个什么东西

其实，AAC就是google提出的一种app开发框架，里面最基础的应该就是Lifecycle-Aware了。在这之前，我们组开发采用的mvvm + data binding 模式。这个模式中，我们常常这样一个需求，我们需要在fragment onDestroy 或 onPause时，反注册ViewModel中释放资源，需要层层经过， ViewModel -- > adapter--> fragment ViewModel ——> fragment等几级回调。而Lifecycle-Aware 这时就有一个优势，就是跟生命周期绑定，直接在相应生命周期逻辑处理好就好。当然，这里也是一种解耦方式，采用观察者模式实现。

三 Lifecycle-Aware 的源码

观察者模式

a 首先找使用入口，一般我们是这样使用Lifecycle-Aware的，

Lifecycle lifecycle = lifecycleOwner.getLifecycle();
        lifecycle.addObserver((GenericLifecycleObserver) (source, event) -> {
           
        });

b lifecycle的addObserver方法，如下：

@Override
    public void addObserver(LifecycleObserver observer) {
        State initialState = mState == DESTROYED ? DESTROYED : INITIALIZED;
        ObserverWithState statefulObserver = new ObserverWithState(observer, initialState);
        ObserverWithState previous = mObserverMap.putIfAbsent(observer, statefulObserver);

      //已添加
        if (previous != null) {
            return;
        }

       //第二次重入
        boolean isReentrance = mAddingObserverCounter != 0 || mHandlingEvent;

        State targetState = calculateTargetState(observer);
        mAddingObserverCounter++;
        while ((statefulObserver.mState.compareTo(targetState) < 0
                && mObserverMap.contains(observer))) {
            pushParentState(statefulObserver.mState);
            statefulObserver.dispatchEvent(mLifecycleOwner, 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--;
    }

statefulObserver.dispatchEvent(mLifecycleOwner, upEvent(statefulObserver.mState))一句就会调用 mLifecycleObserver.onStateChanged(owner, event);即我们上述入口的GenericLifecycleObserver的 void onStateChanged(LifecycleOwner source, Lifecycle.Event event)方法。

c 特别说明下，ObserverWithState statefulObserver = new ObserverWithState(observer, initialState);

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

d 包装Observer为 GenericLifecycleObserver。这里我们注意到，使用了反射。同事在使用时，遇到一个bug就是正式包会crash，原因就是此版本的混淆文件没对Lifecycling类进行keep。

static GenericLifecycleObserver getCallback(Object object) {
        if (object instanceof GenericLifecycleObserver) {
            return (GenericLifecycleObserver) object;
        }
        //noinspection TryWithIdenticalCatches
        try {
            final Class<?> klass = object.getClass();
            Constructor<? extends GenericLifecycleObserver> cachedConstructor = sCallbackCache.get(
                    klass);
            if (cachedConstructor != null) {
                return cachedConstructor.newInstance(object);
            }
            cachedConstructor = getGeneratedAdapterConstructor(klass);
            if (cachedConstructor != null) {
                if (!cachedConstructor.isAccessible()) {
                    cachedConstructor.setAccessible(true);
                }
            } else {
                cachedConstructor = sREFLECTIVE;
            }
            sCallbackCache.put(klass, cachedConstructor);
            return cachedConstructor.newInstance(object);
        } catch (IllegalAccessException e) {
            throw new RuntimeException(e);
        } catch (InstantiationException e) {
            throw new RuntimeException(e);
        } catch (InvocationTargetException e) {
            throw new RuntimeException(e);
        }
    }

e 另外， sync()方法如下：

// happens only on the top of stack (never in reentrance),
    // so it doesn't have to take in account parents
    private void sync() {
        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();
            }
            Entry<LifecycleObserver, ObserverWithState> newest = mObserverMap.newest();
            if (!mNewEventOccurred && newest != null
                    && mState.compareTo(newest.getValue().mState) > 0) {
                forwardPass();
            }
        }
        mNewEventOccurred = false;
    }

f 如果处于初始化周期，onResume及以前forwardPass()，正向回调之前的生命周期。 onPause及以后，逆向回调backwardPass()。

这时，特别数据结构FastSafeIterableMap就比较优势，支持正逆向遍历。如果当前状态小于以前状态，正向回调，否则逆向回调。这里处理与fragment生命周期状态类似。状态机，贴一下官方盗图：

image.png

trick 1 无ui fragment 绑定命周期

a 目前Android sdk 26.1.0 已支持了 Lifecycle-Aware。我们先选取Activity 来看。Lifecycle-Aware的初始化逻辑在SupportActivity ， BaseFragmentActivityApi14 extends SupportActivity，可以看出最低版本是sdk 14 ，最后继承的是我们最熟悉的子类是FragmentActivity extends BaseFragmentActivityApi16。

image.png

b 这里是整个AAC常见的套路，使用无ui fragment来同步activity的生命周期。具体ReportFragment inject的代码如下。

 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();
        }
    }

c 代码比较简单在create等回调，dispatch对应的事件。

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

给对应实现了的LifecycleRegistryOwner或LifecycleOwner activity回调

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);
            }
        }
    }

 public void handleLifecycleEvent(Lifecycle.Event event) {
        mState = getStateAfter(event);
        if (mHandlingEvent || mAddingObserverCounter != 0) {
            mNewEventOccurred = true;
            // we will figure out what to do on upper level.
            return;
        }
        mHandlingEvent = true;
        sync();
        mHandlingEvent = false;
    }

这时会去同步一遍状态

sample trick 2

另外一个小的点是，在BaseSample中使用ContentProvider onCreate中绑定activity和fragment生命周期，这点就不详述了。

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

image.png

继续盗图，左边部分就是目前的回调。

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原始发表：2018.04.09 ，如有侵权请联系 cloudcommunity@tencent.com 删除

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