Android Jetpack架构组件——Lifecycle原理篇

Posted 2023-02-24 我就是马云飞

概述

前面我们讲到了lifecycle是如何使用的，但是我们单单知道如何使用它还是不够的，我们需要知道它到底是如何绑定生命周期的。那么本篇我们就讲一下lifecycle的原理篇。

本篇基于android 9.0源码进行分析，如有不同，注意sdk版本。

Lifecycle是什么？

官方文档是这么描述的，Lifecycle它是一个类，用来存储相关组件的生命周期状态，如Activity或者Fragment等。并且允许其他组件对这些状态进行观察。

我们先简单看一下Lifecycle这个类的源码：

public abstract class Lifecycle 

    @RestrictTo(RestrictTo.Scope.LIBRARY_GROUP)
    @NonNull
    AtomicReference<Object> mInternalScopeRef = new AtomicReference<>();

    @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
    

    @SuppressWarnings("WeakerAccess")
    public enum State 
        DESTROYED,
        INITIALIZED,
        CREATED,
        STARTED,
        RESUMED;
        public boolean isAtLeast(@NonNull State state) 
            return compareTo(state) >= 0;
        
    

从源码可以看出，Lifecycle主要是定义了2个枚举，一个是Event，它代表着Lifecycle分配的生命周期事件，并且它会映射到activity或者fragment的生命周期回调事件里。还有一个是State，它是指Lifecycle所处的生命周期状态。我们可以通过下图来了解State和Event的关系。

[外链图片转存失败,源站可能有防盗链机制,建议将图片保存下来直接上传(img-3DqVJLcD-1613807661989)(https://cdn.jsdelivr.net/gh/sw950729/img_cdn/img/20210220130924.svg)]

如何与Activity和Fragment建立联系？

我们知道一般来说我们的activity是继承于FragmentActivity的。那么FragmentActivity又是继承于ComponentActivity。那我们看一下ComponentActivity的源码：

@RestrictTo(LIBRARY_GROUP_PREFIX)
public class ComponentActivity extends Activity implements
        LifecycleOwner,
        KeyEventDispatcher.Component 
        
    @SuppressWarnings("deprecation")
    private SimpleArrayMap<Class<? extends ExtraData>, ExtraData> mExtraDataMap =
            new SimpleArrayMap<>();
    
    private LifecycleRegistry mLifecycleRegistry = new LifecycleRegistry(this);

    @SuppressWarnings("deprecation")
    @RestrictTo(LIBRARY_GROUP_PREFIX)
    @Deprecated
    public void putExtraData(ExtraData extraData) 
        mExtraDataMap.put(extraData.getClass(), extraData);
    

    @SuppressLint("RestrictedApi")
    @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);
    
    
    @RestrictTo(LIBRARY_GROUP_PREFIX)
    @SuppressWarnings("unchecked", "deprecation")
    @Deprecated
    public <T extends ExtraData> T getExtraData(Class<T> extraDataClass) 
        return (T) mExtraDataMap.get(extraDataClass);
    

    @NonNull
    @Override
    public Lifecycle getLifecycle() 
        return mLifecycleRegistry;
    
    
    ...

我们可以看到ComponentActivity实现了LifecycleOwner的接口。并且定义了LifecycleRegistry。LifecycleRegistry是Lifecycle的实现类。具体逻辑我们就不看了，有兴趣可以自己关注下。

上述代码中我们可以看到在onSaveInstanceState方法中，我们通过makeState把状态设置成Lifecycle.State.CREATED。那么为什么找不到设置其他状态的逻辑呢？我们可以发现在onCreate方法中有一个inject方法。我们进入此方法，哎，它就找到了。我们看下它的源码：

    ...
    
    static void dispatch(@NonNull Activity activity, @NonNull Lifecycle.Event event) 
        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);
            
        
    
    
    @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(@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);
        
    
    
    ...

我们可以看到在对应的生命周期调用了dispatch方法，而在dispatch方法中又调用了一个多参的dispatch方法。在此方法中会判断activity是属于LifecycleRegistryOwner还是LifecycleRegistry。最终都会调到LifecycleRegistry的handleLifecycleEvent方法中去。

   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;
    
    
    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);
    

我们可以看到它是通过getStateAfter去获取event的状态，具体可以参考一下上面的时序图。而在moveToState方法中它又回调用sync方法：

 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.");
        
        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;
    

这里会通过mObserverMap的eldest和newest拿到的状态做对比，判断是向前还是向后。比如ON_CREATE=>ON_START是向前，ON_START=>ON_CREATE是向后。整体流程是差不多的。具体我们看代码示例：

 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();
            
        
    

    private void backwardPass(LifecycleOwner lifecycleOwner) 
        Iterator<Entry<LifecycleObserver, ObserverWithState>> descendingIterator =
                mObserverMap.descendingIterator();
        while (descendingIterator.hasNext() && !mNewEventOccurred) 
            Entry<LifecycleObserver, ObserverWithState> entry = descendingIterator.next();
            ObserverWithState observer = entry.getValue();
            while ((observer.mState.compareTo(mState) > 0 && !mNewEventOccurred
                    && mObserverMap.contains(entry.getKey()))) 
                Event event = downEvent(observer.mState);
                pushParentState(getStateAfter(event));
                observer.dispatchEvent(lifecycleOwner, event);
                popParentState();
            
        
    

最终它都会走到observer.dispatchEvent(lifecycleOwner, event);方法中。
并且最终会调用mLifecycleObserver.onStateChanged(owner, event);也就是LifecycleEventObserver的方法。我们看基于LifecycleEventObserver的实现类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) 
        mInfo.invokeCallbacks(source, event, mWrapped);
    

在onStateChanged方法中调用了CallbackInfo的invokeCallbacks方法：

    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--) 
                    handlers.get(i).invokeCallback(source, event, mWrapped);
                
            
        
    static class MethodReference 
        final int mCallType;
        final Method mMethod;

        MethodReference(int callType, Method method) 
            mCallType = callType;
            mMethod = method;
            mMethod.setAccessible(true);
        

        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);
            
        
    ...
    
    ...

最后走到的是MethodReference的invokeCallback方法，而MethodReference中有两个变量，一个是callType，它表示调用方法的类型，另一个是Method，表示方法，不管是哪种callType都会通过invoke对方法进行反射。

总结

最后我们以一张调用的时序图来对上述流程做个总结。

参考

官方文档

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