Lifecycle

作者: Android_Gleam | 来源:发表于2022-01-23 14:32 被阅读0次

    首先们要明确Lifecycle中两个枚举值,这两个枚举值和生命周期的状态有着直接关系,我们可以先记住,后面会看到使用的地方

    public enum Event {
         
            ON_CREATE,
           
            ON_START,
           
            ON_RESUME,
          
            ON_PAUSE,
           
            ON_STOP,
          
            ON_DESTROY,
           
            ON_ANY;
      //与state的对应关系
      public State getTargetState() {
                switch (this) {
                    //这里看到ON_CREATE和ON_STOP都对应State.CREATED ,在回退生命周期的时候就会用到
                    case ON_CREATE:    
                    case ON_STOP:
                        return State.CREATED;
                    case ON_START:
                    case ON_PAUSE:
                        return State.STARTED;
                    case ON_RESUME:
                        return State.RESUMED;
                    case ON_DESTROY:
                        return State.DESTROYED;
                    case ON_ANY:
                        break;
                }
                throw new IllegalArgumentException(this + " has no target state");
            }
      ...
    }
    
     public enum State {
         
            DESTROYED,
    
            INITIALIZED,
    
            CREATED,
    
            STARTED,
    
            RESUMED;
    
            public boolean isAtLeast(@NonNull State state) {
                return compareTo(state) >= 0;
            }
        }
    

    下面我们先看下,我们日常开发中是如何使用Lifecycle来监听声明周期的,首先我们需要创建一个监听器

    //实现LifecycleObserver接口
    class MyLifecycleObserver : LifecycleObserver {
        
        //必须声明对应生命周期的注解 才会回调对应方法 以下同理
        @OnLifecycleEvent(Lifecycle.Event.ON_CREATE)
        fun onCreate() {}
    
        @OnLifecycleEvent(Lifecycle.Event.ON_START)
        fun onStart() {}
    
        @OnLifecycleEvent(Lifecycle.Event.ON_RESUME)
        fun onResume() {}
    
        @OnLifecycleEvent(Lifecycle.Event.ON_PAUSE)
        fun onPause() {}
    
        @OnLifecycleEvent(Lifecycle.Event.ON_STOP)
        fun onStop() {}
    
        @OnLifecycleEvent(Lifecycle.Event.ON_DESTROY)
        fun onDestroy() {}
    }
    

    然后在Activity中通过以下代码添加监听

    //Activity
     override fun onCreate(savedInstanceState: Bundle?) {
            super.onCreate(savedInstanceState)
            val observer = MyLifecycleObserver()
            lifecycle.addObserver(observer)
        }
    

    当Activity执行到相应的生命周期方法的时候就会回调监听中定义的方法。那具体的实现是怎么做到的呢?首先我们看下lifecycle,定义在ComponentActivity中

    //持有了Activity对象
    private final LifecycleRegistry mLifecycleRegistry = new LifecycleRegistry(this);
    
    //enforceMainThread默认true
    private LifecycleRegistry(@NonNull LifecycleOwner provider, boolean enforceMainThread) {
            //Activity对象 用软引用存储
            mLifecycleOwner = new WeakReference<>(provider);
            mState = INITIALIZED;
            mEnforceMainThread = enforceMainThread;
        }
    
     public Lifecycle getLifecycle() {
            return mLifecycleRegistry;
        }
    

    我们发现lifecycle其实是一个LifecycleRegistry对象,addObserver调用的也是其中的方法,继续看代码

     @Override
        public void addObserver(@NonNull LifecycleObserver observer) {
            //检查是否是主线程,也就是说我们添加监听只能在主线程中调用
            enforceMainThreadIfNeeded("addObserver");
            //初始状态INITIALIZED
            State initialState = mState == DESTROYED ? DESTROYED : INITIALIZED;
            //注意这个对象 可以把它看成一个监听对象
            ObserverWithState statefulObserver = new ObserverWithState(observer, initialState);
            //存储到mObserverMap中 因为可能存在多个监听 这里做处理
            ObserverWithState previous = mObserverMap.putIfAbsent(observer, statefulObserver);
            //如果注册过 直接返回
            if (previous != null) {
                return;
            }
            //如果Activity对象为null直接返回
            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);
                final Event event = Event.upFrom(statefulObserver.mState);
                if (event == null) {
                    throw new IllegalStateException("no event up from " + statefulObserver.mState);
                }
                statefulObserver.dispatchEvent(lifecycleOwner, event);
                popParentState();
                // mState / subling may have been changed recalculate
                targetState = calculateTargetState(observer);
            }
    
            if (!isReentrance) {
                // we do sync only on the top level.
                sync();
            }
            mAddingObserverCounter--;
        }
    

    下面看下ObserverWithState这个对象的初始化

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

    继续Lifecycling

    static LifecycleEventObserver lifecycleEventObserver(Object object) {
            boolean isLifecycleEventObserver = object instanceof LifecycleEventObserver;
            boolean isFullLifecycleObserver = object instanceof FullLifecycleObserver;
            if (isLifecycleEventObserver && isFullLifecycleObserver) {
                return new FullLifecycleObserverAdapter((FullLifecycleObserver) object,
                        (LifecycleEventObserver) object);
            }
            if (isFullLifecycleObserver) {
                return new FullLifecycleObserverAdapter((FullLifecycleObserver) object, null);
            }
    
            if (isLifecycleEventObserver) {
                return (LifecycleEventObserver) object;
            }
            
            //获取class对象
            final Class<?> klass = object.getClass();
            int type = getObserverConstructorType(klass);
            //这里应该是回退的声明周期方法 先不管这里 有兴趣可以自己看看 我们目前只分析正常的流程
            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);
            }
            //正常添加监听 走到这里
            return new ReflectiveGenericLifecycleObserver(object);
        }
    

    继续看ReflectiveGenericLifecycleObserver

    class ReflectiveGenericLifecycleObserver implements LifecycleEventObserver {
        private final Object mWrapped;
        private final CallbackInfo mInfo;
    
        ReflectiveGenericLifecycleObserver(Object wrapped) {
            mWrapped = wrapped;
            //解析监听器对象信息(我们声明的对应生命周期注解的方法) 存储到mInfo中,具体解析流程不做分析 可自行查看源码
            mInfo = ClassesInfoCache.sInstance.getInfo(mWrapped.getClass());
        }
    
        @Override
        public void onStateChanged(@NonNull LifecycleOwner source, @NonNull Event event) {
            //生命周期改变时候调用该方法 event为对应的生命周期
            mInfo.invokeCallbacks(source, event, mWrapped);
        }
    }
    

    目前为止做的工作就是,把我们声明的监听器的信息进行解析并存储了起来。
    监听已经注册好了,那么下来就要看看当 生命周期发生改变的时候,是怎么进行分发的了,我们查看ComponentActivity的onCreate()

    protected void onCreate(@Nullable Bundle savedInstanceState) {
            // Restore the Saved State first so that it is available to
            // OnContextAvailableListener instances
            mSavedStateRegistryController.performRestore(savedInstanceState);
            mContextAwareHelper.dispatchOnContextAvailable(this);
            super.onCreate(savedInstanceState);
            //关键点就是这里
            ReportFragment.injectIfNeededIn(this);
            if (mContentLayoutId != 0) {
                setContentView(mContentLayoutId);
            }
        }
    

    下面看ReportFragment

    public static void injectIfNeededIn(Activity activity) {
            //版本判断进行teshu 处理
            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
            //声明了一个没有UI页面的Fragment
            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();
            }
        }
    

    Glide大家应该都用过,对于生命周期的感知就是通过添加Fragment的方式来完成的,这里其实也是一样的,我们看下29及以后的处理

    @RequiresApi(29)
        static class LifecycleCallbacks implements Application.ActivityLifecycleCallbacks {
    
            static void registerIn(Activity activity) {
                //直接注册了回调 29以前应该是没有吧 没有去深究 
                activity.registerActivityLifecycleCallbacks(new LifecycleCallbacks());
            }
    
            @Override
            public void onActivityCreated(@NonNull Activity activity,
                    @Nullable Bundle bundle) {
            }
    
            @Override
            public void onActivityPostCreated(@NonNull Activity activity,
                    @Nullable Bundle savedInstanceState) {
                //直接进行分发 下面再看dispatch的具体实现
                dispatch(activity, Lifecycle.Event.ON_CREATE);
            }
    
            @Override
            public void onActivityStarted(@NonNull Activity activity) {
            }
    
            @Override
            public void onActivityPostStarted(@NonNull Activity activity) {
                dispatch(activity, Lifecycle.Event.ON_START);
            }
    
            @Override
            public void onActivityResumed(@NonNull Activity activity) {
            }
    
            @Override
            public void onActivityPostResumed(@NonNull Activity activity) {
                dispatch(activity, Lifecycle.Event.ON_RESUME);
            }
    
            @Override
            public void onActivityPrePaused(@NonNull Activity activity) {
                dispatch(activity, Lifecycle.Event.ON_PAUSE);
            }
    
            @Override
            public void onActivityPaused(@NonNull Activity activity) {
            }
    
            @Override
            public void onActivityPreStopped(@NonNull Activity activity) {
                dispatch(activity, Lifecycle.Event.ON_STOP);
            }
    
            @Override
            public void onActivityStopped(@NonNull Activity activity) {
            }
    
            @Override
            public void onActivitySaveInstanceState(@NonNull Activity activity,
                    @NonNull Bundle bundle) {
            }
    
            @Override
            public void onActivityPreDestroyed(@NonNull Activity activity) {
                dispatch(activity, Lifecycle.Event.ON_DESTROY);
            }
    
            @Override
            public void onActivityDestroyed(@NonNull Activity activity) {
            }
        }
    

    既然是通过Fragment进行分发,那么显然我们直接看ReportFragment的生命周期就好了,我们以onResume为例,其余都是一样的处理。

    public void onResume() {
            super.onResume();
            dispatchResume(mProcessListener);
            //上面我们看29及之后的处理的时候 也看到该方法的调用了,这就是分发的方法,参数是对应的生命周期枚举
            dispatch(Lifecycle.Event.ON_RESUME);
        }
    

    下面看下diapatch的实现

    private void dispatch(@NonNull Lifecycle.Event event) {
            //再次判断版本 
            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);
            }
        }
    
     static void dispatch(@NonNull Activity activity, @NonNull Lifecycle.Event event) {
            //这里已经弃用 我们不用关心
            if (activity instanceof LifecycleRegistryOwner) {
                ((LifecycleRegistryOwner) activity).getLifecycle().handleLifecycleEvent(event);
                return;
            }
    
            if (activity instanceof LifecycleOwner) {
                //获取LifecycleRegistry对象 我们上面添加监听的时候就是通过LifecycleRegistry添加的
                Lifecycle lifecycle = ((LifecycleOwner) activity).getLifecycle();
                if (lifecycle instanceof LifecycleRegistry) {
                    ((LifecycleRegistry) lifecycle).handleLifecycleEvent(event);
                }
            }
        }
    

    然后我们看LifecycleRegistry的handleLifecycleEvent方法

        public void handleLifecycleEvent(@NonNull Lifecycle.Event event) {
            //主线程判断
            enforceMainThreadIfNeeded("handleLifecycleEvent");
            //注意这里这里是根据Event来判断当前state状态的 这里可以看最开始我们提到的枚举值
            moveToState(event.getTargetState());
        }
        //生命周期状态的处理
        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) {
                throw new IllegalStateException("LifecycleOwner of this LifecycleRegistry is already"
                        + "garbage collected. It is too late to change lifecycle state.");
            }
            //是否完成同步 如果完成同步 不进行分发
            while (!isSynced()) {
                mNewEventOccurred = false;
                // no need to check eldest for nullability, because isSynced does it for us.
                //如果当前状态值小于监听器状态值 则说明是销毁流程 
                //比如我们目前是onResume 走到了onPause State.STARTED < State.RESUMED
                if (mState.compareTo(mObserverMap.eldest().getValue().mState) < 0) {    
                    //回退生命周期 销毁时调用
                    backwardPass(lifecycleOwner);
                }
                Map.Entry<LifecycleObserver, ObserverWithState> newest = mObserverMap.newest();
                if (!mNewEventOccurred && newest != null
                        && mState.compareTo(newest.getValue().mState) > 0) {      
                    //正常生命周期 显示时调用
                    forwardPass(lifecycleOwner);
                }
            }
            mNewEventOccurred = false;
        }
    
    private void forwardPass(LifecycleOwner lifecycleOwner) {
            Iterator<Map.Entry<LifecycleObserver, ObserverWithState>> ascendingIterator =
                    mObserverMap.iteratorWithAdditions();
            //遍历监听者
            while (ascendingIterator.hasNext() && !mNewEventOccurred) {
                Map.Entry<LifecycleObserver, ObserverWithState> entry = ascendingIterator.next();
                ObserverWithState observer = entry.getValue();
                while ((observer.mState.compareTo(mState) < 0 && !mNewEventOccurred
                        && mObserverMap.contains(entry.getKey()))) {
                    pushParentState(observer.mState);
                    final Event event = Event.upFrom(observer.mState);
                    if (event == null) {
                        throw new IllegalStateException("no event up from " + observer.mState);
                    }
                    //进行分发 observer是ObserverWithState对象 我们上面注册时候 注册的就是这个
                    observer.dispatchEvent(lifecycleOwner, event);
                    popParentState();
                }
            }
        }
    

    然后我们再看下ObserverWithState

    static class ObserverWithState {
            State mState;
            LifecycleEventObserver mLifecycleObserver;
    
            ObserverWithState(LifecycleObserver observer, State initialState) {
                //ReflectiveGenericLifecycleObserver 对象上面注册时候说过
                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;
            }
        }
    

    然后看ReflectiveGenericLifecycleObserver

     @Override
        public void onStateChanged(@NonNull LifecycleOwner source, @NonNull Event event) {
            //上面说过mInfo 存储了监听器的信息 我们解析的注解方法都在这里
            mInfo.invokeCallbacks(source, event, mWrapped);
        }
    

    继续看CallbackInfo

     void invokeCallbacks(LifecycleOwner source, Lifecycle.Event event, Object target) {
                //event 通过Event找到对应的方法
                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) {
                   //调用执行对应方法 这时我们的监听器声明的对应注解方法就被调用了
                    for (int i = handlers.size() - 1; i >= 0; i--) {
                        handlers.get(i).invokeCallback(source, event, mWrapped);
                    }
                }
            }
    

    到这里我们就已经分析完Lifecycle生命周期的分发流程了,总结一下其实很简单:
    解析监听器中声明的对应生命周期的注解方法存储起来,在Activity生命周期改变的时候获取对应的注解方法进行调用。
    细节点

    • 生命周期分发
      29以前是通过添加空的Frgament,然后在Fragment中进行分发;
      29之后是通过LifecycleCallbacks直接进行分发。
    • State判断
      不同的Event枚举对应相同的State值,在onResume后复用之前的状态来节省判断变量,达到生命周期正序和逆序的处理。

    可能有些细节没有讲到,但是整体的思路和处理应该很清晰,如有哪里讲的不对,请留言。

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