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Android Jetpack系列(三): Lifecycle(

Android Jetpack系列(三): Lifecycle(

作者: 程序老秃子 | 来源:发表于2022-07-29 14:24 被阅读0次

    前言

    在上一篇文章中,我们学习了如何去使用Lifecycle; 当然之会使用是不够的,还需要了解它的原理,这是成为优秀工程师必备的;这篇文章就来学习Lifecycle的基本原理

    1.Lifecycle的生命周期状态事件和状态

    Lifecycle使用两个枚举来跟踪其关联组件的生命周期状态,这两个枚举分别是Event和State;State指的是Lifecycle的生命周期所处的状态;Event代表Lifecycle生命周期对应的事件,这些事件会映射到Activity和Fragment中的回调事件中

    Android 9.0的Lifecycle的源码如下所示

    public abstract class Lifecycle {
    
        @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;
            }
        }
    }
    
    

    Lifecycle是一个抽象类; 其内部不仅包括了添加和移除观察者的方法,还包括了此前说到的Event和State枚举。可以看到Event中的事件和Activity的生命周期几乎是对应的,除了ON_ANY,它可用于匹配所有事件

    2.Lifecycle如何观察Activity和Fragment的生命周期

    在Android Support Library 26.1.0 及其之后的版本,Activity和Fragment已经默认实现了LifecycleOwner接口,LifecycleOwner可以理解为被观察者,那么Lifecycle是如何观察Activity和Fragment的生命周期的呢?

    在上一篇文章举的例子中,MainActivity继承了AppCompatActivity,而AppCompatActivity继承了FragmentActivity。在Android 8.0时,FragmentActivity继承自SupportActivity,而在Android 9.0,FragmentActivity继承自ComponentActivity 。SupportActivity和ComponentActivity的代码区别不大,这里以ComponentActivity举例,如下所示

    @RestrictTo(LIBRARY_GROUP)
    public class ComponentActivity extends Activity implements LifecycleOwner {
        private SimpleArrayMap<Class<? extends ExtraData>, ExtraData> mExtraDataMap =
                new SimpleArrayMap<>();
    
        private LifecycleRegistry mLifecycleRegistry = new LifecycleRegistry(this);//1
    
        @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);//2
        }
    
        @CallSuper
        @Override
        protected void onSaveInstanceState(Bundle outState) {
            mLifecycleRegistry.markState(Lifecycle.State.CREATED);//3
            super.onSaveInstanceState(outState);
        }
    
        @RestrictTo(LIBRARY_GROUP)
        public <T extends ExtraData> T getExtraData(Class<T> extraDataClass) {
            return (T) mExtraDataMap.get(extraDataClass);
        }
    
        @Override
        public Lifecycle getLifecycle() {
            return mLifecycleRegistry;//4
        }
    
        @RestrictTo(LIBRARY_GROUP)
        public static class ExtraData {
        }
    }
    

    注释1处创建了LifecycleRegistry,它是Lifecycle的实现类;注释4处实现了LifecycleOwner接口定义的getLifecycle方法,返回了LifecycleRegistry。在注释3处,将Lifecycle的State设置为CREATED;

    正常来说应该在ComponentActivity的各个生命周期方法中改变Lifecycle的State,显然在ComponentActivity中没有做这些,而是将这个任务交给了ReportFragment,注释2处的将ComponentActivity注入到ReportFragment中

    @RestrictTo(RestrictTo.Scope.LIBRARY_GROUP)
    public class ReportFragment extends Fragment {
        private static final String REPORT_FRAGMENT_TAG = "androidx.lifecycle"
                + ".LifecycleDispatcher.report_fragment_tag";
        public static void injectIfNeededIn(Activity activity) {
            android.app.FragmentManager manager = activity.getFragmentManager();
            if (manager.findFragmentByTag(REPORT_FRAGMENT_TAG) == null) {
                manager.beginTransaction().add(new ReportFragment(), REPORT_FRAGMENT_TAG).commit();
                manager.executePendingTransactions();
            }
        }
        static ReportFragment get(Activity activity) {
            return (ReportFragment) activity.getFragmentManager().findFragmentByTag(
                    REPORT_FRAGMENT_TAG);
        }
       ...
        @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);//1
        }
    
        @Override
        public void onResume() {
            super.onResume();
            dispatchResume(mProcessListener);
            dispatch(Lifecycle.Event.ON_RESUME);
        }
    
      ...
        private void dispatch(Lifecycle.Event event) {
            Activity activity = getActivity();
            if (activity instanceof LifecycleRegistryOwner) {//2
                ((LifecycleRegistryOwner) activity).getLifecycle().handleLifecycleEvent(event);
                return;
            }
    
            if (activity instanceof LifecycleOwner) {//3
                Lifecycle lifecycle = ((LifecycleOwner) activity).getLifecycle();
                if (lifecycle instanceof LifecycleRegistry) {
                    ((LifecycleRegistry) lifecycle).handleLifecycleEvent(event);
                }
            }
        }
      ...
    }
    

    ReportFragment的onStart方法中会调用注释1处的dispatch方法; 在dispatch方法的注释2处,判断Activity是否实现了LifecycleRegistryOwner接口,LifecycleRegistryOwner继承了LifecycleOwner接口

    这两个接口不同的是: LifecycleRegistryOwner定义的getLifecycle方法返回的是LifecycleRegistry类型,而LifecycleOwner定义的getLifecycle方法返回的是Lifecycle类型。注释3处如果Activity实现了LifecycleOwner接口,会调用LifecycleRegistry的handleLifecycleEvent方法

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

    getStateAfter方法会获取“即将的事件” :当前事件执行后,即将会处于什么事件,代码如下所示

     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);
        }
    
    

    这个和文章开头给出的State与Event关系的时序图对照看会比较好理解; 比如当前执行了ON_CREATE事件或者ON_STOP事件,那么状态就会处于CREATED;回到handleLifecycleEvent方法,其内部还会调用moveToState方法

       private void moveToState(State next) {
            if (mState == next) {
                return;
            }
            mState = next;
            if (mHandlingEvent || mAddingObserverCounter != 0) {
                mNewEventOccurred = true;
                return;
            }
            mHandlingEvent = true;
            sync();
            mHandlingEvent = false;
        }
    

    如果当前所处的状态和即将要处于的状态一样就不做任何操作,sync方法如下所示

      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;
                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;
        }
    

    sync方法中会根据当前状态和mObserverMap中的eldest和newest的状态做对比 ,判断当前状态是向前还是向后; 比如由STARTED到RESUMED是状态向前,反过来就是状态向后,这个不要和Activity的生命周期搞混;向前还是向后的代码大同小异,这里以向后为例

        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();//1
                while ((observer.mState.compareTo(mState) < 0 && !mNewEventOccurred
                        && mObserverMap.contains(entry.getKey()))) {
                    pushParentState(observer.mState);
                    observer.dispatchEvent(lifecycleOwner, upEvent(observer.mState));//2
                    popParentState();
                }
            }
        }
    

    注释1处的用于获取ObserverWithState,后面会在提到它。 注释2处的upEvent方法会得到当前状态的向前状态

    ObserverWithState的dispatchEvent方法如下所示

        static class ObserverWithState {
            State mState;
            GenericLifecycleObserver mLifecycleObserver;
    
            ObserverWithState(LifecycleObserver observer, State initialState) {
                mLifecycleObserver = Lifecycling.getCallback(observer);//1
                mState = initialState;
            }
    
            void dispatchEvent(LifecycleOwner owner, Event event) {
                State newState = getStateAfter(event);
                mState = min(mState, newState);
                mLifecycleObserver.onStateChanged(owner, event);
                mState = newState;
            }
        }
    

    从名称就可以看出来,它内部包括了State和GenericLifecycleObserver,GenericLifecycleObserver是一个接口,它继承了LifecycleObserver接口; ReflectiveGenericLifecycleObserver和CompositeGeneratedAdaptersObserver是GenericLifecycleObserver的实现类,这里主要查看ReflectiveGenericLifecycleObserver的onStateChanged方法是如何实现的

    class ReflectiveGenericLifecycleObserver implements GenericLifecycleObserver {
        private final Object mWrapped;
        private final CallbackInfo mInfo;
    
        ReflectiveGenericLifecycleObserver(Object wrapped) {
            mWrapped = wrapped;
            mInfo = ClassesInfoCache.sInstance.getInfo(mWrapped.getClass());
        }
    
        @Override
        public void onStateChanged(LifecycleOwner source, Event event) {
            mInfo.invokeCallbacks(source, event, mWrapped);//1
        }
    }
    

    注释1处会调用CallbackInfo的invokeCallbacks方法,在讲这个方法前,需要先了解CallbackInfo是怎么创建的,是由createInfo方法创建的,如下所示

     private CallbackInfo createInfo(Class klass, @Nullable Method[] declaredMethods) {
            Class superclass = klass.getSuperclass();
            Map<MethodReference, Lifecycle.Event> handlerToEvent = new HashMap<>();
           ...
            Method[] methods = declaredMethods != null ? declaredMethods : getDeclaredMethods(klass);
            boolean hasLifecycleMethods = false;
            for (Method method : methods) {
                OnLifecycleEvent annotation = method.getAnnotation(OnLifecycleEvent.class);//1
                if (annotation == null) {
                    continue;
                }
                hasLifecycleMethods = true;
                Class<?>[] params = method.getParameterTypes();
                int callType = CALL_TYPE_NO_ARG;
                if (params.length > 0) {
                    callType = CALL_TYPE_PROVIDER;
                    if (!params[0].isAssignableFrom(LifecycleOwner.class)) {
                        throw new IllegalArgumentException(
                                "invalid parameter type. Must be one and instanceof LifecycleOwner");
                    }
                }
                Lifecycle.Event event = annotation.value();//2
                ...
                MethodReference methodReference = new MethodReference(callType, method);//3
                verifyAndPutHandler(handlerToEvent, methodReference, event, klass);//4
            }
            CallbackInfo info = new CallbackInfo(handlerToEvent);//5
            mCallbackMap.put(klass, info);
            mHasLifecycleMethods.put(klass, hasLifecycleMethods);
            return info;
        }
    

    关键点在注释1处; 不断的遍历各个方法,获取方法上的名为OnLifecycleEvent的注解,这个注解正是实现LifecycleObserver接口时用到的。

    注释2处获取该注解的值; 也就是在@OnLifecycleEvent中定义的事件

    注释3处新建了一个MethodReference; 其内部包括了使用了该注解的方法

    注释4处的verifyAndPutHandler方法用于将MethodReference和对应的Event存在类型为Map<MethodReference, Lifecycle.Event> 的handlerToEvent中

    注释5处新建CallbackInfo,并将handlerToEvent传进去

    接着回头看CallbackInfo的invokeCallbacks方法,代码如下所示

    static class CallbackInfo {
            final Map<Lifecycle.Event, List<MethodReference>> mEventToHandlers;
            final Map<MethodReference, Lifecycle.Event> mHandlerToEvent;
            CallbackInfo(Map<MethodReference, Lifecycle.Event> handlerToEvent) {
                mHandlerToEvent = handlerToEvent;
                mEventToHandlers = new HashMap<>();
                for (Map.Entry<MethodReference, Lifecycle.Event> entry : handlerToEvent.entrySet()) {//1
                    Lifecycle.Event event = entry.getValue();
                    List<MethodReference> methodReferences = mEventToHandlers.get(event);
                    if (methodReferences == null) {
                        methodReferences = new ArrayList<>();
                        mEventToHandlers.put(event, methodReferences);
                    }
                    methodReferences.add(entry.getKey());
                }
            }
            @SuppressWarnings("ConstantConditions")
            void invokeCallbacks(LifecycleOwner source, Lifecycle.Event event, Object target) {
                invokeMethodsForEvent(mEventToHandlers.get(event), source, event, target);//2
                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);//1
                    }
                }
            }
    

    注释1处的循环的意义在于将handlerToEvent进行数据类型转换,转化为一个HashMap,key的值为事件,value的值为MethodReference。注释2处的invokeMethodsForEvent方法会传入mEventToHandlers.get(event),也就是事件对应的MethodReference的集合。invokeMethodsForEvent方法中会遍历MethodReference的集合,调用MethodReference的invokeCallback方法

     @SuppressWarnings("WeakerAccess")
        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) {
                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);
                }
            }
          ...
        }
    

    MethodReference类中有两个变量,一个是callType,它代表调用方法的类型,另一个是Method; 它代表方法,不管是哪种callType都会通过invoke对方法进行反射。 简单来说,实现LifecycleObserver接口的类中,注解修饰的方法和事件会被保存起来,通过反射对事件的对应方法进行调用

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