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Jetpack学习-Lifecycle

Lifecycle是什么

Lifecycle是Jetpack提供的一个组件，可以感知Activity,Fragment的生命周期变化。

简单使用

定义一个类继承自LifecycleObserver，根据业务需要，在这个类中重写相应的方法

public class LifecycleObserverImpl implements LifecycleObserver {

    @OnLifecycleEvent(Lifecycle.Event.ON_CREATE)
    public void onCreate() {
        Log.d(MainActivity.TAG, "onCreate");
    }

    @OnLifecycleEvent(Lifecycle.Event.ON_START)
    public void onStart() {
        Log.d(MainActivity.TAG, "onStart");
    }

    @OnLifecycleEvent(Lifecycle.Event.ON_RESUME)
    public void onResume() {
        Log.d(MainActivity.TAG, "onResume");
    }

    @OnLifecycleEvent(Lifecycle.Event.ON_PAUSE)
    public void onPause() {
        Log.d(MainActivity.TAG, "onPause");
    }

    @OnLifecycleEvent(Lifecycle.Event.ON_STOP)
    public void onStop() {
        Log.d(MainActivity.TAG, "onStop");
    }

    @OnLifecycleEvent(Lifecycle.Event.ON_DESTROY)
    public void onDestroy() {
        Log.d(MainActivity.TAG, "onDestroy");
    }

}

在Activity中使用

public class LifeCycleActivity extends AppCompatActivity {

    LifecycleObserverImpl observer;

    @Override
    protected void onCreate(@Nullable Bundle savedInstanceState) {
        super.onCreate(savedInstanceState);
        setContentView(R.layout.activity_lifecycle);
        //实例化
        observer = new LifecycleObserverImpl();
        //添加observer
        getLifecycle().addObserver(observer);
    }

    @Override
    protected void onDestroy() {
        super.onDestroy();
        //移除observer
        getLifecycle().removeObserver(observer);
    }
}

这样，当Activity的生命周期变化时，我们自定义的observer就可以获取到变化。

运行应用，输出日志如下：

2020-04-14 11:08:06.579 22908-22908/com.wangyz.jetpack D/Jetpack: onCreate
2020-04-14 11:08:06.580 22908-22908/com.wangyz.jetpack D/Jetpack: onStart
2020-04-14 11:08:06.584 22908-22908/com.wangyz.jetpack D/Jetpack: onResume
2020-04-14 11:08:11.350 22908-22908/com.wangyz.jetpack D/Jetpack: onPause
2020-04-14 11:08:11.711 22908-22908/com.wangyz.jetpack D/Jetpack: onStop
2020-04-14 11:08:11.713 22908-22908/com.wangyz.jetpack D/Jetpack: onDestroy

一个最简单的Demo就写好了。

原理

Lifecycle为什么可以感知生命周期变化，并通知到observer，我们来看一下。

添加observer

首先从添加observer这里看起:getLifecycle().addObserver(observer)

我们的Activity是继承自AppCompatActivity,调用getLifecycle()后，其实是调用了FragmentActivity的getLifecycle方法

@Override
    public Lifecycle getLifecycle() {
        return super.getLifecycle();
    }

而FragmentActivity中的getLifecycle方法又调用了父类的ComponentActivity的getLifecycle方法

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

最终是返回mLifecycleRegistry。

调用getLifecycle().addObserver(observer)方法,其实就是调用mLifecycleRegistry的addObserver方法

@Override
    public void addObserver(@NonNull LifecycleObserver observer) {
        //设置初始状态
        State initialState = mState == DESTROYED ? DESTROYED : INITIALIZED;
        //1.将observer和state组装成一个ObserverWithState对象
        ObserverWithState statefulObserver = new ObserverWithState(observer, initialState);
        //2.以observer为key,ObserverWithState为value，保存到map中
        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--;
    }

在上面的注释1处，将observer和state组装成一个ObserverWithState对象

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

在这个构造方法里，调用了 Lifecycling.getCallback(observer)方法

@NonNull
    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);
        }
        //1
        return new ReflectiveGenericLifecycleObserver(object);
    }

在这个方法里，由于我们传入的是继承自LifecycleObserver的observer,最终返回的是ReflectiveGenericLifecycleObserver。

ReflectiveGenericLifecycleObserver(Object wrapped) {
        mWrapped = wrapped;
        mInfo = ClassesInfoCache.sInstance.getInfo(mWrapped.getClass());
    }

在ReflectiveGenericLifecycleObserver的构造方法中，创建了CallbackInfo信息

CallbackInfo getInfo(Class klass) {
        CallbackInfo existing = mCallbackMap.get(klass);
        if (existing != null) {
            return existing;
        }
        existing = createInfo(klass, null);
        return existing;
    }

如果没有缓存过，则创建

private CallbackInfo createInfo(Class klass, @Nullable Method[] declaredMethods) {
        Class superclass = klass.getSuperclass();
        Map<MethodReference, Lifecycle.Event> handlerToEvent = new HashMap<>();
        if (superclass != null) {
            CallbackInfo superInfo = getInfo(superclass);
            if (superInfo != null) {
                handlerToEvent.putAll(superInfo.mHandlerToEvent);
            }
        }

        Class[] interfaces = klass.getInterfaces();
        for (Class intrfc : interfaces) {
            for (Map.Entry<MethodReference, Lifecycle.Event> entry : getInfo(
                    intrfc).mHandlerToEvent.entrySet()) {
                verifyAndPutHandler(handlerToEvent, entry.getKey(), entry.getValue(), klass);
            }
        }

        Method[] methods = declaredMethods != null ? declaredMethods : getDeclaredMethods(klass);
        boolean hasLifecycleMethods = false;
        for (Method method : methods) {
            OnLifecycleEvent annotation = method.getAnnotation(OnLifecycleEvent.class);
            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();

            if (params.length > 1) {
                callType = CALL_TYPE_PROVIDER_WITH_EVENT;
                if (!params[1].isAssignableFrom(Lifecycle.Event.class)) {
                    throw new IllegalArgumentException(
                            "invalid parameter type. second arg must be an event");
                }
                if (event != Lifecycle.Event.ON_ANY) {
                    throw new IllegalArgumentException(
                            "Second arg is supported only for ON_ANY value");
                }
            }
            if (params.length > 2) {
                throw new IllegalArgumentException("cannot have more than 2 params");
            }
            MethodReference methodReference = new MethodReference(callType, method);
            verifyAndPutHandler(handlerToEvent, methodReference, event, klass);
        }
        CallbackInfo info = new CallbackInfo(handlerToEvent);
        mCallbackMap.put(klass, info);
        mHasLifecycleMethods.put(klass, hasLifecycleMethods);
        return info;
    }

在这个方法里，通过注解，将方法添加到了map中。

在注释2处，以observer为key,ObserverWithState为value，保存到map中

添加observer先看到这里。

生命周期变化时的通知

由于我们的Activity继承自AppCompatActivity,而AppCompatActivity最终继承自ComponentActivity,那么在oncreate执行时，会执行ComponentActivity的onCreate方法

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

在这个方法中，执行了ReportFragment.injectIfNeededIn(this)方法;

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();
        }
    }

可以看到，其实就是创建了一个Fragment，然后关联了Activity，这里Activity生命周期变化时，Fragment也会感知到。

下面以onResume为例，来看下onResume时，我们定义的observer是怎样感知到的。

//ReportFragment
public void onResume() {
        super.onResume();
        dispatchResume(mProcessListener);
        dispatch(Lifecycle.Event.ON_RESUME);
    }

我们在observer中注册了Lifecycle.Event.ON_RESUME事件监听，看下dispatch方法

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);
            }
        }
    }

在这个方法中，都会调用到handleLifecycleEvent方法

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

这个方法调用了moveToState方法

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;
    }

在这个方法里，调用了sync()方法

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;
    }

在sync方法中，经过判断，会调用backwardPass或forwardPass方法。我们选择forwardPass方法来看

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();
            }
        }
    }

在forwardPass方法中，会取出之前添加的observer,再依次调用dispatchEvent方法

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

这个方法调用到ReflectiveGenericLifecycleObserver的onStateChanged方法

public void onStateChanged(LifecycleOwner source, Event event) {
        mInfo.invokeCallbacks(source, event, mWrapped);
    }

这个方法调用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);
        }

在这里又调用了invokeMethodsForEvent方法

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);
                }
            }
        }

调用invokeCallback方法

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);
            }
        }

这个方法最终是反射调用。而method在addObserver的时候已经通过解析注解，保存了起来。

最后，附一张简单的时序图: