Android開(kāi)發(fā)Jetpack組件Lifecycle原理篇

更新時(shí)間：2022年08月11日 11:41:10 作者：Android技術(shù)棧

這一篇文章來(lái)介紹Android?Jetpack架構(gòu)組件的Lifecycle；?Lifecycle用于幫助開(kāi)發(fā)者管理Activity和Fragment?的生命周期，?由于Lifecycle是LiveData和ViewModel的基礎(chǔ)；所以需要先學(xué)習(xí)它

前言

在上一篇文章中，我們學(xué)習(xí)了如何去使用Lifecycle；當(dāng)然之會(huì)使用是不夠的，還需要了解它的原理，這是成為優(yōu)秀工程師必備的；這篇文章就來(lái)學(xué)習(xí)Lifecycle的基本原理

1.Lifecycle的生命周期狀態(tài)事件和狀態(tài)

**Lifecycle使用兩個(gè)枚舉來(lái)跟蹤其關(guān)聯(lián)組件的生命周期狀態(tài)，這兩個(gè)枚舉分別是Event和State；**State指的是Lifecycle的生命周期所處的狀態(tài)；Event代表Lifecycle生命周期對(duì)應(yīng)的事件，這些事件會(huì)映射到Activity和Fragment中的回調(diào)事件中

Android 9.0的Lifecycle的源碼如下所示

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是一個(gè)抽象類(lèi)；其內(nèi)部不僅包括了添加和移除觀察者的方法，還包括了此前說(shuō)到的Event和State枚舉?？梢钥吹紼vent中的事件和Activity的生命周期幾乎是對(duì)應(yīng)的，除了ON_ANY，它可用于匹配所有事件

2.Lifecycle如何觀察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已經(jīng)默認(rèn)實(shí)現(xiàn)了LifecycleOwner接口，LifecycleOwner可以理解為被觀察者，那么Lifecycle是如何觀察Activity和Fragment的生命周期的呢？

在上一篇文章舉的例子中，MainActivity繼承了AppCompatActivity，而AppCompatActivity繼承了FragmentActivity。在Android 8.0時(shí)，F(xiàn)ragmentActivity繼承自SupportActivity，而在Android 9.0，F(xiàn)ragmentActivity繼承自ComponentActivity 。SupportActivity和ComponentActivity的代碼區(qū)別不大，這里以ComponentActivity舉例，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 {
    }
}

注釋1處創(chuàng)建了LifecycleRegistry，它是Lifecycle的實(shí)現(xiàn)類(lèi)；注釋4處實(shí)現(xiàn)了LifecycleOwner接口定義的getLifecycle方法，返回了LifecycleRegistry。在注釋3處，將Lifecycle的State設(shè)置為CREATED；

正常來(lái)說(shuō)應(yīng)該在ComponentActivity的各個(gè)生命周期方法中改變Lifecycle的State，顯然在ComponentActivity中沒(méi)有做這些，而是將這個(gè)任務(wù)交給了ReportFragment，注釋2處的將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方法中會(huì)調(diào)用注釋1處的dispatch方法；在dispatch方法的注釋2處，判斷Activity是否實(shí)現(xiàn)了LifecycleRegistryOwner接口，LifecycleRegistryOwner繼承了LifecycleOwner接口

這兩個(gè)接口不同的是： LifecycleRegistryOwner定義的getLifecycle方法返回的是LifecycleRegistry類(lèi)型，而LifecycleOwner定義的getLifecycle方法返回的是Lifecycle類(lèi)型。注釋3處如果Activity實(shí)現(xiàn)了LifecycleOwner接口，會(huì)調(diào)用LifecycleRegistry的handleLifecycleEvent方法

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

getStateAfter方法會(huì)獲取“即將的事件” ：當(dāng)前事件執(zhí)行后，即將會(huì)處于什么事件，代碼如下所示

 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);
    }

這個(gè)和文章開(kāi)頭給出的State與Event關(guān)系的時(shí)序圖對(duì)照看會(huì)比較好理解；比如當(dāng)前執(zhí)行了ON_CREATE事件或者ON_STOP事件，那么狀態(tài)就會(huì)處于CREATED；回到handleLifecycleEvent方法，其內(nèi)部還會(huì)調(diào)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;
    }

如果當(dāng)前所處的狀態(tài)和即將要處于的狀態(tài)一樣就不做任何操作，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方法中會(huì)根據(jù)當(dāng)前狀態(tài)和mObserverMap中的eldest和newest的狀態(tài)做對(duì)比，判斷當(dāng)前狀態(tài)是向前還是向后；比如由STARTED到RESUMED是狀態(tài)向前，反過(guò)來(lái)就是狀態(tài)向后，這個(gè)不要和Activity的生命周期搞混；向前還是向后的代碼大同小異，這里以向后為例

    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();
            }
        }
    }

注釋1處的用于獲取ObserverWithState，后面會(huì)在提到它。注釋2處的upEvent方法會(huì)得到當(dāng)前狀態(tài)的向前狀態(tài)

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;
        }
    }

從名稱(chēng)就可以看出來(lái)，它內(nèi)部包括了State和GenericLifecycleObserver，GenericLifecycleObserver是一個(gè)接口，它繼承了LifecycleObserver接口； ReflectiveGenericLifecycleObserver和CompositeGeneratedAdaptersObserver是GenericLifecycleObserver的實(shí)現(xiàn)類(lèi)，這里主要查看ReflectiveGenericLifecycleObserver的onStateChanged方法是如何實(shí)現(xiàn)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
    }
}

注釋1處會(huì)調(diào)用CallbackInfo的invokeCallbacks方法，在講這個(gè)方法前，需要先了解CallbackInfo是怎么創(chuàng)建的，是由createInfo方法創(chuàng)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;
    }

關(guān)鍵點(diǎn)在注釋1處；不斷的遍歷各個(gè)方法，獲取方法上的名為OnLifecycleEvent的注解，這個(gè)注解正是實(shí)現(xiàn)LifecycleObserver接口時(shí)用到的。

注釋2處獲取該注解的值；也就是在@OnLifecycleEvent中定義的事件

注釋3處新建了一個(gè)MethodReference；其內(nèi)部包括了使用了該注解的方法

注釋4處的verifyAndPutHandler方法用于將MethodReference和對(duì)應(yīng)的Event存在類(lèi)型為Map<MethodReference, Lifecycle.Event> 的handlerToEvent中

注釋5處新建CallbackInfo，并將handlerToEvent傳進(jìn)去

接著回頭看CallbackInfo的invokeCallbacks方法，代碼如下所示

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
                }
            }
        }

注釋1處的循環(huán)的意義在于將handlerToEvent進(jìn)行數(shù)據(jù)類(lèi)型轉(zhuǎn)換，轉(zhuǎn)化為一個(gè)HashMap，key的值為事件，value的值為MethodReference。注釋2處的invokeMethodsForEvent方法會(huì)傳入mEventToHandlers.get(event)，也就是事件對(duì)應(yīng)的MethodReference的集合。invokeMethodsForEvent方法中會(huì)遍歷MethodReference的集合，調(diào)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類(lèi)中有兩個(gè)變量，一個(gè)是callType，它代表調(diào)用方法的類(lèi)型，另一個(gè)是Method；它代表方法，不管是哪種callType都會(huì)通過(guò)invoke對(duì)方法進(jìn)行反射。簡(jiǎn)單來(lái)說(shuō)，實(shí)現(xiàn)LifecycleObserver接口的類(lèi)中，注解修飾的方法和事件會(huì)被保存起來(lái)，通過(guò)反射對(duì)事件的對(duì)應(yīng)方法進(jìn)行調(diào)用

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