Android開發Jetpack組件Lifecycle原理篇

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前言

在上一篇文章中，我們學習瞭如何去使用Lifecycle； 當然之會使用是不夠的，還需要瞭解它的原理，這是成為優秀工程師必備的；這篇文章就來學習Lifecycle的基本原理

1.Lifecycle的生命周期狀態事件和狀態

**Lifecycle使用兩個枚舉來跟蹤其關聯組件的生命周期狀態，這兩個枚舉分別是Event和State；**State指的是Lifecycle的生命周期所處的狀態；Event代表Lifecycle生命周期對應的事件，這些事件會映射到Activity和Fragment中的回調事件中

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是一個抽象類； 其內部不僅包括瞭添加和移除觀察者的方法，還包括瞭此前說到的Event和State枚舉。可以看到Event中的事件和Activity的生命周期幾乎是對應的，除瞭ON_ANY，它可用於匹配所有事件

2.Lifecycle如何觀察Activity和Fragment的生命周期

在Android Support Library 26.1.0 及其之後的版本，Activity和Fragment已經默認實現瞭LifecycleOwner接口，LifecycleOwner可以理解為被觀察者，那麼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繼承瞭AppCompatActivity，而AppCompatActivity繼承瞭FragmentActivity。在Android 8.0時，FragmentActivity繼承自SupportActivity，而在Android 9.0，FragmentActivity繼承自ComponentActivity 。SupportActivity和ComponentActivity的代碼區別不大，這裡以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 {
    }
}

註釋1處創建瞭LifecycleRegistry，它是Lifecycle的實現類；註釋4處實現瞭LifecycleOwner接口定義的getLifecycle方法，返回瞭LifecycleRegistry。在註釋3處，將Lifecycle的State設置為CREATED；

正常來說應該在ComponentActivity的各個生命周期方法中改變Lifecycle的State，顯然在ComponentActivity中沒有做這些，而是將這個任務交給瞭ReportFragment，註釋2處的將ComponentActivity註入到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方法中會調用註釋1處的dispatch方法； 在dispatch方法的註釋2處，判斷Activity是否實現瞭LifecycleRegistryOwner接口，LifecycleRegistryOwner繼承瞭LifecycleOwner接口

這兩個接口不同的是： LifecycleRegistryOwner定義的getLifecycle方法返回的是LifecycleRegistry類型，而LifecycleOwner定義的getLifecycle方法返回的是Lifecycle類型。註釋3處如果Activity實現瞭LifecycleOwner接口，會調用LifecycleRegistry的handleLifecycleEvent方法

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

getStateAfter方法會獲取“即將的事件” ：當前事件執行後，即將會處於什麼事件，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);
    }

這個和文章開頭給出的State與Event關系的時序圖對照看會比較好理解； 比如當前執行瞭ON_CREATE事件或者ON_STOP事件，那麼狀態就會處於CREATED；回到handleLifecycleEvent方法，其內部還會調用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;
    }

如果當前所處的狀態和即將要處於的狀態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方法中會根據當前狀態和mObserverMap中的eldest和newest的狀態做對比 ，判斷當前狀態是向前還是向後； 比如由STARTED到RESUMED是狀態向前，反過來就是狀態向後，這個不要和Activity的生命周期搞混；向前還是向後的代碼大同小異，這裡以向後為例

    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();
            }
        }
    }

註釋1處的用於獲取ObserverWithState，後面會在提到它。 註釋2處的upEvent方法會得到當前狀態的向前狀態

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;
        }
    }

從名稱就可以看出來，它內部包括瞭State和GenericLifecycleObserver，GenericLifecycleObserver是一個接口，它繼承瞭LifecycleObserver接口； ReflectiveGenericLifecycleObserver和CompositeGeneratedAdaptersObserver是GenericLifecycleObserver的實現類，這裡主要查看ReflectiveGenericLifecycleObserver的onStateChanged方法是如何實現的

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
    }
}

註釋1處會調用CallbackInfo的invokeCallbacks方法，在講這個方法前，需要先瞭解CallbackInfo是怎麼創建的，是由createInfo方法創建的，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;
    }

關鍵點在註釋1處； 不斷的遍歷各個方法，獲取方法上的名為OnLifecycleEvent的註解，這個註解正是實現LifecycleObserver接口時用到的。

註釋2處獲取該註解的值； 也就是在@OnLifecycleEvent中定義的事件

註釋3處新建瞭一個MethodReference； 其內部包括瞭使用瞭該註解的方法

註釋4處的verifyAndPutHandler方法用於將MethodReference和對應的Event存在類型為Map<MethodReference, Lifecycle.Event> 的handlerToEvent中

註釋5處新建CallbackInfo，並將handlerToEvent傳進去

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
                }
            }
        }

註釋1處的循環的意義在於將handlerToEvent進行數據類型轉換，轉化為一個HashMap，key的值為事件，value的值為MethodReference。註釋2處的invokeMethodsForEvent方法會傳入mEventToHandlers.get(event)，也就是事件對應的MethodReference的集合。invokeMethodsForEvent方法中會遍歷MethodReference的集合，調用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類中有兩個變量，一個是callType，它代表調用方法的類型，另一個是Method； 它代表方法，不管是哪種callType都會通過invoke對方法進行反射。 簡單來說，實現LifecycleObserver接口的類中，註解修飾的方法和事件會被保存起來，通過反射對事件的對應方法進行調用

到此這篇關於Android開發Jetpack組件Lifecycle使用原理篇的文章就介紹到這瞭,更多相關Android Jetpack組件Lifecycle內容請搜索WalkonNet以前的文章或繼續瀏覽下面的相關文章希望大傢以後多多支持WalkonNet！

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