1. 原理

基于Android10.0

源码

Lifecycle.java

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，内部有Event和State两种枚举类，用于对应生命周期，其中ON_ANY可用于对应任何事件。也有addObserver和removeObserver方法用于添加和删除观察者

由于Activity和Fragment默认实现了LifecycleOwner接口，LifecycleOwner即被观察者。以Activity举例，MainActivity继承了AppCompatActivity，而AppCompatActivity又继承于FragmentActivity。在Android 8.0时，FragmentActivity继承于SupportActivity，在Android 9.0时继承于ComponentActivity，两者相差不大，这里分析后者

ComponentActivity.java

@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);

   
    @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);
    }

    @CallSuper
    @Override
    protected void onSaveInstanceState(Bundle outState) {
        mLifecycleRegistry.markState(Lifecycle.State.CREATED);
        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;
    }

    
    @RestrictTo(LIBRARY_GROUP)
    public static class ExtraData {
    }
}

开头的变量声明了LifecycleRegistry，是Lifecycle的实现类，并且在倒数第2个方法getLifecycle里返回实现类对象

在中间onSaveInstanceState方法里设置了最初的声明周期状态CREATED，其余状态由上面onCreate方法里的ReportFragment设置

ReportFragment.java

/**
 * Internal class that dispatches initialization events.
 *
 * @hide
 */
@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) {
        // 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();
        }
    }

    static ReportFragment get(Activity activity) {
        return (ReportFragment) activity.getFragmentManager().findFragmentByTag(
                REPORT_FRAGMENT_TAG);
    }

    private ActivityInitializationListener mProcessListener;

    private void dispatchCreate(ActivityInitializationListener listener) {
        if (listener != null) {
            listener.onCreate();
        }
    }

    private void dispatchStart(ActivityInitializationListener listener) {
        if (listener != null) {
            listener.onStart();
        }
    }

    private void dispatchResume(ActivityInitializationListener listener) {
        if (listener != null) {
            listener.onResume();
        }
    }

    @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);
    }

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

    @Override
    public void onPause() {
        super.onPause();
        dispatch(Lifecycle.Event.ON_PAUSE);
    }

    @Override
    public void onStop() {
        super.onStop();
        dispatch(Lifecycle.Event.ON_STOP);
    }

    @Override
    public void onDestroy() {
        super.onDestroy();
        dispatch(Lifecycle.Event.ON_DESTROY);
        // just want to be sure that we won't leak reference to an activity
        mProcessListener = null;
    }

    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);
            }
        }
    }

    void setProcessListener(ActivityInitializationListener processListener) {
        mProcessListener = processListener;
    }

    interface ActivityInitializationListener {
        void onCreate();

        void onStart();

        void onResume();
    }
}

开头的注释说明“分发初始化事件的内部类”。观察每一个声明周期方法里都会调用dispatch方法

dispatch方法里会先判断当前activity是否实现了LifecycleRegistryOwner接口或LifecycleOwner接口。与LifecycleOwner接口不同的是：LifecycleRegistryOwner接口定义的getLifecycle方法返回的是LifecycleRegistry，而LifecycleOwner接口定义的getLifecycle方法返回的是Lifecycle

回到方法里，判断activity实现了LifecycleOwner接口后，那么会调用LifecycleRegistry的handleLifecycleEvent方法

LifecycleRegistry.java的handleLifecycleEvent

/**
     * Sets the current state and notifies the observers.
     * <p>
     * Note that if the {@code currentState} is the same state as the last call to this method,
     * calling this method has no effect.
     *
     * @param event The event that was received
     */
    public void handleLifecycleEvent(@NonNull Lifecycle.Event event) {
        State next = getStateAfter(event);
        moveToState(next);
    }

这个方法用于设置当前的状态，并且提醒观察者，如果当前状态和最后一次调用这个方法时的状态是一样的，那么这个方法没有任何作用

getStateAfter方法将会获取即将到来的事件，当前事件执行后，即将会处于什么样的事件

LifecycleRegistry.java的getStateAfter

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);
    }

这个方法说明了event为ON_STOP的时候会返回CREATED，等等状态的声明时机，回到刚刚的handleLifecycleEvent，还有一个方法moveToState

LifecycleRegistry.java的moveToState

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;
    }

如果当前的状态和下一个状态一样，直接返回，否则会进行sync操作

LifecycleRegistry.java的sync

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;
    }

开头进行判空操作，如果为空则警告LifecycleOwner已经被垃圾回收

在while循环里进行当前State和mObserverMap里的eldest状态和newest状态进行对比，判断当前状态向前还是向后，分别调用不同的方法

LifecycleRegistry.java的backwardPass和forwardPass

private void backwardPass(LifecycleOwner lifecycleOwner) {
        Iterator<Entry<LifecycleObserver, ObserverWithState>> descendingIterator =
                mObserverMap.descendingIterator();
        while (descendingIterator.hasNext() && !mNewEventOccurred) {
            Entry<LifecycleObserver, ObserverWithState> entry = descendingIterator.next();
            ObserverWithState observer = entry.getValue();
            while ((observer.mState.compareTo(mState) > 0 && !mNewEventOccurred
                    && mObserverMap.contains(entry.getKey()))) {
                Event event = downEvent(observer.mState);
                pushParentState(getStateAfter(event));
                observer.dispatchEvent(lifecycleOwner, event);
                popParentState();
            }
        }
    }

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();
            }
        }
    }

这里大体逻辑相同，将状态封装为一个迭代器，当迭代器中还有元素，并且新的事件没有发生时，拿到ObserverWithState，并且调用其dispatchEvent方法进行事件的up或down，先看dispatchEvent方法

ObserverWithState的dispatchEvent

static class ObserverWithState {
        State mState;
        GenericLifecycleObserver mLifecycleObserver;

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

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

ObserverWithState是LifecycleRegistry的一个静态内部类，包括了State，GenericLifecycleObserver。后者是一个接口，继承了LifecycleObserver接口

ReflectiveGenericLifecycleObserver 和 CompositeGeneratedAdaptersObserver是GenericLifecycleObserver的实现类，我们主要看ReflectiveGenericLifecycleObserver的onStateChanged方法

ReflectiveGenericLifecycleObserver.java的onStateChanged

/**
 * An internal implementation of {@link GenericLifecycleObserver} that relies on reflection.
 */
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);
    }
}

方法里会调用CallbackInfo的invokeCallbacks方法，先来看看CallbackInfo是怎么实现的

CallbackInfo.java

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);
            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();

···
            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;
    }

开头，for循环遍历方法，获取注解OnLifecycleEvent，就是我们实现LifecycleObserver时用到的。而后将annotaion的value赋值给event，即@OnLifecycleEvent中定义的事件

新建一个MethodReference，其内部包括使用了该注解的方法。下一行verifyAndPutHandler方法用于将MethodReference和对应的Event存入类型为Map<MethodReference, Lifecycle.Event>的handlerToEvent中

最后创建CallbackInfo，传入handlerToEvent。回到上面的onStateChanged，查看invokeCallbacks

CallbackInfo.java的invokeCallbacks

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

第一个for循环处，handlerToEvent调用entrySet进行数据类型转换，转换为HashMap，其中key为event事件，value为MethodReference

invokeCallbacks方法里会调用invokeMethodsForEvent方法，传入mEventToHandlers.get(event)参数，也就是事件对应的MethodReference集合

在invokeMethodsForEvent方法中会遍历MethodReference的集合，调用MethodReference的invokeCallback方法

ClassesInfoCache.java的MethodReference

@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) {
            //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);
            }
        }
···

开头处有两个变量，mCallType，表示调用方法的类型。mMethod就代表方法

在invokeCallback方法中，无论是哪种callType都会通过invoke对方法进行反射

简单来说，就是实现LifecycleObserver接口的类中，注解修饰的方法和事件会被保存起来，通过反射对事件进行调用

2. 流程总结

从ComponentActivity开始，在onCreate方法中进入ReportFragment的injectIfNeededIn方法

在ReportFragment中，dispatch方法里调用LifecycleRegistry的handleLifecycleEvent设置状态

设置好状态后会通过moveToState，进行sync同步，判断此时的状态是向前还是向后，分别调用不同的方法

以forwardPass举例，内部创建ObserverWithState，以upEvent或downEvent作为参数传入ObserverWithState的dispatchEvent方法

由GenericLifecycleObserver的实现类ReflectiveGenericLifecycleObserver在dispatchEvent方法中调用onStateChanged方法，进入CallbackInfo的invokeCallbacks方法

由于CallbackInfo通过for循环获取我们声明的OnLifecycleEvent注解，拿到event。新建MethodReference包括了使用该注解的方法进行保存

所以最后invokeCallbacks就是通过反射对上面保存过的方法和事件进行调用

3. 有关类名区分

有些类名和作用有些相似，容易引起困惑，这里记录一下

Lifecycle

抽象类，感知生命周期的组件

LifecycleRegistry

普通类，继承Lifecycle，处理多个观察者

LifecycleObserver

观察者接口

LifecycleOwner

被观察者接口，Activity和Fragment默认实现此接口

LifecycleRegistryOwner

接口，继承LifecycleOwner接口，已被废弃

4. 应用举例

MyChronometer

public class MyChronometer extends Chronometer implements LifecycleObserver {
    private long eplasedTime;

    public MyChronometer(Context context, AttributeSet attrs) {
        super(context, attrs);
    }

    @OnLifecycleEvent(Lifecycle.Event.ON_RESUME)
    private void startMeter(){
        setBase(SystemClock.elapsedRealtime() - eplasedTime);
        start();
    }

    @OnLifecycleEvent(Lifecycle.Event.ON_PAUSE)
    private void stopMeter(){
        eplasedTime = SystemClock.elapsedRealtime() - getBase();
        stop();
    }
}

实现一个自定义的Chronometer，计时器，继承于TextView用于显示当前时间，通过注解与生命周期绑定方法，解耦于MainActivity

activity_main.xml

<androidx.constraintlayout.widget.ConstraintLayout xmlns:android="http://schemas.android.com/apk/res/android"
    xmlns:app="http://schemas.android.com/apk/res-auto"
    xmlns:tools="http://schemas.android.com/tools"
    android:layout_width="match_parent"
    android:layout_height="match_parent"
    tools:context=".baseLifecycle.MainActivity">

    <com.example.lifecycle.baseLifecycle.MyChronometer
        android:id="@+id/chronometer"
        android:textSize="30sp"
        android:layout_width="wrap_content"
        android:layout_height="wrap_content"
        android:text="Hello World!"
        app:layout_constraintBottom_toBottomOf="parent"
        app:layout_constraintLeft_toLeftOf="parent"
        app:layout_constraintRight_toRightOf="parent"
        app:layout_constraintTop_toTopOf="parent" />

</androidx.constraintlayout.widget.ConstraintLayout>

将自定义布局添加进xml

MainActivity

public class MainActivity extends AppCompatActivity {

    private MyChronometer chronometer;

    @Override
    protected void onCreate(Bundle savedInstanceState) {
        super.onCreate(savedInstanceState);
        setContentView(R.layout.activity_main);
        chronometer = findViewById(R.id.chronometer);
        getLifecycle().addObserver(chronometer);
    }
}

Activity中先拿到Lifecycle对象，再把观察者添加进布局即可

流程图

image.png