Lifecycle

Lifecycle 组件指的是 android.arch.lifecycle 包下提供的各种类与接口，可以让开发者构建能感知其他组件（主要指Activity 、Fragment）生命周期（lifecycle-aware）的类。

使用

引用依赖

def lifecycle_version = "2.1.0"
compile "android.arch.lifecycle:runtime:$lifecycle_version"
annotationProcessor "android.arch.lifecycle:compiler:$lifecycle_version"

使用步骤

1.新建类实现LifecycleObserver接口，通过注解关联生命周期方法。

public class ActivityLifeObserver implements BasePresenter,LifecycleObserver {

    private final static String TAG = "ActivityLifeObserver";
    
    @OnLifecycleEvent(Lifecycle.Event.ON_CREATE)
    @Override
    public void onCreate() {
    }

    @OnLifecycleEvent(Lifecycle.Event.ON_START)
    @Override
    public void onStart() {
    }
    @OnLifecycleEvent(Lifecycle.Event.ON_RESUME)
    @Override
    public void onResume() {
    }
    @OnLifecycleEvent(Lifecycle.Event.ON_PAUSE)
    @Override
    public void onPause() {
    }
    @OnLifecycleEvent(Lifecycle.Event.ON_STOP)
    @Override
    public void onStop() {
    }
    @OnLifecycleEvent(Lifecycle.Event.ON_DESTROY)
    @Override
    public void onDestroy() {
    }
}

BasePresenter是回调方法可以随意，OnLifecycleEvent的参数是枚举类Lifecycle.Event，对应生命周期方法。

2.Activity实现LifecycleOwner接口，声明全局变量LifecycleRegistry，并重写getLifecycle方法返回。通过LifecycleRegistry注册监听。详细代码如下：

public class MainActivity extends AppCompatActivity implements LifecycleOwner {
    private LifecycleRegistry mRegistry;
    private LifecycleObserver mLifecycleObserver;
    @Override
    protected void onCreate(Bundle savedInstanceState) {
        super.onCreate(savedInstanceState);
        setContentView(R.layout.activity_main);
        if (mRegistry == null){
            mRegistry = new LifecycleRegistry(this);
        }
        //绑定
        mRegistry.addObserver(mLifecycleObserver = new ActivityLifeObserver());
    }
    @NonNull
    @Override
    public Lifecycle getLifecycle() {
        //getLifecycle在oncreate前，因此需要判断
        if (null == mRegistry){
            mRegistry = new LifecycleRegistry(this);
        }
        return mRegistry;
    }
    @Override
    protected void onDestroy() {
        super.onDestroy();
        if (null != mRegistry){
            //解绑
            mRegistry.removeObserver(mLifecycleObserver);
        }
    }
}

此时运行就可以监听声明周期了,效果如下:

D/ActivityLifeObserver: onCreate
D/ActivityLifeObserver: onStart: 
D/ActivityLifeObserver: onResume: 
D/ActivityLifeObserver: onPause: 
D/ActivityLifeObserver: onStop: 
D/ActivityLifeObserver: onDestroy: 

原理

Lifecycle是如何感知生命周期的？

Lifecycle是如何完成注解与生命周期方法绑定的？

带着这两个问题我们来看一下Lifecycle源码。

首先我们来看一下Lifecycle是如何感知生命周期的？
从MainActivity来看一共做了三件事

1.初始化LifecycleRegistry
2.绑定LifecycleObserver
3.通过getLifecycle返回LifecycleRegistry

第一二步是基础的数据绑定操作，那么涉及生命周期感知的应该就是第三步了。点击getLifecycle方法跳出引用的地方，我们发现有个类叫ReportFragment。有过glide分析经验的同志应该记得Glide就是向Activity添加无界面的Fragment从而感知生命周期，那么Lifecycle是否如此，我们来看一下ReportFragment源码：

@SuppressWarnings("UnknownNullness") // TODO https://issuetracker.google.com/issues/112197238
@RestrictTo(RestrictTo.Scope.LIBRARY_GROUP_PREFIX)
public class ReportFragment extends Fragment {
    private static final String REPORT_FRAGMENT_TAG = "androidx.lifecycle"
            + ".LifecycleDispatcher.report_fragment_tag";
    ...
    @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) {
            ((LifecycleRegistryOwner) activity).getLifecycle().handleLifecycleEvent(event);
            return;
        }
        if (activity instanceof LifecycleOwner) {
            Lifecycle lifecycle = ((LifecycleOwner) activity).getLifecycle();
            if (lifecycle instanceof LifecycleRegistry) {
                ((LifecycleRegistry) lifecycle).handleLifecycleEvent(event);
            }
        }
    }
}

我们发现ReportFragment的确是用来感知生命周期的，在对应的生命周期方法中调用dispatch将生命周期状态分发下去。最后事件走到LifecycleRegistry的handleLifecycleEvent方法里面。我们看一下ReportFragment是在哪里被添加的：

    //向activity添加ReportFragment
    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();
        }
    }

injectIfNeededIn是ReportFragment添加进Activity的方法，调用的地方在ComponentActivity中，而ComponentActivity是AppCompatActivity的父类：

    @Override
    protected void onCreate(@Nullable Bundle savedInstanceState) {
        super.onCreate(savedInstanceState);
        mSavedStateRegistryController.performRestore(savedInstanceState);
        ReportFragment.injectIfNeededIn(this);
        if (mContentLayoutId != 0) {
            setContentView(mContentLayoutId);
        }
    }

所以在activity刚刚被创建的时候，ReportFragment就已经同步添加了。这时候第一个问题就可以回答了：Lifecycle是通过向Activity添加ReportFragment从而达到感知生命周期的作用。
下面我们来看第二个问题，上面我们看到ReportFragment生命周期方法走到LifecycleRegistry的handleLifecycleEvent方法里面，下面我们来看下该方法：

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

    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;
    }

handleLifecycleEvent会依据Event找到下一个状态，并将同步至该状态，核心方法是 sync()：

    private void sync() {
        LifecycleOwner lifecycleOwner = mLifecycleOwner.get();
        if (lifecycleOwner == null) {
            throw new IllegalStateException("LifecycleOwner of this LifecycleRegistry is already"
                    + "garbage collected. It is too late to change lifecycle state.");
        }
        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;
    }

sync方法对比此状态与map最后一个状态，如果小于则回退，对比最新的状态，如果大于则前进状态，则前进，以下是backwardPass代码：

    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();
            }
        }
    }

代码核心为ObserverWithState ，调用其dispatchEvent完成状态分发，ObserverWithState 是内部类代码为：

    static class ObserverWithState {
        State mState;
        LifecycleEventObserver mLifecycleObserver;

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

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

可以看到ObserverWithState 将LifecycleObserver封装为LifecycleEventObserver ，然后调用onStateChanged了，下面我们看下封装的代码Lifecycling.lifecycleEventObserver(observer)：

    static LifecycleEventObserver lifecycleEventObserver(Object object) {
        boolean isLifecycleEventObserver = object instanceof LifecycleEventObserver;
        boolean isFullLifecycleObserver = object instanceof FullLifecycleObserver;
        if (isLifecycleEventObserver && isFullLifecycleObserver) {
            return new FullLifecycleObserverAdapter((FullLifecycleObserver) object,
                    (LifecycleEventObserver) object);
        }
        if (isFullLifecycleObserver) {
            return new FullLifecycleObserverAdapter((FullLifecycleObserver) object, null);
        }

        if (isLifecycleEventObserver) {
            return (LifecycleEventObserver) 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);
        }
        return new ReflectiveGenericLifecycleObserver(object);
    }

lifecycleEventObserver有4层判定：

1、FullLifecycleObserver的子类，返回FullLifecycleObserverAdapter，FullLifecycleObserver是官方实现，默认有生命周期方法。不用注解。
2、LifecycleEventObserver子类，返回LifecycleEventObserver子类本身，改类提供了onStateChanged回调方法，用于检测生命周期变换。
3、如果APT生成了对应的类，则返回CompositeGeneratedAdaptersObserver，其内部也是调用反射执行对应的注解方法。
4、如都不满足则返回ReflectiveGenericLifecycleObserver，内部onStateChanged调用CallbackInfo的invokeCallbacks方法。

我们来看下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);
}
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);
                }
            }
        }
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);
            }
        }

一路追寻，发现最终是通过反射来完成方法的执行的。那么Method是如下关联的呢？我们发现在前面的lifecycleEventObserver方法中有个代码语句：

int type = getObserverConstructorType(klass);

private static int getObserverConstructorType(Class<?> klass) {
        Integer callbackCache = sCallbackCache.get(klass);
        if (callbackCache != null) {
            return callbackCache;
        }
        int type = resolveObserverCallbackType(klass);
        sCallbackCache.put(klass, type);
        return type;
    }

该代码块是判断是否是APT还是默认的反射方式，其中resolveObserverCallbackType做了一些操作：

...
ClassesInfoCache.sInstance.hasLifecycleMethods(klass);
...
hasLifecycleMethods方法：
...
Method[] methods = getDeclaredMethods(klass);
for (Method method : methods) {
        OnLifecycleEvent annotation = method.getAnnotation(OnLifecycleEvent.class);
        if (annotation != null) {
                // Optimization for reflection, we know that this method is called
                // when there is no generated adapter. But there are methods with @OnLifecycleEvent
                // so we know that will use ReflectiveGenericLifecycleObserver,
                // so we createInfo in advance.
                // CreateInfo always initialize mHasLifecycleMethods for a class, so we don't do it
                // here.
         createInfo(klass, methods);
         return true;
  }
}
...

hasLifecycleMethods获取被OnLifecycleEvent 注解的方法，然后一一创建CallbackInfo对象：

for (Method method : methods) {
            OnLifecycleEvent annotation = method.getAnnotation(OnLifecycleEvent.class);
            ....
            Class<?>[] params = method.getParameterTypes();
            int callType = CALL_TYPE_NO_ARG;
            Lifecycle.Event event = annotation.value();
           ....
            MethodReference methodReference = new MethodReference(callType, method);
            verifyAndPutHandler(handlerToEvent, methodReference, event, klass);
        }
        CallbackInfo info = new CallbackInfo(handlerToEvent);

到这里我们就可以知道，原来onStateChanged执行的CallbackInfo对象实际上就是封装了OnLifecycleEvent 注解方法的对象。所以当状态变更时，对应的方法也会调用。

总结：

Lifecycle是如何感知生命周期的？

Lifecycle向Activity添加ReportFragment，然后通过ReportFragment感知生命周期。

Lifecycle是如何完成注解与生命周期方法绑定的？

生命周期分发时，最终会执行ObserverWithState的dispatchEvent方法，dispatchEvent方法会调用LifecycleEventObserver的onStateChanged方法。
ObserverWithState通过Lifecycling获取不同类型的LifecycleEventObserver，获取过程中会遍历被OnLifecycleEvent 注解修饰的方法，并保存。
当onStateChanged执行时，通过反射执行对应状态的方法，从而完成注解与生命周期方法的绑定。
如果配置了APT依赖，APT分支在于默认反射分支前。