【Android Jetpack】Lifecycle 从入门到精通

背景

Android Jetpack 是一个由多个库组成的套件，可帮助开发者遵循最佳做法，减少样板代码并编写可在各种 Android 版本和设备中一致运行的代码，让开发者精力集中编写重要的代码。也就是说，Google 开发了一套通用的库让我们使用，帮助开发者高效的开发应用的工具集。详细内容可以参考这里。

介绍

Lifecycle 是 Android Jetpack 的一部分，是生命周期感知型组件，可执行操作来响应另一个组件（Activity 和 Fragment）的生命周期状态的变化。Lifecycle 是一个类，用于存储有关组件（如 Activity 或 Fragment）的生命周期状态的信息，并允许其他对象观察此状态。它是 LiveData 和 ViewModel 的基础，如果你想更深入的了解 Android Jetpack 的其他组件，你应该从它学起。

为什么我们需要它？

举个例子，假设我们有一个在屏幕上显示设备位置的 Activity。常见的实现可能如下所示：

internal class MyLocationListener(
        private val context: Context,
        private val callback: (Location) -> Unit
) {

    fun start() {
        // connect to system location service
    }

    fun stop() {
        // disconnect from system location service
    }
}

class MyActivity : AppCompatActivity() {
    private lateinit var myLocationListener: MyLocationListener

    override fun onCreate(...) {
        myLocationListener = MyLocationListener(this) { location ->
            // update UI
        }
    }

    public override fun onStart() {
        super.onStart()
        myLocationListener.start()
        // manage other components that need to respond
        // to the activity lifecycle
    }

    public override fun onStop() {
        super.onStop()
        myLocationListener.stop()
        // manage other components that need to respond
        // to the activity lifecycle
    }
}

虽然这个例子看起来没有太大的问题，但是在实际应用中，最终会有太多管理界面和其他组件的调用，以响应生命周期的当前状态。比如多个组件会在生命周期方法（onStart() 和 onStop()）中放置大量代码，不利于维护。

此外，无法保证组件会在 Activity 或 Fragment 停止之前启动。在我们需要执行长时间运行的操作（如 onStart() 中的某种配置检查）时尤其如此。这可能会导致出现一种竞态条件，在这种条件下，onStop() 方法会在 onStart() 之前结束，这使得组件留存的时间比所需的时间要长。

class MyActivity : AppCompatActivity() {
    private lateinit var myLocationListener: MyLocationListener

    override fun onCreate(...) {
        myLocationListener = MyLocationListener(this) { location ->
            // update UI
        }
    }

    public override fun onStart() {
        super.onStart()
        Util.checkUserStatus { result ->
            // 如果在 activity stop 后调用此回调该怎么办？,通常我们的处理是注销回调，要手动处理，使用	             Lifecycle 就自动帮助我们处理了。
            if (result) {
                myLocationListener.start()
            }
        }
    }

    public override fun onStop() {
        super.onStop()
        myLocationListener.stop()
    }

}

androidx.lifecycle 软件包提供的类和接口可帮助您以弹性和隔离的方式解决这些问题。

基础使用

导入依赖

implementation "androidx.lifecycle:lifecycle-extensions:2.2.0"
annotationProcessor "androidx.lifecycle:lifecycle-compiler:2.0.0"

Lifecycle 使用

然后我们来看看怎么解决上面的问题？我们可以通过添加注解来监控组件的生命周期状态，您可以通过调用 Lifecycle 类的 addObserver()方法并传递观察者的实例来添加观察者，如以下示例中所示：

class MyObserver : LifecycleObserver {

    @OnLifecycleEvent(Lifecycle.Event.ON_RESUME)
    fun connectListener() {
        ...
    }

    @OnLifecycleEvent(Lifecycle.Event.ON_PAUSE)
    fun disconnectListener() {
        ...
    }
}

myLifecycleOwner.getLifecycle().addObserver(MyObserver())

LifecycleOwner 是单一方法接口，表示类具有 Lifecycle。它具有一种方法（即 getLifecycle()，该方法必须由类实现。此接口从各个类（如 Fragment 和 AppCompatActivity）抽象化 Lifecycle 的所有权，并允许编写与这些类搭配使用的组件。任何自定义应用类均可实现 LifecycleOwner 接口。

实现 LifecycleObserver 的组件可与实现 LifecycleOwner 的组件完美配合，因为所有者可以提供生命周期，而观察者可以注册以观察生命周期。

class MyActivity : AppCompatActivity() {
    private lateinit var myLocationListener: MyLocationListener

    override fun onCreate(...) {
        myLocationListener = MyLocationListener(this, lifecycle) { location ->
            // update UI
        }
        Util.checkUserStatus { result ->
            if (result) {
                myLocationListener.enable()
            }
        }
    }
}

对于未知跟踪实例，我们可以让MyLocationListener 类实现 LifecycleObserver，然后在 onCreate 方法中实现 Activity 的 Lifecycle 对其进行初始化，然后配合注解。这意味着对生命周期状态的变化做出响应的逻辑会在MyLocationListener（而不是在 Activity）中进行声明。让各个组件存储自己的逻辑可使 Activity 和 Fragment 逻辑更易于管理。

同时还要注意的是，如果 Lifecycle 现在未处于良好的状态，则应避免调用某些回调。例如，如果回调在 Activity 状态保存后运行 Fragment 事务，就会触发崩溃，因此我们绝不能调用该回调。为简化此使用场景，Lifecycle 类允许其他对象查询当前状态。

对应到 MyLocationListener 的做法如下：

internal class MyLocationListener(
        private val context: Context,
        private val lifecycle: Lifecycle,
        private val callback: (Location) -> Unit
): LifecycleObserver {

    private var enabled = false

    @OnLifecycleEvent(Lifecycle.Event.ON_START)
    fun start() {
        if (enabled) {
            // connect
        }
    }

    fun enable() {
        enabled = true
        if (lifecycle.currentState.isAtLeast(Lifecycle.State.STARTED)) {
            // connect if not connected
        }
    }

    @OnLifecycleEvent(Lifecycle.Event.ON_STOP)
    fun stop() {
        // disconnect if connected
    }
}

对于此实现，LocationListener 类可以完全感知生命周期。如果我们需要从另一个 Activity 或 Fragment 使用 LocationListener，只需对其进行初始化。所有设置和拆解操作都由类本身管理。

如果您尝试管理整个应用进程的生命周期，请参阅 ProcessLifecycleOwner。

Applicaiton 生命周期 ProcessLifecycleOwner

通常我们判断在应用是不是处于前后台的做法是：注册一个 registerActivityLifecycleCallbacks(callback)，然后在 callback 中利用一个全局变量做计数，在onActivityStarted()中计数加1，在onActivityStopped方法中计数减1，从而判断前后台切换。

而使用ProcessLifecycleOwner可以直接获取应用前后台切换状态（记得先引入lifecycle-process依赖）。使用方式和Activity 中类似，只不过要使用 ProcessLifecycleOwner.get() 获取 ProcessLifecycleOwner，代码如下：

public class MyApplication extends Application {

    @Override
    public void onCreate() {
        super.onCreate();

	//注册App生命周期观察者
        ProcessLifecycleOwner.get().getLifecycle().addObserver(new ApplicationLifecycleObserver());
    }
    
    /**
     * Application生命周期观察，提供整个应用进程的生命周期
     *
     * Lifecycle.Event.ON_CREATE只会分发一次，Lifecycle.Event.ON_DESTROY不会被分发。
     *
     * 第一个Activity进入时，ProcessLifecycleOwner将分派Lifecycle.Event.ON_START, Lifecycle.Event.ON_RESUME。
     * 而Lifecycle.Event.ON_PAUSE, Lifecycle.Event.ON_STOP，将在最后一个Activit退出后后延迟分发。如果由于配置更改而销毁并重新创建活动，则此延迟足以保证ProcessLifecycleOwner不会发送任何事件。
     *
     * 作用：监听应用程序进入前台或后台
     */
    private static class ApplicationLifecycleObserver implements LifecycleObserver {
        @OnLifecycleEvent(Lifecycle.Event.ON_START)
        private void onAppForeground() {
            Log.w(TAG, "ApplicationObserver: app moved to foreground");
        }

        @OnLifecycleEvent(Lifecycle.Event.ON_STOP)
        private void onAppBackground() {
            Log.w(TAG, "ApplicationObserver: app moved to background");
        }
    }
}

实现自定义 LifecycleOwner

支持库 26.1.0 及更高版本中的 Fragment 和 Activity 已实现 LifecycleOwner接口。如果你有一个自定义类并希望使其成为 LifecycleOwner，可以使用 LifecycleRegistry类，但需要将事件转发到该类，代码如下：

class MyActivity : Activity(), LifecycleOwner {

    private lateinit var lifecycleRegistry: LifecycleRegistry

    override fun onCreate(savedInstanceState: Bundle?) {
        super.onCreate(savedInstanceState)

        lifecycleRegistry = LifecycleRegistry(this)
        lifecycleRegistry.markState(Lifecycle.State.CREATED)
    }

    public override fun onStart() {
        super.onStart()
        lifecycleRegistry.markState(Lifecycle.State.STARTED)
    }

    override fun getLifecycle(): Lifecycle {
        return lifecycleRegistry
    }
}

该 MainActivty 是实现了 Activity 类，而不是 AppcompatActivity（FragmentActivity）类，所以需要这样的处理。我们来看看 FragmentActivity 中的源码，比如 onCreate方法和 onPause方法

public class FragmentActivity extends ComponentActivity implements
        ActivityCompat.OnRequestPermissionsResultCallback,
        ActivityCompat.RequestPermissionsRequestCodeValidator {          

/**
     * A {@link Lifecycle} that is exactly nested outside of when the FragmentController
     * has its state changed, providing the proper nesting of Lifecycle callbacks
     * <p>
     * TODO(b/127528777) Drive Fragment Lifecycle with LifecycleObserver
     */
  final LifecycleRegistry mFragmentLifecycleRegistry = new LifecycleRegistry(this);


	protected void onCreate(@Nullable Bundle savedInstanceState) {
  
  	···
    mFragmentLifecycleRegistry.handleLifecycleEvent(Lifecycle.Event.ON_CREATE);
  	···
	}

		@Override
    protected void onPause() {
        super.onPause();
       	···
        mFragmentLifecycleRegistry.handleLifecycleEvent(Lifecycle.Event.ON_PAUSE);
    }
}          
//ComponentActivity.java
public class ComponentActivity extends androidx.core.app.ComponentActivity implements
        LifecycleOwner,
        ViewModelStoreOwner,
        SavedStateRegistryOwner,
        OnBackPressedDispatcherOwner {

明白了基本的操作，我们再看看看源码是如何实现的，注意，本文不会仔细深究源码，而是梳理一个大概的思路，如果想要讲具体深入每个细节，则可以照着我这个思路深入研究。

源码分析

Lifecycle 五种状态对应六个事件（生命周期）

首先我们来看 Lifecycle 这个类，让脑子里面有一个大致的概念

public abstract class Lifecycle {

    /**
     * Lifecycle coroutines extensions stashes the CoroutineScope into this field.
     *
     * @hide used by lifecycle-common-ktx
     */
    @RestrictTo(RestrictTo.Scope.LIBRARY_GROUP)
    @NonNull
    AtomicReference<Object> mInternalScopeRef = new AtomicReference<>();
		// 添加观察者
    @MainThread
    public abstract void addObserver(@NonNull LifecycleObserver observer);
  	// 移除观察者  
  	@MainThread
    public abstract void removeObserver(@NonNull LifecycleObserver observer);
    @MainThread
    @NonNull
    public abstract State getCurrentState(); // 获取当前状态，共五种状态

    @SuppressWarnings("WeakerAccess")
    public enum Event {  //对应六个生命周期方法，还多个一个 ON_ANY，可以响应任何事件
        ON_CREATE,
        ON_START,
        ON_RESUME,
        ON_PAUSE,
        ON_STOP,
        ON_DESTROY,
        ON_ANY
    }

    /**
     * Lifecycle states. You can consider the states as the nodes in a graph and
     * {@link Event}s as the edges between these nodes.
     */
    @SuppressWarnings("WeakerAccess")
    public enum State {  //五种状态，对应生命周期的方法/Event 方法
        DESTROYED,
        INITIALIZED,
        CREATED,
        STARTED,
        RESUMED;
        public boolean isAtLeast(@NonNull State state) {
            return compareTo(state) >= 0;
        }
    }
}

如果你觉得还不够清楚，只需要要记住五种状态对应六个生命周期方法（六个事件，ON_ANY 可以忽略）。然后我们再来看看官方的图，五个状态对应六个生命周期方法，六个事件，这句话一定要记住。

Event 表示生命周期事件，与 Activity/Fragment 生命周期方法对应。State 是 Lifecycle 这些生命周期对应的状态，为什么不直接用对应的生命周期设置对应的状态呢，而是设置了五个状态呢？原因很简单，因为 Lifecycle 不仅仅只是给自己用这些状态，还有 LiveData，ViewMode 等控件也会用到这些状态的进行逻辑判断，所以 Google 工程师做了更好的封装和管理。

//Lifecycle.class
* {@link Event#ON_CREATE}, {@link Event#ON_START}, {@link Event#ON_RESUME} events in this class
 * are dispatched <b>after</b> the {@link LifecycleOwner}'s related method returns.
 * {@link Event#ON_PAUSE}, {@link Event#ON_STOP}, {@link Event#ON_DESTROY} events in this class
 * are dispatched <b>before</b> the {@link LifecycleOwner}'s related method is called.

Event 中的 ON_CREATE，ON_START，ON_RESUME 是在 LifecyclerOwner 对应的方法执行之后分发，ON_PAUSE，ON_STOP，ON_DESTROY 是在 LifecyclerOwner 对应的方法执行之前分发。

我们的 Activity 因为实现了 LifecycleOwner 才能直接调用 getLifecycle()，从前面的分析我们可以看到是在 ComponentActivity 中实现了该接口

public class ComponentActivity extends androidx.core.app.ComponentActivity implements
        LifecycleOwner,
        ViewModelStoreOwner,
        SavedStateRegistryOwner,
        OnBackPressedDispatcherOwner {
private final LifecycleRegistry mLifecycleRegistry = new LifecycleRegistry(this);
		@Override
    protected void onCreate(@Nullable Bundle savedInstanceState) {
        super.onCreate(savedInstanceState);
        mSavedStateRegistryController.performRestore(savedInstanceState);
        ReportFragment.injectIfNeededIn(this);
        if (mContentLayoutId != 0) {
            setContentView(mContentLayoutId);
        }
    }  
    @NonNull
    @Override
    public Lifecycle getLifecycle() {
        return mLifecycleRegistry;
    } 
// LifecycleRegistry.java
public class LifecycleRegistry extends Lifecycle {

getLifecycle().addObserver(MyObserver()) 是我们添加观察者的方式，我们来看看addObserver方法的实现，因为的实现是LifecycleRegistry

//LifecycleRegistry.java
    @Override
    public void addObserver(@NonNull LifecycleObserver observer) {
        State initialState = mState == DESTROYED ? DESTROYED : INITIALIZED;
      	//带状态的观察者，这个状态的作用：新的事件触发后 遍历通知所有观察者时，判断是否已经通知这个观察者了
        ObserverWithState statefulObserver = new ObserverWithState(observer, initialState);
        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;
        }
				//下面代码的逻辑：通过while循环，把新的观察者的状态 连续地 同步到最新状态mState。
    		//意思就是：虽然可能添加的晚，但把之前的事件一个个分发给你(upEvent方法)，即粘性
        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--;
    }

通过statefulObserver.dispatchEvent(lifecycleOwner, upEvent(statefulObserver.mState));分发事件，我们先来看ObserverWithState

    static class ObserverWithState {
        State mState;
        LifecycleEventObserver mLifecycleObserver;

        ObserverWithState(LifecycleObserver observer, State initialState) {
            mLifecycleObserver = Lifecycling.lifecycleEventObserver(observer);
            mState = initialState;
        }
      //分发，通知观察者
      void dispatchEvent(LifecycleOwner owner, Event event) {
            State newState = getStateAfter(event);
            mState = min(mState, newState);
            mLifecycleObserver.onStateChanged(owner, event);
            mState = newState;
        }

mLifecycleObserver的值为Lifecycling.lifecycleEventObserver(observer);，然后查看方法lifecycleEventObserver

    @NonNull
    static LifecycleEventObserver lifecycleEventObserver(Object object) {
        boolean isLifecycleEventObserver = object instanceof LifecycleEventObserver;
        boolean isFullLifecycleObserver = object instanceof FullLifecycleObserver;
       	···
        return new ReflectiveGenericLifecycleObserver(object);
    }

查看最后一行中的ReflectiveGenericLifecycleObserver

class ReflectiveGenericLifecycleObserver implements LifecycleEventObserver {
    private final Object mWrapped;
    private final CallbackInfo mInfo;

    ReflectiveGenericLifecycleObserver(Object wrapped) {
        mWrapped = wrapped;
        mInfo = ClassesInfoCache.sInstance.getInfo(mWrapped.getClass()); //存放了注解方法的信息
    }

    @Override
    public void onStateChanged(LifecycleOwner source, Event event) {
        mInfo.invokeCallbacks(source, event, mWrapped); //执行状态变化对应的方法
    }
}

getInfo的内容如下：

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

处理注解

接下来是重要方法createInfo，在该方法中进行注解的处理，并存储到 mCallbackMap。提供给后面的观察者调用

   //createInfo,遍历方法 处理注解，将
       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();

						···
            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;
    }

被观察者通知观察者

观察者进行回调的时候调用了ReflectiveGenericLifecycleObserver的mInfo.invokeCallbacks(source, event, mWrapped);，最终会调用到MethodReference的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);
            }
        }

到这里观察者的注册和注解的处理，以及通知被观察者的流程就已经分析完毕了。

如何监听生命周期

我们回到之前的ComponentActivity的onCreate方法中可以看到调用了ReportFragment.injectIfNeededIn(this);，这个 ReportFragment 和 Glide 中使用透明的 Fragment 获取界面生命周期是一样的。

    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();
        }
    }

然后通过dispatch方法分发生命周期

    @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;
    }

dispatch 的具体逻辑如下，最终都是调用 handleLifecycleEvent来处理

    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);
            }
        }
    }

因为我们这里是LifecycleRegistryOwner，所以来看看其handleLifecycleEvent方法

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

紧接着调用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(); //把生命周期状态同步给所有观察者
    mHandlingEvent = false;
}

接着调用sync方法，然后把生命周期同步给所有观察者

    // happens only on the top of stack (never in reentrance),
    // so it doesn't have to take in account parents
    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;
    }