3. Jetpack源码解析---用Lifecycles管理生命周期

Hankkin

发布于 2022-01-11 15:26:02

5650

发布于 2022-01-11 15:26:02

文章被收录于专栏：Android干货园

1. 背景

上一篇我门对Jetpack组件中的Navigation做了介绍，并且对其做了源码分析，相信看过之后已经对此有了一定的了解，本篇文章我们会对Lifecycles进行使用及源码的介绍，还没看上篇的可以看一下：

系列文章：

1. Jetpack源码解析—看完你就知道Navigation是什么了？

2. Jetpack源码解析—Navigation为什么切换Fragment会重绘？

2. 基础

2.1 简介

Lifecycles是一个持有组件生命周期状态（如Activity、Fragment）信息的类，并允许其他对象观察此状态。可以帮助我们方便的管理Activity和Fragment的生命周期。

Lifecycle组件中通过两个枚举类来跟踪其关联组件的生命周期：

2.2 基本使用

在我们的日常开发中，经常需要我们在Activity或者Fragment的生命周期方法中手动去管理一下资源的释放等行为。举个简单的例子，当我们做自定义相机扫描的时候，camera相关的资源就需要我们手动的去释放及开启预览；或者我们在使用MVP模式去开发的时候，P的创建和销毁也需要我们在生命周期方法中去操作。

通过Lifecycles组件我们可以这样使用：

我们可以定义一个Observer实现LifecycleObserver，并且在Activity或者Fragment中进行观察：

/**
 * created by ${Hankkin}
 * on 2019-06-10
 */

class MyObserver : LifecycleObserver {

    @OnLifecycleEvent(Lifecycle.Event.ON_START)
    fun onStart() {
        Log.e(javaClass.name, "-------onStart")
    }

    @OnLifecycleEvent(Lifecycle.Event.ON_CREATE)
    fun onCreate() {
        Log.e(javaClass.name, "-------onCreate")
    }

    @OnLifecycleEvent(Lifecycle.Event.ON_RESUME)
    fun onResume() {
        Log.e(javaClass.name, "-------onResume")
    }

    @OnLifecycleEvent(Lifecycle.Event.ON_PAUSE)
    fun onPause() {
        Log.e(javaClass.name, "-------onPause")
    }


    @OnLifecycleEvent(Lifecycle.Event.ON_STOP)
    fun onStop() {
        Log.e(javaClass.name, "-------onStop")
    }


    @OnLifecycleEvent(Lifecycle.Event.ON_DESTROY)
    fun onDestroy() {
        Log.e(javaClass.name, "-------onDestroy")
    }

}

下面是Activity：

class MainActivity : AppCompatActivity() {
    
    override fun onCreate(savedInstanceState: Bundle?) {
        super.onCreate(savedInstanceState)
        setContentView(R.layout.activity_main)


        lifecycle.addObserver(MyObserver())
        Log.e(javaClass.name, "-------onCreate")

    }

    override fun onStart() {
        super.onStart()
        Log.e(javaClass.name, "-------onStart")
    }

    override fun onResume() {
        super.onResume()
        Log.e(javaClass.name, "-------onResume")
    }

    override fun onPause() {
        super.onPause()
        Log.e(javaClass.name, "-------onPause")
    }

    override fun onStop() {
        super.onStop()
        Log.e(javaClass.name, "-------onStop")
    }

    override fun onDestroy() {
        super.onDestroy()
        Log.e(javaClass.name, "-------onDestroy")
    }
}

启动Activity，我们可以看到控制台中的打印日志：

MainActivity: -------onCreate
MyObserver: -------onCreate
MainActivity: -------onStart
MyObserver: -------onStart
MainActivity: -------onResume
MyObserver: -------onResume
......

通过控制台打印我们可以看到我们的观察者Activity和被观察者的日志均被打印了。具体是怎么实现的呢？

3. 源码分析

通过代码我们大概能看出来Lifecycles组件是通过观察者模式来实现的，接下来我们具体分析实现原理，我们发现在Activity和Fragment中可以直接通过getLifecycle()方法获取到Lifecycle，那么我们就从这里入手:

3.1 getLifecycle()

我们点击进去发现ComponentActivity中实现了LifecycleOwner接口，LifecycleOwner接口中则声明了getLifecycle()方法，而ComponentActivity中直接返回了mLifecycleRegistry：

public class ComponentActivity extends androidx.core.app.ComponentActivity implements
        LifecycleOwner,
        ViewModelStoreOwner,
        SavedStateRegistryOwner,
        OnBackPressedDispatcherOwner {

    private final LifecycleRegistry mLifecycleRegistry = new LifecycleRegistry(this);
    ......
    @NonNull
    @Override
    public Lifecycle getLifecycle() {
        return mLifecycleRegistry;
    }

LifecycleRegistry是个什么东西呢？原来它继承了Lifecycle

public class LifecycleRegistry extends Lifecycle

3.2 Lifecycle和LifecycleRegistry

我们看一下Lifecycle类：

public abstract class Lifecycle {

        //注册LifecycleObserver （比如Presenter）
        public abstract void addObserver(@NonNull LifecycleObserver observer);
        //移除LifecycleObserver 
        public abstract void removeObserver(@NonNull LifecycleObserver observer);
        //获取当前状态
        public abstract State getCurrentState();

        public enum Event {
            ON_CREATE,
            ON_START,
            ON_RESUME,
            ON_PAUSE,
            ON_STOP,
            ON_DESTROY,
            ON_ANY
        }
        
       public enum State {
            DESTROYED,
            INITIALIZED,
            CREATED,
            STARTED,
            RESUMED;

            public boolean isAtLeast(@NonNull State state) {
                return compareTo(state) >= 0;
            }
       }
}

Lifecycle中就是声明了一些抽象方法和两个状态的枚举类，具体的实现看LifecycleRegistry：

public class LifecycleRegistry extends Lifecycle {

// LifecycleObserver Map，每一个Observer都有一个State
    private FastSafeIterableMap<LifecycleObserver, ObserverWithState> mObserverMap = new FastSafeIterableMap<>();
    // 当前的状态
    private State mState;
    // 生命周期拥有者，ComponentActivity继承了LifecycleOwner
    private final WeakReference<LifecycleOwner> mLifecycleOwner;

    //修改State值
    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;
    }

    /**
    * 添加LifecycleObserver观察者，并将之前的状态分发给这个Observer,例如我们在onResume之后注册这个Observer，
    * 该Observer依然能收到ON_CREATE事件
    */
    @Override
    public void addObserver(@NonNull LifecycleObserver observer) {
        State initialState = mState == DESTROYED ? DESTROYED : INITIALIZED;
        //新建带有状态的观察者
        ObserverWithState statefulObserver = new ObserverWithState(observer, initialState);
        ObserverWithState previous = mObserverMap.putIfAbsent(observer, statefulObserver);
        ......
        // 例如：Observer初始状态是INITIALIZED，当前状态是RESUMED，需要将INITIALIZED到RESUMED之间的
        // 所有事件分发给Observer
        while ((statefulObserver.mState.compareTo(targetState) < 0
                && mObserverMap.contains(observer))) {
            pushParentState(statefulObserver.mState);
            statefulObserver.dispatchEvent(lifecycleOwner, upEvent(statefulObserver.mState));
            popParentState();
            targetState = calculateTargetState(observer);
        }
        ......
}

/**
     * 同步Observer状态，并分发事件
     */
    private void sync() {
        LifecycleOwner lfecycleOwner = 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;
            // State中，状态值是从DESTROYED-INITIALIZED-CREATED-STARTED-RESUMED增大
            // 如果当前状态值 < Observer状态值，需要通知Observer减小状态值，直到等于当前状态值
            if (mState.compareTo(mObserverMap.eldest().getValue().mState) < 0) {
                backwardPass(lifecycleOwner);
            }
            Entry<LifecycleObserver, ObserverWithState> newest = mObserverMap.newest();
            // 如果当前状态值 > Observer状态值，需要通知Observer增大状态值，直到等于当前状态值
            if (!mNewEventOccurred && newest != null
                    && mState.compareTo(newest.getValue().mState) > 0) {
                forwardPass(lifecycleOwner);
            }
        }
        mNewEventOccurred = false;
    }

    /**
     * 向前传递事件，对应图中的INITIALIZED -> RESUMED
     * 增加Observer的状态值，直到状态值等于当前状态值
     */
    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);
                // 分发状态改变事件
                observer.dispatchEvent(lifecycleOwner, upEvent(observer.mState));
                popParentState();
            }
        }
    }

    /**
     * 向后传递事件，对应图中的RESUMED -> DESTROYED
     * 减小Observer的状态值，直到状态值等于当前状态值
     */
    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);
                // 分发状态改变事件
                pushParentState(getStateAfter(event));
                observer.dispatchEvent(lifecycleOwner, event);
                popParentState();
            }
        }
    }

LifecycleRegistry代码我门看完了，注释也已经很清楚了，基本作用就是添加观察者，响应生命周期事件，分发生命周期事件的作用。

3.3 ReportFragment

接下来我们继续分析ComponentActivity，我们在onCreate()声明周期中发现一个比较熟悉ReportFragment，这个fragment我以前在做内存泄漏优化的时候经常碰到，在leakcanary中经常会报出这个类，所以这里面看看ReportFragment.injectIfNeededIn(this);到底做了什么？

@RestrictTo(RestrictTo.Scope.LIBRARY_GROUP_PREFIX)
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();
        }
    }
    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);
            }
        }
    }

查看源码我们发现，ReportFragment在每个生命周期中都做了分发事件的处理，通过调用getLifecycle().handleLifecycleEvent(event)来做生命周期的分发。意思也就是在ComponentActivity中添加了一个没有页面的ReportFragment，当Activity生命周期变化的时候，通过调用LifecycleRegistry.handleLifecycleEvent()方法通知LifecycleRegistry改变状态，LifecycleRegistry内部调用moveToState()改变状态，并调用每个LifecycleObserver.onStateChange()方法通知生命周期变化。

我们通过查看ReportFragment调用，发现还有两个类也调用了它，一个是LifecycleDispatcher，一个是ProcessLifecycleOwner，这两个究竟是做什么的呢？

[外链图片转存失败(img-Kmu4o9EL-1562754821909)(http://psv0rasgi.bkt.clouddn.com/jetpack/_image/2019-06-10/屏幕快照 2019-06-11 下午2.19.20 (2).png#align=left&display=inline&height=324&originHeight=648&originWidth=1112&status=done&width=556)]

3.4 LifecycleDispatcher

class LifecycleDispatcher {

    private static AtomicBoolean sInitialized = new AtomicBoolean(false);

    static void init(Context context) {
        if (sInitialized.getAndSet(true)) {
            return;
        }
        ((Application) context.getApplicationContext())
                .registerActivityLifecycleCallbacks(new DispatcherActivityCallback());
    }

    @SuppressWarnings("WeakerAccess")
    @VisibleForTesting
    static class DispatcherActivityCallback extends EmptyActivityLifecycleCallbacks {

        @Override
        public void onActivityCreated(Activity activity, Bundle savedInstanceState) {
            ReportFragment.injectIfNeededIn(activity);
        }

        @Override
        public void onActivityStopped(Activity activity) {
        }

        @Override
        public void onActivitySaveInstanceState(Activity activity, Bundle outState) {
        }
    }

    private LifecycleDispatcher() {
    }
}

我们查看源码，发现在init()方法中，通过Application注册了DispatcherActivityCallback，并且在onActivityCreated中将ReportFragment注入到Activity中。

3.5 ProcessLifecycleOwner

public class ProcessLifecycleOwner implements LifecycleOwner {

       private final LifecycleRegistry mRegistry = new LifecycleRegistry(this);

    private Runnable mDelayedPauseRunnable = new Runnable() {
        @Override
        public void run() {
            dispatchPauseIfNeeded();
            dispatchStopIfNeeded();
        }
    };

    private ActivityInitializationListener mInitializationListener =
            new ActivityInitializationListener() {
                @Override
                public void onCreate() {
                }

                @Override
                public void onStart() {
                    activityStarted();
                }

                @Override
                public void onResume() {
                    activityResumed();
                }
            };

    private static final ProcessLifecycleOwner sInstance = new ProcessLifecycleOwner();

   
    public static LifecycleOwner get() {
        return sInstance;
    }

    static void init(Context context) {
        sInstance.attach(context);
    }

    void activityStarted() {
        mStartedCounter++;
        if (mStartedCounter == 1 && mStopSent) {
            mRegistry.handleLifecycleEvent(Lifecycle.Event.ON_START);
            mStopSent = false;
        }
    }

    void activityResumed() {
        mResumedCounter++;
        if (mResumedCounter == 1) {
            if (mPauseSent) {
                mRegistry.handleLifecycleEvent(Lifecycle.Event.ON_RESUME);
                mPauseSent = false;
            } else {
                mHandler.removeCallbacks(mDelayedPauseRunnable);
            }
        }
    }

   ......
    private void dispatchStopIfNeeded() {
        if (mStartedCounter == 0 && mPauseSent) {
            mRegistry.handleLifecycleEvent(Lifecycle.Event.ON_STOP);
            mStopSent = true;
        }
    }

    private ProcessLifecycleOwner() {
    }
    //监听Application生命周期，并分发给Activity
    void attach(Context context) {
        mHandler = new Handler();
        mRegistry.handleLifecycleEvent(Lifecycle.Event.ON_CREATE);
        Application app = (Application) context.getApplicationContext();
        app.registerActivityLifecycleCallbacks(new EmptyActivityLifecycleCallbacks() {
            @Override
            public void onActivityCreated(Activity activity, Bundle savedInstanceState) {
                ReportFragment.get(activity).setProcessListener(mInitializationListener);
            }

            @Override
            public void onActivityPaused(Activity activity) {
                activityPaused();
            }

            @Override
            public void onActivityStopped(Activity activity) {
                activityStopped();
            }
        });
    }
}

根据官方注释，我们可以了解到：

ProcessLifecycleOwner是用来监听Application生命周期的，因此它只会分发一次ON_CREATE事件，并且不会分发ON_DESTROY事件。
ProcessLifecycleOwner在Activity的onResume和onStop方法中都采用了Handle.postDelayed()方法，是为了处理Activity重建时比如横竖屏幕切换时，不会发送事件。
ProcessLifecycleOwner一般用来判断应用是在前台还是后台。但由于使用了Handle.postDelayed()，因此这个判断不是即时的，有默认700ms的延迟。
ProcessLifecycleOwner与LifecycleDispatcher一样，都是通过注册Application.registerActivityLifecycleCallbacks来监听Activity的生命周期回调，来给每个Activity添加ReportFragment的。

看了着两个类我们发现它们的入口均为init(),所以看一下谁调用了？

3.6 ProcessLifecycleOwnerInitializer

public class ProcessLifecycleOwnerInitializer extends ContentProvider {
    @Override
    public boolean onCreate() {
        LifecycleDispatcher.init(getContext());
        ProcessLifecycleOwner.init(getContext());
        return true;
    }

    @Nullable
    @Override
    public Cursor query(@NonNull Uri uri, String[] strings, String s, String[] strings1,
            String s1) {
        return null;
    }

    @Nullable
    @Override
    public String getType(@NonNull Uri uri) {
        return null;
    }

    @Nullable
    @Override
    public Uri insert(@NonNull Uri uri, ContentValues contentValues) {
        return null;
    }

    @Override
    public int delete(@NonNull Uri uri, String s, String[] strings) {
        return 0;
    }

    @Override
    public int update(@NonNull Uri uri, ContentValues contentValues, String s, String[] strings) {
        return 0;
    }
}

果真，在ProcessLifecycleOwnerInitializer 的onCreate()中对这两个进行了初始化，看类名可以翻译成_进程生命周期初始化_，到这里我们对该类就找不到调用者或者使用者了，所以不得不百度一下，发现有人说这个类是在AndroidManifest.xml中声明的，在构建APK过程中，AS会将多个模块的AndroidManifest.xml合并到一起，所以查看build目录，具体路径为build/intermediates/bundle_manifest/debug/processDebugManifest/bundle-manifest/AndroidManifest.xml，果真在里面：

<?xml version="1.0" encoding="utf-8"?>
<manifest xmlns:android="http://schemas.android.com/apk/res/android"
    package="com.hankkin.reading_aac"
    android:versionCode="1"
    android:versionName="1.0" >

    <uses-sdk
        android:minSdkVersion="19"
        android:targetSdkVersion="28" />

    <application
        android:appComponentFactory="androidx.core.app.CoreComponentFactory"
        android:debuggable="true"
        android:icon="@mipmap/ic_launcher"
        android:label="@string/app_name"
        android:roundIcon="@mipmap/ic_launcher_round"
        android:supportsRtl="true"
        android:testOnly="true"
        android:theme="@style/AppTheme" >
        <activity android:name="com.hankkin.reading_aac.ui.LoginActivity" >
        </activity>
        <activity android:name="com.hankkin.reading_aac.MainActivity" >
            <intent-filter>
                <action android:name="android.intent.action.MAIN" />

                <category android:name="android.intent.category.LAUNCHER" />
            </intent-filter>
        </activity>

        <provider
            android:name="androidx.lifecycle.ProcessLifecycleOwnerInitializer"
            android:authorities="com.hankkin.reading_aac.lifecycle-process"
            android:exported="false"
            android:multiprocess="true" />
    </application>

</manifest>

到这里整个Lifecycle初始化的过程就结束了。

4. 总结

经过上面的源码分析，我们可以大概给整个Lifecycle组件分为三部分：

4.1 Lifecycle的初始化

通过在manifest中声明provider，`ProcessLifecycleOwnerInitializer`注册Activity和fragment.的声明周期回调并做监听，同时向Activity中添加一个空白的`ReportFragment`，使用它作为生命周期的事件分发。而当Activity和Fragment生命周期状态发生改变时，都通过LifecycleRegistryOwner来处理生命周期状态的改变。

4.2 Lifecycle的状态改变及状态分发

在`ReportFragment`中调用`LifecycleRegister.handleLifecycleEvent(Lifecycle.Event)`，通过**获取事件对应的下一个状态**以及**改变当前状态到下一个状态**，同步分发事件；最后将各个观察者的生命周期状态依次移动到正确状态。

可以查看下面的时序图：

4.3 Lifecycle解析生命周期

我们声明的MyObserver中的方法都是带有注解的，查看OnLifecycleEvent：

@Retention(RetentionPolicy.RUNTIME)
@Target(ElementType.METHOD)
public @interface OnLifecycleEvent {
    Lifecycle.Event value();
}

注解修饰的方法会通过反射的方式获取，并且保存在ClassesInfoCache中，然后在生命周期发生改变的时候再找到对应 Event 的方法，通过反射来调用方法。

5. 参考链接：

https://juejin.im/post/5cd81634e51d453af7192b87#heading-10

https://yuqirong.me/2018/07/15/Android%20Architecture%20Component%E4%B9%8BLifecycle%E8%A7%A3%E6%9E%90/

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