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？
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。
