【Jetpack】Lifecycle生命周期感知组件
Lifecycle生命周期感知组件
安卓开发过程中业务和应用的生命周期息息相关，这会导致业务和生命周期的耦合。尽管通过MVP等架构设计项目代码稍微有一些好转，但是还是依赖生命周期的各种回调。Google提供的Lifecycle组件，是一种可以感知应用程序生命周期的组件，开发者专注自己的业务，降低代码的耦合。
1、LifecycleObserver和LifecycleOwner
Lifecycle使用之前需要先来了解两个接口：LifecycleObserver和LifecycleOwner。从名字不难看出，一个是观察者，另一个是被观察者。是设计模式中的观察者模式，对生命周期进行监听。
1.1、LifecycleObserver
LifecycleObserver的接口定义如下，这是需要开发者实现的接口，虽然没有任何方法。
public interface LifecycleObserver {
}1.2、LifecycleOwner
LifecycleOwner的接口定义如下，有一个抽象方法getLifecycle()，返回非空的Lifecycle对象。
public interface LifecycleOwner {
/**
* Returns the Lifecycle of the provider.
*
* @return The lifecycle of the provider.
*/
@NonNull
Lifecycle getLifecycle();
}目前的项目中使用的是AndroidX库中的AppCompatActivity
public class MainActivity extends AppCompatActivity {
...
}查看源码，可以发现AppCompatActivity继承自ComponentActivity类，该类已经实现了LifecycleOwner接口。所以观察者模式中的被观察者已经由Google实现好了，剩下的观察者就由开发者自己实现LifecycleObserver接口。
public class ComponentActivity extends androidx.core.app.ComponentActivity implements
LifecycleOwner,
ViewModelStoreOwner,
SavedStateRegistryOwner,
OnBackPressedDispatcherOwner {
...
}LifecycleOwner接口中的getLifecycle()方法实现如下，
@NonNull
@Override
public Lifecycle getLifecycle() {
return mLifecycleRegistry;
}返回LifecycleRegistry类型的mLifecycleRegistry实例，LifecycleRegistry继承Lifecycle抽象类。进入第二部分
2、Lifecycle
Lifecycle是组件的核心。
dependencies {
def lifecycle_version = "2.2.0"
// Lifecycles only (without ViewModel or LiveData)
implementation "androidx.lifecycle:lifecycle-runtime:$lifecycle_version"
}2.1、颇析Lifecycle抽象类
定义了注册和反注册观察者的接口
addObserver(LifecycleObserver observer) removeObserver(LifecycleObserver observer)
以及一个获取当前生命周期状态的方法
State getCurrentState()
State是内部定义的枚举，标识Android组件的不同生命周期状态
public enum State {
DESTROYED,
INITIALIZED,
CREATED,
STARTED,
RESUMED;
}还有另外一个枚举Event，定义生命周期状态的回调事件。后面会在实现LifecycleObserver的接口中注解方法时用到。
public enum Event {
ON_CREATE,
ON_START,
ON_RESUME,
ON_PAUSE,
ON_STOP,
ON_DESTROY,
/**
* An {@link Event Event} constant that can be used to match all events.
*/
ON_ANY
}2.2、实现类LifecycleRegistry
Lifecycle是抽象类，定义了接口和基本数据类型。优秀的设计模式往往是这样，对扩展开放对修改闭合。Lifecycle的实现类是LifecycleRegistry，可以处理多个观察者LifecycleObserver。它的构造方法如下：
public LifecycleRegistry(LifecycleOwner provider) {//传入LifecycleOwner被观察者
mLifecycleOwner = new WeakReference<>(provider);
mState = INITIALIZED;
}AndroidX库中的Activity都自行实现了被观察者部分的代码，实现LifecycleOwner接口。并将将自身作为被观察对象，传递给LifecycleRegistry。
public class ComponentActivity extends ComponentActivity implements LifecycleOwner ,...{
private final LifecycleRegistry mLifecycleRegistry = new LifecycleRegistry(this);
...
}LifecycleRegistry被AndroidX库中fragment和Activity使用。
2.3、使用Lifecycle组件解耦Activity
自定义类实现LifecycleObserver接口，定义业务自己的组件。因为该接口是空方法所以不需要实现任何方法，里面的方法都是和业务相关的(资源释放、业务逻辑、动画等)，并且放在不同的方法中，通过@OnLifecycleEvent注解对应执行的生命周期，当Android生命周期发生变化的时候自动回调对应方法。
public class Listener implements LifecycleObserver {
private static final String TAG = "TAG_Listener";
@OnLifecycleEvent(Lifecycle.Event.ON_RESUME)
private void start() {
Log.d(TAG, "start");
}
@OnLifecycleEvent(Lifecycle.Event.ON_STOP)
private void stop() {
Log.d(TAG, "stop");
}
}①现在的项目基本上都是使用AndroidX中的Activity作为基础类(如果不是，赶紧适配升级！)，它们都实现了LifecycleOwner接口，并且拥有Lifecycle实例，可以方便的获取该被观察者实例，注册观察者。
public class MainActivity extends AppCompatActivity {
private Lifecycle lifecycle;
@Override
protected void onCreate(Bundle savedInstanceState) {
super.onCreate(savedInstanceState);
setContentView(R.layout.activity_main);
//获取mLifecycleRegistry对象
lifecycle = getLifecycle();
//注册观察者
lifecycle.addObserver(new LocationListener(this));
}
}当Activity生命周期发生变化的时候会自动通过注册调用LifecycleObserver实例中的对应方法。
②开发者也可以自定义前面介绍的LifecycleOwner被观察者接口，直接创建LifecycleRegistry实例并使用它。
public class MyActivity extends Activity implements LifecycleOwner {//当前Activity继承自基类Activity，实现LifecycleOwner接口
private LifecycleRegistry lifecycle;
@Override
protected void onCreate(@Nullable Bundle savedInstanceState) {
super.onCreate(savedInstanceState);
//创建LifecycleRegistry实例，持有当前实现了LifecycleOwner的Activity
lifecycle = new LifecycleRegistry(this);
lifecycle.addObserver(new Listener(this));
lifecycle.setCurrentState(Lifecycle.State.CREATED);
}
@Override
public Lifecycle getLifecycle() {
return lifecycle;
}
@Override
protected void onResume() {
super.onResume();
//在Activity各个生命周期设置当前生命周期State状态，会回调对应的方法
lifecycle.setCurrentState(Lifecycle.State.RESUMED);
}
@Override
protected void onDestroy() {
super.onDestroy();
lifecycle.setCurrentState(Lifecycle.State.DESTROYED);
}
}每次重新设置当前被观察者的State，都会执行如下的方法回调moveToState(state)，在第三节中详细分析该方法的回调流程。
/**
* Moves the Lifecycle to the given state and dispatches necessary events to the observers.
*/
@MainThread
public void setCurrentState(State state) {
moveToState(state);
}不使用AndroidX中的Activity，自己实现LifecycleOwner接口，结合LifecycleRegistry也能实现生命周期监听的组件。
3、LifecycleService
拥有生命周期的组件不止Activity，Service同样拥有生命周期。关于Service详见四大组件之服务：Service。这就用到Lifecycle系列组件的中的LifecycleService。
dependencies {
def lifecycle_version = "2.2.0"
implementation "androidx.lifecycle:lifecycle-service:$lifecycle_version"
...
}3.1、LifecycleService
LifecycleService继承自Service，实现了LifecycleOwner接口和实现getLifecycle()方法
public class LifecycleService extends Service implements LifecycleOwner {
private final ServiceLifecycleDispatcher mDispatcher = new ServiceLifecycleDispatcher(this);
public void onCreate() {
mDispatcher.onServicePreSuperOnCreate();
super.onCreate();
}
public IBinder onBind(Intent intent) {
mDispatcher.onServicePreSuperOnBind();
return null;
}
public void onStart(Intent intent, int startId) {
mDispatcher.onServicePreSuperOnStart();
super.onStart(intent, startId);
}
public int onStartCommand(Intent intent, int flags, int startId) {
return super.onStartCommand(intent, flags, startId);
}
public void onDestroy() {
mDispatcher.onServicePreSuperOnDestroy();
super.onDestroy();
}
public Lifecycle getLifecycle() {
return mDispatcher.getLifecycle();
}
}之所以LifecycleService如此简洁，是因为将生命周期相关的回调又抽取到ServiceLifecycleDispatcher类中了。在ServiceLifecycleDispatcher 中进行各种生命周期的分发回调给观察者LifecycleObserver。
public ServiceLifecycleDispatcher(LifecycleOwner provider) {
mRegistry = new LifecycleRegistry(provider);
mHandler = new Handler();
}在Service不同的生命周期都会进行回调，通过最后的Runnable中执行的handleLifecycleEvent(Lifecycle.Event event)方法
public void handleLifecycleEvent(Lifecycle.Event event) {
State next = getStateAfter(event);
moveToState(next);
}来看看moveToState(State next)方法：
private void moveToState(State next) {
if (mState == next) {
return;
}
...
mHandlingEvent = true;
sync();
mHandlingEvent = false;
}关键是其中的sync()同步，
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;
}根据情况适宜的向后推、向前推分别调用backwardPass(lifecycleOwner)和forwardPass(lifecycleOwner)方法。以forwardPass(lifecycleOwner)方法为例，代码如下：
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();
}
}
}调用ObserverWithState类的dispatchEvent(LifecycleOwner owner, Event event)方法
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;
}
}mLifecycleObserver对象的onStateChanged()方法，mLifecycleObserver是LifecycleEventObserver类型，该接口类型的定义如下
public interface LifecycleEventObserver extends LifecycleObserver {
void onStateChanged(LifecycleOwner source,Lifecycle.Event event);
}它的实现类是ReflectiveGenericLifecycleObserver。具体代码就不往下看了，后面会通过反射拿到注解，根据注解反射调用对应的方法，从而实现代码层面的解耦。
3.2、使用LifecycleService解耦Service
自定义观察者，实现LifecycleObserver接口，注解不同生命周期对应的回调方法
class MyServiceObserver implements LifecycleObserver {
@OnLifecycleEvent(Lifecycle.Event.ON_START)
private void startService() {
Log.d("TAG_QUIBBLER", "startService Lifecycle.Event.ON_START");
}
@OnLifecycleEvent(Lifecycle.Event.ON_STOP)
private void stopService() {
Log.d("TAG_QUIBBLER", "stopService Lifecycle.Event.ON_STOP");
}
}只需自定义Service继承自LifecycleService即可，在Service的生命周期中专注业务处理，不用考虑生命周期的回调而耦合。
public class MyService extends LifecycleService {
public MyService() {
getLifecycle().addObserver(new MyServiceObserver());
}
}别忘了在AndroidManifest中注册Service！
4、ProcessLifecycleOwner
Application也是有生命周期的，只不过生命周期没有Activity或Service状态那么多，只有创建、启动等。通过之前提到的Activity生命周期监听ActivityLifecycleCallbacks回调可以方便的在Application中监听所有Activity的生命周期创建。这里通过Lifecycle组件中的ProcessLifecycleOwner组件可以方便的监听整个应用进程的状态，比如前后台。
dependencies {
def lifecycle_version = "2.2.0"
// optional - ProcessLifecycleOwner provides a lifecycle for the whole application process
implementation "androidx.lifecycle:lifecycle-process:$lifecycle_version"
}4.1、使用ProcessLifecycleOwner监听应用程序生命周期
使用ProcessLifecycleOwner监听应用程序生命周期，首先自定义观察者实现LifecycleObserver接口。
public static class MyApplicationObserver implements LifecycleObserver {
private static final String TAG = "TAG_MyApplicationObserver";
//调用一次
@OnLifecycleEvent(Lifecycle.Event.ON_CREATE)
public void onCreate() {
Log.d(TAG, "onCreate");
}
//进入前台调用
@OnLifecycleEvent(Lifecycle.Event.ON_START)
public void onStart() {
Log.d(TAG, "onStart");
}
@OnLifecycleEvent(Lifecycle.Event.ON_RESUME)
public void onResume() {
Log.d(TAG, "onResume");
}
//进入后台调用
@OnLifecycleEvent(Lifecycle.Event.ON_PAUSE)
public void onPause() {
Log.d(TAG, "onPause");
}
@OnLifecycleEvent(Lifecycle.Event.ON_STOP)
public void onStop() {
Log.d(TAG, "onStop");
}
//不会调用
@OnLifecycleEvent(Lifecycle.Event.ON_DESTROY)
public void onDestroy() {
Log.d(TAG, "onDestroy");
}
}在自定义的Application中使用ProcessLifecycleOwner注册观察者，ProcessLifecycleOwner是单例。
public class MyApplication extends Application {
@Override
public void onCreate() {
super.onCreate();
ProcessLifecycleOwner.get().getLifecycle().addObserver(new MyApplicationObserver());
}
}4.2、使用注意
①ProcessLifecycleOwner监听整个应用程序的生命周期，和Activity数量无关
②ON_CREATE只会调用一次，因为应用程序启动只会被创建一次。ON_DESTORY永远不会调用。
③应用程序前台退出到后台：ON_PAUSE -> ON_STOP
④应用程序从后台进入前台：ON_START -> ONRESUME
⑤方法回调有延迟
⑥屏幕旋转可能会导致Activity重新创建，但是应用仍然在前台
4.3、ProcessLifecycleOwner颇析
使用过LeakCanary新版本的会发现，只需要添加依赖，什么不做，就能自动的添加各种监控内存泄漏，代码真正的0耦合，是如何做到的呢？
ProcessLifecycleOwnerInitializer是单例：
private static final ProcessLifecycleOwner sInstance = new ProcessLifecycleOwner();
/**
* The LifecycleOwner for the whole application process. Note that if your application
* has multiple processes, this provider does not know about other processes.
*
*/
@NonNull
public static LifecycleOwner get() {
return sInstance;
}这里的ProcessLifecycleOwner也是用类似的方法，借助ProcessLifecycleOwnerInitializer继承自ContentProvider。
public class ProcessLifecycleOwnerInitializer extends ContentProvider {
@Override
public boolean onCreate() {
LifecycleDispatcher.init(getContext());
ProcessLifecycleOwner.init(getContext());
return true;
}
}通过ContentProvider初始化绑定到Application。
static void init(Context context) {
sInstance.attach(context);
}将Application同ProcessLifecycleOwner单例绑定，这里看源码可以发现用到了ActivityLifecycleCallbacks
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 onActivityPreCreated(Activity activity,Bundle savedInstanceState) {
activity.registerActivityLifecycleCallbacks(new EmptyActivityLifecycleCallbacks() {
@Override
public void onActivityPostStarted(Activity activity) {
activityStarted();
}
@Override
public void onActivityPostResumed(Activity activity) {
activityResumed();
}
});
}
@Override
public void onActivityCreated(Activity activity, Bundle savedInstanceState) {
...
}
@Override
public void onActivityPaused(Activity activity) {
activityPaused();
}
@Override
public void onActivityStopped(Activity activity) {
activityStopped();
}
});
}源码不继续往下分析了，和前面几节提到的差不多，借助LifecycleRegistry被观察者。
相关资料：
Jetpack > Libraries > Lifecycle
Android Jetpack架构组件之Lifecycle源码分析
