Lifecycle是Android Architecture Components的一员,这玩意儿挺方便的,它是能感知Activity和Fragment的生命周期的.
一、使用Lifecycle
1. 引入Lifecycle
我们来看一下如何引入:
1.非androidX项目引入:
//运行时 implementation "android.arch.lifecycle:runtime:1.1.1" // 编译期 annotationProcessor "android.arch.lifecycle:compiler:1.1.1"
2.androidX项目引入:
androidX是support库的新时代,Google正在将support迁移到androidx中.
implementation "androidx.lifecycle:lifecycle-runtime:2.0.0" implementation "androidx.lifecycle:lifecycle-extensions:2.0.0" implementation "androidx.lifecycle:lifecycle-common-java8:2.0.0" annotationProcessor "androidx.lifecycle:lifecycle-compiler:2.0.0"
2. 创建生命周期观察者
public class MyObserver implements LifecycleObserver { private static final String TAG = "MyObserver"; @OnLifecycleEvent(Lifecycle.Event.ON_CREATE) public void onCreate() { Log.w(TAG, "onCreate: "); } @OnLifecycleEvent(Lifecycle.Event.ON_START) public void onStart() { Log.w(TAG, "onStart: "); } @OnLifecycleEvent(Lifecycle.Event.ON_RESUME) public void onResume() { Log.w(TAG, "onResume: "); } @OnLifecycleEvent(Lifecycle.Event.ON_PAUSE) public void onPause() { Log.w(TAG, "onPause: "); } @OnLifecycleEvent(Lifecycle.Event.ON_STOP) public void onStop() { Log.w(TAG, "onStop: "); } @OnLifecycleEvent(Lifecycle.Event.ON_DESTROY) public void onDestroy() { Log.w(TAG, "onDestroy: "); } }
我们首先创建了一个类,它实现了LifecycleObserver接口,并且我写了几个模拟生命周期的方法,并在每个方法上加上了注解.
3. 观察生命周期
然后我在Activity中这样写:
public class MainActivity extends AppCompatActivity { @Override protected void onCreate(Bundle savedInstanceState) { super.onCreate(savedInstanceState); setContentView(R.layout.activity_main); //添加一个生命周期观察者 getLifecycle()是FragmentActivity中的方法 MyObserver observer = new MyObserver(); getLifecycle().addObserver(observer); } }
我们将项目运行起来,观察结果:
2019-03-12 22:14:26.672 15790-15790/? W/MyObserver: onCreate: 2019-03-12 22:14:26.676 15790-15790/? W/MyObserver: onStart: 2019-03-12 22:14:26.679 15790-15790/? W/MyObserver: onResume: 2019-03-12 22:15:13.054 15790-15790/? W/MyObserver: onPause: 2019-03-12 22:15:13.234 15790-15790/? W/MyObserver: onStop: 2019-03-12 22:15:13.241 15790-15790/? W/MyObserver: onDestroy:
我们发现,不管Activity的生命周期如何变化,我创建的观察者总是能够监听到响应的生命周期变化,并且变化时还会回调我写的生命周期方法(比如:public void onDestroy()).
方不方便? 你可能会问,这有啥用? 用处大了,比如我现在Presenter中就可以很方便的监听Activity中的生命周期,从而进行一些相应的操作和处理.
二、Lifecycle原理解析
1. 从使用处入手
我们从使用的地方入手
MyObserver observer = new MyObserver();
getLifecycle().addObserver(observer);
getLifecycle()方法点进去是FragmentActivity,看注释意思是返回生命周期提供者的Lifecycle
/** * Returns the Lifecycle of the provider. */ @Override public Lifecycle getLifecycle() { return super.getLifecycle(); }
再跟着super.getLifecycle();进入,来到了androidx.core.app.ComponentActivity,可以看到,ComponentActivity是继承自Activity并实现了LifecycleOwner(该接口的作用是标记类有Android的生命周期的,比如Activity和Fragment)接口.
public class ComponentActivity extends Activity implements LifecycleOwner, KeyEventDispatcher.Component { private LifecycleRegistry mLifecycleRegistry = new LifecycleRegistry(this); @Override public Lifecycle getLifecycle() { return mLifecycleRegistry; } } /** * A class that has an Android lifecycle */ public interface LifecycleOwner { @NonNull Lifecycle getLifecycle(); }
那么其实最终是返回的LifecycleRegistry,它是Lifecycle的子类
Lifecycle是一个抽象类,里面有3个方法(添加观察者和移除观察者,获取当前的状态),还有一些状态的枚举定义.
public abstract class Lifecycle { @MainThread public abstract void addObserver(@NonNull LifecycleObserver observer); @MainThread public abstract void removeObserver(@NonNull LifecycleObserver observer); @MainThread @NonNull public abstract State getCurrentState(); @SuppressWarnings("WeakerAccess") public enum Event { /** * Constant for onCreate event of the {@link LifecycleOwner}. */ ON_CREATE, /** * Constant for onStart event of the {@link LifecycleOwner}. */ ON_START, /** * Constant for onResume event of the {@link LifecycleOwner}. */ ON_RESUME, /** * Constant for onPause event of the {@link LifecycleOwner}. */ ON_PAUSE, /** * Constant for onStop event of the {@link LifecycleOwner}. */ ON_STOP, /** * Constant for onDestroy event of the {@link LifecycleOwner}. */ ON_DESTROY, /** * An {@link Event Event} constant that can be used to match all events. */ ON_ANY } @SuppressWarnings("WeakerAccess") public enum State { DESTROYED, INITIALIZED, CREATED, STARTED, RESUMED; public boolean isAtLeast(@NonNull State state) { return compareTo(state) >= 0; } } }
LifecycleRegistry是Lifecycle的一个实现,它是用在Fragment和Activity上的,它可以处理多个生命周期观察者. 具体它有什么作用,后面再讲.
2. ReportFragment的由来
下面是ComponentActivity的onCreate()方法.
protected void onCreate(@Nullable Bundle savedInstanceState) { super.onCreate(savedInstanceState); ReportFragment.injectIfNeededIn(this); }
可以看到,在上面搞了一句,注入一个ReportFragment(报告Fragment? 纪检委员? 打小报告的? 当然,我只是猜测).
有一点眉目了,其实就是在Activity中搞了一个Fragment,Fragment的生命周期我们知道了,当然就知道了Activity的生命周期,接着通知相关的观察者即可.当然,这个Fragment是没有界面的. 我们来看看,这个注入的方法干了啥.
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(); } }
其实这个injectIfNeededIn()看起来像是注入的方法干的就是将Fragment添加到Activity中,
来看看这个ReportFragment的生命周期方法都干了些啥,
@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); } private void dispatchCreate(ActivityInitializationListener listener) { if (listener != null) { listener.onCreate(); } }
通过调用dispatchCreate(mProcessListener)方法,感觉从命名上(是不是有点像dispatchTouchEvent())看就知道是在干啥了: 分发当前的生命周期事件.
dispatch(Lifecycle.Event.ON_START); 感觉这个方法也像是在分发事件.
我们跟着这个mProcessListener来看看是在哪里设置的
/** * Class that provides lifecycle for the whole application process. */ public class ProcessLifecycleOwner implements LifecycleOwner { //注意,我是一个单例 private static final ProcessLifecycleOwner sInstance = new ProcessLifecycleOwner(); static void init(Context context) { sInstance.attach(context); } 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(); } }); } } //Activity的监听器 ActivityInitializationListener mInitializationListener = new ActivityInitializationListener() { @Override public void onCreate() { } @Override public void onStart() { activityStarted(); } @Override public void onResume() { activityResumed(); } private final LifecycleRegistry mRegistry = new LifecycleRegistry(this); //Activity创建的时候,分发Lifecycle.Event.ON_START事件 void activityStarted() { mStartedCounter++; if (mStartedCounter == 1 && mStopSent) { mRegistry.handleLifecycleEvent(Lifecycle.Event.ON_START); mStopSent = false; } }
ReportFragment.java static ReportFragment get(Activity activity) { return (ReportFragment) activity.getFragmentManager().findFragmentByTag( REPORT_FRAGMENT_TAG); }
ProcessLifecycleOwner看起来就像是老大哥,给整个APP提供lifecycle的,也就是说通过它我们可以观察到整个应用程序的生命周期. 如何做到的? ProcessLifecycleOwner的attach()中registerActivityLifecycleCallbacks()注册了一个监听器,一旦有Activity创建就给它设置一个Listener.这样就保证了每个ReportFragment都有Listener.
既然是一个全局的单例,并且可以监听整个应用程序的生命周期,那么,肯定一开始就需要初始化.
既然没有让我们在Application里面初始化,那么肯定就是在ContentProvider里面初始化的.
3. 初始化
ps: 这里穿插一个小知识点: ContentProvider的onCreate()方法执行时间比Application的onCreate()执行时间还要早,而且肯定会执行.所以在ContentProvider的onCreate()方法里面初始化几个特殊的小东西是没啥问题的.
我们跟着ProcessLifecycleOwner的init()方法的调用处,来到了ProcessLifecycleOwnerInitializer,果不其然,它是一个ContentProvider.并且,在这里,真的就初始化了2个小东西.
public class ProcessLifecycleOwnerInitializer extends ContentProvider { @Override public boolean onCreate() { LifecycleDispatcher.init(getContext()); ProcessLifecycleOwner.init(getContext()); return true; } }
1.ProcessLifecycleOwner初始化就不说了,是拿来观察整个应用的生命周期的,其原理就是利用ReportFragment,我们稍后详细到来.
2.LifecycleDispatcher尤其重要.
class LifecycleDispatcher { static void init(Context context) { ... //registerActivityLifecycleCallbacks 注册一个监听器 ((Application) context.getApplicationContext()) .registerActivityLifecycleCallbacks(new DispatcherActivityCallback()); } } static class DispatcherActivityCallback extends EmptyActivityLifecycleCallbacks { @Override public void onActivityCreated(Activity activity, Bundle savedInstanceState) { //又来注入咯 ReportFragment.injectIfNeededIn(activity); } @Override public void onActivityStopped(Activity activity) { } @Override public void onActivitySaveInstanceState(Activity activity, Bundle outState) { } }
初始化的时候,就注册了一个监听器,每个创建的时候都给它注入一个ReportFragment.咦?这里又来注入一次,不是每个Activity都注册了一次么,在ComponentActivity中,搞啥玩意儿?
我猜,可能是为了兼容吧.2次注入,确保万无一失.而且这个injectIfNeededIn()方法,内部实现是只会成功注入一次的,所以多调用一次,无所谓.
4. 分发事件
相当于,到了这里,应用程序里面的任何一个Activity都会被注入一个ReportFragment.而注入的这个无界面的ReportFragment是可以观察到当然Activity的生命周期的.
下面我们来仔细看一下,这个事件是如何一步步分发下去的.
ReportFragment.java @Override public void onActivityCreated(Bundle savedInstanceState) { super.onActivityCreated(savedInstanceState); dispatchCreate(mProcessListener); dispatch(Lifecycle.Event.ON_CREATE); } private void dispatch(Lifecycle.Event event) { Activity activity = getActivity(); if (activity instanceof LifecycleRegistryOwner) { ((LifecycleRegistryOwner) activity).getLifecycle().handleLifecycleEvent(event); return; } if (activity instanceof LifecycleOwner) { //获取Activity中的LifecycleRegistry Lifecycle lifecycle = ((LifecycleOwner) activity).getLifecycle(); if (lifecycle instanceof LifecycleRegistry) { ((LifecycleRegistry) lifecycle).handleLifecycleEvent(event); } } }
不知道小伙伴儿们是否记得ComponentActivity是实现了LifecycleOwner的.
public class ComponentActivity extends Activity implements LifecycleOwner
下面我们获取到Activity中的LifecycleRegistry,下面的代码做了精简,只保留关键代码
public void handleLifecycleEvent(@NonNull Lifecycle.Event event) { State next = getStateAfter(event); moveToState(next); } private void moveToState(State next) { ...... sync(); ...... } private void sync() { LifecycleOwner lifecycleOwner = mLifecycleOwner.get(); //循环 遍历所有观察者 while (...) { .... //分发事件 forwardPass(lifecycleOwner); } } 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); //分发事件 observer.dispatchEvent(lifecycleOwner, upEvent(observer.mState)); popParentState(); } } }
上面的observer其实是一个ObserverWithState对象,
static class ObserverWithState { State mState; GenericLifecycleObserver mLifecycleObserver; ObserverWithState(LifecycleObserver observer, State initialState) { mLifecycleObserver = Lifecycling.getCallback(observer); mState = initialState; } void dispatchEvent(LifecycleOwner owner, Event event) { State newState = getStateAfter(event); mState = min(mState, newState); //生命周期变了.... 关键代码 mLifecycleObserver.onStateChanged(owner, event); mState = newState; } }
在ObserverWithState的构造方法中，通过 Lifecycling.getCallback(observer)根据传进来的 observer ，构造了一个 GenericLifecycleObserver 类型的 mLifecycleObserver ,我们跟进去看一下.
static GenericLifecycleObserver getCallback(Object object) { if (object instanceof FullLifecycleObserver) { return new FullLifecycleObserverAdapter((FullLifecycleObserver) object); } if (object instanceof GenericLifecycleObserver) { return (GenericLifecycleObserver) 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); }
这个方法大概意思就是,根据传进的LifecycleObserver进行判断,构造一个GenericLifecycleObserver(目前是只有4个子类:FullLifecycleObserverAdapter、SingleGeneratedAdapterObserver、CompositeGeneratedAdaptersObserver、ReflectiveGenericLifecycleObserver)的对象.
5. 依赖注入
首先,这里穿插一点.我们在引入lifecycle时添加了语句annotationProcessor "android.arch.lifecycle:compiler:1.1.1",这个其实是注解处理器的依赖.
引入这个之后,会自动生成xxx_LifecycleAdapter的文件,上面的demo中生成的是MyObserver_LifecycleAdapter文件,其内容如下:
public class MyObserver_LifecycleAdapter implements GeneratedAdapter { final MyObserver mReceiver; MyObserver_LifecycleAdapter(MyObserver receiver) { this.mReceiver = receiver; } @Override public void callMethods(LifecycleOwner owner, Lifecycle.Event event, boolean onAny, MethodCallsLogger logger) { boolean hasLogger = logger != null; if (onAny) { return; } if (event == Lifecycle.Event.ON_CREATE) { if (!hasLogger || logger.approveCall("onCreate", 1)) { mReceiver.onCreate(); } return; } if (event == Lifecycle.Event.ON_START) { if (!hasLogger || logger.approveCall("onStart", 1)) { mReceiver.onStart(); } return; } if (event == Lifecycle.Event.ON_RESUME) { if (!hasLogger || logger.approveCall("onResume", 1)) { mReceiver.onResume(); } return; } if (event == Lifecycle.Event.ON_PAUSE) { if (!hasLogger || logger.approveCall("onPause", 1)) { mReceiver.onPause(); } return; } if (event == Lifecycle.Event.ON_STOP) { if (!hasLogger || logger.approveCall("onStop", 1)) { mReceiver.onStop(); } return; } if (event == Lifecycle.Event.ON_DESTROY) { if (!hasLogger || logger.approveCall("onDestroy", 1)) { mReceiver.onDestroy(); } return; } } }
因为我们的事件是声明在MyObserver的方法注解上面的,每次去反射取这些东西,比较耗性能.那么我们通过该依赖库,把这些标注了的方法进行预处理,然后直接回调这些方法,避免反射,进行提高性能.666,佩服.
有了上面的知识之后,分析getCallback()方法,不难发现,因为MyObserver_LifecycleAdapter只有一个构造方法,那么就会构造出SingleGeneratedAdapterObserver.而SingleGeneratedAdapterObserver内部其实就是调用一下方法而已.
public class SingleGeneratedAdapterObserver implements GenericLifecycleObserver { private final GeneratedAdapter mGeneratedAdapter; SingleGeneratedAdapterObserver(GeneratedAdapter generatedAdapter) { mGeneratedAdapter = generatedAdapter; } @Override public void onStateChanged(LifecycleOwner source, Lifecycle.Event event) { mGeneratedAdapter.callMethods(source, event, false, null); mGeneratedAdapter.callMethods(source, event, true, null); } }
上面的mGeneratedAdapter其实就是我们的MyObserver_LifecycleAdapter.好了,结束了. 生命周期事件从Activity开始,然后到打小报告的ReportFragment那里出来,辗转发侧,终于到了我们定义的观察者,不容易啊.谷歌工程师写的代码就是牛逼。
最后我想说：对于程序员来说，要学习的知识内容、技术有太多太多，要想不被环境淘汰就只有不断提升自己，从来都是我们去适应环境，而不是环境来适应我们！
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