如何绑定页面生命周期（二）－基于Android Architecture Components的Lifecycle实现

上篇文章如何绑定页面生命周期（一）－Glide实现介绍了Glide实现生命周期感知的原理，这里咱们再介绍基于Android Architecture Components的Lifecycle实现页面生命周期感知。

Lifecycle是Android Architecture Components(以后简称AAC)的一个组件，用于将系统组件（Activity、Fragment等等）的生命周期分离到Lifecycle类，Lifecycle容许其余类做为观察者，观察组件生命周期的变化。

基于AAC实现组件生命周期观察实践

• 控件实现LifecycleObserver接口，内部经过@OnLifecycleEvent注解声明生命周期事件

public class LifecycleObserverDemo implements LifecycleObserver {

    @OnLifecycleEvent(Lifecycle.Event.ON_ANY)
    void onAny(LifecycleOwner owner, Lifecycle.Event event) {
        System.out.println("onAny:" + event.name());
    }
    @OnLifecycleEvent(Lifecycle.Event.ON_CREATE)
    void onCreate() {
        System.out.println("onCreate");
    }

    @OnLifecycleEvent(Lifecycle.Event.ON_DESTROY)
    void onDestroy() {
        System.out.println("onDestroy");
    }
}
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• 在LifecycleRegistryOwner，好比在实现了LifecycleRegistryOwner接口的Activity中。定义LifecycleRegistry实例，并将控件lifecycleRegistry实例中的监听集合中。

public class MainActivity extends AppCompatActivity implements LifecycleRegistryOwner {
	// 定义LifecycleRegistry实例
    private LifecycleRegistry lifecycleRegistry = new LifecycleRegistry(this);

    @Override
    protected void onCreate(Bundle savedInstanceState) {
        super.onCreate(savedInstanceState);
        setContentView(R.layout.activity_main);
        // 加入监听集合
        getLifecycle().addObserver(new LifecycleObserverDemo());        
    }

    @Override
    public LifecycleRegistry getLifecycle() {
        return lifecycleRegistry;
    }
}
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只须要如上两步，当Activity页面生命周期发生变化时，都会通知到LifecycleObserverDemo。一样，本文以Activity为例，介绍Lifecycle感知生命周期的原理。

生命周期绑定实现原理

实现原理简介

经过在对指定activity注册无UI的Fragment，传递页面Activity生命周期到Fragment。而后经过Fragment绑定LifecycleRegistry，当Fragment的生命周期变化时，回调LifecycleRegistry中LifecycleObserver对象相应的生命周期回调方法。

如何传递生命周期

下图是文章Android Architecture Component -- Lifecycle 浅析中关于Lifecycle生命周期传递的一幅图，我以为很清晰地展现了生命周期的传递过程。下面咱们跟着这幅图，来一步步看一下生命周期是如何传递的。

• 如何在Activity上注册无UI的ReportFragment

  首先看下LifecycleDispatcher初始化的过程：

  • 利用 ContentProvider 的特色在应用程序初始化时，向其注入两行代码：

  LifecycleDispatcher.init(getContext());
  ProcessLifecycleOwner.init(getContext());  // 监听整个应用先后台切换
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  • 这个ContentProvider从哪里来？查看apk中的AndroidManifest.xml文件，发现多了一个ContentProvider声明：

  <provider
      android:name="android.arch.lifecycle.LifecycleRuntimeTrojanProvider"
      android:authorities="${applicationId}.lifecycle-trojan"
      android:exported="false"
      android:multiprocess="true" />
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在这个LifecycleRuntimeTrojanProvider（低版本的AAC里，这个类叫ProcessLifecycleOwnerInitializer）的初始化方法中，实现了LifecycleDispatcher的相应初始化操做。

下面再来看一下LifecycleDispatcher的init方法：

static void init(Context context) {
    if (sInitialized.getAndSet(true)) {
        return;
    }
    ((Application) context.getApplicationContext())
            .registerActivityLifecycleCallbacks(new DispatcherActivityCallback());
}
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在 LifecycleDispatcher#init(Context) 中，它经过 registerActivityLifecycleCallbacks 方法，向当前的 Application 注册一个 DispatcherActivityCallback。但 Lifecycle 并没使用 ActivityLifecycleCallbacks 来监听并派发生命周期事件。而是经过一个无 UI 的 Fragment，在 DispatcherActivityCallback#onActivityCreated 能够看到它在 Activity#onCreate 时，为 Activity 添加一个 ReportFragment。最终由 ReportFragment 来监听各个生命周期事件，而后传递给 LifecycleRegistry。

• 无UI的Fragment与LifecycleRegistry创建联系

查看ReportFragment的生命周期回调方法：

@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; } 复制代码

回调生命周期方法时，会调用dispatch(Lifecycle.Event event)方法。看下dispatch(Lifecycle.Event event)方法的源码：

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);
        }
    }
}
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这里会经过ReportFragment注册的Activity的getLifecycle()方法获取LifecycleRegistry，而后调用LifecycleRegistry的handleLifecycleEvent(@NonNull Lifecycle.Event event)处理传递的生命周期Event。

• _LifecycleAdapter如何与LifecycleRegistry创建联系

  在LifecycleRegistry中，定义了以下的map：

  private FastSafeIterableMap<LifecycleObserver, ObserverWithState> mObserverMap =
          new FastSafeIterableMap<>();
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  当咱们在页面Activity中将观察者加入集合时，加入的就是上面定义的mObserverMap。ObserverWithState对象构造函数初始化时，经过Lifecycling.getCallback(observer)方法返回GenericLifecycleObserver对象，实际上就是_LifecycleAdapter对象。由于_LifecycleAdapter实现了GenericLifecycleObserver。

  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);
  }
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基于注解生成了_LifecycleAdapter 的class，经过反射生成_LifecycleAdapter对象

• _LifecycleAdapter回调生命周期方法，继续传递生命周期给最终的观察者

LifecycleRegistry和_LifecycleAdapter创建联系后，生命周期会经过调用ObserverWithState的dispatchEvent方法：

void dispatchEvent(LifecycleOwner owner, Event event) {
    State newState = getStateAfter(event);
    mState = min(mState, newState);
    mLifecycleObserver.onStateChanged(owner, event);
    mState = newState;
}
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最终，会调用mLifecycleObserver，即咱们前面返回的_LifecycleAdapter的onStateChanged方法。下面看下_LifecycleAdapter的实现：

public class LifecycleObserverDemo_LifecycleAdapter implements GenericLifecycleObserver {
  final LifecycleObserverDemo mReceiver;

  LifecycleObserverDemo_LifecycleAdapter(LifecycleObserverDemo receiver) {
    this.mReceiver = receiver;
  }

  @Override
  public void onStateChanged(LifecycleOwner owner, Lifecycle.Event event) {
    mReceiver.onAny(owner,event);
    if (event == Lifecycle.Event.ON_CREATE) {
      mReceiver.onCreate();
    }
    if (event == Lifecycle.Event.ON_START) {
      mReceiver.onStart();
    }
    if (event == Lifecycle.Event.ON_PAUSE) {
      mReceiver.onPause();
    }
    if (event == Lifecycle.Event.ON_DESTROY) {
      mReceiver.onDestroy();
    }
  }

  public Object getReceiver() {
    return mReceiver;
  }
}
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上面的类，能够在 build 目录下找到。这是注解处理器为咱们生成了 LifecycleObserverDemo_LifecycleAdapter，不过这只是一个适配器，用于将生命周期事件派发到 LifecycleObserverDemo 对应的方法。至此，LifecycleObserverDemo实现了对页面Activity生命周期的感知。

核心类介绍

• LifecycleObserver：接口，标记一个类是可观察的，基于注解实现相应回调方法
• Lifecycle：抽象类，拥有android生命周期
• LifecycleRegistry：继承Lifecycle，能够处理多LifecycleObserver
• LifecycleOwner：接口，持有一个android lifecycle
• LifecycleRegistryOwner：接口，继承LifecycleOwner，返回LifecycleRegistry
• LifecycleDispatcher：在Application中hook，观察activity的生命周期并分发
• LifecycleRuntimeTrojanProvider：LifecycleDispatcher等初始化

生命周期管理框架实践

Demo省略了注解相关步骤，须要观察者本身去实现一个ZRLifecycleObserver接口。虽然稍有不一样，可是不妨碍理解。

Demo的框架图以下所示：

使用的话也比较简单，主要进行如下一些设置。

• 观察者实现ZRLifecycleObserver接口

public class MyView extends View implements ZRLifecycleObserver {

    public MyView(Context context) {
        this(context, null);
    }

    public MyView(Context context,
            @Nullable AttributeSet attrs) {
        this(context, attrs, 0);
    }

    public MyView(Context context, @Nullable AttributeSet attrs, int defStyleAttr) {
        super(context, attrs, defStyleAttr);
    }

    @Override
    public void onCreate() {
        System.out.println("MyView onCreate");
    }

    @Override
    public void onStart() {
        System.out.println("MyView onStart");
    }

    @Override
    public void onResume() {
        System.out.println("MyView onResume");
    }

    @Override
    public void onPause() {
        System.out.println("MyView onPause");
    }

    @Override
    public void onStop() {
        System.out.println("MyView onStop");
    }

    @Override
    public void onDestroy() {
        System.out.println("MyView onDestroy");
    }

    @Override
    public void onRestart() {
        System.out.println("MyView onRestart");
    }
}
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• 应用启动时初始化ZRLifecycleDispatcher

public class MyApplication extends Application {

    @Override
    public void onCreate() {
        super.onCreate();
        ZRLifecycleDispatcher.init(this);
    }
}
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• 被观察页面实现ZRLifecycleRegistryOwner，并将要要观察此页面生命周期的观察者对象加入集合

public class MainActivity extends Activity implements ZRLifecycleRegistryOwner {

    private ZRLifecycleRegistry lifecycleRegistry = new ZRLifecycleRegistry(this);

    @Override
    protected void onCreate(Bundle savedInstanceState) {
        super.onCreate(savedInstanceState);
        setContentView(R.layout.activity_main);

        MyView myView = findViewById(R.id.view_test);

        getLifecycle().addObserver(myView);

    }

    @Override
    public ZRLifecycleRegistry getLifecycle() {
        return lifecycleRegistry;
    }
}
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具体工程代码能够从这里获取：CustomAACLifecycleDemo

结束

至此，关于AAC如何绑定页面生命周期的原理讲解结束。在上一篇文章如何绑定页面生命周期（一）－Glide实现，介绍了Glide绑定生命周期的原理。两种绑定页面生命周期的方式，你们能够对比着看，相信确定会对绑定页面生命周期有更加深刻的了解。

参考

Android Architecture Component -- Lifecycle 浅析