1. 简介

很早就听说了Google的Lifecycle组件,因为项目没有使用过,所以并没有过多的接触。不过最近看到了一篇文章,其中的一条评论提到了LiveData。恰巧这两天工作内容不多,所以赶紧研究一波!

不过在看LiveData之前,我觉得还是先看下Lifecycle吧(Lifecycle更像是LiveData的基础)。

2. Lifecycle的简单介绍

Lifecycle的介绍,我们还是拿Google的官方文档作为参考吧。

Lifecycle主要解决的是业务和Activity/Fragment生命周期相关的问题。例如:我们在onResume()/onStart()中请求定位,在onPause()/onStop()中停止定位。那么我们一般的做法:

class MyLocationListener {
  public MyLocationListener(Context context, Callback callback) {
    // ...
  }

  void start() {
    // connect to system location service
  }

  void stop() {
    // disconnect from system location service
  }
}

class MyActivity extends AppCompatActivity {
  private MyLocationListener myLocationListener;

  @Override
  public void onCreate(...) {
    myLocationListener = new MyLocationListener(this, (location) -> {
      // update UI
    });
  }

  @Override
  public void onStart() {
    super.onStart();
    myLocationListener.start();
    // manage other components that need to respond
    // to the activity lifecycle
  }

  @Override
  public void onStop() {
    super.onStop();
    myLocationListener.stop();
    // manage other components that need to respond
    // to the activity lifecycle
  }
}

上面的代码虽然看起来还可以,但在真实的应用程序中,可能会有很多的方法都需要根据当前Activity/Fragment的生命周期来进行不同的操作。因此其生命周期方法中可能会被放置大量代码,例如onStart()和 onStop()中,这使得它们难以维护。因此Lifecycle应运而生!

3. Lifecycle的使用

Lifecycle已经发布了release版,所以其中的一些默认支持已经包含到了support-v7:26.1.0以及更高。

添加依赖:

implementation "android.arch.lifecycle:extensions:1.1.1"
annotationProcessor "android.arch.lifecycle:compiler:1.1.1"

使用方式1:实现LifecycleObserver接口,使用@OnLifecycleEvent注解,通过编译时注解生成代码:

public class MyLifecycleObserver implements LifecycleObserver {

  private static final String TAG = MyLifecycleObserver.class.getSimpleName();

  @OnLifecycleEvent(Lifecycle.Event.ON_CREATE)
  public void start(LifecycleOwner lifecycleOwner) {
    Lifecycle.State currentState = lifecycleOwner.getLifecycle().getCurrentState();
    Log.d(TAG, "start: " + currentState);
  }

  @OnLifecycleEvent(Lifecycle.Event.ON_PAUSE)
  public void stop(LifecycleOwner lifecycleOwner) {
    Log.d(TAG, "stop: " + lifecycleOwner.getLifecycle().getCurrentState());
  }
}

 

APT生成的代码

 使用方式2:实现GenericLifecycleObserver接口,实现其onStateChanged方法:

public class MyLifecycleObserver implements GenericLifecycleObserver {

  private static final String TAG = MyLifecycleObserver.class.getSimpleName();

  @Override
  public void onStateChanged(LifecycleOwner source, Lifecycle.Event event) {
    Log.d(TAG, event.name());
  }
}

创建完成后,我们需要将其添加:

public class MainActivity extends AppCompatActivity {
  @Override
  protected void onCreate(Bundle savedInstanceState) {
    super.onCreate(savedInstanceState);
    setContentView(R.layout.activity_main);
    getLifecycle().addObserver(new MyLifecycleObserver());
  }
}

结果图:

方法1

方法2

4. 源码分析

1.Lifecycle的获取

根据源码追踪我们可以看到Lifecycle是在SupportActivity中:

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

这里还有个挺重要的代码:

SupportActivity.java

protected void onCreate(@Nullable Bundle savedInstanceState) {
  super.onCreate(savedInstanceState);
  ReportFragment.injectIfNeededIn(this);
}

ReportFragment.java

public static void injectIfNeededIn(Activity activity) {
  android.app.FragmentManager manager = activity.getFragmentManager();
  if (manager.findFragmentByTag(REPORT_FRAGMENT_TAG) == null) {
    manager.beginTransaction().add(new ReportFragment(), REPORT_FRAGMENT_TAG).commit();
    manager.executePendingTransactions();
  }
}

 

这里在Activity中添加了一个Fragment,至于作用,留到后面在讲。

2.addObserver

addObserver方法的话,还是简单来看吧:

LifecycleRegistry.java

@Override
public void addObserver(@NonNull LifecycleObserver observer) {
  State initialState = mState == DESTROYED ? DESTROYED : INITIALIZED;
  ObserverWithState statefulObserver = new ObserverWithState(observer, initialState);
  ObserverWithState previous = mObserverMap.putIfAbsent(observer, statefulObserver);
  ......
  // 对状态的修正,这里会对已经过去的状态进行分发。也就是说如果添加observer时状态已经改变,那么也会被通知到!
}

// 有状态的Observer
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);
    mLifecycleObserver.onStateChanged(owner, event);
    mState = newState;
  }
}

我们将实现的接口通过装饰者(我认为是这样)模式转成ObserverWithState对象,并将该对象添加到mObserverMap中。
 在ObserverWithState的构造方法中,有对我们传入的LifecycleObserver进行包装:

Lifecycling.java

@NonNull
static GenericLifecycleObserver getCallback(Object object) {
  // 这里应该算第三种实现方式,然而FullLifecycleObserver不是public,所以不能使用
  if (object instanceof FullLifecycleObserver) {
    return new FullLifecycleObserverAdapter((FullLifecycleObserver) object);
  }
  // 我们的第一种方式
  if (object instanceof GenericLifecycleObserver) {
    return (GenericLifecycleObserver) object;
  }
  // 通过注解生成的
  final Class<?> klass = object.getClass();
  // 将生成的MyLifecycleObserver_LifecycleAdapter放到Map中
  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);
}

根据我们传入的对象进行解析,最终返回结果为:GenericLifecycleObserver或者GenericLifecycleObserver的实现类。附获取生成MyLifecycleObserver_LifecycleAdapter代码:

// 通过名称获取
public static String getAdapterName(String className) {
  return className.replace(".", "_") + "_LifecycleAdapter";
}

最终通过Class.forName方式获得Class。

3.事件分发

Lifecycle的事件分发在ObserverWithState类中:

// 事件分发
void dispatchEvent(LifecycleOwner owner, Event event) {
  State newState = getStateAfter(event);
  mState = min(mState, newState);
  mLifecycleObserver.onStateChanged(owner, event);
  mState = newState;
}

看下哪里调用了dispatchEvent方法:

 

调用

 我们看这个方法,其中一个在addObserver中调用,另外的两个都会通过sync()方法调用:

// happens only on the top of stack (never in reentrance),
// so it doesn't have to take in account parents
private void sync() {
  LifecycleOwner lifecycleOwner = mLifecycleOwner.get();
  if (lifecycleOwner == null) {
    Log.w(LOG_TAG, "LifecycleOwner is garbage collected, you shouldn't try dispatch "
        + "new events from it.");
    return;
  }
  // 最新的状态和当前状态不一致,则需要进行状态修改
  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;
}
// 是否状态同步的
private boolean isSynced() {
  if (mObserverMap.size() == 0) {
    return true;
  }
  // 最新的和最后添加的Observer状态一致,并且当前的状态和最新状态一致,则已经同步了
  State eldestObserverState = mObserverMap.eldest().getValue().mState;
  State newestObserverState = mObserverMap.newest().getValue().mState;
  return eldestObserverState == newestObserverState && mState == newestObserverState;
}

这里的同步方法用于同步当前Observer的状态,如果最新的和最老的Observer的状态不一致或者当前的状态和最新的状态不一致时,那么需要进行状态同步。同步包括了向前同步和向后同步。

调用sync()方法一共就两处,一处在addObserver方法,另一处是moveToState方法,而调用moveToState方法也有两处:

@SuppressWarnings("WeakerAccess")
@MainThread
public void markState(@NonNull State state) {
  moveToState(state);
}

public void handleLifecycleEvent(@NonNull Lifecycle.Event event) {
  State next = getStateAfter(event);
  moveToState(next);
}

handleLifecycleEvent方法有很多处调用:

handleLifecycleEvent调用

 看图的话,就可以知道Fragment在不同的生命周期调用了handleLifecycleEvent方法,随便看一个吧:

 

ON_CREATE

还记得上面说的ReportFragment吗?这里也出现了,我们看下ReportFragment到底做什么的:

// 调用Activity的
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);
    }
  }
}

// 生命周期
@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;
}

可以看到,ReportFragment只是作为一个中间层,通过它来分发各种事件!

由于篇幅原因(已经啰嗦了很多了),这里就不写LifecycleDispatcher和ProcessLifecycleOwner了,这两个通过自定义的内容提供者ProcessLifecycleOwnerInitializer进行初始化,并且通过registerActivityLifecycleCallbacks和registerFragmentLifecycleCallbacks注册统一的观察回调。有兴趣的话,自己看看吧。

5 总结

Lifecycle简单来说就是用于处理和生命周期相关的业务,其原理以及实现还是很简单的。当然了,项目中并没有使用到Lifecycle,所以实际应用效果怎样只能靠猜想了😂。后面呢,应该会分析下与Lifecycle相关的一个东西LievData,要抓紧时间了!

以上就是本文的全部内容,希望对大家的学习有所帮助,也希望大家多多支持悠悠之家。

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