Glide之生命周期管理

 关于Glide的生命周期设计是Glide的一个很大亮点，现在就来看一下Glide生命周期的实现。
 在Glide源码解析这篇文章中就说过在Glide.with(context)中就实现了生命周期管理，with根据传入的参数有不同的实现。

  //传入一个Context
  public static RequestManager with(@NonNull Context context)
  //传入一个activity
  public static RequestManager with(@NonNull Activity activity)
  //传入一个FragmentActivity 
  public static RequestManager with(@NonNull FragmentActivity activity)
  //传入一个Fragment
  public static RequestManager with(@NonNull Fragment fragment)
  //传入一个View
  public static RequestManager with(@NonNull View view)

 虽然有这么多类型，但其实可以分为两类的。

• 传入一个ApplicationContext，Glide的生命周期就相当于绑定了整个应用，只要应用不退出，任何时候都能够加载，也可以理解为不对Glide生命周期进行管理。
• 传入activity、FragmentActivity 、Fragment 及View ，这样就会创建一个看不见的fragment，Glide的生命周期就随着该Fragment的变化而变化。

 当传入参数为ApplicationContext时，代码实现如下。

  public static RequestManager with(@NonNull Context context) {
    return getRetriever(context).get(context);
  }
  //由于传入参数是ApplicationContext，所以最终调用getApplicationManager方法。
  public RequestManager get(@NonNull Context context) {
    if (context == null) {
      throw new IllegalArgumentException("You cannot start a load on a null Context");
    } else if (Util.isOnMainThread() && !(context instanceof Application)) {
      if (context instanceof FragmentActivity) {
        //判断context类型是不是FragmentActivity
        return get((FragmentActivity) context);
      } else if (context instanceof Activity) {
        //判断context类型是不是Activity
        return get((Activity) context);
      } else if (context instanceof ContextWrapper) {
        //判断context类型是不是ContextWrapper
        return get(((ContextWrapper) context).getBaseContext());
      }
    }
    //context类型属于ApplicationContext
    return getApplicationManager(context);
  }

 由于传入参数是ApplicationContext，所以最终调用getApplicationManager方法。

  private RequestManager getApplicationManager(@NonNull Context context) {
    // Either an application context or we're on a background thread.
    if (applicationManager == null) {
      synchronized (this) {
        if (applicationManager == null) {
          Glide glide = Glide.get(context.getApplicationContext());
          applicationManager =
              factory.build(
                  glide,
                  new ApplicationLifecycle(),
                  new EmptyRequestManagerTreeNode(),
                  context.getApplicationContext());
        }
      }
    }

    return applicationManager;
  }

 这里就直接创建一个ApplicationLifecycle来管理生命周期，但ApplicationLifecycle并不受控制，所以就无法对Glide生命周期进行管理。
 以传入参数类型为Activity为例，代码实现如下。

  public static RequestManager with(@NonNull Activity activity) {
    return getRetriever(activity).get(activity);
  }
  public RequestManager get(@NonNull Activity activity) {
    //如果在子线程，则不对Glide生命周期就那些管理
    if (Util.isOnBackgroundThread()) {
      return get(activity.getApplicationContext());
    } else {
      assertNotDestroyed(activity);
      //拿到FragmentManager对象
      android.app.FragmentManager fm = activity.getFragmentManager();
      //获取fragment对象，并返回一个RequestManager 对象
      return fragmentGet(
          activity, fm, /*parentHint=*/ null, isActivityVisible(activity));
    }
  }

 如果当前是在子线程，则不需要对Glide生命周期进行管理，否则将通过fragmentGet方法创建一个fragment。

  private RequestManager fragmentGet(@NonNull Context context,
      @NonNull android.app.FragmentManager fm,
      @Nullable android.app.Fragment parentHint,
      boolean isParentVisible) {
    //创建一个fragment对象
    RequestManagerFragment current = getRequestManagerFragment(fm, parentHint, isParentVisible);
    RequestManager requestManager = current.getRequestManager();
    if (requestManager == null) {
      // TODO(b/27524013): Factor out this Glide.get() call.
      //防止Glide对象被异常回收
      Glide glide = Glide.get(context);
      //创建一个RequestManager对象
      requestManager =
          factory.build(
              glide, current.getGlideLifecycle(), current.getRequestManagerTreeNode(), context);
      current.setRequestManager(requestManager);
    }
    return requestManager;
  }

 在该方法中，通过getRequestManagerFragment来获得一个Fragment对象。然后拿到该Fragment对应的RequestManager 对象，如果该对象为null则创建一个RequestManager对象并将fragment中的ActivityFragmentLifecycle对象传递给RequestManager。先来看getRequestManagerFragment方法的实现。

  private RequestManagerFragment getRequestManagerFragment(
      @NonNull final android.app.FragmentManager fm,
      @Nullable android.app.Fragment parentHint,
      boolean isParentVisible) {
    //查找tag为FRAGMENT_TAG的fragment
    RequestManagerFragment current = (RequestManagerFragment) fm.findFragmentByTag(FRAGMENT_TAG);
    if (current == null) {
      //从HashMap中取出fm
      current = pendingRequestManagerFragments.get(fm);
      if (current == null) {
        //创建fragment对象
        current = new RequestManagerFragment();
        //当fragment嵌套fragment时才会使用，否则parentHint是null
        current.setParentFragmentHint(parentHint);
        if (isParentVisible) {
          //开始执行请求
          current.getGlideLifecycle().onStart();
        }
        //将fm添加到HashMap中，防止fragment的重复创建
        pendingRequestManagerFragments.put(fm, current);
        //添加fragment
        fm.beginTransaction().add(current, FRAGMENT_TAG).commitAllowingStateLoss();
        //从HashMap集合从移除fm
        handler.obtainMessage(ID_REMOVE_FRAGMENT_MANAGER, fm).sendToTarget();
      }
    }
    return current;
  }

 从fm中去查找tag为FRAGMENT_TAG的fragment是否存在，如果不存在就从pendingRequestManagerFragments这个HashMap中去取，如果没有就创建一个fragment，添加到pendingRequestManagerFragments并且将该fragment绑定到activity，绑定成功后则从pendingRequestManagerFragments移除fragment。这里的pendingRequestManagerFragments主要是防止fragment重复创建（Glide生命周期管理），因为每个activity必须对应一个唯一的fragment。来看一下这个fragment的实现RequestManagerFragment。

public class RequestManagerFragment extends Fragment {
  private final ActivityFragmentLifecycle lifecycle;
  public SupportRequestManagerFragment() {
    this(new ActivityFragmentLifecycle());
  }
  public SupportRequestManagerFragment(@NonNull ActivityFragmentLifecycle lifecycle) {
    this.lifecycle = lifecycle;
  }
  ...
  @NonNull
  ActivityFragmentLifecycle getGlideLifecycle() {
    return lifecycle;
  }
  ...
  @Override
  public void onStart() {
    super.onStart();
    lifecycle.onStart();
  }

  @Override
  public void onStop() {
    super.onStop();
    lifecycle.onStop();
  }

  @Override
  public void onDestroy() {
    super.onDestroy();
    lifecycle.onDestroy();
    unregisterFragmentWithRoot();
  }
  ...
}

 再回到fragmentGet方法，fragment创建成功后，在创建RequestManager时会传入fragment中的ActivityFragmentLifecycle，再来看RequestManager的实现。

public class RequestManager implements LifecycleListener,
    ModelTypes<RequestBuilder<Drawable>> {
  private final Runnable addSelfToLifecycle = new Runnable() {
    @Override
    public void run() {
      lifecycle.addListener(RequestManager.this);
    }
  };
  public RequestManager(
      @NonNull Glide glide, @NonNull Lifecycle lifecycle,
      @NonNull RequestManagerTreeNode treeNode, @NonNull Context context) {
    this(
        glide,
        lifecycle,
        treeNode,
        new RequestTracker(),
        glide.getConnectivityMonitorFactory(),
        context);
  }

  // Our usage is safe here.
  @SuppressWarnings("PMD.ConstructorCallsOverridableMethod")
  RequestManager(
      Glide glide,
      Lifecycle lifecycle,
      RequestManagerTreeNode treeNode,
      RequestTracker requestTracker,
      ConnectivityMonitorFactory factory,
      Context context) {
    this.glide = glide;
    this.lifecycle = lifecycle;
    this.treeNode = treeNode;
    this.requestTracker = requestTracker;
    this.context = context;
    ...
    if (Util.isOnBackgroundThread()) {
      //当在子线程时通过Handler将当前对象注册到ActivityFragmentLifecycle
      mainHandler.post(addSelfToLifecycle);
    } else {
      //将当前对象注册到ActivityFragmentLifecycle
      lifecycle.addListener(this);
    }
    //网络变化监听
    lifecycle.addListener(connectivityMonitor);
    ...
  }
  //开始加载
  @Override
  public synchronized void onStart() {
    resumeRequests();
    //如果有动画则开始动画
    targetTracker.onStart();
  }
  //停止加载
  @Override
  public synchronized void onStop() {
    pauseRequests();
    //如果有动画则动画停止
    targetTracker.onStop();
  }
  //销毁
  @Override
  public synchronized void onDestroy() {
    //如果有动画则动画结束并销毁
    targetTracker.onDestroy();
    ...
  }
  //开始请求数据
  synchronized void track(@NonNull Target<?> target, @NonNull Request request) {
    targetTracker.track(target);
    requestTracker.runRequest(request);
  }
 ...
}

 可以看见在RequestManager的构造函数将RequestManager注册到ActivityFragmentLifecycle中，再来看看ActivityFragmentLifecycle的实现。

class ActivityFragmentLifecycle implements Lifecycle {
  private final Set<LifecycleListener> lifecycleListeners =
      Collections.newSetFromMap(new WeakHashMap<LifecycleListener, Boolean>());
  private boolean isStarted;
  private boolean isDestroyed;
  @Override
  public void addListener(@NonNull LifecycleListener listener) {
    lifecycleListeners.add(listener);

    if (isDestroyed) {
      listener.onDestroy();
    } else if (isStarted) {
      listener.onStart();
    } else {
      listener.onStop();
    }
  }

  @Override
  public void removeListener(@NonNull LifecycleListener listener) {
    lifecycleListeners.remove(listener);
  }
  //每个RequestManager对应一个LifecycleListener 
  void onStart() {
    isStarted = true;
    for (LifecycleListener lifecycleListener : Util.getSnapshot(lifecycleListeners)) {
      lifecycleListener.onStart();
    }
  }
  //每个RequestManager对应一个LifecycleListener 
  void onStop() {
    isStarted = false;
    for (LifecycleListener lifecycleListener : Util.getSnapshot(lifecycleListeners)) {
      lifecycleListener.onStop();
    }
  }
  //每个RequestManager对应一个LifecycleListener 
  void onDestroy() {
    isDestroyed = true;
    for (LifecycleListener lifecycleListener : Util.getSnapshot(lifecycleListeners)) {
      lifecycleListener.onDestroy();
    }
  }
}

 由于ActivityFragmentLifecycle对象是在fragment中创建并且它的onStart、onStop、onDestory方法与fragment一一对应，这样就将RequestManager的生命周期就与fragment关联起来了，也就与当前activity关联起来。总体流程如下：

 当fragment生命周期发生变化时，通过ActivityFragmentLifecycle将变化告诉给RequestManager与DefaultConnectivityMonitor。而RequestManager又将此变化告诉给ImageViewTarget。
 至于传入参数为其他类型的实现基本上与activity的类似，就不在叙述。

【参考资料】
Glide生命周期管理
Glide源码分析(一) 图片加载的生命周期