Glide生命周期绑定

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Glide中如何实现生命周期绑定呢？

添加RequestManagerFragment

RequestManagerFragment是实现Glide生命周期的关键。RequestManagerFragment是一个没有界面的Fragment，用于存放RequestManager。RequestManagerFragment可以管理(包括启动和停止)它自身所在的Fragment或Activity中的Target上关联的Glide Request。

当你调用Glide.with(activity)时，Glide已经偷偷在这个activity中添加了一个RequestManagerFragment。

注意：Android中系统内置了一种Activity，support库提供了另一种Activity(FragmentActivity)。为了兼容不同的Activity，Glide.with()方法有几个重载版本，对前者而言是Glide.with(android.app.Activity)，对后者是Glide.with(android.support.v4.app.FragmentActivity)。不同的Glide.with()方法添加的Fragment有所不同。

Activity类型 添加的Fragment类型
android.support.v4.app.FragmentActivity SupportRequestManagerFragment
android.app.Activity RequestManagerFragment

Glide已经注意到其中的差异，它并不会弄混。我们只要知道其中的差异即可。下文的讨论中关注的重点是Glide”偷偷”添加Fragment的流程，而忽略了Fragment类型的差异。

Glide.with()最终会调用到RequestManagerRetriever.get()方法。RequestManagerRetriever.get()源码如下：

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 // RequestManagerRetriever.java
 public RequestManager get(Activity activity) {
...
   android.app.FragmentManager fm = activity.getFragmentManager();
   return fragmentGet(activity, fm, null /*parentHint*/);
 }

 private RequestManager fragmentGet(Context context, android.app.FragmentManager fm,
     android.app.Fragment parentHint) {
   RequestManagerFragment current = getRequestManagerFragment(fm, parentHint);
   RequestManager requestManager = current.getRequestManager();
   if (requestManager == null) {
     Glide glide = Glide.get(context);
     requestManager =
         factory.build(glide, current.getGlideLifecycle(), current.getRequestManagerTreeNode());
     current.setRequestManager(requestManager);
   }
   return requestManager;
 }

 RequestManagerFragment getRequestManagerFragment(
     final android.app.FragmentManager fm, android.app.Fragment parentHint) {
   RequestManagerFragment current = (RequestManagerFragment) fm.findFragmentByTag(FRAGMENT_TAG);
   if (current == null) {
     current = pendingRequestManagerFragments.get(fm);
     if (current == null) {
       current = new RequestManagerFragment();
       current.setParentFragmentHint(parentHint);
       pendingRequestManagerFragments.put(fm, current);
       fm.beginTransaction().add(current, FRAGMENT_TAG).commitAllowingStateLoss();
       handler.obtainMessage(ID_REMOVE_FRAGMENT_MANAGER, fm).sendToTarget();
     }
   }
   return current;
 }

从代码可以看到调用Glide.with(activity)时Glide的确添加了Fragment。很容易用以下代码验证是否真的添加了Fragment。

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private static final String FRAGMENT_TAG = "com.bumptech.glide.manager";

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

    Glide.with(this);

    findViewById(R.id.button).setOnClickListener(new View.OnClickListener() {
        @Override
        public void onClick(View v) {
            Fragment fragment = getSupportFragmentManager().findFragmentByTag(FRAGMENT_TAG);
            if (fragment instanceof SupportRequestManagerFragment) {
                System.out.println(fragment);
            }
        }
    });
}

点击Button输出如下：

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07-15 03:53:37.272 26036-26036/? I/System.out: SupportRequestManagerFragment{ccdb477 #0 com.bumptech.glide.manager}{parent=null}

RequestManagerFragment生命周期

  • RequestManagerFragment - 有一个类型为ActivityFragmentLifecycle的lifecycle成员。RequestManagerFragment生命周期事件会通知给lifecycle相应的方法。
  • ActivityFragmentLifecycle - 有一个类型为Set的lifecycleListeners成员。onStart/onStop/onDestroy被调用时会回调lifecycleListeners中的每个listener。
  • RequestManager - 它实现了LifecycleListener接口，有一个类型为TargetTracker的成员。RequestManager的onStart/onStop/onDestroy被调用时会通知TargetTracker。RequestManager自身作为LifecycleListener被添加到ActivityFragmentLifecycle。
  • TargetTracker - 它实现了LifecycleListener接口，用于管理多个Target。TargetTracker的onStart/onStop/onDestory被调用时会通知Target。
  • Target - Target继承自LifecycleListener。

UI上有一个ImageView，调用Glide给这个ImageView加载一张图片，代码如下：

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Glide.with(this).load(...).into(imageView);

into()方法原型是RequestBuilder.into(ImageView)。这个方法中，ImageView会被封装成BitmapImageViewTarget。然后调用RequestBuilder.into(Target)方法。代码如下：

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public <Y extends Target<TranscodeType>> Y into(@NonNull Y target) {
  Util.assertMainThread();
  Preconditions.checkNotNull(target);
  if (!isModelSet) {
    throw new IllegalArgumentException("You must call #load() before calling #into()");
  }

  Request previous = target.getRequest();

  if (previous != null) {
    requestManager.clear(target);
  }

  requestOptions.lock();
  Request request = buildRequest(target);
  target.setRequest(request);
  requestManager.track(target, request);

  return target;
}

整理出来的流程是这样的：

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Glide.with() ->
RequestManagerRetriever.get() ->
RequestManager.load() ->
RequestBuilder.into() ->
RequestManager.track() ->
TargetTracker.track() ->

通过应用中的一个场景来理解RequestManagerFragment生命周期过程。假设用户按Home键退出应用中某个正在加载图片的页面，有以下流程：

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RequestManagerFragment.onStop() ->
ActivityFragmentLifecycle.onStop() ->
RequestManager.onStop() ->
TargetTracker.onStop() ->
Target.onStop() ->

ActivityFragmentLifecycle如何跟RequestManager关联上的呢？关键在于这段代码：

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  private RequestManager fragmentGet(Context context, android.app.FragmentManager fm,
      android.app.Fragment parentHint) {
    RequestManagerFragment current = getRequestManagerFragment(fm, parentHint);
    RequestManager requestManager = current.getRequestManager();
    if (requestManager == null) {
      // TODO(b/27524013): Factor out this Glide.get() call.
      Glide glide = Glide.get(context);
      requestManager =
          factory.build(glide, current.getGlideLifecycle(), current.getRequestManagerTreeNode());
      current.setRequestManager(requestManager);
    }
    return requestManager;
  }

  private static final RequestManagerFactory DEFAULT_FACTORY = new RequestManagerFactory() {
    @Override
    public RequestManager build(Glide glide, Lifecycle lifecycle,
        RequestManagerTreeNode requestManagerTreeNode) {
      return new RequestManager(glide, lifecycle, requestManagerTreeNode);
    }
  };

public class RequestManagerFragment extends Fragment {
  private final ActivityFragmentLifecycle lifecycle;

  public RequestManagerFragment() {
    this(new ActivityFragmentLifecycle());
  }

  RequestManagerFragment(ActivityFragmentLifecycle lifecycle) {
    this.lifecycle = lifecycle;
  }

  ActivityFragmentLifecycle getGlideLifecycle() {
    return lifecycle;
  }
}

梳理一下我们不难发现，RequestManager的lifecycle成员跟RequestManagerFragment的lifecycle是同一个对象！

RequestManagerFragment的数量

  1. 有几个RequestManagerFragment?
  2. 有几个RequestManager？
  3. 多个RequestManagerFragment之间是什么关系?
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public class RequestManagerRetriever implements Handler.Callback {
  final Map<FragmentManager, SupportRequestManagerFragment> pendingSupportRequestManagerFragments =
      new HashMap<>();

  private RequestManager fragmentGet(Context context, android.app.FragmentManager fm,
      android.app.Fragment parentHint) {
    RequestManagerFragment current = getRequestManagerFragment(fm, parentHint);
    RequestManager requestManager = current.getRequestManager();
    if (requestManager == null) {
      // TODO(b/27524013): Factor out this Glide.get() call.
      Glide glide = Glide.get(context);
      requestManager =
          factory.build(glide, current.getGlideLifecycle(), current.getRequestManagerTreeNode());
      current.setRequestManager(requestManager);
    }
    return requestManager;
  }
}

先来回答前两个问题。有n个Activity/Fragment使用Glide加载图片的话，则有n个RequestManagerFragment和n个RequestManager。即，每个Activity/Fragment中会被添加一个TAG为”com.bumptech.glide.manager”的RequestManageFragment，RequestManageFragment和RequestManager是一一对应关系。

第3个问题稍麻烦一点。考虑这样一种情况，一个Activity中有两个Fragment，它们都在使用Glide加载图片。一共有3个RequestManagerFragment，它们的关系如下：

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public class RequestManagerFragment extends Fragment {
  private final HashSet<RequestManagerFragment> childRequestManagerFragments =
      new HashSet<>();

  @Nullable private RequestManagerFragment rootRequestManagerFragment;
}

Glide 如何触发 onTrimMemory()

流程如下图：

Glide 如何触发 onStop()

其他事件的传递过程也类似。如下图：