这篇摸清一个问题,Glide是怎么实现生命周期管理的?
前面已经分析过Glide通过创建一个隐藏的fragment添加到Activity中通过系统生命周期方法的回调,来管理图片加载生命周期.
我们来看下这个隐藏fragment的创建过程:
在RequestManagerRetriever.java中,RequestManagerRetriever是一个创建RequestManager和RequestManagerFragment(隐藏fragment),并将两者联系起来的类.

@NonNull
  private RequestManager fragmentGet(@NonNull Context context,
      @NonNull android.app.FragmentManager fm,
      @Nullable 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(), context);
      current.setRequestManager(requestManager);
    }
    return requestManager;
  }

@NonNull
  private RequestManager supportFragmentGet(@NonNull Context context, @NonNull FragmentManager fm,
      @Nullable Fragment parentHint) {
    //获取隐藏fragment
    SupportRequestManagerFragment current = getSupportRequestManagerFragment(fm, parentHint);
    RequestManager requestManager = current.getRequestManager();
    if (requestManager == null) {
      // TODO(b/27524013): Factor out this Glide.get() call.
      Glide glide = Glide.get(context);
      //创建requestmanager
      requestManager =
          factory.build(
              glide, current.getGlideLifecycle(), current.getRequestManagerTreeNode(), context);
      //将requestManager和隐藏fragment联系起来
      current.setRequestManager(requestManager);
    }
    return requestManager;
  }

上面两个方法是创建RequestManager,分别在context对应Activity和FragmentActivity时调用,分析第二个方法.

SupportRequestManagerFragment getSupportRequestManagerFragment(
      @NonNull final FragmentManager fm, @Nullable Fragment parentHint) {
    SupportRequestManagerFragment current =
        (SupportRequestManagerFragment) fm.findFragmentByTag(FRAGMENT_TAG);
    if (current == null) {
      //从Map缓存中取
      current = pendingSupportRequestManagerFragments.get(fm);
      if (current == null) {
        current = new SupportRequestManagerFragment();
        current.setParentFragmentHint(parentHint);
        //存到Map缓存
        pendingSupportRequestManagerFragments.put(fm, current);
        //添加到当前Activity
        fm.beginTransaction().add(current, FRAGMENT_TAG).commitAllowingStateLoss();
        //发送一个消息移除在Map缓存中的当前fm对应的RequestManagerFragment
        handler.obtainMessage(ID_REMOVE_SUPPORT_FRAGMENT_MANAGER, fm).sendToTarget();
      }
    }
    return current;
  }

来看下隐藏fragment -----SupportRequestManagerFragment:

/**
 * A view-less {@link android.support.v4.app.Fragment} used to safely store an {@link
 * com.bumptech.glide.RequestManager} that can be used to start, stop and manage Glide requests
 * started for targets within the fragment or activity this fragment is a child of.
 *
 * @see com.bumptech.glide.manager.RequestManagerFragment
 * @see com.bumptech.glide.manager.RequestManagerRetriever
 * @see com.bumptech.glide.RequestManager
 */
public class SupportRequestManagerFragment extends Fragment {
  private static final String TAG = "SupportRMFragment";
  private final ActivityFragmentLifecycle lifecycle;
  private final RequestManagerTreeNode requestManagerTreeNode =
      new SupportFragmentRequestManagerTreeNode();
  private final Set<SupportRequestManagerFragment> childRequestManagerFragments = new HashSet<>();

  @Nullable private SupportRequestManagerFragment rootRequestManagerFragment;
  @Nullable private RequestManager requestManager;
  @Nullable private Fragment parentFragmentHint;

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

  // For testing only.
  @SuppressLint("ValidFragment")
  public SupportRequestManagerFragment(ActivityFragmentLifecycle lifecycle) {
    this.lifecycle = lifecycle;
  }

  /**
   * Sets the current {@link com.bumptech.glide.RequestManager}.
   *
   * @param requestManager The manager to put.
   */
  public void setRequestManager(@Nullable RequestManager requestManager) {
    this.requestManager = requestManager;
  }

  ActivityFragmentLifecycle getGlideLifecycle() {
    return lifecycle;
  }

  /**
   * Returns the current {@link com.bumptech.glide.RequestManager} or null if none is put.
   */
  @Nullable
  public RequestManager getRequestManager() {
    return requestManager;
  }

  /**
   * Returns the {@link RequestManagerTreeNode} that provides tree traversal methods relative
   * to the
   * associated {@link RequestManager}.
   */
  public RequestManagerTreeNode getRequestManagerTreeNode() {
    return requestManagerTreeNode;
  }

  private void addChildRequestManagerFragment(SupportRequestManagerFragment child) {
    childRequestManagerFragments.add(child);
  }

  private void removeChildRequestManagerFragment(SupportRequestManagerFragment child) {
    childRequestManagerFragments.remove(child);
  }

  /**
   * Returns the set of fragments that this RequestManagerFragment's parent is a parent to. (i.e.
   * our parent is the fragment that we are annotating).
   */
  @Synthetic Set<SupportRequestManagerFragment> getDescendantRequestManagerFragments() {
    if (rootRequestManagerFragment == null) {
      return Collections.emptySet();
    } else if (this.equals(rootRequestManagerFragment)) {
      return Collections.unmodifiableSet(childRequestManagerFragments);
    } else {
      Set<SupportRequestManagerFragment> descendants = new HashSet<>();
      for (SupportRequestManagerFragment fragment : rootRequestManagerFragment
          .getDescendantRequestManagerFragments()) {
        if (isDescendant(fragment.getParentFragmentUsingHint())) {
          descendants.add(fragment);
        }
      }
      return Collections.unmodifiableSet(descendants);
    }
  }

  /**
   * Sets a hint for which fragment is our parent which allows the fragment to return correct
   * information about its parents before pending fragment transactions have been executed.
   */
  void setParentFragmentHint(@Nullable Fragment parentFragmentHint) {
    this.parentFragmentHint = parentFragmentHint;
    if (parentFragmentHint != null && parentFragmentHint.getActivity() != null) {
      registerFragmentWithRoot(parentFragmentHint.getActivity());
    }
  }

  private Fragment getParentFragmentUsingHint() {
    Fragment fragment = getParentFragment();
    return fragment != null ? fragment : parentFragmentHint;
  }

  /**
   * Returns true if the fragment is a descendant of our parent.
   */
  private boolean isDescendant(Fragment fragment) {
    Fragment root = this.getParentFragmentUsingHint();
    Fragment parentFragment;
    while ((parentFragment = fragment.getParentFragment()) != null) {
      if (parentFragment.equals(root)) {
        return true;
      }
      fragment = fragment.getParentFragment();
    }
    return false;
  }

  private void registerFragmentWithRoot(FragmentActivity activity) {
    unregisterFragmentWithRoot();
    rootRequestManagerFragment = Glide.get(activity).getRequestManagerRetriever()
        .getSupportRequestManagerFragment(activity.getSupportFragmentManager(), null);
    if (!this.equals(rootRequestManagerFragment)) {
      rootRequestManagerFragment.addChildRequestManagerFragment(this);
    }
  }

  private void unregisterFragmentWithRoot() {
    if (rootRequestManagerFragment != null) {
      rootRequestManagerFragment.removeChildRequestManagerFragment(this);
      rootRequestManagerFragment = null;
    }
  }

  @Override
  public void onAttach(Context context) {
    super.onAttach(context);
    try {
      registerFragmentWithRoot(getActivity());
    } catch (IllegalStateException e) {
      // OnAttach can be called after the activity is destroyed, see #497.
      if (Log.isLoggable(TAG, Log.WARN)) {
        Log.w(TAG, "Unable to register fragment with root", e);
      }
    }
  }

  @Override
  public void onDetach() {
    super.onDetach();
    parentFragmentHint = null;
    unregisterFragmentWithRoot();
  }

  @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();
  }

  @Override
  public String toString() {
    return super.toString() + "{parent=" + getParentFragmentUsingHint() + "}";
  }

  private class SupportFragmentRequestManagerTreeNode implements RequestManagerTreeNode {

    @Synthetic
    SupportFragmentRequestManagerTreeNode() { }
    
    //获取当前隐藏fragment所有的RequestManager
    @Override
    public Set<RequestManager> getDescendants() {
      Set<SupportRequestManagerFragment> descendantFragments =
          getDescendantRequestManagerFragments();
      Set<RequestManager> descendants = new HashSet<>(descendantFragments.size());
      for (SupportRequestManagerFragment fragment : descendantFragments) {
        if (fragment.getRequestManager() != null) {
          descendants.add(fragment.getRequestManager());
        }
      }
      return descendants;
    }

    @Override
    public String toString() {
      return super.toString() + "{fragment=" + SupportRequestManagerFragment.this + "}";
    }
  }
}

这个fragment还是比较简单的,介绍下几个比较重要的成员变量

• lifecycle : 保存着需要监听当前fragment或Activity的RequestManager,在隐藏fragment的生命周期改变时去通知保存的每一个RequestManager
• requestManagerTreeNode: 可以通过这个对象获取到在当前隐藏fragment里面的所有的RequestManager.
• childRequestManagerFragments: 当前隐藏fragment所有的子隐藏fragment的集合.
• rootRequestManagerFragment: 当前隐藏fragment的根隐藏fragment,就是Activity中第一个创建的隐藏fragment.
• parentFragmentHint: 当前隐藏fragment的父fragment.
  接下来看下这个隐藏fragment是怎么通知Glide管理生命周期的,就拿onStart()方法分析

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

lifecycle的onStart()方法

 void onStart() {
    isStarted = true;
    //通知每一个lifecycleListener
    for (LifecycleListener lifecycleListener : Util.getSnapshot(lifecycleListeners)) {
      lifecycleListener.onStart();
    }
  }

lifecycleListener通过下面方法加入进来:

@Override
  public void addListener(LifecycleListener listener) {
    lifecycleListeners.add(listener);

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

我们来看下addListener()的调用位置:

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;

    connectivityMonitor =
        factory.build(
            context.getApplicationContext(),
            new RequestManagerConnectivityListener(requestTracker));

    // If we're the application level request manager, we may be created on a background thread.
    // In that case we cannot risk synchronously pausing or resuming requests, so we hack around the
    // issue by delaying adding ourselves as a lifecycle listener by posting to the main thread.
    // This should be entirely safe.
    if (Util.isOnBackgroundThread()) {
      mainHandler.post(addSelfToLifecycle);
    } else {
      lifecycle.addListener(this);//RequesManager实现了LifecycleListener这里就是将当前RequesManager
//加入lifecycle的管理中,典型的观察者模式,被观察者-----隐藏fragment生命周期调用,通过ActivityFragmentLifecycle通知
//每一个观察者RequesManager
    }
    lifecycle.addListener(connectivityMonitor);

    setRequestOptions(glide.getGlideContext().getDefaultRequestOptions());

    glide.registerRequestManager(this);
  }

这里的lifecycle就是ActivityFragmentLifecycle,在隐藏fragment创建的时候创建,并且在创建RequesManager的时候被传入.所以上面的lifecycleListener.onStart();就是调用RequesManager的onStart()方法:

/**
   * Lifecycle callback that registers for connectivity events (if the
   * android.permission.ACCESS_NETWORK_STATE permission is present) and restarts failed or paused
   * requests.
   */
  @Override
  public void onStart() {
    resumeRequests();
    targetTracker.onStart();
  }

/**
   * Restarts any loads that have not yet completed.
   *
   * @see #isPaused()
   * @see #pauseRequests()
   */
  public void resumeRequests() {
    Util.assertMainThread();
    requestTracker.resumeRequests();
  }

RequestTracker: 跟踪request,负责对request取消,重新启动,暂停,运行等.

/**
   * Starts any not yet completed or failed requests.
   */
  public void resumeRequests() {
    isPaused = false;
    for (Request request : Util.getSnapshot(requests)) {
      if (!request.isComplete() && !request.isCancelled() && !request.isRunning()) {
        request.begin();//调用每个request的开始方法,这个方法就是加载的开始,上篇文章分析过
      }
    }
    pendingRequests.clear();
  }

那么request是怎么和RequestTracker联系起来的呢?
它们是在into()这个方法中关联起来的:

private <Y extends Target<TranscodeType>> Y into(
      @NonNull Y target,
      @Nullable RequestListener<TranscodeType> targetListener,
      @NonNull RequestOptions options) {

    ....省略...

    options = options.autoClone();
    Request request = buildRequest(target, targetListener, options);

    Request previous = target.getRequest();

   ...省略...

    requestManager.clear(target);
    target.setRequest(request);
    requestManager.track(target, request);//重点

    return target;
  }

RequestManager

void track(Target<?> target, Request request) {
    targetTracker.track(target);
    requestTracker.runRequest(request);
  }

RequestTracker

/**
   * Starts tracking the given request.
   */
  public void runRequest(Request request) {
    requests.add(request);//加入tracking集合
    if (!isPaused) {
      request.begin();
    } else {
      pendingRequests.add(request);
    }
  }

上面分析了onStart()方法,可以看出来Glide是在隐藏fragment的onStart()方法被调用时通过ActivityFragmentLifecycle通知每一个requestManager调用onStart(),随后调用RequestTracker的onStart()方法开启每一个RequestTracker跟踪的request的begin()方法开启或者继续加载.