Glide-4.11.0 浅析

Glide-4.11.0 浅析

Glide 是Google推荐使用的图片加载框架，在开发过程中我们也会经常使用到，使用起来也比较简单，一般一行代码就可以完成图片加载；如：

Glide.with(mThumbIv).load(url).into(mThumbIv);

今天我们来阅读一下Glide的源码，了解一下Glide 加载图标的原理，我们今天阅读的Glide源码是Glide:4.11.0版本的，是当前最新版本。源码获取地址为 https://github.com/bumptech/glide/releases/tag/v4.11.0。

源码的阅读，是比较枯燥的；但是掌握了阅读代码的方法之后，一个人，一杯茶，一个电脑，也是一种特别的享受。

开始阅读代码

上面说了Glide加载的方法，一般使用时，在load之前，我们还会有其他的一些设置想，但是总体来说Glide加载图片就可以分为三步，第一步：with,第二步：load,第三步：into，接下来我们就从这三步来阅读代码

1. with

Glide.with(context) ;with是Glide类的静态方法，在 Glide类中有多个with方法的重载，代码如下：

@NonNull
public static RequestManager with(@NonNull Context context) {
  return getRetriever(context).get(context);
}


@NonNull
public static RequestManager with(@NonNull Activity activity) {
  return getRetriever(activity).get(activity);
}


@NonNull
public static RequestManager with(@NonNull FragmentActivity activity) {
  return getRetriever(activity).get(activity);
}


@NonNull
public static RequestManager with(@NonNull androidx.fragment.app.Fragment fragment) {
  return getRetriever(fragment.getContext()).get(fragment);
}


@SuppressWarnings("deprecation")
@Deprecated
@NonNull
public static RequestManager with(@NonNull android.app.Fragment fragment) {
  return getRetriever(fragment.getActivity()).get(fragment);
}

@NonNull
public static RequestManager with(@NonNull View view) {
  return getRetriever(view.getContext()).get(view);
}

从上面代码中，我们可以看到，在调用Glide.with方法时，我们可以传入的参数有Context,Activity,FragmentActivity,androidx包下的Fragment和android包下的Fragment,View,无论传入什么参数，最终都调用了getRetriever().get()方法来返回了RequestManager对象;

我们来看一下 getRetriever()方法的源码，

@NonNull
private static RequestManagerRetriever getRetriever(@Nullable Context context) {
  // Context could be null for other reasons (ie the user passes in null), but in practice it will
  // only occur due to errors with the Fragment lifecycle.
  Preconditions.checkNotNull(
      context,
      "You cannot start a load on a not yet attached View or a Fragment where getActivity() "
          + "returns null (which usually occurs when getActivity() is called before the Fragment "
          + "is attached or after the Fragment is destroyed).");
  return Glide.get(context).getRequestManagerRetriever();
}

getRetriever()方法时通过调用Glide 类的get()方法来获取Glide的单例对象。然后通过初始化之后的Glide对象，获取RequestManagerRetriever对象；所以getRetriever()方法最终返回了一个RequestManagerRetriever对象。

with 方法中，在调用getRetriever()方法，拿到RequestManagerRetriever对象之后，调用了他的get()方法。传入了with方法传入的参数。我们来看一下RequestManagerRetriever对象中的get方法：

public RequestManagerRetriever(@Nullable RequestManagerFactory factory) {
    this.factory = factory != null ? factory : DEFAULT_FACTORY;
    handler = new Handler(Looper.getMainLooper(), this /* Callback */);
  }

  @NonNull
  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) {
          // Normally pause/resume is taken care of by the fragment we add to the fragment or
          // activity. However, in this case since the manager attached to the application will not
          // receive lifecycle events, we must force the manager to start resumed using
          // ApplicationLifecycle.

          // TODO(b/27524013): Factor out this Glide.get() call.
          Glide glide = Glide.get(context.getApplicationContext());
          applicationManager =
              factory.build(
                  glide,
                  new ApplicationLifecycle(),
                  new EmptyRequestManagerTreeNode(),
                  context.getApplicationContext());
        }
      }
    }
    return applicationManager;
  }

  @NonNull
  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) {
        return get((FragmentActivity) context);
      } else if (context instanceof Activity) {
        return get((Activity) context);
      } else if (context instanceof ContextWrapper
          // Only unwrap a ContextWrapper if the baseContext has a non-null application context.
          // Context#createPackageContext may return a Context without an Application instance,
          // in which case a ContextWrapper may be used to attach one.
          && ((ContextWrapper) context).getBaseContext().getApplicationContext() != null) {
        return get(((ContextWrapper) context).getBaseContext());
      }
    }

    return getApplicationManager(context);
  }

  @NonNull
  public RequestManager get(@NonNull FragmentActivity activity) {
    if (Util.isOnBackgroundThread()) {
      return get(activity.getApplicationContext());
    } else {
      assertNotDestroyed(activity);
      FragmentManager fm = activity.getSupportFragmentManager();
      return supportFragmentGet(activity, fm, /*parentHint=*/ null, isActivityVisible(activity));
    }
  }

  @NonNull
  public RequestManager get(@NonNull Fragment fragment) {
    Preconditions.checkNotNull(
        fragment.getContext(),
        "You cannot start a load on a fragment before it is attached or after it is destroyed");
    if (Util.isOnBackgroundThread()) {
      return get(fragment.getContext().getApplicationContext());
    } else {
      FragmentManager fm = fragment.getChildFragmentManager();
      return supportFragmentGet(fragment.getContext(), fm, fragment, fragment.isVisible());
    }
  }

  @SuppressWarnings("deprecation")
  @NonNull
  public RequestManager get(@NonNull Activity activity) {
    if (Util.isOnBackgroundThread()) {
      return get(activity.getApplicationContext());
    } else {
      assertNotDestroyed(activity);
      android.app.FragmentManager fm = activity.getFragmentManager();
      return fragmentGet(activity, fm, /*parentHint=*/ null, isActivityVisible(activity));
    }
  }

  @SuppressWarnings("deprecation")
  @NonNull
  public RequestManager get(@NonNull View view) {
    if (Util.isOnBackgroundThread()) {
      return get(view.getContext().getApplicationContext());
    }

    Preconditions.checkNotNull(view);
    Preconditions.checkNotNull(
        view.getContext(), "Unable to obtain a request manager for a view without a Context");
    Activity activity = findActivity(view.getContext());
    // The view might be somewhere else, like a service.
    if (activity == null) {
      return get(view.getContext().getApplicationContext());
    }

    if (activity instanceof FragmentActivity) {
      Fragment fragment = findSupportFragment(view, (FragmentActivity) activity);
      return fragment != null ? get(fragment) : get((FragmentActivity) activity);
    }

    // Standard Fragments.
    android.app.Fragment fragment = findFragment(view, activity);
    if (fragment == null) {
      return get(activity);
    }
    return get(fragment);
  }

  private static void findAllSupportFragmentsWithViews(
      @Nullable Collection topLevelFragments, @NonNull Map result) {
    if (topLevelFragments == null) {
      return;
    }
    for (Fragment fragment : topLevelFragments) {
      // getFragment()s in the support FragmentManager may contain null values, see #1991.
      if (fragment == null || fragment.getView() == null) {
        continue;
      }
      result.put(fragment.getView(), fragment);
      findAllSupportFragmentsWithViews(fragment.getChildFragmentManager().getFragments(), result);
    }
  }

  @Nullable
  private Fragment findSupportFragment(@NonNull View target, @NonNull FragmentActivity activity) {
    tempViewToSupportFragment.clear();
    findAllSupportFragmentsWithViews(
        activity.getSupportFragmentManager().getFragments(), tempViewToSupportFragment);
    Fragment result = null;
    View activityRoot = activity.findViewById(android.R.id.content);
    View current = target;
    while (!current.equals(activityRoot)) {
      result = tempViewToSupportFragment.get(current);
      if (result != null) {
        break;
      }
      if (current.getParent() instanceof View) {
        current = (View) current.getParent();
      } else {
        break;
      }
    }

    tempViewToSupportFragment.clear();
    return result;
  }

  @SuppressWarnings({"deprecation", "DeprecatedIsStillUsed"})
  @Deprecated
  @Nullable
  private android.app.Fragment findFragment(@NonNull View target, @NonNull Activity activity) {
    tempViewToFragment.clear();
    findAllFragmentsWithViews(activity.getFragmentManager(), tempViewToFragment);

    android.app.Fragment result = null;

    View activityRoot = activity.findViewById(android.R.id.content);
    View current = target;
    while (!current.equals(activityRoot)) {
      result = tempViewToFragment.get(current);
      if (result != null) {
        break;
      }
      if (current.getParent() instanceof View) {
        current = (View) current.getParent();
      } else {
        break;
      }
    }
    tempViewToFragment.clear();
    return result;
  }

  // TODO: Consider using an accessor class in the support library package to more directly retrieve
  // non-support Fragments.
  @SuppressWarnings("deprecation")
  @Deprecated
  @TargetApi(Build.VERSION_CODES.O)
  private void findAllFragmentsWithViews(
      @NonNull android.app.FragmentManager fragmentManager,
      @NonNull ArrayMap result) {
    if (Build.VERSION.SDK_INT >= Build.VERSION_CODES.O) {
      for (android.app.Fragment fragment : fragmentManager.getFragments()) {
        if (fragment.getView() != null) {
          result.put(fragment.getView(), fragment);
          findAllFragmentsWithViews(fragment.getChildFragmentManager(), result);
        }
      }
    } else {
      findAllFragmentsWithViewsPreO(fragmentManager, result);
    }
  }

  @SuppressWarnings("deprecation")
  @Deprecated
  private void findAllFragmentsWithViewsPreO(
      @NonNull android.app.FragmentManager fragmentManager,
      @NonNull ArrayMap result) {
    int index = 0;
    while (true) {
      tempBundle.putInt(FRAGMENT_INDEX_KEY, index++);
      android.app.Fragment fragment = null;
      try {
        fragment = fragmentManager.getFragment(tempBundle, FRAGMENT_INDEX_KEY);
      } catch (Exception e) {
        // This generates log spam from FragmentManager anyway.
      }
      if (fragment == null) {
        break;
      }
      if (fragment.getView() != null) {
        result.put(fragment.getView(), fragment);
        if (VERSION.SDK_INT >= VERSION_CODES.JELLY_BEAN_MR1) {
          findAllFragmentsWithViews(fragment.getChildFragmentManager(), result);
        }
      }
    }
  }

  @Nullable
  private static Activity findActivity(@NonNull Context context) {
    if (context instanceof Activity) {
      return (Activity) context;
    } else if (context instanceof ContextWrapper) {
      return findActivity(((ContextWrapper) context).getBaseContext());
    } else {
      return null;
    }
  }

  @TargetApi(Build.VERSION_CODES.JELLY_BEAN_MR1)
  private static void assertNotDestroyed(@NonNull Activity activity) {
    if (Build.VERSION.SDK_INT >= Build.VERSION_CODES.JELLY_BEAN_MR1 && activity.isDestroyed()) {
      throw new IllegalArgumentException("You cannot start a load for a destroyed activity");
    }
  }

  @SuppressWarnings("deprecation")
  @Deprecated
  @NonNull
  @TargetApi(Build.VERSION_CODES.JELLY_BEAN_MR1)
  public RequestManager get(@NonNull android.app.Fragment fragment) {
    if (fragment.getActivity() == null) {
      throw new IllegalArgumentException(
          "You cannot start a load on a fragment before it is attached");
    }
    if (Util.isOnBackgroundThread() || Build.VERSION.SDK_INT < Build.VERSION_CODES.JELLY_BEAN_MR1) {
      return get(fragment.getActivity().getApplicationContext());
    } else {
      android.app.FragmentManager fm = fragment.getChildFragmentManager();
      return fragmentGet(fragment.getActivity(), fm, fragment, fragment.isVisible());
    }
  }

  @SuppressWarnings("deprecation")
  @Deprecated
  @NonNull
  RequestManagerFragment getRequestManagerFragment(Activity activity) {
    return getRequestManagerFragment(
        activity.getFragmentManager(), /*parentHint=*/ null, isActivityVisible(activity));
  }

  @SuppressWarnings("deprecation")
  @NonNull
  private RequestManagerFragment getRequestManagerFragment(
      @NonNull final android.app.FragmentManager fm,
      @Nullable android.app.Fragment parentHint,
      boolean isParentVisible) {
    RequestManagerFragment current = (RequestManagerFragment) fm.findFragmentByTag(FRAGMENT_TAG);
    if (current == null) {
      current = pendingRequestManagerFragments.get(fm);
      if (current == null) {
        current = new RequestManagerFragment();
        current.setParentFragmentHint(parentHint);
        if (isParentVisible) {
          current.getGlideLifecycle().onStart();
        }
        pendingRequestManagerFragments.put(fm, current);
        fm.beginTransaction().add(current, FRAGMENT_TAG).commitAllowingStateLoss();
        handler.obtainMessage(ID_REMOVE_FRAGMENT_MANAGER, fm).sendToTarget();
      }
    }
    return current;
  }

  @SuppressWarnings({"deprecation", "DeprecatedIsStillUsed"})
  @Deprecated
  @NonNull
  private RequestManager fragmentGet(
      @NonNull Context context,
      @NonNull android.app.FragmentManager fm,
      @Nullable android.app.Fragment parentHint,
      boolean isParentVisible) {
    RequestManagerFragment current = getRequestManagerFragment(fm, parentHint, isParentVisible);
    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
  SupportRequestManagerFragment getSupportRequestManagerFragment(
      Context context, FragmentManager fragmentManager) {
    return getSupportRequestManagerFragment(
        fragmentManager, /*parentHint=*/ null, isActivityVisible(context));
  }

  private static boolean isActivityVisible(Context context) {
    // This is a poor heuristic, but it's about all we have. We'd rather err on the side of visible
    // and start requests than on the side of invisible and ignore valid requests.
    Activity activity = findActivity(context);
    return activity == null || !activity.isFinishing();
  }

  @NonNull
  private SupportRequestManagerFragment getSupportRequestManagerFragment(
      @NonNull final FragmentManager fm, @Nullable Fragment parentHint, boolean isParentVisible) {
    SupportRequestManagerFragment current =
        (SupportRequestManagerFragment) fm.findFragmentByTag(FRAGMENT_TAG);
    if (current == null) {
      current = pendingSupportRequestManagerFragments.get(fm);
      if (current == null) {
        current = new SupportRequestManagerFragment();
        current.setParentFragmentHint(parentHint);
        if (isParentVisible) {
          current.getGlideLifecycle().onStart();
        }
        pendingSupportRequestManagerFragments.put(fm, current);
        fm.beginTransaction().add(current, FRAGMENT_TAG).commitAllowingStateLoss();
        handler.obtainMessage(ID_REMOVE_SUPPORT_FRAGMENT_MANAGER, fm).sendToTarget();
      }
    }
    return current;
  }

  @NonNull
  private RequestManager supportFragmentGet(
      @NonNull Context context,
      @NonNull FragmentManager fm,
      @Nullable Fragment parentHint,
      boolean isParentVisible) {
    SupportRequestManagerFragment current =
        getSupportRequestManagerFragment(fm, parentHint, isParentVisible);
    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;
  }

  @Override
  public boolean handleMessage(Message message) {
    boolean handled = true;
    Object removed = null;
    Object key = null;
    switch (message.what) {
      case ID_REMOVE_FRAGMENT_MANAGER:
        android.app.FragmentManager fm = (android.app.FragmentManager) message.obj;
        key = fm;
        removed = pendingRequestManagerFragments.remove(fm);
        break;
      case ID_REMOVE_SUPPORT_FRAGMENT_MANAGER:
        FragmentManager supportFm = (FragmentManager) message.obj;
        key = supportFm;
        removed = pendingSupportRequestManagerFragments.remove(supportFm);
        break;
      default:
        handled = false;
        break;
    }
    if (handled && removed == null && Log.isLoggable(TAG, Log.WARN)) {
      Log.w(TAG, "Failed to remove expected request manager fragment, manager: " + key);
    }
    return handled;
  }

  /** Used internally to create {@link RequestManager}s. */
  public interface RequestManagerFactory {
    @NonNull
    RequestManager build(
        @NonNull Glide glide,
        @NonNull Lifecycle lifecycle,
        @NonNull RequestManagerTreeNode requestManagerTreeNode,
        @NonNull Context context);
  }

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

从第33行，我们可以看到，在该get()方法中，根据Context类型，整体分为两类，一类是非Application的Context，一类是Application的Context，Application的Context，直接执行第7行getApplicationManager(context)方法，来创建和Application同生命周期的RequestManager对象；当应用关闭后，Glide也就停止加载；

传入非Application的Context，无论实Activity,Fragment,最终都是根据是否是FragmentActivity的子类或者Activity的子类来创建一个隐藏的SupportRequestManagerFragment 或 RequestManagerFragment对象（第302行和第356行），该对象继承自Fragment；来实现对应的Activity的生命周期的控制；当该Activity销毁时，对应的Glide也停止加载。

总体来说：with方法就是创建一个RequestManager 对象，然后根据传入的context来控制图片加载的生命周期

2. load

接下来我们来看load方法，在上一步中，我们通过with方法，获取到了RequestManager 对象；那么load方法，就在RequestManager类中，我们来看一下RequestManager类

  @NonNull
  @CheckResult
  public RequestBuilder asBitmap() {
    return as(Bitmap.class).apply(DECODE_TYPE_BITMAP);
  }

  @NonNull
  @CheckResult
  public RequestBuilder asGif() {
    return as(GifDrawable.class).apply(DECODE_TYPE_GIF);
  }

  @NonNull
  @CheckResult
  public RequestBuilder asDrawable() {
    return as(Drawable.class);
  }

  @NonNull
  @CheckResult
  @Override
  public RequestBuilder load(@Nullable Bitmap bitmap) {
    return asDrawable().load(bitmap);
  }

  
  @NonNull
  @CheckResult
  @Override
  public RequestBuilder load(@Nullable Drawable drawable) {
    return asDrawable().load(drawable);
  }

  @NonNull
  @CheckResult
  @Override
  public RequestBuilder load(@Nullable String string) {
    return asDrawable().load(string);
  }

  
  @NonNull
  @CheckResult
  @Override
  public RequestBuilder load(@Nullable Uri uri) {
    return asDrawable().load(uri);
  }

  @NonNull
  @CheckResult
  @Override
  public RequestBuilder load(@Nullable File file) {
    return asDrawable().load(file);
  }

  
  @SuppressWarnings("deprecation")
  @NonNull
  @CheckResult
  @Override
  public RequestBuilder load(@RawRes @DrawableRes @Nullable Integer resourceId) {
    return asDrawable().load(resourceId);
  }

  
  @SuppressWarnings("deprecation")
  @CheckResult
  @Override
  @Deprecated
  public RequestBuilder load(@Nullable URL url) {
    return asDrawable().load(url);
  }

 
  @NonNull
  @CheckResult
  @Override
  public RequestBuilder load(@Nullable byte[] model) {
    return asDrawable().load(model);
  }

 
  @NonNull
  @CheckResult
  @Override
  public RequestBuilder load(@Nullable Object model) {
    return asDrawable().load(model);
  }

  @NonNull
  @CheckResult
  public RequestBuilder downloadOnly() {
    return as(File.class).apply(DOWNLOAD_ONLY_OPTIONS);
  }

  
  @NonNull
  @CheckResult
  public RequestBuilder download(@Nullable Object model) {
    return downloadOnly().load(model);
  }

  @NonNull
  @CheckResult
  public RequestBuilder asFile() {
    return as(File.class).apply(skipMemoryCacheOf(true));
  }

  @NonNull
  @CheckResult
  public  RequestBuilder as(
      @NonNull Class resourceClass) {
    return new RequestBuilder<>(glide, this, resourceClass, context);
  }
  

在RequestManager类中，我们可以看到有很多load方法的重载方法，可以传入不同的图片资源来加载图片资源；在load方法中我们可以看到都是调用了asDrawable()方法返回一个RequestBuilder对象，再调用RequestBuilder对象中的load方法，传入对应的图片资源或地址，进行加载图片；我们来看一下asDrawable() 方法,在第15行，看到asDrawable() 方法中调用了as()方法（第111行）生成了RequestBuilder对象 ，传入了glide对象，当前的 RequestManager 对象，泛型的具体类，和context； 除了asDrawable() 方法，还有三个方法asGif()，asBitmap()和asFile()也调用了as()方法生成了对应类型的RequestBuilder对象。

可以看到，最终是将图片资源传入到了RequestBuilder类中的load方法中，接下来我们看一下RequestBuilder类中的load方法:

 public RequestBuilder load(@Nullable Bitmap bitmap) {
    return loadGeneric(bitmap).apply(diskCacheStrategyOf(DiskCacheStrategy.NONE));
  }

  public RequestBuilder load(@Nullable Drawable drawable) {
    return loadGeneric(drawable).apply(diskCacheStrategyOf(DiskCacheStrategy.NONE));
  }
  
  public RequestBuilder load(@Nullable String string) {
    return loadGeneric(string);
  }
 
  public RequestBuilder load(@Nullable Uri uri) {
    return loadGeneric(uri);
  }
 
  public RequestBuilder load(@Nullable File file) {
    return loadGeneric(file);
  }

  public RequestBuilder load(@RawRes @DrawableRes @Nullable Integer resourceId) {
    return loadGeneric(resourceId).apply(signatureOf(AndroidResourceSignature.obtain(context)));
  }


  public RequestBuilder load(@Nullable URL url) {
    return loadGeneric(url);
  }


  public RequestBuilder load(@Nullable byte[] model) {
    RequestBuilder result = loadGeneric(model);
    if (!result.isDiskCacheStrategySet()) {
      result = result.apply(diskCacheStrategyOf(DiskCacheStrategy.NONE));
    }
    if (!result.isSkipMemoryCacheSet()) {
      result = result.apply(skipMemoryCacheOf(true /*skipMemoryCache*/));
    }
    return result;
  }
  
public RequestBuilder load(@Nullable Object model) {
  return loadGeneric(model);
}

@NonNull
private RequestBuilder loadGeneric(@Nullable Object model) {
  this.model = model;
  isModelSet = true;
  return this;
}

可以看到在load方法中都调用了该类中的loadGeneric方法，传入了Object类型的model参数，这个model参数就是我吗传入的图片资源或者图片地址；看到这里，我们是不是有点疑惑，在load方法中调用了loadGeneric方法赋值后，返回RequestBuilder对象，Glide的load方法到这里就结束了他的工作。

3. into

在上面的with和load方法中，我们还没有看到真正的图片加载和显示的代码，所以，可以想到，图片的加载和显示，都是通过into方法来调用执行的，所以，into方法是相对比较难分析的一个方法；接下来我们就一起看一下into方法所承担的重要职责。

执行完load方法，我们获得了RequestBuilder对象，所以，into方法，就在RequestBuilder类中，我们一起看一张这个类：

public class RequestBuilder extends BaseRequestOptions>
    implements Cloneable, ModelTypes> {
  
@NonNull
public > Y into(@NonNull Y target) {
  return into(target, /*targetListener=*/ null, Executors.mainThreadExecutor());
}

@NonNull
@Synthetic
> Y into(
    @NonNull Y target,
    @Nullable RequestListener targetListener,
    Executor callbackExecutor) {
  return into(target, targetListener, /*options=*/ this, callbackExecutor);
}

private > Y into(
    @NonNull Y target,
    @Nullable RequestListener targetListener,
    BaseRequestOptions options,
    Executor callbackExecutor) {
  Preconditions.checkNotNull(target);
  if (!isModelSet) {
    throw new IllegalArgumentException("You must call #load() before calling #into()");
  }

  Request request = buildRequest(target, targetListener, options, callbackExecutor);

  Request previous = target.getRequest();
  if (request.isEquivalentTo(previous)
      && !isSkipMemoryCacheWithCompletePreviousRequest(options, previous)) {
    if (!Preconditions.checkNotNull(previous).isRunning()) {
      previous.begin();
    }
    return target;
  }

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

  return target;
}


private boolean isSkipMemoryCacheWithCompletePreviousRequest(
    BaseRequestOptions options, Request previous) {
  return !options.isMemoryCacheable() && previous.isComplete();
}

@NonNull
public ViewTarget into(@NonNull ImageView view) {
  Util.assertMainThread();
  Preconditions.checkNotNull(view);

  BaseRequestOptions requestOptions = this;
  if (!requestOptions.isTransformationSet()
      && requestOptions.isTransformationAllowed()
      && view.getScaleType() != null) {
    switch (view.getScaleType()) {
      case CENTER_CROP:
        requestOptions = requestOptions.clone().optionalCenterCrop();
        break;
      case CENTER_INSIDE:
        requestOptions = requestOptions.clone().optionalCenterInside();
        break;
      case FIT_CENTER:
      case FIT_START:
      case FIT_END:
        requestOptions = requestOptions.clone().optionalFitCenter();
        break;
      case FIT_XY:
        requestOptions = requestOptions.clone().optionalCenterInside();
        break;
      case CENTER:
      case MATRIX:
      default:
        // Do nothing.
    }
  }

  return into(
      glideContext.buildImageViewTarget(view, transcodeClass),
      /*targetListener=*/ null,
      requestOptions,
      Executors.mainThreadExecutor());
}


@Deprecated
public FutureTarget into(int width, int height) {
  return submit(width, height);
}


@NonNull
public FutureTarget submit() {
  return submit(Target.SIZE_ORIGINAL, Target.SIZE_ORIGINAL);
}


@NonNull
public FutureTarget submit(int width, int height) {
  final RequestFutureTarget target = new RequestFutureTarget<>(width, height);
  return into(target, target, Executors.directExecutor());
}


@NonNull
public Target preload(int width, int height) {
  final PreloadTarget target = PreloadTarget.obtain(requestManager, width, height);
  return into(target);
}


@NonNull
public Target preload() {
  return preload(Target.SIZE_ORIGINAL, Target.SIZE_ORIGINAL);
}
}

可以看到，我们调用的into方法，是第53行参数为ImageView的方法，在这个方法中根据apply方法中传的RequestOptions对象和图片显示类型scaleType来设置RequestOptions对象（当前对象，RequestBuilder 继承自BaseRequestOptions）的Transform类型。并且通过glideContext.buildImageViewTarget(view, transcodeClass)来创建了一个ViewTarget对象（BitmapImageViewTarget(view)或DrawableImageViewTarget(view)）,然后将ViewTarget对象，RequestOptions对象和Executors.mainThreadExecutor()（主线程的一个线程执行器，内部有一个主线程的handler对象用了执行runnable对象）传入到第18行的into方法，接下来我们看一下，在这个into方法中做了什么操作。

在第28行可以看到通过buildRequest(target, targetListener, options, callbackExecutor)方法得到了一个Request对象 request,稍后来看这个方法。继续往下看，第30行，通过传入的target.getRequest()方法也获得了一个Request对象 previous然后判断这两个Request对象是否是同一个请求（判断请求的参数和资源地址），如果是同一个请求，并且未跳过内存缓存完成请求，则target的Request对象开始请求previous.begin();

如果两个Request对象不满足上面的条件，则将该request对象设置给target的Request对象，并且将该request对象通过RequestManager对象的track(target, request)方法，将该request对象添加到请求追踪器RequestTracker对象中进行请求（runRequest(Request request)）；粘一下代码

RequestManager对象的track(target, request)方法

synchronized void track(@NonNull Target target, @NonNull Request request) {
  targetTracker.track(target);
  requestTracker.runRequest(request);
}

RequestTracker对象中的runRequest(Request request)方法

public void runRequest(@NonNull Request request) {
  requests.add(request);
  if (!isPaused) {
    request.begin();
  } else {
    request.clear();
    if (Log.isLoggable(TAG, Log.VERBOSE)) {
      Log.v(TAG, "Paused, delaying request");
    }
    pendingRequests.add(request);
  }
}

可以看到在runRequest(Request request)方法中，先将该请求添加到请求的集合中，并且判断当前是否处于暂停加载的状态，如果不处于暂停加载状态，则执行请求对象的begin()方法开始请求图片，如果处于暂停加载状态，则清除加载列表，并将该请求加入等待请求的队列，待下次重新开始请求时进行请求。

接下来我们来了解一下buildRequest方法的内容

private Request buildRequest(
    Target target,
    @Nullable RequestListener targetListener,
    BaseRequestOptions requestOptions,
    Executor callbackExecutor) {
  return buildRequestRecursive(
      /*requestLock=*/ new Object(),
      target,
      targetListener,
      /*parentCoordinator=*/ null,
      transitionOptions,
      requestOptions.getPriority(),
      requestOptions.getOverrideWidth(),
      requestOptions.getOverrideHeight(),
      requestOptions,
      callbackExecutor);
}

private Request buildRequestRecursive(
    Object requestLock,
    Target target,
    @Nullable RequestListener targetListener,
    @Nullable RequestCoordinator parentCoordinator,
    TransitionOptions transitionOptions,
    Priority priority,
    int overrideWidth,
    int overrideHeight,
    BaseRequestOptions requestOptions,
    Executor callbackExecutor) {

  // Build the ErrorRequestCoordinator first if necessary so we can update parentCoordinator.
  ErrorRequestCoordinator errorRequestCoordinator = null;
  if (errorBuilder != null) {
    errorRequestCoordinator = new ErrorRequestCoordinator(requestLock, parentCoordinator);
    parentCoordinator = errorRequestCoordinator;
  }

  Request mainRequest =
      buildThumbnailRequestRecursive(
          requestLock,
          target,
          targetListener,
          parentCoordinator,
          transitionOptions,
          priority,
          overrideWidth,
          overrideHeight,
          requestOptions,
          callbackExecutor);

  if (errorRequestCoordinator == null) {
    return mainRequest;
  }

  int errorOverrideWidth = errorBuilder.getOverrideWidth();
  int errorOverrideHeight = errorBuilder.getOverrideHeight();
  if (Util.isValidDimensions(overrideWidth, overrideHeight) && !errorBuilder.isValidOverride()) {
    errorOverrideWidth = requestOptions.getOverrideWidth();
    errorOverrideHeight = requestOptions.getOverrideHeight();
  }

  Request errorRequest =
      errorBuilder.buildRequestRecursive(
          requestLock,
          target,
          targetListener,
          errorRequestCoordinator,
          errorBuilder.transitionOptions,
          errorBuilder.getPriority(),
          errorOverrideWidth,
          errorOverrideHeight,
          errorBuilder,
          callbackExecutor);
  errorRequestCoordinator.setRequests(mainRequest, errorRequest);
  return errorRequestCoordinator;
}

private Request buildThumbnailRequestRecursive(
    Object requestLock,
    Target target,
    RequestListener targetListener,
    @Nullable RequestCoordinator parentCoordinator,
    TransitionOptions transitionOptions,
    Priority priority,
    int overrideWidth,
    int overrideHeight,
    BaseRequestOptions requestOptions,
    Executor callbackExecutor) {
  if (thumbnailBuilder != null) {
    // Recursive case: contains a potentially recursive thumbnail request builder.
    if (isThumbnailBuilt) {
      throw new IllegalStateException(
          "You cannot use a request as both the main request and a "
              + "thumbnail, consider using clone() on the request(s) passed to thumbnail()");
    }

    TransitionOptions thumbTransitionOptions =
        thumbnailBuilder.transitionOptions;

    // Apply our transition by default to thumbnail requests but avoid overriding custom options
    // that may have been applied on the thumbnail request explicitly.
    if (thumbnailBuilder.isDefaultTransitionOptionsSet) {
      thumbTransitionOptions = transitionOptions;
    }

    Priority thumbPriority =
        thumbnailBuilder.isPrioritySet()
            ? thumbnailBuilder.getPriority()
            : getThumbnailPriority(priority);

    int thumbOverrideWidth = thumbnailBuilder.getOverrideWidth();
    int thumbOverrideHeight = thumbnailBuilder.getOverrideHeight();
    if (Util.isValidDimensions(overrideWidth, overrideHeight)
        && !thumbnailBuilder.isValidOverride()) {
      thumbOverrideWidth = requestOptions.getOverrideWidth();
      thumbOverrideHeight = requestOptions.getOverrideHeight();
    }

    ThumbnailRequestCoordinator coordinator =
        new ThumbnailRequestCoordinator(requestLock, parentCoordinator);
    Request fullRequest =
        obtainRequest(
            requestLock,
            target,
            targetListener,
            requestOptions,
            coordinator,
            transitionOptions,
            priority,
            overrideWidth,
            overrideHeight,
            callbackExecutor);
    isThumbnailBuilt = true;
    // Recursively generate thumbnail requests.
    Request thumbRequest =
        thumbnailBuilder.buildRequestRecursive(
            requestLock,
            target,
            targetListener,
            coordinator,
            thumbTransitionOptions,
            thumbPriority,
            thumbOverrideWidth,
            thumbOverrideHeight,
            thumbnailBuilder,
            callbackExecutor);
    isThumbnailBuilt = false;
    coordinator.setRequests(fullRequest, thumbRequest);
    return coordinator;
  } else if (thumbSizeMultiplier != null) {
    // Base case: thumbnail multiplier generates a thumbnail request, but cannot recurse.
    ThumbnailRequestCoordinator coordinator =
        new ThumbnailRequestCoordinator(requestLock, parentCoordinator);
    Request fullRequest =
        obtainRequest(
            requestLock,
            target,
            targetListener,
            requestOptions,
            coordinator,
            transitionOptions,
            priority,
            overrideWidth,
            overrideHeight,
            callbackExecutor);
    BaseRequestOptions thumbnailOptions =
        requestOptions.clone().sizeMultiplier(thumbSizeMultiplier);

    Request thumbnailRequest =
        obtainRequest(
            requestLock,
            target,
            targetListener,
            thumbnailOptions,
            coordinator,
            transitionOptions,
            getThumbnailPriority(priority),
            overrideWidth,
            overrideHeight,
            callbackExecutor);

    coordinator.setRequests(fullRequest, thumbnailRequest);
    return coordinator;
  } else {
    // Base case: no thumbnail.
    return obtainRequest(
        requestLock,
        target,
        targetListener,
        requestOptions,
        parentCoordinator,
        transitionOptions,
        priority,
        overrideWidth,
        overrideHeight,
        callbackExecutor);
  }
}

private Request obtainRequest(
    Object requestLock,
    Target target,
    RequestListener targetListener,
    BaseRequestOptions requestOptions,
    RequestCoordinator requestCoordinator,
    TransitionOptions transitionOptions,
    Priority priority,
    int overrideWidth,
    int overrideHeight,
    Executor callbackExecutor) {
  return SingleRequest.obtain(
      context,
      glideContext,
      requestLock,
      model,
      transcodeClass,
      requestOptions,
      overrideWidth,
      overrideHeight,
      priority,
      target,
      targetListener,
      requestListeners,
      requestCoordinator,
      glideContext.getEngine(),
      transitionOptions.getTransitionFactory(),
      callbackExecutor);
}

可以看到在第1行buildRequest()方法中调用了buildRequestRecursive方法（第19行），在该方法中，根据在RequestBuilder类对象中设置的errorBuilder（类型为RequestBuilder）来创建不同的Request对象，buildThumbnailRequestRecursive方法中判断是否存在缩略图来创建不同的Request对象（创建一个拥有请求原图的Request对象和请求缩略图的Request对象的ThumbnailRequestCoordinator对象，或者只有单个请求的SingleRequest对象），最终都是执行了Request对象的begin()方法取请求图片资源；在上面的方法中可以看到，多次执行之后，最终大部分都Request对象中包含了SingleRequest对象，其中的begin()方法也是调用了SingleRequest对象中的begin()方法，接下来我们来看一下SingleRequest对象中的begin()方法；分析到现在为止，我们还没有看到请求图片资源的代码，所以猜想SingleRequest对象中的begin()方法接下来应该就是调起网络请求的方法。

@Override
public void begin() {
  synchronized (requestLock) {
    assertNotCallingCallbacks();
    stateVerifier.throwIfRecycled();
    startTime = LogTime.getLogTime();
    if (model == null) {
      if (Util.isValidDimensions(overrideWidth, overrideHeight)) {
        width = overrideWidth;
        height = overrideHeight;
      }
      int logLevel = getFallbackDrawable() == null ? Log.WARN : Log.DEBUG;
      onLoadFailed(new GlideException("Received null model"), logLevel);
      return;
    }

    if (status == Status.RUNNING) {
      throw new IllegalArgumentException("Cannot restart a running request");
    }

    if (status == Status.COMPLETE) {
      onResourceReady(resource, DataSource.MEMORY_CACHE);
      return;
    }

    status = Status.WAITING_FOR_SIZE;
    if (Util.isValidDimensions(overrideWidth, overrideHeight)) {
      onSizeReady(overrideWidth, overrideHeight);
    } else {
      target.getSize(this);
    }

    if ((status == Status.RUNNING || status == Status.WAITING_FOR_SIZE)
        && canNotifyStatusChanged()) {
      target.onLoadStarted(getPlaceholderDrawable());
    }
    if (IS_VERBOSE_LOGGABLE) {
      logV("finished run method in " + LogTime.getElapsedMillis(startTime));
    }
  }
}
 private void onLoadFailed(GlideException e, int maxLogLevel) {
    stateVerifier.throwIfRecycled();
    synchronized (requestLock) {
      e.setOrigin(requestOrigin);
      int logLevel = glideContext.getLogLevel();
      if (logLevel <= maxLogLevel) {
        Log.w(
            GLIDE_TAG, "Load failed for " + model + " with size [" + width + "x" + height + "]", e);
        if (logLevel <= Log.INFO) {
          e.logRootCauses(GLIDE_TAG);
        }
      }

      loadStatus = null;
      status = Status.FAILED;

      isCallingCallbacks = true;
      try {
        // TODO: what if this is a thumbnail request?
        boolean anyListenerHandledUpdatingTarget = false;
        if (requestListeners != null) {
          for (RequestListener listener : requestListeners) {
            anyListenerHandledUpdatingTarget |=
                listener.onLoadFailed(e, model, target, isFirstReadyResource());
          }
        }
        anyListenerHandledUpdatingTarget |=
            targetListener != null
                && targetListener.onLoadFailed(e, model, target, isFirstReadyResource());

        if (!anyListenerHandledUpdatingTarget) {
          setErrorPlaceholder();
        }
      } finally {
        isCallingCallbacks = false;
      }

      notifyLoadFailed();
    }
  }

 private void setErrorPlaceholder() {
    if (!canNotifyStatusChanged()) {
      return;
    }

    Drawable error = null;
    if (model == null) {
      error = getFallbackDrawable();
    }
    // Either the model isn't null, or there was no fallback drawable set.
    if (error == null) {
      error = getErrorDrawable();
    }
    // The model isn't null, no fallback drawable was set or no error drawable was set.
    if (error == null) {
      error = getPlaceholderDrawable();
    }
    target.onLoadFailed(error);
  }

可以看到在这个方法里，stateVerifier.throwIfRecycled(),中判断了生命周期，如果没销毁，则抛出异常。

然后判断了model == null，这里的model就是我们之前传入的图片资源（图片地址），为空的时候，则不需要加载，直接执行onLoadFailed方法，在onLoadFailed方法中则是判断加载的监听器是否处理了onLoadFailed事件，如果没有处理，则设置加载错误的占位图片（第73行），最终则执行target的target.onLoadFailed(error);回调方法将错误占位图显示出来

当model不为空，继续向下执行，判断了当前状态是否在运行中(第17行)，如果是，抛出异常，如果不是，继续判读状态是否已完成，如果已完成，则执行onResourceReady回调方法（第22行），这个方法我们稍后在看，如果状态不是已完成状态，则继续向下执行，判断宽度和高度是否有效（>0或者为自适应），如果有效执行onSizeReady方法，如果无效，则执行target.getSize(this);方法，重新获取宽度和高度，target.getSize(this)是ViewTarget中的方法，在该方法中获取了当前target的宽度和高度，获取有效宽度和高度之后，回调继续执行onSizeReady方法。这里的cb就是在target.getSize(this);方法中传递过来的this，也就是SingleRequest类对象

target.getSize(this) 方法

void getSize(@NonNull SizeReadyCallback cb) {
  int currentWidth = getTargetWidth();
  int currentHeight = getTargetHeight();
  if (isViewStateAndSizeValid(currentWidth, currentHeight)) {
    cb.onSizeReady(currentWidth, currentHeight);
    return;
  }

  // We want to notify callbacks in the order they were added and we only expect one or two
  // callbacks to be added a time, so a List is a reasonable choice.
  if (!cbs.contains(cb)) {
    cbs.add(cb);
  }
  if (layoutListener == null) {
    ViewTreeObserver observer = view.getViewTreeObserver();
    layoutListener = new SizeDeterminerLayoutListener(this);
    observer.addOnPreDrawListener(layoutListener);
  }
}

可以看到target.getSize(this)最终还是执行了onSizeReady方法，接下来我们看一下在onSizeReady方法中做了什么

SingleRequest类对象中的onSizeReady方法

public void onSizeReady(int width, int height) {
  stateVerifier.throwIfRecycled();
  synchronized (requestLock) {
    if (IS_VERBOSE_LOGGABLE) {
      logV("Got onSizeReady in " + LogTime.getElapsedMillis(startTime));
    }
    if (status != Status.WAITING_FOR_SIZE) {
      return;
    }
    status = Status.RUNNING;

    float sizeMultiplier = requestOptions.getSizeMultiplier();
    this.width = maybeApplySizeMultiplier(width, sizeMultiplier);
    this.height = maybeApplySizeMultiplier(height, sizeMultiplier);

    if (IS_VERBOSE_LOGGABLE) {
      logV("finished setup for calling load in " + LogTime.getElapsedMillis(startTime));
    }
    loadStatus =
        engine.load(
            glideContext,
            model,
            requestOptions.getSignature(),
            this.width,
            this.height,
            requestOptions.getResourceClass(),
            transcodeClass,
            priority,
            requestOptions.getDiskCacheStrategy(),
            requestOptions.getTransformations(),
            requestOptions.isTransformationRequired(),
            requestOptions.isScaleOnlyOrNoTransform(),
            requestOptions.getOptions(),
            requestOptions.isMemoryCacheable(),
            requestOptions.getUseUnlimitedSourceGeneratorsPool(),
            requestOptions.getUseAnimationPool(),
            requestOptions.getOnlyRetrieveFromCache(),
            this,
            callbackExecutor);

    // This is a hack that's only useful for testing right now where loads complete synchronously
    // even though under any executor running on any thread but the main thread, the load would
    // have completed asynchronously.
    if (status != Status.RUNNING) {
      loadStatus = null;
    }
    if (IS_VERBOSE_LOGGABLE) {
      logV("finished onSizeReady in " + LogTime.getElapsedMillis(startTime));
    }
  }
}

可以看到，在第20行，调用了engine.load方法，传入了很多的参数，大部分都是根图片相关的倒数第二款参数this，可前面一样，也是ResourceCallback类型的，用于回调；接下来进入到engine.load方法看一下

public  LoadStatus load(
    GlideContext glideContext,
    Object model,
    Key signature,
    int width,
    int height,
    Class resourceClass,
    Class transcodeClass,
    Priority priority,
    DiskCacheStrategy diskCacheStrategy,
    Map, Transformation> transformations,
    boolean isTransformationRequired,
    boolean isScaleOnlyOrNoTransform,
    Options options,
    boolean isMemoryCacheable,
    boolean useUnlimitedSourceExecutorPool,
    boolean useAnimationPool,
    boolean onlyRetrieveFromCache,
    ResourceCallback cb,
    Executor callbackExecutor) {
  long startTime = VERBOSE_IS_LOGGABLE ? LogTime.getLogTime() : 0;

  EngineKey key =
      keyFactory.buildKey(
          model,
          signature,
          width,
          height,
          transformations,
          resourceClass,
          transcodeClass,
          options);

  EngineResource memoryResource;
  synchronized (this) {
    memoryResource = loadFromMemory(key, isMemoryCacheable, startTime);

    if (memoryResource == null) {
      return waitForExistingOrStartNewJob(
          glideContext,
          model,
          signature,
          width,
          height,
          resourceClass,
          transcodeClass,
          priority,
          diskCacheStrategy,
          transformations,
          isTransformationRequired,
          isScaleOnlyOrNoTransform,
          options,
          isMemoryCacheable,
          useUnlimitedSourceExecutorPool,
          useAnimationPool,
          onlyRetrieveFromCache,
          cb,
          callbackExecutor,
          key,
          startTime);
    }
  }

  // Avoid calling back while holding the engine lock, doing so makes it easier for callers to
  // deadlock.
  cb.onResourceReady(memoryResource, DataSource.MEMORY_CACHE);
  return null;
}
private EngineResource loadFromMemory(
      EngineKey key, boolean isMemoryCacheable, long startTime) {
    if (!isMemoryCacheable) {
      return null;
    }

    EngineResource active = loadFromActiveResources(key);
    if (active != null) {
      if (VERBOSE_IS_LOGGABLE) {
        logWithTimeAndKey("Loaded resource from active resources", startTime, key);
      }
      return active;
    }

    EngineResource cached = loadFromCache(key);
    if (cached != null) {
      if (VERBOSE_IS_LOGGABLE) {
        logWithTimeAndKey("Loaded resource from cache", startTime, key);
      }
      return cached;
    }

    return null;
  }

  private static void logWithTimeAndKey(String log, long startTime, Key key) {
    Log.v(TAG, log + " in " + LogTime.getElapsedMillis(startTime) + "ms, key: " + key);
  }

  @Nullable
  private EngineResource loadFromActiveResources(Key key) {
    EngineResource active = activeResources.get(key);
    if (active != null) {
      active.acquire();
    }

    return active;
  }

  private EngineResource loadFromCache(Key key) {
    EngineResource cached = getEngineResourceFromCache(key);
    if (cached != null) {
      cached.acquire();
      activeResources.activate(key, cached);
    }
    return cached;
  }

 private EngineResource getEngineResourceFromCache(Key key) {
    Resource cached = cache.remove(key);

    final EngineResource result;
    if (cached == null) {
      result = null;
    } else if (cached instanceof EngineResource) {
      // Save an object allocation if we've cached an EngineResource (the typical case).
      result = (EngineResource) cached;
    } else {
      result =
          new EngineResource<>(
              cached, /*isMemoryCacheable=*/ true, /*isRecyclable=*/ true, key, /*listener=*/ this);
    }
    return result;
  }

在engine.load方法中，可以看到，第23行，根据传入的请求地址，宽度和高度，签名等其他请求参数，生成了一个EngineKey对象key，这个对象就是用了标识每一个请求，不难想到，如果参数全部相同，则生成的key对象也相同；在第36行中，看到执行了loadFromMemory方法，在该方法中先是通过loadFromActiveResources方法，从内存中查找是否有对象key 的缓存资源，如果有返回；如果没有，则通过loadFromCache方法从缓存中查找是否有对象key的缓存资源，在getEngineResourceFromCache方法中，通过cache.remove(key)方法来获取到一个缓存对象，这里的cahce对象就是LruResourceCache的类对象，是通过LruResourceCache的类对象来缓存资源。

看完了获取缓存的方法之后，我们回过头来继续看加载的方法，第38行，当获取的缓存为空时，返回了waitForExistingOrStartNewJob方法的结果，我们来看一下waitForExistingOrStartNewJob方法

private  LoadStatus waitForExistingOrStartNewJob(
    GlideContext glideContext,
    Object model,
    Key signature,
    int width,
    int height,
    Class resourceClass,
    Class transcodeClass,
    Priority priority,
    DiskCacheStrategy diskCacheStrategy,
    Map, Transformation> transformations,
    boolean isTransformationRequired,
    boolean isScaleOnlyOrNoTransform,
    Options options,
    boolean isMemoryCacheable,
    boolean useUnlimitedSourceExecutorPool,
    boolean useAnimationPool,
    boolean onlyRetrieveFromCache,
    ResourceCallback cb,
    Executor callbackExecutor,
    EngineKey key,
    long startTime) {

  EngineJob current = jobs.get(key, onlyRetrieveFromCache);
  if (current != null) {
    current.addCallback(cb, callbackExecutor);
    if (VERBOSE_IS_LOGGABLE) {
      logWithTimeAndKey("Added to existing load", startTime, key);
    }
    return new LoadStatus(cb, current);
  }

  EngineJob engineJob =
      engineJobFactory.build(
          key,
          isMemoryCacheable,
          useUnlimitedSourceExecutorPool,
          useAnimationPool,
          onlyRetrieveFromCache);

  DecodeJob decodeJob =
      decodeJobFactory.build(
          glideContext,
          model,
          key,
          signature,
          width,
          height,
          resourceClass,
          transcodeClass,
          priority,
          diskCacheStrategy,
          transformations,
          isTransformationRequired,
          isScaleOnlyOrNoTransform,
          onlyRetrieveFromCache,
          options,
          engineJob);

  jobs.put(key, engineJob);

  engineJob.addCallback(cb, callbackExecutor);
  engineJob.start(decodeJob);

  if (VERBOSE_IS_LOGGABLE) {
    logWithTimeAndKey("Started new load", startTime, key);
  }
  return new LoadStatus(cb, engineJob);
}

前面看了load方法中，已经从缓存中取了资源，没有取到，在执行该方法，可想而知，真正的网络请求，则是在该方法中执行的；看到在第24行中，通过jobs对象获取了EngineJob对象current，如果有取到，则说明有正在请求的对象，将回调加入到当前对象中，返回当前请求状态；如果没有取到，则创建一个engineJob对象和一个decodeJob对象；然后为该请求对象添加回调接口对象，并且传入decodeJob进行启动加载（第63行）；看下engineJob.start方法.

public synchronized void start(DecodeJob decodeJob) {
  this.decodeJob = decodeJob;
  GlideExecutor executor =
      decodeJob.willDecodeFromCache() ? diskCacheExecutor : getActiveSourceExecutor();
  executor.execute(decodeJob);
}

在该方法中根据decodeJob.willDecodeFromCache()获取到了一个GlideExecutor对象，该对象实现了ExecutorService接口类，是一个用了处理请求任务的线程池，在看一下executor.execute(decodeJob)方法需要传入一个Runnable对象，然后我吗回去看DecodeJob类是实现了Runnable接口，所以可以想到上面的engineJob对象是一个处理线程的线程池对象，真正的请求工作是在decodeJob对象的run方法中执行。

@Override
public void run() {
  GlideTrace.beginSectionFormat("DecodeJob#run(model=%s)", model);
  DataFetcher localFetcher = currentFetcher;
  try {
    if (isCancelled) {
      notifyFailed();
      return;
    }
    runWrapped();
  } catch (CallbackException e) {
    throw e;
  } catch (Throwable t) {
    if (Log.isLoggable(TAG, Log.DEBUG)) {
      Log.d(
          TAG,
          "DecodeJob threw unexpectedly" + ", isCancelled: " + isCancelled + ", stage: " + stage,
          t);
    }
    if (stage != Stage.ENCODE) {
      throwables.add(t);
      notifyFailed();
    }
    if (!isCancelled) {
      throw t;
    }
    throw t;
  } finally {
    if (localFetcher != null) {
      localFetcher.cleanup();
    }
    GlideTrace.endSection();
  }
}

private void runWrapped() {
  switch (runReason) {
    case INITIALIZE:
      stage = getNextStage(Stage.INITIALIZE);
      currentGenerator = getNextGenerator();
      runGenerators();
      break;
    case SWITCH_TO_SOURCE_SERVICE:
      runGenerators();
      break;
    case DECODE_DATA:
      decodeFromRetrievedData();
      break;
    default:
      throw new IllegalStateException("Unrecognized run reason: " + runReason);
  }
}

private DataFetcherGenerator getNextGenerator() {
  switch (stage) {
    case RESOURCE_CACHE:
      return new ResourceCacheGenerator(decodeHelper, this);
    case DATA_CACHE:
      return new DataCacheGenerator(decodeHelper, this);
    case SOURCE:
      return new SourceGenerator(decodeHelper, this);
    case FINISHED:
      return null;
    default:
      throw new IllegalStateException("Unrecognized stage: " + stage);
  }
}

private void runGenerators() {
  currentThread = Thread.currentThread();
  startFetchTime = LogTime.getLogTime();
  boolean isStarted = false;
  while (!isCancelled
      && currentGenerator != null
      && !(isStarted = currentGenerator.startNext())) {
    stage = getNextStage(stage);
    currentGenerator = getNextGenerator();

    if (stage == Stage.SOURCE) {
      reschedule();
      return;
    }
  }
  if ((stage == Stage.FINISHED || isCancelled) && !isStarted) {
    notifyFailed();
  }

}

可以看到，在run方法中，除了一些判断和释放资源的操作，主要是执行了runWrapped()方法；在runWrapped()方法中，根据不同的runReason,执行了getNextGenerator()和runGenerators()方法，getNextGenerator()方法根据不同的stage生成不同的DataFetcherGenerator对象赋值给currentGenerator；在runGenerators()方法中，循环调用了http请求，来请求图片数据，调用方法为currentGenerator.startNext()，在startNext方法中可以看到loadData.fetcher.loadData()方法来获取图片数据

ResourceCacheGenerator类下的startNext()；第61行

@Override
public boolean startNext() {
  List sourceIds = helper.getCacheKeys();
  if (sourceIds.isEmpty()) {
    return false;
  }
  List> resourceClasses = helper.getRegisteredResourceClasses();
  if (resourceClasses.isEmpty()) {
    if (File.class.equals(helper.getTranscodeClass())) {
      return false;
    }
    throw new IllegalStateException(
        "Failed to find any load path from "
            + helper.getModelClass()
            + " to "
            + helper.getTranscodeClass());
  }
  while (modelLoaders == null || !hasNextModelLoader()) {
    resourceClassIndex++;
    if (resourceClassIndex >= resourceClasses.size()) {
      sourceIdIndex++;
      if (sourceIdIndex >= sourceIds.size()) {
        return false;
      }
      resourceClassIndex = 0;
    }

    Key sourceId = sourceIds.get(sourceIdIndex);
    Class resourceClass = resourceClasses.get(resourceClassIndex);
    Transformation transformation = helper.getTransformation(resourceClass);
    // PMD.AvoidInstantiatingObjectsInLoops Each iteration is comparatively expensive anyway,
    // we only run until the first one succeeds, the loop runs for only a limited
    // number of iterations on the order of 10-20 in the worst case.
    currentKey =
        new ResourceCacheKey( // NOPMD AvoidInstantiatingObjectsInLoops
            helper.getArrayPool(),
            sourceId,
            helper.getSignature(),
            helper.getWidth(),
            helper.getHeight(),
            transformation,
            resourceClass,
            helper.getOptions());
    cacheFile = helper.getDiskCache().get(currentKey);
    if (cacheFile != null) {
      sourceKey = sourceId;
      modelLoaders = helper.getModelLoaders(cacheFile);
      modelLoaderIndex = 0;
    }
  }

  loadData = null;
  boolean started = false;
  while (!started && hasNextModelLoader()) {
    ModelLoader modelLoader = modelLoaders.get(modelLoaderIndex++);
    loadData =
        modelLoader.buildLoadData(
            cacheFile, helper.getWidth(), helper.getHeight(), helper.getOptions());
    if (loadData != null && helper.hasLoadPath(loadData.fetcher.getDataClass())) {
      started = true;
      loadData.fetcher.loadData(helper.getPriority(), this);
    }
  }

  return started;
}

DataCacheGenerator类下的startNext()；第31行

@Override
public boolean startNext() {
  while (modelLoaders == null || !hasNextModelLoader()) {
    sourceIdIndex++;
    if (sourceIdIndex >= cacheKeys.size()) {
      return false;
    }

    Key sourceId = cacheKeys.get(sourceIdIndex);
    // PMD.AvoidInstantiatingObjectsInLoops The loop iterates a limited number of times
    // and the actions it performs are much more expensive than a single allocation.
    @SuppressWarnings("PMD.AvoidInstantiatingObjectsInLoops")
    Key originalKey = new DataCacheKey(sourceId, helper.getSignature());
    cacheFile = helper.getDiskCache().get(originalKey);
    if (cacheFile != null) {
      this.sourceKey = sourceId;
      modelLoaders = helper.getModelLoaders(cacheFile);
      modelLoaderIndex = 0;
    }
  }

  loadData = null;
  boolean started = false;
  while (!started && hasNextModelLoader()) {
    ModelLoader modelLoader = modelLoaders.get(modelLoaderIndex++);
    loadData =
        modelLoader.buildLoadData(
            cacheFile, helper.getWidth(), helper.getHeight(), helper.getOptions());
    if (loadData != null && helper.hasLoadPath(loadData.fetcher.getDataClass())) {
      started = true;
      loadData.fetcher.loadData(helper.getPriority(), this);
    }
  }
  return started;
}

SourceGenerator类下的startNext()；第29行

@Override
public boolean startNext() {
  if (dataToCache != null) {
    Object data = dataToCache;
    dataToCache = null;
    cacheData(data);
  }

  if (sourceCacheGenerator != null && sourceCacheGenerator.startNext()) {
    return true;
  }
  sourceCacheGenerator = null;

  loadData = null;
  boolean started = false;
  while (!started && hasNextModelLoader()) {
    loadData = helper.getLoadData().get(loadDataListIndex++);
    if (loadData != null
        && (helper.getDiskCacheStrategy().isDataCacheable(loadData.fetcher.getDataSource())
            || helper.hasLoadPath(loadData.fetcher.getDataClass()))) {
      started = true;
      startNextLoad(loadData);
    }
  }
  return started;
}

private void startNextLoad(final LoadData toStart) {
  loadData.fetcher.loadData(
      helper.getPriority(),
      new DataCallback