Glide4.0源码全解析（三），into()方法背后的故事

public class RequestBuilder<TranscodeType> implements Cloneable {
      ...
      ...省略
      ...

  /**
    * 对当前的ImageView加载资源，取消该View已加载过的资源，并释放资源。
    * @param view 此视图将取消先前加载并加载新资源的视图。
    * @return 一个可以使Glide加载资源并通知相关生命周期事件的接口。（后面详细介绍Target）
    */
  public Target<TranscodeType> into(ImageView view) {
   /**
     * 保证当前在主线程（更新视图需要在主线程，这个应该不用多说吧）
     */
    Util.assertMainThread();

   /**
     * 保证view不为null
     * 如果为努力了，会抛NullPointerException
     */
    Preconditions.checkNotNull(view);

   /**
     * requestOptions相关设置
     * RequestOptions ：提供Glide自定义加载选项
     * 暂时略过，后面章节详细讲解
     */
    if (!requestOptions.isTransformationSet()
        && requestOptions.isTransformationAllowed()
        && view.getScaleType() != null) {
      if (requestOptions.isLocked()) {
        requestOptions = requestOptions.clone();
      }
      switch (view.getScaleType()) {
        case CENTER_CROP:
          requestOptions.optionalCenterCrop();
          break;
        case CENTER_INSIDE:
          requestOptions.optionalCenterInside();
          break;
        case FIT_CENTER:
        case FIT_START:
        case FIT_END:
          requestOptions.optionalFitCenter();
          break;
        case FIT_XY:
          requestOptions.optionalCenterInside();
          break;
        case CENTER:
        case MATRIX:
        default:
          // Do nothing.
      }
    }

    return into(context.buildImageViewTarget(view, transcodeClass));
  }
      ...
      ...省略
      ...

  /**
   * 将资源加载到传入的target
   *
   * @param target 将资源记载到target
   * @return 传入的target.
   * （后面我们会去详细看一下target）
   */
  public <Y extends Target<TranscodeType>> Y into(@NonNull Y target) {
    Util.assertMainThread(); //判断是否是主线程
    Preconditions.checkNotNull(target);  //判断传入的target是否为空
    /**
     * 这里在上一篇讲过，在调用 .load() 之后isModelSet会被设为true
     * 这里是为了保证记载的资源不会为空，上面的方法是是为了保证被加载的view不会为空
     */
    if (!isModelSet) {
      throw new IllegalArgumentException("You must call #load() before calling #into()");
    }

    /**
     * 锁定requestOptions，防止被再次修改，如果此时修改会抛出异常
     * 为了防止加载资源的统统是还在修改图片的自定义属性
     */
    requestOptions.lock();
    /**
     * 这个方法很重要哦，⚠️ 先记住，后面讲解哦 ！！！
     * 因为在这里创建了request
     */
    Request request = buildRequest(target);

    Request previous = target.getRequest();
    /**
     * 这里修复了一个bug，详见 https://github.com/bumptech/glide/issues/2270
     */
    if (request.isEquivalentTo(previous)
      && (Preconditions.checkNotNull(previous).isComplete()
         || Preconditions.checkNotNull(previous).isRunning())) {
      request.recycle();
      /**
       * 防止请求被中断
       */
      if (!Preconditions.checkNotNull(previous).isRunning()) {
        previous.begin();
      }
      return target;
    }

    requestManager.clear(target);
    target.setRequest(request);
    /**
     * 这个方法很重要哦，⚠️ 先记住，后面讲解哦 ！！！
     * 因为在这里执行了request
     */
    requestManager.track(target, request);

   /**
     * 返回一个target
     * 该target就是传入的target
     */
    return target;
  }
      ...
      ...省略
      ...
}

public class RequestBuilder<TranscodeType> implements Cloneable {
  ...

  public Target<TranscodeType> into(ImageView view) {
    ...

    return into(context.buildImageViewTarget(view, transcodeClass));

    ...    
  }

  ...
}


public class GlideContext extends ContextWrapper {
  ...

  public <X> Target<X> buildImageViewTarget(ImageView imageView, Class<X> transcodeClass) {
    return imageViewTargetFactory.buildTarget(imageView, transcodeClass);
  }

  ...
}

/**
 * 负责给传入的子类产生正确的类型的工厂
 */
public class ImageViewTargetFactory {

  @SuppressWarnings("unchecked")
  public <Z> Target<Z> buildTarget(ImageView view, Class<Z> clazz) {
    if (Bitmap.class.equals(clazz)) {
      return (Target<Z>) new BitmapImageViewTarget(view);
    } else if (Drawable.class.isAssignableFrom(clazz)) {
      DrawableImageViewTarget drawableImageViewTarget = new DrawableImageViewTarget(view);
      return (Target<Z>) new DrawableImageViewTarget(view);
    } else {
      throw new IllegalArgumentException(
          "Unhandled class: " + clazz + ", try .as*(Class).transcode(ResourceTranscoder)");
    }
  }
}

public <Y extends Target<TranscodeType>> Y into(@NonNull Y target) {
  Request request = buildRequest(target);
}

private Request buildRequest(Target<TranscodeType> target) {
    return buildRequestRecursive(target, null, transitionOptions, requestOptions.getPriority(),
        requestOptions.getOverrideWidth(), requestOptions.getOverrideHeight());
  }

  private Request buildRequestRecursive(Target<TranscodeType> target,
      @Nullable ThumbnailRequestCoordinator parentCoordinator,
      TransitionOptions<?, ? super TranscodeType> transitionOptions,
      Priority priority, int overrideWidth, int overrideHeight) {
    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<?, ? super TranscodeType> 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.requestOptions.isPrioritySet()
          ? thumbnailBuilder.requestOptions.getPriority() : getThumbnailPriority(priority);

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

      ThumbnailRequestCoordinator coordinator = new ThumbnailRequestCoordinator(parentCoordinator);
      Request fullRequest = obtainRequest(target, requestOptions, coordinator,
          transitionOptions, priority, overrideWidth, overrideHeight);
      isThumbnailBuilt = true;
      // Recursively generate thumbnail requests.
      Request thumbRequest = thumbnailBuilder.buildRequestRecursive(target, coordinator,
          thumbTransitionOptions, thumbPriority, thumbOverrideWidth, thumbOverrideHeight);
      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(parentCoordinator);
      Request fullRequest = obtainRequest(target, requestOptions, coordinator, transitionOptions,
          priority, overrideWidth, overrideHeight);
      RequestOptions thumbnailOptions = requestOptions.clone()
          .sizeMultiplier(thumbSizeMultiplier);

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

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

private Request obtainRequest(Target<TranscodeType> target,
      RequestOptions requestOptions, RequestCoordinator requestCoordinator,
      TransitionOptions<?, ? super TranscodeType> transitionOptions, Priority priority,
      int overrideWidth, int overrideHeight) {
    requestOptions.lock();

    return SingleRequest.obtain(
        context,
        model,
        transcodeClass,
        requestOptions,
        overrideWidth,
        overrideHeight,
        priority,
        target,
        requestListener,
        requestCoordinator,
        context.getEngine(),
        transitionOptions.getTransitionFactory());
  }

public static <R> SingleRequest<R> obtain(
      GlideContext glideContext,
      Object model,
      Class<R> transcodeClass,
      RequestOptions requestOptions,
      int overrideWidth,
      int overrideHeight,
      Priority priority,
      Target<R> target,
      RequestListener<R> requestListener,
      RequestCoordinator requestCoordinator,
      Engine engine,
      TransitionFactory<? super R> animationFactory) {
    @SuppressWarnings("unchecked") SingleRequest<R> request =
        (SingleRequest<R>) POOL.acquire();
    if (request == null) {
      request = new SingleRequest<>();
    }
    request.init(
        glideContext,
        model,
        transcodeClass,
        requestOptions,
        overrideWidth,
        overrideHeight,
        priority,
        target,
        requestListener,
        requestCoordinator,
        engine,
        animationFactory);
    return request;
  }

  private void init(
      GlideContext glideContext,
      Object model,
      Class<R> transcodeClass,
      RequestOptions requestOptions,
      int overrideWidth,
      int overrideHeight,
      Priority priority,
      Target<R> target,
      RequestListener<R> requestListener,
      RequestCoordinator requestCoordinator,
      Engine engine,
      TransitionFactory<? super R> animationFactory) {
    this.glideContext = glideContext;
    this.model = model;
    this.transcodeClass = transcodeClass;
    this.requestOptions = requestOptions;
    this.overrideWidth = overrideWidth;
    this.overrideHeight = overrideHeight;
    this.priority = priority;
    this.target = target;
    this.requestListener = requestListener;
    this.requestCoordinator = requestCoordinator;
    this.engine = engine;
    this.animationFactory = animationFactory;
    status = Status.PENDING;
  }

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

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

  @Override
  public void begin() {
    stateVerifier.throwIfRecycled();
    startTime = LogTime.getLogTime();
    if (model == null) {
      if (Util.isValidDimensions(overrideWidth, overrideHeight)) {
        width = overrideWidth;
        height = overrideHeight;
      }
      // Only log at more verbose log levels if the user has set a fallback drawable, because
      // fallback Drawables indicate the user expects null models occasionally.
      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 we're restarted after we're complete (usually via something like a notifyDataSetChanged
    // that starts an identical request into the same Target or View), we can simply use the
    // resource and size we retrieved the last time around and skip obtaining a new size, starting a
    // new load etc. This does mean that users who want to restart a load because they expect that
    // the view size has changed will need to explicitly clear the View or Target before starting
    // the new load.
    if (status == Status.COMPLETE) {
      onResourceReady(resource, DataSource.MEMORY_CACHE);
      return;
    }

    // Restarts for requests that are neither complete nor running can be treated as new requests
    // and can run again from the beginning.

    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 (Log.isLoggable(TAG, Log.VERBOSE)) {
      logV("finished run method in " + LogTime.getElapsedMillis(startTime));
    }
  }

  private void onLoadFailed(GlideException e, int maxLogLevel) {
    stateVerifier.throwIfRecycled();
    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;
    //TODO: what if this is a thumbnail request?
    if (requestListener == null
        || !requestListener.onLoadFailed(e, model, target, isFirstReadyResource())) {
      setErrorPlaceholder();
    }
  }

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);
  }

  /**
   * 
   *
   * @param errorDrawable {@inheritDoc}
   */
  @Override
  public void onLoadFailed(@Nullable Drawable errorDrawable) {
    super.onLoadFailed(errorDrawable);
    setResourceInternal(null);
    setDrawable(errorDrawable);
  }

private void setResourceInternal(@Nullable Z resource) {
    maybeUpdateAnimatable(resource);
    setResource(resource);
  }

@Override
  public void onSizeReady(int width, int height) {
    /**
     * 如果对象可回收(即当前在对象池中)，则抛出一个异常。
     */
    stateVerifier.throwIfRecycled();
    if (Log.isLoggable(TAG, Log.VERBOSE)) {
      logV("Got onSizeReady in " + LogTime.getElapsedMillis(startTime));
    }
    /**
     * 如果Status为WAITING_FOR_SIZE说明没有设置大小或者没有获取到计算后的大小
     */
    if (status != Status.WAITING_FOR_SIZE) {
      return;
    }
    status = Status.RUNNING;

    /**
     * 一个乘数适用于在加载资源之前，目标的大小。
     * 用于加载缩略图或尝试避免加载巨大资源特别是在屏幕过于密集的设备上
     *
     *  默认为1
     *  float sizeMultiplier = 1f;
     */
    float sizeMultiplier = requestOptions.getSizeMultiplier();

    /**
     * 根据上面的乘数计算需要加载的资源宽高
     */
    this.width = maybeApplySizeMultiplier(width, sizeMultiplier);
    this.height = maybeApplySizeMultiplier(height, sizeMultiplier);

    if (Log.isLoggable(TAG, Log.VERBOSE)) {
      logV("finished setup for calling load in " + LogTime.getElapsedMillis(startTime));
    }

    /**
     * 好熟悉的参数，基本都是equestOptions的属性
     * 都是经常用到的参数，就不详细说明了
     * 剩下的是事情在engine.load（）里,接下来我们去看一下
     */
    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.getOnlyRetrieveFromCache(),
        this);
    if (Log.isLoggable(TAG, Log.VERBOSE)) {
      logV("finished onSizeReady in " + LogTime.getElapsedMillis(startTime));
    }
  }

/**
 * 所有的请求流程都如下：
 * 1.检查内存缓存并提供缓存的资源
 * 2.检查当前使用的资源，并返回当前的活跃资源
 * 3.检查当前的加载进度，并将cb添加到正在进行的加载进度中
 * 4.开始一个新的加载
 */
public <R> LoadStatus load(
      GlideContext glideContext,
      Object model,
      Key signature,
      int width,
      int height,
      Class<?> resourceClass,
      Class<R> transcodeClass,
      Priority priority,
      DiskCacheStrategy diskCacheStrategy,
      Map<Class<?>, Transformation<?>> transformations,
      boolean isTransformationRequired,
      boolean isScaleOnlyOrNoTransform,
      Options options,
      boolean isMemoryCacheable,
      boolean useUnlimitedSourceExecutorPool,
      boolean onlyRetrieveFromCache,
      ResourceCallback cb) {
    Util.assertMainThread();
    long startTime = LogTime.getLogTime();

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

   /**
    * 1.检查内存缓存并提供缓存的资源
    */
    EngineResource<?> cached = loadFromCache(key, isMemoryCacheable);
    if (cached != null) {
      cb.onResourceReady(cached, DataSource.MEMORY_CACHE);
      if (Log.isLoggable(TAG, Log.VERBOSE)) {
        logWithTimeAndKey("Loaded resource from cache", startTime, key);
      }
      return null;
    }

   /**
    * 2.检查当前使用的资源，并返回当前的活跃资源
    */
    EngineResource<?> active = loadFromActiveResources(key, isMemoryCacheable);
    if (active != null) {
      cb.onResourceReady(active, DataSource.MEMORY_CACHE);
      if (Log.isLoggable(TAG, Log.VERBOSE)) {
        logWithTimeAndKey("Loaded resource from active resources", startTime, key);
      }
      return null;
    }

   /**
    *  3.检查当前的加载进度，并将cb添加到正在进行的加载进度中
    */
    EngineJob<?> current = jobs.get(key);
    if (current != null) {
      current.addCallback(cb);
      if (Log.isLoggable(TAG, Log.VERBOSE)) {
        logWithTimeAndKey("Added to existing load", startTime, key);
      }
      return new LoadStatus(cb, current);
    }

    EngineJob<R> engineJob = engineJobFactory.build(key, isMemoryCacheable,
        useUnlimitedSourceExecutorPool);
    DecodeJob<R> 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);
    engineJob.start(decodeJob);

    if (Log.isLoggable(TAG, Log.VERBOSE)) {
      logWithTimeAndKey("Started new load", startTime, key);
    }

   /**
    *   4.开始一个新的加载
    */
    return new LoadStatus(cb, engineJob);
  }

EngineJob<R> engineJob = engineJobFactory.build(key, isMemoryCacheable,
        useUnlimitedSourceExecutorPool);

DecodeJob<R> decodeJob = decodeJobFactory.build()

engineJob.start(decodeJob);

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

@Override
  public void execute(Runnable command) {
    if (executeSynchronously) {
      command.run();
    } else {
      super.execute(command);
    }
  }

  @Override
  public void run() {
    // This should be much more fine grained, but since Java's thread pool implementation silently
    // swallows all otherwise fatal exceptions, this will at least make it obvious to developers
    // that something is failing.
    TraceCompat.beginSection("DecodeJob#run");
    try {
      if (isCancelled) {
        notifyFailed();
        return;
      }
      runWrapped();
    } catch (RuntimeException e) {
      if (Log.isLoggable(TAG, Log.DEBUG)) {
        Log.d(TAG, "DecodeJob threw unexpectedly"
            + ", isCancelled: " + isCancelled
            + ", stage: " + stage, e);
      }
      // When we're encoding we've already notified our callback and it isn't safe to do so again.
      if (stage != Stage.ENCODE) {
        notifyFailed();
      }
      if (!isCancelled) {
        throw e;
      }
    } finally {
      if (currentFetcher != null) {
        currentFetcher.cleanup();
      }
      TraceCompat.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 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;
      }
    }
    // We've run out of stages and generators, give up.
    if ((stage == Stage.FINISHED || isCancelled) && !isStarted) {
      notifyFailed();
    }

    // Otherwise a generator started a new load and we expect to be called back in
    // onDataFetcherReady.
  }

  @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;
        loadData.fetcher.loadData(helper.getPriority(), this);
      }
    }
    return started;
  }

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

      Key sourceId = cacheKeys.get(sourceIdIndex);
      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<File, ?> 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;
  }

  List<ModelLoader<File, ?>> getModelLoaders(File file)
      throws Registry.NoModelLoaderAvailableException {
    return glideContext.getRegistry().getModelLoaders(file);
  }

    registry = new Registry();
    registry.register(new DefaultImageHeaderParser());

    Downsampler downsampler = new Downsampler(registry.getImageHeaderParsers(),
        resources.getDisplayMetrics(), bitmapPool, arrayPool);
    ByteBufferGifDecoder byteBufferGifDecoder =
        new ByteBufferGifDecoder(context, registry.getImageHeaderParsers(), bitmapPool, arrayPool);

    registry.register(ByteBuffer.class, new ByteBufferEncoder())
        .register(InputStream.class, new StreamEncoder(arrayPool))
        /* Bitmaps */
        .append(ByteBuffer.class, Bitmap.class,
            new ByteBufferBitmapDecoder(downsampler))
        .append(InputStream.class, Bitmap.class,
            new StreamBitmapDecoder(downsampler, arrayPool))
        .append(ParcelFileDescriptor.class, Bitmap.class, new VideoBitmapDecoder(bitmapPool))
        .register(Bitmap.class, new BitmapEncoder())
        /* GlideBitmapDrawables */
        .append(ByteBuffer.class, BitmapDrawable.class,
            new BitmapDrawableDecoder<>(resources, bitmapPool,
                new ByteBufferBitmapDecoder(downsampler)))
        .append(InputStream.class, BitmapDrawable.class,
            new BitmapDrawableDecoder<>(resources, bitmapPool,
                new StreamBitmapDecoder(downsampler, arrayPool)))
        .append(ParcelFileDescriptor.class, BitmapDrawable.class,
            new BitmapDrawableDecoder<>(resources, bitmapPool, new VideoBitmapDecoder(bitmapPool)))
        .register(BitmapDrawable.class, new BitmapDrawableEncoder(bitmapPool, new BitmapEncoder()))
        /* GIFs */
        .prepend(InputStream.class, GifDrawable.class,
            new StreamGifDecoder(registry.getImageHeaderParsers(), byteBufferGifDecoder, arrayPool))
        .prepend(ByteBuffer.class, GifDrawable.class, byteBufferGifDecoder)
        .register(GifDrawable.class, new GifDrawableEncoder())
        /* GIF Frames */
        .append(GifDecoder.class, GifDecoder.class, new UnitModelLoader.Factory<GifDecoder>())
        .append(GifDecoder.class, Bitmap.class, new GifFrameResourceDecoder(bitmapPool))
        /* Files */
        .register(new ByteBufferRewinder.Factory())
        .append(File.class, ByteBuffer.class, new ByteBufferFileLoader.Factory())
        .append(File.class, InputStream.class, new FileLoader.StreamFactory())
        .append(File.class, File.class, new FileDecoder())
        .append(File.class, ParcelFileDescriptor.class, new FileLoader.FileDescriptorFactory())
        .append(File.class, File.class, new UnitModelLoader.Factory<File>())
        /* Models */
        .register(new InputStreamRewinder.Factory(arrayPool))
        .append(int.class, InputStream.class, new ResourceLoader.StreamFactory(resources))
        .append(
                int.class,
                ParcelFileDescriptor.class,
                new ResourceLoader.FileDescriptorFactory(resources))
        .append(Integer.class, InputStream.class, new ResourceLoader.StreamFactory(resources))
        .append(
                Integer.class,
                ParcelFileDescriptor.class,
                new ResourceLoader.FileDescriptorFactory(resources))
        .append(String.class, InputStream.class, new DataUrlLoader.StreamFactory())
        .append(String.class, InputStream.class, new StringLoader.StreamFactory())
        .append(String.class, ParcelFileDescriptor.class, new StringLoader.FileDescriptorFactory())
        .append(Uri.class, InputStream.class, new HttpUriLoader.Factory())
        .append(Uri.class, InputStream.class, new AssetUriLoader.StreamFactory(context.getAssets()))
        .append(
                Uri.class,
                ParcelFileDescriptor.class,
                new AssetUriLoader.FileDescriptorFactory(context.getAssets()))
        .append(Uri.class, InputStream.class, new MediaStoreImageThumbLoader.Factory(context))
        .append(Uri.class, InputStream.class, new MediaStoreVideoThumbLoader.Factory(context))
        .append(
            Uri.class,
             InputStream.class,
             new UriLoader.StreamFactory(context.getContentResolver()))
        .append(Uri.class, ParcelFileDescriptor.class,
             new UriLoader.FileDescriptorFactory(context.getContentResolver()))
        .append(Uri.class, InputStream.class, new UrlUriLoader.StreamFactory())
        .append(URL.class, InputStream.class, new UrlLoader.StreamFactory())
        .append(Uri.class, File.class, new MediaStoreFileLoader.Factory(context))
        .append(GlideUrl.class, InputStream.class, new HttpGlideUrlLoader.Factory())
        .append(byte[].class, ByteBuffer.class, new ByteArrayLoader.ByteBufferFactory())
        .append(byte[].class, InputStream.class, new ByteArrayLoader.StreamFactory())
        /* Transcoders */
        .register(Bitmap.class, BitmapDrawable.class,
            new BitmapDrawableTranscoder(resources, bitmapPool))
        .register(Bitmap.class, byte[].class, new BitmapBytesTranscoder())
        .register(GifDrawable.class, byte[].class, new GifDrawableBytesTranscoder());

  public <Model, Data> Registry append(Class<Model> modelClass, Class<Data> dataClass,
      ModelLoaderFactory<Model, Data> factory) {
    modelLoaderRegistry.append(modelClass, dataClass, factory);
    return this;
  }

  public synchronized <Model, Data> void append(Class<Model> modelClass, Class<Data> dataClass,
      ModelLoaderFactory<Model, Data> factory) {
    multiModelLoaderFactory.append(modelClass, dataClass, factory);
    cache.clear();
  }

  synchronized <Model, Data> void append(Class<Model> modelClass, Class<Data> dataClass,
      ModelLoaderFactory<Model, Data> factory) {
    add(modelClass, dataClass, factory, true /*append*/);
  }

  private <Model, Data> void add(Class<Model> modelClass, Class<Data> dataClass,
      ModelLoaderFactory<Model, Data> factory, boolean append) {
    Entry<Model, Data> entry = new Entry<>(modelClass, dataClass, factory);
    entries.add(append ? entries.size() : 0, entry);
  }

    public Entry(Class<Model> modelClass, Class<Data> dataClass,
        ModelLoaderFactory<Model, Data> factory) {
      this.modelClass = modelClass;
      this.dataClass = dataClass;
      this.factory = factory;
    }

  public <Model> List<ModelLoader<Model, ?>> getModelLoaders(Model model) {
    List<ModelLoader<Model, ?>> result = modelLoaderRegistry.getModelLoaders(model);
    if (result.isEmpty()) {
      throw new NoModelLoaderAvailableException(model);
    }
    return result;
  }

  public synchronized <A> List<ModelLoader<A, ?>> getModelLoaders(A model) {
    List<ModelLoader<A, ?>> modelLoaders = getModelLoadersForClass(getClass(model));
    int size = modelLoaders.size();
    List<ModelLoader<A, ?>> filteredLoaders = new ArrayList<>(size);
    for (int i = 0; i < size; i++) {
      ModelLoader<A, ?> loader = modelLoaders.get(i);
      if (loader.handles(model)) {
        filteredLoaders.add(loader);
      }
    }
    return filteredLoaders;
  }

  private <A> List<ModelLoader<A, ?>> getModelLoadersForClass(Class<A> modelClass) {
    List<ModelLoader<A, ?>> loaders = cache.get(modelClass);
    if (loaders == null) {
      loaders = Collections.unmodifiableList(multiModelLoaderFactory.build(modelClass));
      cache.put(modelClass, loaders);
    }
    return loaders;
  }

  synchronized <Model> List<ModelLoader<Model, ?>> build(Class<Model> modelClass) {
    try {
      List<ModelLoader<Model, ?>> loaders = new ArrayList<>();
      for (Entry<?, ?> entry : entries) {
        // Avoid stack overflow recursively creating model loaders by only creating loaders in
        // recursive requests if they haven't been created earlier in the chain. For example:
        // A Uri loader may translate to another model, which in turn may translate back to a Uri.
        // The original Uri loader won't be provided to the intermediate model loader, although
        // other Uri loaders will be.
        if (alreadyUsedEntries.contains(entry)) {
          continue;
        }
        if (entry.handles(modelClass)) {
          alreadyUsedEntries.add(entry);
          loaders.add(this.<Model, Object>build(entry));
          alreadyUsedEntries.remove(entry);
        }
      }
      return loaders;
    } catch (Throwable t) {
      alreadyUsedEntries.clear();
      throw t;
    }
  }

    public boolean handles(Class<?> modelClass, Class<?> dataClass) {
      return handles(modelClass) && this.dataClass.isAssignableFrom(dataClass);
    }

this.modelClass.isAssignableFrom(modelClass); 
和
this.dataClass.isAssignableFrom(dataClass);

  private <Model, Data> ModelLoader<Model, Data> build(Entry<?, ?> entry) {
    return (ModelLoader<Model, Data>) Preconditions.checkNotNull(entry.factory.build(this));
  }

  /**
   * 为这种模型类型构建一个具体的模型加载器。
   *
   * @param multiFactory A map of classes to factories that can be used to construct additional
   *                     {@link ModelLoader}s that this factory's {@link ModelLoader} may depend on
   * @return A new {@link ModelLoader}
   */
  ModelLoader<T, Y> build(MultiModelLoaderFactory multiFactory);

  /**
   * A lifecycle method that will be called when this factory is about to replaced.
   */
  void teardown();

public class HttpUriLoader implements ModelLoader<Uri, InputStream> {
  private static final Set<String> SCHEMES =
      Collections.unmodifiableSet(new HashSet<>(Arrays.asList("http", "https")));

  private final ModelLoader<GlideUrl, InputStream> urlLoader;

  public HttpUriLoader(ModelLoader<GlideUrl, InputStream> urlLoader) {
    this.urlLoader = urlLoader;
  }

  @Override
  public LoadData<InputStream> buildLoadData(Uri model, int width, int height, Options options) {
    return urlLoader.buildLoadData(new GlideUrl(model.toString()), width, height, options);
  }

  @Override
  public boolean handles(Uri model) {
    return SCHEMES.contains(model.getScheme());
  }

  /**
   * Factory for loading {@link InputStream}s from http/https {@link Uri}s.
   */
  public static class Factory implements ModelLoaderFactory<Uri, InputStream> {

    @Override
    public ModelLoader<Uri, InputStream> build(MultiModelLoaderFactory multiFactory) {
      return new HttpUriLoader(multiFactory.build(GlideUrl.class, InputStream.class));
    }

    @Override
    public void teardown() {
      // Do nothing.
    }
  }
}

public HttpUriLoader(ModelLoader<GlideUrl, InputStream> urlLoader) {
    this.urlLoader = urlLoader;
  }

private <A> List<ModelLoader<A, ?>> getModelLoadersForClass(Class<A> modelClass) {
    List<ModelLoader<A, ?>> loaders = cache.get(modelClass);
    if (loaders == null) {
      loaders = Collections.unmodifiableList(multiModelLoaderFactory.build(modelClass));
      cache.put(modelClass, loaders);
    }
    return loaders;
  }

final class DecodeHelper<Transcode> {
  ...
  List<ModelLoader<File, ?>> getModelLoaders(File file)
      throws Registry.NoModelLoaderAvailableException {
    return glideContext.getRegistry().getModelLoaders(file);
  }
  ...
}

class ResourceCacheGenerator{
    @Override
  public boolean startNext() {
    List<Key> sourceIds = helper.getCacheKeys();
    if (sourceIds.isEmpty()) {
      return false;
    }
    List<Class<?>> resourceClasses = helper.getRegisteredResourceClasses();
    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);

      currentKey = new ResourceCacheKey(sourceId, helper.getSignature(), helper.getWidth(),
          helper.getHeight(), transformation, resourceClass, helper.getOptions());
      cacheFile = helper.getDiskCache().get(currentKey);
      if (cacheFile != null) {
        this.sourceKey = sourceId;
        modelLoaders = helper.getModelLoaders(cacheFile);
        modelLoaderIndex = 0;
      }
    }

    loadData = null;
    boolean started = false;
    while (!started && hasNextModelLoader()) {
      ModelLoader<File, ?> 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;
  }
}

  /**
   *返回一个ModelLoader。LoadData包含一个DataFetcher，它需要解码这个模型所表示的资源，
   *以及一组密钥，这些键识别DataFetcher加载的数据，以及一个可选的可加载等价数据的备用键列
   *表。如果资源已经被缓存，DataFetcher将不会被使用。
   */
@Nullable
LoadData<Data> buildLoadData(Model model, int width, int height, Options options);

public class HttpUrlFetcher implements DataFetcher<InputStream> {
  private static final String TAG = "HttpUrlFetcher";
  private static final int MAXIMUM_REDIRECTS = 5;
  // Visible for testing.
  static final HttpUrlConnectionFactory DEFAULT_CONNECTION_FACTORY =
      new DefaultHttpUrlConnectionFactory();

  private final GlideUrl glideUrl;
  private final int timeout;
  private final HttpUrlConnectionFactory connectionFactory;

  private HttpURLConnection urlConnection;
  private InputStream stream;
  private volatile boolean isCancelled;

  public HttpUrlFetcher(GlideUrl glideUrl, int timeout) {
    this(glideUrl, timeout, DEFAULT_CONNECTION_FACTORY);
  }

  // Visible for testing.
  HttpUrlFetcher(GlideUrl glideUrl, int timeout, HttpUrlConnectionFactory connectionFactory) {
    this.glideUrl = glideUrl;
    this.timeout = timeout;
    this.connectionFactory = connectionFactory;
  }

  @Override
  public void loadData(Priority priority, DataCallback<? super InputStream> callback) {
    long startTime = LogTime.getLogTime();
    final InputStream result;
    try {
      result = loadDataWithRedirects(glideUrl.toURL(), 0 /*redirects*/, null /*lastUrl*/,
          glideUrl.getHeaders());
    } catch (IOException e) {
      if (Log.isLoggable(TAG, Log.DEBUG)) {
        Log.d(TAG, "Failed to load data for url", e);
      }
      callback.onLoadFailed(e);
      return;
    }

    if (Log.isLoggable(TAG, Log.VERBOSE)) {
      Log.v(TAG, "Finished http url fetcher fetch in " + LogTime.getElapsedMillis(startTime)
          + " ms and loaded " + result);
    }
    callback.onDataReady(result);
  }

  private InputStream loadDataWithRedirects(URL url, int redirects, URL lastUrl,
      Map<String, String> headers) throws IOException {
    if (redirects >= MAXIMUM_REDIRECTS) {
      throw new HttpException("Too many (> " + MAXIMUM_REDIRECTS + ") redirects!");
    } else {
      // Comparing the URLs using .equals performs additional network I/O and is generally broken.
      // See http://michaelscharf.blogspot.com/2006/11/javaneturlequals-and-hashcode-make.html.
      try {
        if (lastUrl != null && url.toURI().equals(lastUrl.toURI())) {
          throw new HttpException("In re-direct loop");

        }
      } catch (URISyntaxException e) {
        // Do nothing, this is best effort.
      }
    }

    urlConnection = connectionFactory.build(url);
    for (Map.Entry<String, String> headerEntry : headers.entrySet()) {
      urlConnection.addRequestProperty(headerEntry.getKey(), headerEntry.getValue());
    }
    urlConnection.setConnectTimeout(timeout);
    urlConnection.setReadTimeout(timeout);
    urlConnection.setUseCaches(false);
    urlConnection.setDoInput(true);

    // Stop the urlConnection instance of HttpUrlConnection from following redirects so that
    // redirects will be handled by recursive calls to this method, loadDataWithRedirects.
    urlConnection.setInstanceFollowRedirects(false);

    // Connect explicitly to avoid errors in decoders if connection fails.
    urlConnection.connect();
    if (isCancelled) {
      return null;
    }
    final int statusCode = urlConnection.getResponseCode();
    if (statusCode / 100 == 2) {
      return getStreamForSuccessfulRequest(urlConnection);
    } else if (statusCode / 100 == 3) {
      String redirectUrlString = urlConnection.getHeaderField("Location");
      if (TextUtils.isEmpty(redirectUrlString)) {
        throw new HttpException("Received empty or null redirect url");
      }
      URL redirectUrl = new URL(url, redirectUrlString);
      return loadDataWithRedirects(redirectUrl, redirects + 1, url, headers);
    } else if (statusCode == -1) {
      throw new HttpException(statusCode);
    } else {
      throw new HttpException(urlConnection.getResponseMessage(), statusCode);
    }
  }

  private InputStream getStreamForSuccessfulRequest(HttpURLConnection urlConnection)
      throws IOException {
    if (TextUtils.isEmpty(urlConnection.getContentEncoding())) {
      int contentLength = urlConnection.getContentLength();
      stream = ContentLengthInputStream.obtain(urlConnection.getInputStream(), contentLength);
    } else {
      if (Log.isLoggable(TAG, Log.DEBUG)) {
        Log.d(TAG, "Got non empty content encoding: " + urlConnection.getContentEncoding());
      }
      stream = urlConnection.getInputStream();
    }
    return stream;
  }

  @Override
  public void cleanup() {
    if (stream != null) {
      try {
        stream.close();
      } catch (IOException e) {
        // Ignore
      }
    }
    if (urlConnection != null) {
      urlConnection.disconnect();
    }
  }

  @Override
  public void cancel() {
    // TODO: we should consider disconnecting the url connection here, but we can't do so
    // directly because cancel is often called on the main thread.
    isCancelled = true;
  }

  @NonNull
  @Override
  public Class<InputStream> getDataClass() {
    return InputStream.class;
  }

  @NonNull
  @Override
  public DataSource getDataSource() {
    return DataSource.REMOTE;
  }

  interface HttpUrlConnectionFactory {
    HttpURLConnection build(URL url) throws IOException;
  }

  private static class DefaultHttpUrlConnectionFactory implements HttpUrlConnectionFactory {

    @Synthetic
    DefaultHttpUrlConnectionFactory() { }

    @Override
    public HttpURLConnection build(URL url) throws IOException {
      return (HttpURLConnection) url.openConnection();
    }
  }
}

  @Override
  public void loadData(Priority priority, DataCallback<? super InputStream> callback) {
    long startTime = LogTime.getLogTime();
    final InputStream result;
    try {
      result = loadDataWithRedirects(glideUrl.toURL(), 0 /*redirects*/, null /*lastUrl*/,
          glideUrl.getHeaders());
    } catch (IOException e) {
      if (Log.isLoggable(TAG, Log.DEBUG)) {
        Log.d(TAG, "Failed to load data for url", e);
      }
      callback.onLoadFailed(e);
      return;
    }

    if (Log.isLoggable(TAG, Log.VERBOSE)) {
      Log.v(TAG, "Finished http url fetcher fetch in " + LogTime.getElapsedMillis(startTime)
          + " ms and loaded " + result);
    }
    callback.onDataReady(result);
  }

class ResourceCacheGenerator{
  ...
  @Override
  public boolean startNext() {
    ...
    loadData.fetcher.loadData(helper.getPriority(), this);
    ...
  }

  ...
  @Override
 public void onDataReady(Object data) {
   cb.onDataFetcherReady(sourceKey, data, loadData.fetcher, DataSource.RESOURCE_DISK_CACHE,
       currentKey);
 }
  ...
}

  public ResourceCacheGenerator(DecodeHelper<?> helper, FetcherReadyCallback cb) {
    this.helper = helper;
    this.cb = cb;
  }

class DecodeJob<R>{
   ...
   ...
  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);
    }
  }
  ...
  ...
    @Override
  public void onDataFetcherReady(Key sourceKey, Object data, DataFetcher<?> fetcher,
      DataSource dataSource, Key attemptedKey) {
    this.currentSourceKey = sourceKey;
    this.currentData = data;
    this.currentFetcher = fetcher;
    this.currentDataSource = dataSource;
    this.currentAttemptingKey = attemptedKey;
    if (Thread.currentThread() != currentThread) {
      runReason = RunReason.DECODE_DATA;
      callback.reschedule(this);
    } else {
      TraceCompat.beginSection("DecodeJob.decodeFromRetrievedData");
      try {
        decodeFromRetrievedData();
      } finally {
        TraceCompat.endSection();
      }
    }
  }
  ...
  ...
}

private void decodeFromRetrievedData() {
    if (Log.isLoggable(TAG, Log.VERBOSE)) {
      logWithTimeAndKey("Retrieved data", startFetchTime,
          "data: " + currentData
          + ", cache key: " + currentSourceKey
          + ", fetcher: " + currentFetcher);
    }
    Resource<R> resource = null;
    try {

        //通过decodeFromData方法把输入流解码并返回给resource
      resource = decodeFromData(currentFetcher, currentData, currentDataSource);
    } catch (GlideException e) {
      e.setLoggingDetails(currentAttemptingKey, currentDataSource);
      exceptions.add(e);
    }
    if (resource != null) {
      notifyEncodeAndRelease(resource, currentDataSource);
    } else {
      runGenerators();
    }
  }

  private <Data> Resource<R> decodeFromData(DataFetcher<?> fetcher, Data data,
      DataSource dataSource) throws GlideException {
    try {
      if (data == null) {
        return null;
      }
      long startTime = LogTime.getLogTime();
      Resource<R> result = decodeFromFetcher(data, dataSource);
      if (Log.isLoggable(TAG, Log.VERBOSE)) {
        logWithTimeAndKey("Decoded result " + result, startTime);
      }
      return result;
    } finally {
      fetcher.cleanup();
    }
  }

  private <Data> Resource<R> decodeFromFetcher(Data data, DataSource dataSource)
      throws GlideException {
    LoadPath<Data, ?, R> path = decodeHelper.getLoadPath((Class<Data>) data.getClass());
    return runLoadPath(data, dataSource, path);
  }

  <Data> LoadPath<Data, ?, Transcode> getLoadPath(Class<Data> dataClass) {
    return glideContext.getRegistry().getLoadPath(dataClass, resourceClass, transcodeClass);
  }

  public <Data, TResource, Transcode> LoadPath<Data, TResource, Transcode> getLoadPath(
      Class<Data> dataClass, Class<TResource> resourceClass, Class<Transcode> transcodeClass) {
    LoadPath<Data, TResource, Transcode> result =
        loadPathCache.get(dataClass, resourceClass, transcodeClass);
    if (result == null && !loadPathCache.contains(dataClass, resourceClass, transcodeClass)) {
      List<DecodePath<Data, TResource, Transcode>> decodePaths =
          getDecodePaths(dataClass, resourceClass, transcodeClass);
      // It's possible there is no way to decode or transcode to the desired types from a given
      // data class.
      if (decodePaths.isEmpty()) {
        result = null;
      } else {
        result = new LoadPath<>(dataClass, resourceClass, transcodeClass, decodePaths,
            exceptionListPool);
      }
      loadPathCache.put(dataClass, resourceClass, transcodeClass, result);
    }
    return result;
  }

  private <Data, TResource, Transcode> List<DecodePath<Data, TResource, Transcode>> getDecodePaths(
      Class<Data> dataClass, Class<TResource> resourceClass, Class<Transcode> transcodeClass) {

    List<DecodePath<Data, TResource, Transcode>> decodePaths = new ArrayList<>();
    List<Class<TResource>> registeredResourceClasses =
        decoderRegistry.getResourceClasses(dataClass, resourceClass);

    for (Class<TResource> registeredResourceClass : registeredResourceClasses) {
      List<Class<Transcode>> registeredTranscodeClasses =
          transcoderRegistry.getTranscodeClasses(registeredResourceClass, transcodeClass);

      for (Class<Transcode> registeredTranscodeClass : registeredTranscodeClasses) {

        List<ResourceDecoder<Data, TResource>> decoders =
            decoderRegistry.getDecoders(dataClass, registeredResourceClass);
        ResourceTranscoder<TResource, Transcode> transcoder =
            transcoderRegistry.get(registeredResourceClass, registeredTranscodeClass);
        decodePaths.add(new DecodePath<>(dataClass, registeredResourceClass,
            registeredTranscodeClass, decoders, transcoder, exceptionListPool));
      }
    }
    return decodePaths;
  }

  public <Data, TResource, Transcode> LoadPath<Data, TResource, Transcode> getLoadPath(
      Class<Data> dataClass, Class<TResource> resourceClass, Class<Transcode> transcodeClass) {
    LoadPath<Data, TResource, Transcode> result =
        loadPathCache.get(dataClass, resourceClass, transcodeClass);
    if (result == null && !loadPathCache.contains(dataClass, resourceClass, transcodeClass)) {
      List<DecodePath<Data, TResource, Transcode>> decodePaths =
          getDecodePaths(dataClass, resourceClass, transcodeClass);
      // It's possible there is no way to decode or transcode to the desired types from a given
      // data class.
      if (decodePaths.isEmpty()) {
        result = null;
      } else {
        result = new LoadPath<>(dataClass, resourceClass, transcodeClass, decodePaths,
            exceptionListPool);
      }
      loadPathCache.put(dataClass, resourceClass, transcodeClass, result);
    }
    return result;
  }

  private <Data> Resource<R> decodeFromFetcher(Data data, DataSource dataSource)
      throws GlideException {
    LoadPath<Data, ?, R> path = decodeHelper.getLoadPath((Class<Data>) data.getClass());
    return runLoadPath(data, dataSource, path);
  }

  private <Data, ResourceType> Resource<R> runLoadPath(Data data, DataSource dataSource,
      LoadPath<Data, ResourceType, R> path) throws GlideException {
    Options options = getOptionsWithHardwareConfig(dataSource);
    DataRewinder<Data> rewinder = glideContext.getRegistry().getRewinder(data);
    try {
      // ResourceType in DecodeCallback below is required for compilation to work with gradle.
      return path.load(
          rewinder, options, width, height, new DecodeCallback<ResourceType>(dataSource));
    } finally {
      rewinder.cleanup();
    }
  }

  private Resource<Transcode> loadWithExceptionList(DataRewinder<Data> rewinder, Options options,
      int width, int height, DecodePath.DecodeCallback<ResourceType> decodeCallback,
      List<Exception> exceptions) throws GlideException {
    int size = decodePaths.size();
    Resource<Transcode> result = null;
    for (int i = 0; i < size; i++) {
      DecodePath<Data, ResourceType, Transcode> path = decodePaths.get(i);
      try {
        result = path.decode(rewinder, width, height, options, decodeCallback);
      } catch (GlideException e) {
        exceptions.add(e);
      }
      if (result != null) {
        break;
      }
    }

    if (result == null) {
      throw new GlideException(failureMessage, new ArrayList<>(exceptions));
    }

    return result;
  }

  public Resource<Transcode> decode(DataRewinder<DataType> rewinder, int width, int height,
      Options options, DecodeCallback<ResourceType> callback) throws GlideException {
    Resource<ResourceType> decoded = decodeResource(rewinder, width, height, options);
    Resource<ResourceType> transformed = callback.onResourceDecoded(decoded);
    return transcoder.transcode(transformed);
  }

  private Resource<ResourceType> decodeResource(DataRewinder<DataType> rewinder, int width,
      int height, Options options) throws GlideException {
    List<Exception> exceptions = listPool.acquire();
    try {
      return decodeResourceWithList(rewinder, width, height, options, exceptions);
    } finally {
      listPool.release(exceptions);
    }
  }

  private Resource<ResourceType> decodeResourceWithList(DataRewinder<DataType> rewinder, int width,
      int height, Options options, List<Exception> exceptions) throws GlideException {
    Resource<ResourceType> result = null;
    /**
     * 遍历decoders集合，分别的获取到ResourceDecoder解码器
     * 主要包含两种：BitmapDrawable.class和GifDrawable.class。
     * 之前的文章讲过，可以自行查阅前两篇文章
     */
    for (int i = 0, size = decoders.size(); i < size; i++) {
      ResourceDecoder<DataType, ResourceType> decoder = decoders.get(i);
      try {
        DataType data = rewinder.rewindAndGet();
        /**
         * 方法来过滤掉不相匹配的解码器
         * 会给我们配对对应的解码器
         * 这就是为什么Gif图，我们不用设置asGif也可以加载的原因。
         */
        if (decoder.handles(data, options)) {
          /**
           * 获取我们的InputStream数据流
           */
          data = rewinder.rewindAndGet();
          /**
           * 得到真正的Resource
           */
          result = decoder.decode(data, width, height, options);
        }
      } catch (IOException e) {
        if (Log.isLoggable(TAG, Log.VERBOSE)) {
          Log.v(TAG, "Failed to decode data for " + decoder, e);
        }
        exceptions.add(e);
      }

      if (result != null) {
        break;
      }
    }

    if (result == null) {
      throw new GlideException(failureMessage, new ArrayList<>(exceptions));
    }
    return result;
  }

public class BitmapDrawableTranscoder{
    ...
    ...
  @Override
  public Resource<BitmapDrawable> transcode(Resource<Bitmap> toTranscode) {
    return LazyBitmapDrawableResource.obtain(resources, bitmapPool, toTranscode.get());
  }
    ...
    ...
}

  private void decodeFromRetrievedData() {
    if (Log.isLoggable(TAG, Log.VERBOSE)) {
      logWithTimeAndKey("Retrieved data", startFetchTime,
          "data: " + currentData
          + ", cache key: " + currentSourceKey
          + ", fetcher: " + currentFetcher);
    }
    Resource<R> resource = null;
    try {
      /**
       * 通过decodeFromData方法把输入流解码并返回给resource
       */
      resource = decodeFromData(currentFetcher, currentData, currentDataSource);
    } catch (GlideException e) {
      e.setLoggingDetails(currentAttemptingKey, currentDataSource);
      exceptions.add(e);
    }
    if (resource != null) {
      notifyEncodeAndRelease(resource, currentDataSource);
    } else {
      runGenerators();
    }
  }

  private void notifyEncodeAndRelease(Resource<R> resource, DataSource dataSource) {
    if (resource instanceof Initializable) {
      ((Initializable) resource).initialize();
    }

    Resource<R> result = resource;
    LockedResource<R> lockedResource = null;
    if (deferredEncodeManager.hasResourceToEncode()) {
      lockedResource = LockedResource.obtain(resource);
      result = lockedResource;
    }

    notifyComplete(result, dataSource);

    stage = Stage.ENCODE;
    try {
      if (deferredEncodeManager.hasResourceToEncode()) {
        deferredEncodeManager.encode(diskCacheProvider, options);
      }
    } finally {
      if (lockedResource != null) {
        lockedResource.unlock();
      }
      onEncodeComplete();
    }
  }

  private void notifyComplete(Resource<R> resource, DataSource dataSource) {
    setNotifiedOrThrow();
    callback.onResourceReady(resource, dataSource);
  }

class EngineJob{
   ...
  @Override
  public void onResourceReady(Resource<R> resource, DataSource dataSource) {
    this.resource = resource;
    this.dataSource = dataSource;
    MAIN_THREAD_HANDLER.obtainMessage(MSG_COMPLETE, this).sendToTarget();
  }
  ...
}

@Synthetic
  void handleResultOnMainThread() {
    stateVerifier.throwIfRecycled();
    if (isCancelled) {
      resource.recycle();
      release(false /*isRemovedFromQueue*/);
      return;
    } else if (cbs.isEmpty()) {
      throw new IllegalStateException("Received a resource without any callbacks to notify");
    } else if (hasResource) {
      throw new IllegalStateException("Already have resource");
    }
    engineResource = engineResourceFactory.build(resource, isCacheable);
    hasResource = true;

    // Hold on to resource for duration of request so we don't recycle it in the middle of
    // notifying if it synchronously released by one of the callbacks.
    engineResource.acquire();
    listener.onEngineJobComplete(key, engineResource);

    for (ResourceCallback cb : cbs) {
      if (!isInIgnoredCallbacks(cb)) {
        engineResource.acquire();
        cb.onResourceReady(engineResource, dataSource);
      }
    }
    // Our request is complete, so we can release the resource.
    engineResource.release();

    release(false /*isRemovedFromQueue*/);
  }

private void notifyComplete(Resource<R> resource, DataSource dataSource) {
    setNotifiedOrThrow();
    callback.onResourceReady(resource, dataSource);
  }

  /**
   * A callback method that should never be invoked directly.
   */
  @SuppressWarnings("unchecked")
  @Override
  public void onResourceReady(Resource<?> resource, DataSource dataSource) {
    stateVerifier.throwIfRecycled();
    loadStatus = null;
    if (resource == null) {
      GlideException exception = new GlideException("Expected to receive a Resource<R> with an "
          + "object of " + transcodeClass + " inside, but instead got null.");
      onLoadFailed(exception);
      return;
    }

    Object received = resource.get();
    if (received == null || !transcodeClass.isAssignableFrom(received.getClass())) {
      releaseResource(resource);
      GlideException exception = new GlideException("Expected to receive an object of "
          + transcodeClass + " but instead" + " got "
          + (received != null ? received.getClass() : "") + "{" + received + "} inside" + " "
          + "Resource{" + resource + "}."
          + (received != null ? "" : " " + "To indicate failure return a null Resource "
          + "object, rather than a Resource object containing null data."));
      onLoadFailed(exception);
      return;
    }

    if (!canSetResource()) {
      releaseResource(resource);
      // We can't put the status to complete before asking canSetResource().
      status = Status.COMPLETE;
      return;
    }

    onResourceReady((Resource<R>) resource, (R) received, dataSource);
  }

public class LazyBitmapDrawableResource{
  ...
  @Override
  public BitmapDrawable get() {
    return new BitmapDrawable(resources, bitmap);
  }
  ...
}

  /**
   * Internal {@link #onResourceReady(Resource, DataSource)} where arguments are known to be safe.
   *
   * @param resource original {@link Resource}, never <code>null</code>
   * @param result   object returned by {@link Resource#get()}, checked for type and never
   *                 <code>null</code>
   */
  private void onResourceReady(Resource<R> resource, R result, DataSource dataSource) {
    // We must call isFirstReadyResource before setting status.
    boolean isFirstResource = isFirstReadyResource();
    status = Status.COMPLETE;
    this.resource = resource;

    if (glideContext.getLogLevel() <= Log.DEBUG) {
      Log.d(GLIDE_TAG, "Finished loading " + result.getClass().getSimpleName() + " from "
          + dataSource + " for " + model + " with size [" + width + "x" + height + "] in "
          + LogTime.getElapsedMillis(startTime) + " ms");
    }

    if (requestListener == null
        || !requestListener.onResourceReady(result, model, target, dataSource, isFirstResource)) {
      Transition<? super R> animation =
          animationFactory.build(dataSource, isFirstResource);
      target.onResourceReady(result, animation);
    }

    notifyLoadSuccess();
  }

public abstract class ImageViewTarget<Z> extends ViewTarget<ImageView, Z>implements Transition.ViewAdapter {
    ...
    ...
      @Override
  public void onResourceReady(Z resource, @Nullable Transition<? super Z> transition) {
    if (transition == null || !transition.transition(resource, this)) {
      setResourceInternal(resource);
    } else {
      maybeUpdateAnimatable(resource);
    }
  }
    ...
    ...
      private void setResourceInternal(@Nullable Z resource) {
    maybeUpdateAnimatable(resource);
    setResource(resource);
  }
    ...
    ...
    protected abstract void setResource(@Nullable Z resource);
    ...
    ...
}