Glide源码解析二---into方法

转载请标明出处，维权必究： https://www.cnblogs.com/tangZH/p/12543154.html

Glide作为一个强大的图片加载框架，已经被android官方使用，所以，明白Glide的加载流程以及原理对加深我们对glide的理解是很重要的。

本文基于glide 4.11

Glide.with(this).load("").into(new ImageView(this));

我们从这一句入手，上次我们看了Glide的初始化过程，也就是Glide.with(this)这个方法。现在我们来看into方法。

    @NonNull
    public ViewTarget<ImageView, TranscodeType> into(@NonNull ImageView view) {
        Util.assertMainThread();
        //检查view是否为null
        Preconditions.checkNotNull(view);

        //根据view.getScaleType()设置不同的transform变换，这个transform变换我们单独讲
        BaseRequestOptions<?> requestOptions = this;
        if (!requestOptions.isTransformationSet()
                && requestOptions.isTransformationAllowed()
                && view.getScaleType() != null) {
            // Clone in this method so that if we use this RequestBuilder to load into a View and then
            // into a different target, we don't retain the transformation applied based on the previous
            // View's scale type.
            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(
                //根据transcodeClass的类型构造不同的Target
                glideContext.buildImageViewTarget(view, transcodeClass),
                /*targetListener=*/ null,
                requestOptions,
                Executors.mainThreadExecutor());
    }

构建不同的target

glideContext.buildImageViewTarget(view, transcodeClass)，跟着代码点进去，最后跟踪到了这里：

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

如果说我们最终要将资源解码为bitmap，那么就构造BitmapImageViewTarget，如果要将资源解码为Drawable，那么就构造DrawableImageViewTarget。

如果你在使用Glide加载图片的时候调用了asBitmap()方法，那么这里就会构建出BitmapImageViewTarget对象，否则的话构建的都是DrawableImageViewTarget对象。

target里面有一些方法，比如失败的回调，设置资源等等。

 

接下来继续看代码，会调用下面这个方法。

    private <Y extends Target<TranscodeType>> Y into(
            @NonNull Y target,
            @Nullable RequestListener<TranscodeType> targetListener,
            BaseRequestOptions<?> options,
            Executor callbackExecutor) {
        Preconditions.checkNotNull(target);
        //检测是否已经调用过load方法
        if (!isModelSet) {
            throw new IllegalArgumentException("You must call #load() before calling #into()");
        }

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

        //获取改target是否已经有绑定的request
        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)
                && !isSkipMemoryCacheWithCompletePreviousRequest(options, previous)) {
            // If the request is completed, beginning again will ensure the result is re-delivered,
            // triggering RequestListeners and Targets. If the request is failed, beginning again will
            // restart the request, giving it another chance to complete. If the request is already
            // running, we can let it continue running without interruption.
            if (!Preconditions.checkNotNull(previous).isRunning()) {
                // Use the previous request rather than the new one to allow for optimizations like skipping
                // setting placeholders, tracking and un-tracking Targets, and obtaining View dimensions
                // that are done in the individual Request.
                previous.begin();
            }
            return target;
        }

        requestManager.clear(target);
        //将该request设置给target
        target.setRequest(request);
        requestManager.track(target, request);

        return target;
    }

先看一下buildRequest(target, targetListener, options, callbackExecutor);做了什么

追踪进去，调用buildRequestRecursive方法。

然后主要是这两个方法：

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

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

最后设置给ErrorRequestCoordinator

errorRequestCoordinator.setRequests(mainRequest, errorRequest);

ErrorRequestCoordinator负责管理这些请求，如果请求失败就运行错误的请求。

我们看这个方法：buildThumbnailRequestRecursive

  private Request buildThumbnailRequestRecursive(
      Object requestLock,
      Target<TranscodeType> target,
      RequestListener<TranscodeType> targetListener,
      @Nullable RequestCoordinator parentCoordinator,
      TransitionOptions<?, ? super TranscodeType> 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<?, ? 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.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);
    }
  }

首先对缩略图及是否对Target设置参数的判断(是否使用了thumbnail()方法和sizeMultiplier()方法)，如果有使用thunmnail()方法，则生成原始图片和缩略图的请求，并由ThumbnailRequestCoordinator对象来协调管理，使用了sizeMultiplier()方法，则同样的处理(前者递归的获得缩略图的Request，后者不递归)，否则就只生成原始图片的请求。

他们最终都会调用obtainRequest方法，追踪进去可以发现该方法最终返回的是SingleRequest对象。初始化request的时候传递的参数很多：

  public static <R> SingleRequest<R> obtain(
      Context context,
      GlideContext glideContext,
      Object requestLock,
      Object model,
      Class<R> transcodeClass,
      BaseRequestOptions<?> requestOptions,
      int overrideWidth,
      int overrideHeight,
      Priority priority,
      Target<R> target,
      RequestListener<R> targetListener,
      @Nullable List<RequestListener<R>> requestListeners,
      RequestCoordinator requestCoordinator,
      Engine engine,
      TransitionFactory<? super R> animationFactory,
      Executor callbackExecutor) {

1.GlideContext glideContext : 全局上下文
2.Object model ：加载的资源类型
3.Class transcodeClass ：转换的类型
4.RequestOptions requestOptions：设置选项（包括：skipMemoryCache，errorDrawable，placeholder，timeoutOf，encodeFormatOf等等）
5.int overrideWidth：目标宽度在所需资源的像素点。
6.int overrideHeight：目标高度在所需资源的像素点。
7. Priority priority：加载的优先级（IMMEDIATE,HIGH,NORMAL,LOW）
8.Target target:上面刚讲过，绑定的target
9.RequestListener requestListener:请求加载时候的监听器
10.RequestCoordinator requestCoordinator:请求协调器（用来协调具有相同Target的协调器）
11.Engine engine:负责启动负载和管理活动和缓存资源。
12.TransitionFactory<? super R> animationFactory:一个工厂类，可以根据请求的状态产生不同的转换。

我们再回到into代码中，获取了request之后我们就要开始请求了。

我们看着一句requestManager.track(target, request);

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

runRequest就是执行请求的代码：

  /** Starts tracking the given request. */
  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);
    }
  }

判断Glide当前是不是处理暂停状态，如果不是暂停状态就调用Request的begin()方法来执行Request，否则的话就先将Request添加到待执行队列里面，等暂停状态解除了之后再执行。

我们来看begin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;
        }
        // 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");
      }

      如果说这个资源已经被加载过了，那么我们直接调用onResourceReady
      // 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 (IS_VERBOSE_LOGGABLE) {
        logV("finished run method in " + LogTime.getElapsedMillis(startTime));
      }
    }
  }

如果model为null，说明我们没有调用load方法，这时候会回调onLoadFailed，将status设置为Status.FAILED，然后调用setErrorPlaceholder，这个方法里面最终调用target.onLoadFailed(error);将资源置空，然后显示错误图片。

    @Override
    public void begin() {
        synchronized (requestLock) {
            assertNotCallingCallbacks();
            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 == SingleRequest.Status.RUNNING) {
                throw new IllegalArgumentException("Cannot restart a running request");
            }

            /**
             * 如果完成后重新启动（通常是通过notifyDataSetChanged之类的方法
             * 将相同的请求发送到相同的Target或View），我们可以简单地使用
             * 我们最后一次检索的资源和大小，然后跳过获取新大小的步骤，
             * 不用开始一个新的加载。这确实意味着要重新加载的用户，因为他们
             * 更改视图大小，那么需要先明确清除view和target，然后
             * 开始新的加载。
             */
            if (status == SingleRequest.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 = SingleRequest.Status.WAITING_FOR_SIZE;
            /**
             * 这里会判断Util.isValidDimensions(overrideWidth, overrideHeight)
             * 如果你在使用时候调用了override() API为图片指定了一个固定的宽高，就会按照你给定的去加载；第二种情况是没有给定的情况，
             * 那么target.getSize()方法的内部会根据ImageView的layout_width和layout_height值做一系列的计算，来算出图片应该的宽高，
             * 具体计算就在getSize里面
             * 但是不管怎样，最后都会调用onSizeReady()。
             */

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

            if ((status == SingleRequest.Status.RUNNING || status == SingleRequest.Status.WAITING_FOR_SIZE)
                    && canNotifyStatusChanged()) {
                target.onLoadStarted(getPlaceholderDrawable());
            }
            if (IS_VERBOSE_LOGGABLE) {
                logV("finished run method in " + LogTime.getElapsedMillis(startTime));
            }
        }
    }

我们进去onSizeReady看看

    @Override
    public void onSizeReady(int width, int height) {
        //如果对象以及被回收了，那么抛出异常
        stateVerifier.throwIfRecycled();
        synchronized (requestLock) {
            if (IS_VERBOSE_LOGGABLE) {
                logV("Got onSizeReady in " + LogTime.getElapsedMillis(startTime));
            }
            //说明没有设置大小或者没有获取到计算后的大小
            if (status != SingleRequest.Status.WAITING_FOR_SIZE) {
                return;
            }
            status = SingleRequest.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 != SingleRequest.Status.RUNNING) {
                loadStatus = null;
            }
            if (IS_VERBOSE_LOGGABLE) {
                logV("finished onSizeReady in " + LogTime.getElapsedMillis(startTime));
            }
        }
    }

主要的代码是engine.load。

    /**
     * /**
     *  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
     *
     * @param glideContext
     * @param model
     * @param signature
     * @param width
     * @param height
     * @param resourceClass
     * @param transcodeClass
     * @param priority
     * @param diskCacheStrategy
     * @param transformations
     * @param isTransformationRequired
     * @param isScaleOnlyOrNoTransform
     * @param options
     * @param isMemoryCacheable
     * @param useUnlimitedSourceExecutorPool
     * @param useAnimationPool
     * @param onlyRetrieveFromCache
     * @param cb
     * @param callbackExecutor
     * @param <R>
     * @return
     */
    public <R> Engine.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 useAnimationPool,
            boolean onlyRetrieveFromCache,
            ResourceCallback cb,
            Executor callbackExecutor) {
        long startTime = VERBOSE_IS_LOGGABLE ? LogTime.getLogTime() : 0;

        //构造一个key
        EngineKey key =
                keyFactory.buildKey(
                        model,
                        signature,
                        width,
                        height,
                        transformations,
                        resourceClass,
                        transcodeClass,
                        options);

        EngineResource<?> memoryResource;
        synchronized (this) {
            //通过这个key去缓存中看是不是存在资源，loadFromMemory里面会先去活跃资源缓存池中获取，
            // 没有的话再去内存缓存中获取，活跃资源即现在正在被其他组件使用的资源。
            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;
    }

如果找得到资源，那么就回调cb.onResourceReady，不然的话会走waitForExistingOrStartNewJob。

我们进去看一下：

    private <R> Engine.LoadStatus waitForExistingOrStartNewJob(
            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 useAnimationPool,
            boolean onlyRetrieveFromCache,
            ResourceCallback cb,
            Executor callbackExecutor,
            EngineKey key,
            long startTime) {

        //通过key获取EngineJob，EngineJob负责开启线程异步加载。
        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 Engine.LoadStatus(cb, current);
        }

        //没有EngineJob则构建一个
        EngineJob<R> engineJob =
                engineJobFactory.build(
                        key,
                        isMemoryCacheable,
                        useUnlimitedSourceExecutorPool,
                        useAnimationPool,
                        onlyRetrieveFromCache);

        //负责给图片解码等一些复杂操作
        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, callbackExecutor);
        //运行
        engineJob.start(decodeJob);

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

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

如果要从磁盘缓存中去解码的话，就获取diskCacheExecutor，否则就用针对原始资源的一个执行器。

在executor.execute(decodeJob)之后便切换到子线程了，我们到DecodeJob里面去看一下。

  @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.
    GlideTrace.beginSectionFormat("DecodeJob#run(model=%s)", model);
    // Methods in the try statement can invalidate currentFetcher, so set a local variable here to
    // ensure that the fetcher is cleaned up either way.
    DataFetcher<?> localFetcher = currentFetcher;
    try {
      if (isCancelled) {
        notifyFailed();
        return;
      }
      runWrapped();
    } catch (CallbackException e) {
      // If a callback not controlled by Glide throws an exception, we should avoid the Glide
      // specific debug logic below.
      throw e;
    } catch (Throwable t) {
      // Catch Throwable and not Exception to handle OOMs. Throwables are swallowed by our
      // usage of .submit() in GlideExecutor so we're not silently hiding crashes by doing this. We
      // are however ensuring that our callbacks are always notified when a load fails. Without this
      // notification, uncaught throwables never notify the corresponding callbacks, which can cause
      // loads to silently hang forever, a case that's especially bad for users using Futures on
      // background threads.
      if (Log.isLoggable(TAG, Log.DEBUG)) {
        Log.d(
            TAG,
            "DecodeJob threw unexpectedly" + ", isCancelled: " + isCancelled + ", stage: " + stage,
            t);
      }
      // When we're encoding we've already notified our callback and it isn't safe to do so again.
      if (stage != Stage.ENCODE) {
        throwables.add(t);
        notifyFailed();
      }
      if (!isCancelled) {
        throw t;
      }
      throw t;
    } finally {
      // Keeping track of the fetcher here and calling cleanup is excessively paranoid, we call
      // close in all cases anyway.
      if (localFetcher != null) {
        localFetcher.cleanup();
      }
      GlideTrace.endSection();
    }
  }

主要是runWrapped();

  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);
    }
  }

当INITIALIZE或者SWITCH_TO_SOURCE_SERVICE的时候，走runGenerators()。这两种是没有缓存的情况下。

runGenerators()：

  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.
  }

重点：currentGenerator.startNext()。实现startNext方法的有三个：

而我们的currentGenerator是哪一个呢？

回头看runWrapped

case INITIALIZE:
        stage = getNextStage(Stage.INITIALIZE);
        currentGenerator = getNextGenerator();
        runGenerators();

点进去getNextGenerator，结合status的值便可以知道返回的是SourceGenerator。（我们讨论的是初次加载没有缓存的情况）

我们来到SourceGenerator的startNext()方法：

    @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;
    }

如果sourceCacheGenerator 不为null，就调用它的startNext，在里面去获取modelLoader，然后去加载资源，modelLoader即模型加载器，Glide初始化的时候注册了很多模型加载器。

registry
        .append(int.class, InputStream.class, resourceLoaderStreamFactory)
        .append(int.class, ParcelFileDescriptor.class, resourceLoaderFileDescriptorFactory)
        .append(Integer.class, InputStream.class, resourceLoaderStreamFactory)
        .append(Integer.class, ParcelFileDescriptor.class, resourceLoaderFileDescriptorFactory)
        .append(Integer.class, Uri.class, resourceLoaderUriFactory)
        .append(int.class, AssetFileDescriptor.class, resourceLoaderAssetFileDescriptorFactory)
        .append(Integer.class, AssetFileDescriptor.class, resourceLoaderAssetFileDescriptorFactory)
        .append(int.class, Uri.class, resourceLoaderUriFactory)
        .append(String.class, InputStream.class, new DataUrlLoader.StreamFactory<String>())
        .append(Uri.class, InputStream.class, new DataUrlLoader.StreamFactory<Uri>())

如append(Uri.class, InputStream.class, new DataUrlLoader.StreamFactory<Uri>())

将Uri对象转换为InputStream，模型加载器为DataUrlLoader.StreamFactory，也就是说我们加载的时候如果传进来的是一个uri对象，那么最终会被转换为InputStream。

我们不进去看，直接看接下来的代码。

loadData = helper.getLoadData().get(loadDataListIndex++);

loadData里面包含着：

sourceKey：标识这个加载的原始资源的key

alternateKeys：备用的缓存key指向相同的数据

DataFetcher：用来获取没有在缓存中发现的数据（即需要去加载的，modelLoader中都包含着这个）

接下来看：

startNextLoad(loadData);

private void startNextLoad(final LoadData<?> toStart) {
  loadData.fetcher.loadData(
      helper.getPriority(),
      new DataCallback<Object>() {
        @Override
        public void onDataReady(@Nullable Object data) {
          if (isCurrentRequest(toStart)) {
            onDataReadyInternal(toStart, data);
          }
        }
        @Override
        public void onLoadFailed(@NonNull Exception e) {
          if (isCurrentRequest(toStart)) {
            onLoadFailedInternal(toStart, e);
          }
        }
      });
}

loadData.fetcher.loadData：这里是我们真正去加载资源的地方。

点击进去loadeData，发现好多实现了该方法的类。那我们这里的Fetcher究竟是哪一个呢？

首先从名字来看，如果我们加载的是网络资源，那么就是：HttpUrlFetcher。

这个HttpUrlFetcher跟我们的modelLoader是什么关系呢

我们可以看出LoadData在ModelLoader类中。

查看HttpUrlFetcher的调用可以追溯到HttpGlideUrlLoader。

  @Override
  public LoadData<InputStream> buildLoadData(
      @NonNull GlideUrl model, int width, int height, @NonNull Options options) {
    // GlideUrls memoize parsed URLs so caching them saves a few object instantiations and time
    // spent parsing urls.
    GlideUrl url = model;
    if (modelCache != null) {
      url = modelCache.get(model, 0, 0);
      if (url == null) {
        modelCache.put(model, 0, 0, model);
        url = model;
      }
    }
    int timeout = options.get(TIMEOUT);
    return new LoadData<>(url, new HttpUrlFetcher(url, timeout));
  }

我们可以看出HttpGlideUrlLoader实现了ModelLoader的方法，buildLoadData，而在buildLoadData中返回了一个LoadData对象，这个对象传入的就是HttpUrlFetcher。

这个buildLoadData什么时候被调用的呢？

我们回到startNext方法，loadData = helper.getLoadData().get(loadDataListIndex++);的getLoadData()里面：

好，那么我们看HttpUrlFetcher的loadData();

  @Override
  public void loadData(
      @NonNull Priority priority, @NonNull DataCallback<? super InputStream> callback) {
    long startTime = LogTime.getLogTime();
    try {
      InputStream result = loadDataWithRedirects(glideUrl.toURL(), 0, null, glideUrl.getHeaders());
      callback.onDataReady(result);
    } catch (IOException e) {
      if (Log.isLoggable(TAG, Log.DEBUG)) {
        Log.d(TAG, "Failed to load data for url", e);
      }
      callback.onLoadFailed(e);
    } finally {
      if (Log.isLoggable(TAG, Log.VERBOSE)) {
        Log.v(TAG, "Finished http url fetcher fetch in " + LogTime.getElapsedMillis(startTime));
      }
    }
  }

loadDataWithRedirects返回了InputStream，然后callback回调。

我们进去loadDataWithRedirects看可以发现这个方法其实是去请求网络，我们就不细看了。

  private void startNextLoad(final LoadData<?> toStart) {
    loadData.fetcher.loadData(
        helper.getPriority(),
        new DataCallback<Object>() {
          @Override
          public void onDataReady(@Nullable Object data) {
            if (isCurrentRequest(toStart)) {
              onDataReadyInternal(toStart, data);
            }
          }

          @Override
          public void onLoadFailed(@NonNull Exception e) {
            if (isCurrentRequest(toStart)) {
              onLoadFailedInternal(toStart, e);
            }
          }
        });
  }

回调后调用onDataReadyInternal(toStart, data);

且看一下：

@SuppressWarnings("WeakerAccess")
@Synthetic
void onDataReadyInternal(LoadData<?> loadData, Object data) {
  DiskCacheStrategy diskCacheStrategy = helper.getDiskCacheStrategy();
  if (data != null && diskCacheStrategy.isDataCacheable(loadData.fetcher.getDataSource())) {
    dataToCache = data;
    // We might be being called back on someone else's thread. Before doing anything, we should
    // reschedule to get back onto Glide's thread.
    cb.reschedule();
  } else {
    cb.onDataFetcherReady(
        loadData.sourceKey,
        data,
        loadData.fetcher,
        loadData.fetcher.getDataSource(),
        originalKey);
  }
}

我们看这个方法：onDataFetcherReady

查看一下便可以知道会回调DecodeJob中的方法：

  @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 {
      GlideTrace.beginSection("DecodeJob.decodeFromRetrievedData");
      try {
        decodeFromRetrievedData();
      } finally {
        GlideTrace.endSection();
      }
    }
  }

到此我们就完成了加网络资源的过程，接下来就是解码等等的操作了。

我们看decodeFromRetrievedData：

里面有一句：

resource = decodeFromData(currentFetcher, currentData, currentDataSource);

将资源解码成Resource。

我们追踪进去：

decodeFromData -> decodeFromFetcher -> runLoadPath -> path.load -> loadWithExceptionList

loadWithExceptionList里面便开始进行我们的解码操作了。

  private Resource<Transcode> loadWithExceptionList(
      DataRewinder<Data> rewinder,
      @NonNull Options options,
      int width,
      int height,
      DecodePath.DecodeCallback<ResourceType> decodeCallback,
      List<Throwable> exceptions)
      throws GlideException {
    Resource<Transcode> result = null;
    //noinspection ForLoopReplaceableByForEach to improve perf
    for (int i = 0, size = decodePaths.size(); 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;
      }
    }

我们看：

result = path.decode(rewinder, width, height, options, decodeCallback);

  public Resource<Transcode> decode(
      DataRewinder<DataType> rewinder,
      int width,
      int height,
      @NonNull 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, options);
  }

我们看decodeResource -> decodeResourceWithList

   @NonNull
    private Resource<ResourceType> decodeResourceWithList(
            DataRewinder<DataType> rewinder,
            int width,
            int height,
            @NonNull Options options,
            List<Throwable> exceptions)
            throws GlideException {
        Resource<ResourceType> result = null;
        //noinspection ForLoopReplaceableByForEach to improve perf
        //遍历获取解码器
        for (int i = 0, size = decoders.size(); i < size; i++) {
            ResourceDecoder<DataType, ResourceType> decoder = decoders.get(i);
            try {
                DataType data = rewinder.rewindAndGet();
                //该解码器是否可以解码该数据
                if (decoder.handles(data, options)) {
                    //获取数据
                    data = rewinder.rewindAndGet();
                    //解码
                    result = decoder.decode(data, width, height, options);
                }
                // Some decoders throw unexpectedly. If they do, we shouldn't fail the entire load path, but
                // instead log and continue. See #2406 for an example.
            } catch (IOException | RuntimeException | OutOfMemoryError 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;
    }

获取对应的解码器后用decode方法进行解码。Glide初始化的时候便注册了一大堆解码器，如：

.append(
            Registry.BUCKET_GIF,
            InputStream.class,
            GifDrawable.class,
            new StreamGifDecoder(imageHeaderParsers, byteBufferGifDecoder, arrayPool))

对于Gif类型，将InputStream解码为GifDrawable，解码器为StreamGifDecoder

接下来我们回到这个方法：

  public Resource<Transcode> decode(
      DataRewinder<DataType> rewinder,
      int width,
      int height,
      @NonNull 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, options);
  }

Resource<ResourceType> transformed = callback.onResourceDecoded(decoded);

这个方法里面便是对我们的资源进行了变换。

往下追溯到这个方法：

  @Synthetic
  @NonNull
  <Z> Resource<Z> onResourceDecoded(DataSource dataSource, @NonNull Resource<Z> decoded) {
    @SuppressWarnings("unchecked")
    Class<Z> resourceSubClass = (Class<Z>) decoded.get().getClass();
    Transformation<Z> appliedTransformation = null;
    Resource<Z> transformed = decoded;
    if (dataSource != DataSource.RESOURCE_DISK_CACHE) {
      appliedTransformation = decodeHelper.getTransformation(resourceSubClass);
      transformed = appliedTransformation.transform(glideContext, decoded, width, height);
    }
    // TODO: Make this the responsibility of the Transformation.
    if (!decoded.equals(transformed)) {
      decoded.recycle();
    }

    final EncodeStrategy encodeStrategy;
    final ResourceEncoder<Z> encoder;
    if (decodeHelper.isResourceEncoderAvailable(transformed)) {
      encoder = decodeHelper.getResultEncoder(transformed);
      encodeStrategy = encoder.getEncodeStrategy(options);
    } else {
      encoder = null;
      encodeStrategy = EncodeStrategy.NONE;
    }

    Resource<Z> result = transformed;
    boolean isFromAlternateCacheKey = !decodeHelper.isSourceKey(currentSourceKey);
    if (diskCacheStrategy.isResourceCacheable(
        isFromAlternateCacheKey, dataSource, encodeStrategy)) {
      if (encoder == null) {
        throw new Registry.NoResultEncoderAvailableException(transformed.get().getClass());
      }
      final Key key;
      switch (encodeStrategy) {
        case SOURCE:
          key = new DataCacheKey(currentSourceKey, signature);
          break;
        case TRANSFORMED:
          key =
              new ResourceCacheKey(
                  decodeHelper.getArrayPool(),
                  currentSourceKey,
                  signature,
                  width,
                  height,
                  appliedTransformation,
                  resourceSubClass,
                  options);
          break;
        default:
          throw new IllegalArgumentException("Unknown strategy: " + encodeStrategy);
      }

      LockedResource<Z> lockedResult = LockedResource.obtain(transformed);
      deferredEncodeManager.init(key, encoder, lockedResult);
      result = lockedResult;
    }
    return result;
  }

首先获取Transformation，然后调用transform方法进行变换处理。

返回resource之后回到decodeFromRetrievedData方法。

  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 {
      resource = decodeFromData(currentFetcher, currentData, currentDataSource);
    } catch (GlideException e) {
      e.setLoggingDetails(currentAttemptingKey, currentDataSource);
      throwables.add(e);
    }
    if (resource != null) {
      notifyEncodeAndRelease(resource, currentDataSource);
    } else {
      runGenerators();
    }
  }

我们沿着方法进入notifyEncodeAndRelease ---> notifyComplete ----> onResourceReady ------>  notifyCallbacksOfResult

。。。。算了写的到这里好累，接下来不写了自己看，就是去设置资源。