Glide源码阅读（一）

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Glide是一个图片加载框架，其他的图片加载框架还有UniversalImageLoader，Picasso，Fresco等，个人还是比较喜欢Glide这个框架的。Glide致力于打造更好的列表图片滑动体验，就如他的名字Glide（滑翔）一样顺畅，Glide还支持加载video，Gif，SVG格式。而且Glide会与你的Activity/Fragment绑定相关的生命周期，有自己的缓存策略，这样就让图片加载操作变得非常简单。本文的Glide源码是4.0的版本（我本来以为是3.7.0来着，后来一看有些代码对不上……)。

准备工作

如果你还没有使用过glide，可以通过下面两句话引入glide：

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compile 'com.github.bumptech.glide:glide:3.7.0'
compile 'com.android.support:support-v4:19.1.0'

编译源码：
在阅读源码的时候，难免希望自己能加上几个log或者自己加点注释之类的来方便自己阅读。可以通过如下方式编译glide源码：

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git clone git@github.com:bumptech/glide.git # use https://github.com/bumptech/glide.git if "Permission Denied"
cd glide
git submodule init && git submodule update
./gradlew jar

不过说实话，我按这个流程操作了几次才成功。编译成功之后你就可以运行sample和尽情的修改glide源码了。

如何打开Glide的log，这个官方仓库里有介绍，直接放上链接：Debugging and Error Handling

从最简流程切入读源码

我们用Glide加载一张图，最简单的代码是怎样的呢？

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Glide.with(Activity).load(url).into(imageview);

因为流式api，整个调用流程显得非常简洁、简单，但是这短短的几个方法调用里，涉及到的东西却是非常之多的，所以看累了的话，可以起来走走，喝点水……

在正式看代码之前，先上一下Glide的设计图：
设计图
这图也不是我画的，我看到了几篇文都有这图，这图画得很好，文末会给出参考资料链接。

上面的东西，刚接触肯定是很陌生的。不过没事，大概知道有这么些个东西就好了。那么正式开始看代码吧，首先是Glide.with()，点进去看看：

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public static RequestManager with(Context context) {
  RequestManagerRetriever retriever = RequestManagerRetriever.get();
  return retriever.get(context);
}

public static RequestManager with(Activity activity) {
  RequestManagerRetriever retriever = RequestManagerRetriever.get();
  return retriever.get(activity);
}

public static RequestManager with(FragmentActivity activity) {
  RequestManagerRetriever retriever = RequestManagerRetriever.get();
  return retriever.get(activity);
}

public static RequestManager with(android.app.Fragment fragment) {
  RequestManagerRetriever retriever = RequestManagerRetriever.get();
  return retriever.get(fragment);
}

public static RequestManager with(Fragment fragment) {
  RequestManagerRetriever retriever = RequestManagerRetriever.get();
  return retriever.get(fragment);
}

可以看到with有多个重载方法，适配了各种可能出现的情况。 继续追踪源码：

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public RequestManager get(Context context) {
  if (context == null) {
    throw new IllegalArgumentException("You cannot start a load on a null Context");
  } else if (Util.isOnMainThread() && !(context instanceof Application)) {
    if (context instanceof FragmentActivity) {
      return get((FragmentActivity) context);
    } else if (context instanceof Activity) {
      return get((Activity) context);
    } else if (context instanceof ContextWrapper) {
      return get(((ContextWrapper) context).getBaseContext());
    }
  }

  return getApplicationManager(context);
}

public RequestManager get(FragmentActivity activity) {
  if (Util.isOnBackgroundThread()) {
    // 不在主线程
    return get(activity.getApplicationContext());
  } else {
    assertNotDestroyed(activity);
    FragmentManager fm = activity.getSupportFragmentManager();
    return supportFragmentGet(activity, fm, null);
  }
}

public RequestManager get(Fragment fragment) {
  if (fragment.getActivity() == null) {
    throw new IllegalArgumentException(
        "You cannot start a load on a fragment before it is attached");
  }
  if (Util.isOnBackgroundThread()) {
      // 不在主线程
    return get(fragment.getActivity().getApplicationContext());
  } else {
    FragmentManager fm = fragment.getChildFragmentManager();
    return supportFragmentGet(fragment.getActivity(), fm, fragment);
  }
}

public RequestManager get(Activity activity) {
    // 不在主线程
  if (Util.isOnBackgroundThread() || Build.VERSION.SDK_INT < Build.VERSION_CODES.HONEYCOMB) {
    return get(activity.getApplicationContext());
  } else {
    assertNotDestroyed(activity);
    android.app.FragmentManager fm = activity.getFragmentManager();
    return fragmentGet(activity, fm, null);
  }
}

先不看参数是context的方法，看一下get(FragmentActivity activity)这个方法，首先判断是否在主线程，如果是的话调用supportFragmentGet方法，看下这个方法的代码：

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RequestManager supportFragmentGet(Context context, FragmentManager fm, Fragment parentHint) {
  SupportRequestManagerFragment current = getSupportRequestManagerFragment(fm, parentHint);
  RequestManager requestManager = current.getRequestManager();
  if (requestManager == null) {
    // TODO(b/27524013): Factor out this Glide.get() call.
    Glide glide = Glide.get(context);
    requestManager =
        new RequestManager(glide, current.getLifecycle(), current.getRequestManagerTreeNode());
    current.setRequestManager(requestManager);
  }
  return requestManager;
}

这个方法里有一个SupportRequestManagerFragment（以下简称SRMFragment，如果参数是Activity或者app包中的Fragment，则是RequestManagerFragment）。这个SRMFragment是一个不可见的Fragment，用来和你的Fragment/Activity的生命周期关联起来。如何关联呢？通过ChildFragmentManager/FragmentManager，将这个Fragment添加到Fragment/Activity中，最终将生命周期传递到RequestManager中，让RequestManager对不同的状态做相应的处理。对于内存较低的情况，SRMFragment也有相应的周期。关于SRMFragment的简单介绍便到此为止。接着获取Glide和RequestManager对象，首先是glide：

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public static Glide get(Context context) {
  if (glide == null) {
    synchronized (Glide.class) {
      if (glide == null) {
        Context applicationContext = context.getApplicationContext();
        List<GlideModule> modules = new ManifestParser(applicationContext).parse();

        GlideBuilder builder = new GlideBuilder(applicationContext);
        for (GlideModule module : modules) {
          module.applyOptions(applicationContext, builder);
        }
        glide = builder.createGlide();
        for (GlideModule module : modules) {
          module.registerComponents(applicationContext, glide.registry);
        }
      }
    }
  }

  return glide;
}

很明显是单例的写法，采用双重校验锁的方式。接着是构造RequestManager对象。在Glide中，加载图片是以Request的形式交给Engine去处理的。而RequestManager是为Glide管理和开启请求的类，可以通过Activity/Fragment/Connectivity（网络连接监听）的生命周期方法进行stop，start，restart。

上面有一个参数为Context的get方法略过了，但是其实跟上面介绍的也差不多，只不过如果不是FragmentActivity、Activity和ContextWrapper的对象的引用，则会调用getApplicationManager方法，看一下这个方法：

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private RequestManager getApplicationManager(Context context) {
  // Either an application context or we're on a background thread.
  if (applicationManager == null) {
    synchronized (this) {
      if (applicationManager == null) {
        // Normally pause/resume is taken care of by the fragment we add to the fragment or
        // activity. However, in this case since the manager attached to the application will not
        // receive lifecycle events, we must force the manager to start resumed using
        // ApplicationLifecycle.

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

  return applicationManager;
}

可以看到也是获取了Glide和RequestManager对象，上面的注释比较简单就不翻译了，简单的解释了用这个context的生命周期的问题。至此简单的分析了Glide.with()方法，这个方法将Glide的生命周期与我们的程序相关联，让我们无需考虑各种复杂的情况。Glide.with()返回的是一个RequestManager方法，接下来就是分析Glide.with().load()了，很显然在RequestManager内：

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public RequestBuilder<Drawable> load(@Nullable Object model) {
    return asDrawable().load(model);
}

我就是在这发现我看的好像是假的源码，我从glide仓库clone编译的源码，发现跟我原来项目里引用的源码有些不一样，后来看了一下……果然版本不一样：

4.0

在3.7.0的源码中有好几个load的重载方法，但是在最新的代码中只有这一个load方法，更多的load的重载方法在RequestBuilder中了，大概是想让各个类的职责更加清晰一点吧。那么就进RequestBuilder看看几个load方法吧：

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public RequestBuilder load(@Nullable Object model) {
 return loadGeneric(model);
}
public RequestBuilder load(@Nullable String string) {
 return loadGeneric(string);
}
public RequestBuilder load(@Nullable Uri uri) {
 return loadGeneric(uri);
}
public RequestBuilder load(@Nullable File file) {
 return loadGeneric(file);
}
public RequestBuilder load(@Nullable Integer resourceId) {
 return loadGeneric(resourceId).apply(signatureOf(ApplicationVersionSignature.obtain(context)));
}
@Deprecated
public RequestBuilder load(@Nullable URL url) {
 return loadGeneric(url);
}
public RequestBuilder load(@Nullable byte[] model) {
 return loadGeneric(model).apply(signatureOf(new ObjectKey(UUID.randomUUID().toString()))
     .diskCacheStrategy(DiskCacheStrategy.NONE).skipMemoryCache(true /*skipMemoryCache*/));
}

这么多重载方法，最终都调用了loadGeneric方法：

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

仅仅是设置了两个字段的值，那么继续看into方法，还是在RequestBuilder中：

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public Target<TranscodeType> into(ImageView view) {
    Util.assertMainThread();
    Preconditions.checkNotNull(view);

    if (!requestOptions.isTransformationSet()
            && requestOptions.isTransformationAllowed()
            && view.getScaleType() != null) {
        if (requestOptions.isLocked()) {
            requestOptions = requestOptions.clone();
        }
        // 根据ImageView的ScaleType配置requestOptions
        switch (view.getScaleType()) {
            case CENTER_CROP:
                requestOptions.optionalCenterCrop(context);
                break;
            case CENTER_INSIDE:
                requestOptions.optionalCenterInside(context);
                break;
            case FIT_CENTER:
            case FIT_START:
            case FIT_END:
                requestOptions.optionalFitCenter(context);
                break;
            //$CASES-OMITTED$
            default:
                // Do nothing.
        }
    }
    return into(context.buildImageViewTarget(view, transcodeClass));
}

根据ImageView的scaleType进行了一些配置，这里通过glideContext生成了一个ViewTarget。在Glide中Target是资源加载的目标，最后调用了into重载方法：

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

    Request previous = target.getRequest();

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

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

    return target;
}

调用了buildRequest方法，看下这个方法和这个方法调用的方法：

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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) {
    // 缩略图请求
    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;
        if (DEFAULT_ANIMATION_OPTIONS.equals(thumbTransitionOptions)) {
            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);
        BaseRequestOptions<?> 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);
    }
}

buildRequest创建了请求，如果配置了缩略图请求，那么会生成一个ThumbnailRequestCoordinator请求。这个请求内部包含了一个FullRequest和ThumbnailRequest。如果没有配置，则通过obtainRequest方法生成一个SingleRequest对象返回。

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private Request obtainRequest(Target<TranscodeType> target,
                              BaseRequestOptions<?> 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());
}

看完了buildRequest方法，接着看requestManager.track(target,requst)，这个方法调用了RequestTrack的runRequest方法：

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

public void runRequest(Request request) {
  requests.add(request);
  if (!isPaused) {
    request.begin();
  } else {
    pendingRequests.add(request);
  }
}

request.biegin调用的是SingleRequest的begin方法，begin调用了onSizeReady方法而onSizeReady又调用了engine.load()方法。至此，终于摸到了真正加载的门槛了：

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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,
    Options options,
    boolean isMemoryCacheable,
    boolean useUnlimitedSourceExecutorPool,
    boolean onlyRetrieveFromCache,
    ResourceCallback cb) {
  Util.assertMainThread();
  long startTime = LogTime.getLogTime();
  // 创建key，资源的唯一标识
  EngineKey key = keyFactory.buildKey(model, signature, width, height, transformations,
      resourceClass, transcodeClass, options);
  // 内存缓存中读取数据
  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;
  }

  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;
  }
  // 根据key获取缓存的job
  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);
  }
  // 创建job
  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,
      onlyRetrieveFromCache,
      options,
      engineJob);
  jobs.put(key, engineJob);
  // 放入线程池，执行
  engineJob.addCallback(cb);
  engineJob.start(decodeJob);

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

可以看到首先从内存缓存中获取资源，其次从存活的资源中加载缓存，这么做可以提高命中率。内存缓存使用的是LruResourceCache继承自LruCache（lru，最近最少使用），当然了，这个LruCache也是glide内部自己实现的一个，现在还不必深入去看怎么实现的知道就行。如果loadFromCache命中，那么会将资源放入activeResources中，同时将资源从LruCache中移除。命中后会直接调用ResourceCallback回调方法onResourceReady，而最终又会调用target的onResourceReady方法，继续看下去就会发现最终的实现类调用了view.setImageBitmap(BitmapImageViewTarget)或者view.setImageDrawable(DrawableImageViewTarget)。另外一个内存缓存用弱引用缓存当前正在使用的资源，回调方式和之前的LruCache是一样的，不做更多的介绍了。

前面说的都是命中的情况，接着看未命中的。内存中读取数据都没有命中的话，则会生成EngineJob和DecodeJob。EngineJob的职责是调度DecodeJob，添加，移除资源回调，并notify回调。DecodeJob负责从缓存资源或者原始数据中读取资源，Glide中的脏累活基本都是这个DecodeJob干的。回到代码，最后调用了engineJob.start方法，看代码：

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

这个GlideExecutor是一个线程池，而DecodeJob是实现了Runnable接口的类，所以看一下DecodeJob的run方法：

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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.
  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;
    }
  }
}

private void runWrapped() {
   switch (runReason) {
     // 初始化 获取下一个阶段状态
    case INITIALIZE:
      stage = getNextStage(Stage.INITIALIZE);
      currentGenerator = getNextGenerator();
      // 运行load数据
      runGenerators();
      break;
    case SWITCH_TO_SOURCE_SERVICE:
      runGenerators();
      break;
    case DECODE_DATA:
      // 处理已经load到的数据
      decodeFromRetrievedData();
      break;
    default:
      throw new IllegalStateException("Unrecognized run reason: " + runReason);
  }

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

private Stage getNextStage(Stage current) {
  switch (current) {
    case INITIALIZE:
      return diskCacheStrategy.decodeCachedResource()
          ? Stage.RESOURCE_CACHE : getNextStage(Stage.RESOURCE_CACHE);
    case RESOURCE_CACHE:
      return diskCacheStrategy.decodeCachedData()
          ? Stage.DATA_CACHE : getNextStage(Stage.DATA_CACHE);
    case DATA_CACHE:
      // Skip loading from source if the user opted to only retrieve the resource from cache.
      return onlyRetrieveFromCache ? Stage.FINISHED : Stage.SOURCE;
    case SOURCE:
    case FINISHED:
      return Stage.FINISHED;
    default:
      throw new IllegalArgumentException("Unrecognized stage: " + current);
  }
}

主要的加载逻辑就在这几个方法里。这个runReason初始值就是INITIALIZE，进入这个case首先调用了getNextStage方法，获取到Stage然后根据Stage获取相应的Generator，最后执行Generator。一共有三种Generator：

  • ResourceCacheGenerator：从处理过的缓存加载数据
  • DataCacheGenerator：从原始缓存加载数据
  • SourceGenerator：从数据源请求数据

上面提到执行Generator，会调用currentGenerator.startNext方法，这里主要看一下SourceGenerator的starNext方法：

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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;
}

前面老长一段先不看了，第一次加载肯定是null，直接看后面，首先是getLoadData：

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List<LoadData<?>> getLoadData() {
  if (!isLoadDataSet) {
    isLoadDataSet = true;
    loadData.clear();
    List<ModelLoader<Object, ?>> modelLoaders = glideContext.getRegistry().getModelLoaders(model);
    int size = modelLoaders.size();
    for (int i = 0; i < size; i++) {
      ModelLoader<Object, ?> modelLoader = modelLoaders.get(i);
      LoadData<?> current =
          modelLoader.buildLoadData(model, width, height, options);
      if (current != null) {
        loadData.add(current);
      }
    }
  }
  return loadData;
}

通过glide的上下文获取到所有能处理Model类型的注册过的ModelLoader，遍历这些ModelLoader，通过buildLoadData生成LoadData，最终返回一个LoadData的列表。loadData咋来的弄清楚了，接着看重点了，loadData，这一看就是加载数据的方法了。由于Glide适配了多种网络请求框架，这基本都是用的接口来解耦，看起来真的挺蛋疼的……关于fetcher的实现类就不去分析了，直接看数据加载成功的回调就好。上面的代码是将SourceGenertor自身传递进去作为回调，直接看回调方法：

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public void onDataReady(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);
  }
}

这里调用了reschedule，重新调度当前任务，这个时候我们前面忽略的代码就起作用了，会进行写缓存的操作。这之后会切换加载的策略，最终获取数据成功时会调用onDataFecherReady，这个方法又会调用decodeFromRetrieveData方法，这个方法又调用了decodeFromData，这个方法又调用了decodeFromFetcher（有完没完了……）

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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 {
  DataRewinder<Data> rewinder = glideContext.getRegistry().getRewinder(data);
  try {
    return path.load(rewinder, options, width, height,
        new DecodeCallback<ResourceType>(dataSource));
  } finally {
    rewinder.cleanup();
  }
}

看到这我真是无语了，又是一堆看不懂的……我能怎么办啊，我也很绝望啊！不过行百里者半九十（虽然可能连60都没……），咬咬牙就挺过去了~

里面涉及到的东西，一个一个的简介：

  • LoadPath：根据给定的数据类型的DataFetcher尝试获取数据，然后尝试通过一个或多个decodePath进行decode
  • DecodePath：根据指定的数据类型对resource进行decode和transcode
  • DataRewinder：负责将流转换成数据类型

最终在run方法里开始了最终的加载，传入的参数是DecodeJob的内部类，实现了DecodePath.DecodeCallback，看一下最终的回调方法：

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public Resource<Z> onResourceDecoded(Resource<Z> decoded) {
  Class<Z> resourceSubClass = getResourceClass(decoded);
  Transformation<Z> appliedTransformation = null;
  Resource<Z> transformed = decoded;
  if (dataSource != DataSource.RESOURCE_DISK_CACHE) {
    appliedTransformation = decodeHelper.getTransformation(resourceSubClass);
    // 资源转换
    transformed = appliedTransformation.transform(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;
    if (encodeStrategy == EncodeStrategy.SOURCE) {
      key = new DataCacheKey(currentSourceKey, signature);
    } else if (encodeStrategy == EncodeStrategy.TRANSFORMED) {
      key = new ResourceCacheKey(currentSourceKey, signature, width, height,
          appliedTransformation, resourceSubClass, options);
    } else {
      throw new IllegalArgumentException("Unknown strategy: " + encodeStrategy);
    }

    LockedResource<Z> lockedResult = LockedResource.obtain(transformed);
    // 根据缓存策略初始化
    deferredEncodeManager.init(key, encoder, lockedResult);
    result = lockedResult;
  }
  return result;
}

最后还是回到了decodeFromRetrievedData：

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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);
    exceptions.add(e);
  }
  if (resource != null) {
    notifyEncodeAndRelease(resource, currentDataSource);
  } else {
    runGenerators();
  }
}

notifyEncodeAndRelease中处理了对处理过的图片的缓存操作。当缓存完成后（如果有需要的话）就通过回调告诉外面加载完成了。至此，整个加载过程完成。

后记

这里只是简单的过了一遍最简的调用所经历的过程，但是这中间涉及到的东西真的很多。Glide用起来还是挺方便的，由于需要考虑各种情况和适配不同的网络请求框架，内部采用了各种接口解耦，读的时候这也算是比较痛苦的一点吧，往往得往前追溯几个类你才能知道这个实现类是什么。不过Glide还是很值得我们去学习一下，阅读一下的，以后我也会更加深入的去阅读Glide。

参考资料

Glide源码之生命周期

Glide源码分析

Glide官方仓库

如何调试Glide加载图片

Glide源码导读