一、客戶端代碼介紹

這里分兩個部分：

1）添加webp動圖解碼組件

@GlideModule
public class WebpGlideLibraryModule extends LibraryGlideModule {

    @Override
   public void registerComponents(Context context, Glide glide, Registry registry) {
       final BitmapPool bitmapPool = glide.getBitmapPool();
       final ArrayPool arrayPool = glide.getArrayPool();
       /* animate webp decoders */
      ByteBufferWebpDecoder byteBufferWebpDecoder = new ByteBufferWebpDecoder(context, arrayPool, bitmapPool);
...
       /* Animated webp images */
       registry.prepend(InputStream.class, WebpDrawable.class, new StreamWebpDecoder(byteBufferWebpDecoder, arrayPool))
...
   }
}

ByteBufferWebpDecoder是最終webp動圖資源解碼器

2）Glide加載webp動圖url

Glide.with(mContext).load(Constants.dynamicWebpUrl).into(mImageView);

二、Glide加載網絡webp數據轉換流程

先給出Glide加載webp動圖的完整調用棧：

    com.bumptech.glide.integration.webp.decoder.ByteBufferWebpDecoder.decode:33
    com.bumptech.glide.load.engine.DecodePath.decodeResourceWithList:72
    com.bumptech.glide.load.engine.DecodePath.decodeResource:55
    com.bumptech.glide.load.engine.DecodePath.decode:45
    com.bumptech.glide.load.engine.LoadPath.loadWithExceptionList:62
    com.bumptech.glide.load.engine.LoadPath.load:47
    com.bumptech.glide.load.engine.DecodeJob.runLoadPath:510
    com.bumptech.glide.load.engine.DecodeJob.decodeFromFetcher:475
    com.bumptech.glide.load.engine.DecodeJob.decodeFromData:461
    com.bumptech.glide.load.engine.DecodeJob.decodeFromRetrievedData:413
    com.bumptech.glide.load.engine.DecodeJob.onDataFetcherReady:382
    com.bumptech.glide.load.engine.SourceGenerator.onDataFetcherReady:139
    com.bumptech.glide.load.engine.DataCacheGenerator.onDataReady:99
    com.bumptech.glide.load.model.ByteBufferFileLoader$ByteBufferFetcher.loadData:74
    com.bumptech.glide.load.engine.DataCacheGenerator.startNext:79
    com.bumptech.glide.load.engine.SourceGenerator.startNext:53
    com.bumptech.glide.load.engine.DecodeJob.runGenerators:305
    com.bumptech.glide.load.engine.DecodeJob.runWrapped:275

    com.bumptech.glide.load.engine.DecodeJob.run:236
    java.util.concurrent.ThreadPoolExecutor.runWorker:1167
    java.util.concurrent.ThreadPoolExecutor$Worker.run:641
    com.bumptech.glide.load.engine.EngineJob.start:118
    com.bumptech.glide.load.engine.Engine.load:233
    com.bumptech.glide.request.SingleRequest.onSizeReady:432
    com.bumptech.glide.request.target.ViewTarget$SizeDeterminer.getSize:389
    com.bumptech.glide.request.target.ViewTarget.getSize:221
    com.bumptech.glide.request.SingleRequest.begin:257
    com.bumptech.glide.manager.RequestTracker.runRequest:44
    com.bumptech.glide.RequestManager.track:616
    com.bumptech.glide.RequestBuilder.into:651
    com.bumptech.glide.RequestBuilder.into:711

整個流程主要分三塊：

• 封裝并發起一個圖片加載Request
• 圖片資源獲取：內存緩存、磁盤文件緩存、網絡請求等。
• 圖片資源解析：對某些數據源進行資源解碼。

2.1 封裝并發起一個圖片加載Request

load ：通過RequestManager加載一個String 類型的model。
into：加載一個ImageView的目標控件作為target，然后通過RequestBuilder開始數據處理流程。

2.2 圖片資源獲取

EngineJob以前的流程非常簡單明確，這里著重看下DecodeJob部分的處理流程：

SourceGenerator.java

@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;
}

這里有個ModelLoader-LoadData-DataFetcher關系需要捋一下：

DecodeHelper.java

List<LoadData<?>> getLoadData() {
  if (!isLoadDataSet) {
    isLoadDataSet = true;
   loadData.clear();
   List<ModelLoader<Object, ?>> modelLoaders = glideContext.getRegistry().getModelLoaders(model);
   //noinspection ForLoopReplaceableByForEach to improve perf
   for (int i = 0, size = modelLoaders.size(); 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;
}

這里modelLoaders是在Registry中由ModelLoaderRegistry來獲取所有的models。這里model對應ByteBufferFileLoader，由他執行buildLoadData。

ByteBufferFileLoader.java

@Override
public LoadData<ByteBuffer> buildLoadData(@NonNull File file, int width, int height,
   @NonNull Options options) {
  return new LoadData<>(new ObjectKey(file), new ByteBufferFetcher(file));
}

而LoadData是ModelLoader的內部類，它的屬性包括一個DataFetcher，它就是最終加載數據的地方。

class LoadData<Data> {
  public final Key sourceKey;
  public final List<Key> alternateKeys;
  public final DataFetcher<Data> fetcher;
...
}

那么總結一下：首先是獲取對應數據源類型的ModelLoader,ModelLoader初始化一個LoadData，然后LoadData通過內部關聯的DataFetcher來正真去執行加載數據的操作！

這里很顯然對應的DataFetcher實例是LoadData初始化時傳入的ByteBufferFetcher。回到SourceGenerator的startNext方法，最終調用ByteBufferFetcher的loadData。

當然這里數據獲取的方式有很多種，有網絡請求、有磁盤文件獲取等等:

DataFetcher子類

這里DataFetcher也可以自定義，舉例：
Glide網絡請求默認使用的是HttpUrlConnection，這里可以替換為Okhttp請求。
做法是：

public class OkHttpUrlLoader implements ModelLoader<GlideUrl, InputStream> {
  private final Call.Factory client;
  // Public API.
  @SuppressWarnings("WeakerAccess")
  public OkHttpUrlLoader(@NonNull Call.Factory client) {
    this.client = client;
  }

  @Override
  public boolean handles(@NonNull GlideUrl url) {
    return true;
  }

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

創建對應的ModelLoader，并且自定義一個OkHttpStreamFetcher來實現Okhttp網絡請求功能，同時通過Registry去替換組件：

@Override
public void registerComponents(@NonNull Context context, @NonNull Glide glide, @NonNull Registry registry) {
        registry.replace(GlideUrl.class, InputStream.class, new OkHttpUrlLoader.Factory());
}

這里基本上圖片資源獲取就介紹完了。

2.3 圖片資源解析

資源獲取成功后，會通過callback.onDataReady(result)進行回調，這個callback是通過參數傳入的loadData.fetcher.loadData(helper.getPriority(), this),這里的this就是SourceGenerator

SourceGenerator.java

@Override
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);
  }
}

這個cb是FetcherReadyCallback,在SourceGenerator初始化時傳入

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

從SourceGenerator初始化出追這個cb，就是DecodeJob,這樣最終獲取的數據源通過兩層callback傳入了DecodeJob，準備進行解碼處理。

DecodeJob.java
onDataFetcherReady() -> decodeFromRetrievedData() ->decodeFromData()->decodeFromFetcher()
這個流程沒什么可分析的，直接到decodeFromFetcher

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);
}

DecodeHelper.java

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

這里很顯然又是去Registry拿的。如果有的話最終會返回一個LoadPath對象

new LoadPath<>(dataClass, resourceClass, transcodeClass, decodePaths, throwableListPool);

繼續往下走：

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();
  }
}

這里調用LoadPath的load方法，該方法調用loadWithExceptionList

LoadPath.java

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;
   }
  }

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

這里獲取了一個decodePath，然后調用它的decode方法去執行具體的解碼工作了。debug一個看看這里path是什么

這里就是文章開頭說的客戶端自定義添加webp動圖解碼組件。具體解碼處理留到下一篇分析。

好的，最后再來簡單總結下整個流程：

上層Glide作為客戶端調用的主入口，通過RequestManager以及RequestBuilder收集圖片源model以及目標控件target，創建一個對應的request交給EngineJob線程池去處理這個request，DecodeJob作為一個執行線程接收這個request任務，然后交由Generator選擇處理方式，包括緩存還是網絡請求等，而它通過獲取Registry注冊的對應的ModelLoader-LoadData-DataFetcher最終去獲取圖片數據。然后由Generator回調給DecodeJob，DecodeJob通過向Registry獲取對應的LoadPath,最終匹配到對應的解碼器DecodePath去執行解碼操作。

后面就是將通過onSourceReady層層回調返回到SingleRequest，最終為目標控件設置webp動圖資源。

整個數據轉換流程為：