Glide源码流程分析（二）

到这里我们就完完整整的创建了一个request，接下开我们就看看如何运行request的：

RequestTracker#runRequest

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

可以看见我这里先判断有没有暂停，如果暂停了的话就加入暂停的队列里，没有的话就掉用request.begin()方法。
继续查看：

GenericRequest#begin()：

@Override
public void begin() {
    startTime = LogTime.getLogTime();
    if (model == null) {
        onException(null);
        return;
    }
    status = Status.WAITING_FOR_SIZE;
    if (Util.isValidDimensions(overrideWidth, overrideHeight)) {
        onSizeReady(overrideWidth, overrideHeight);
    } else {
        target.getSize(this);
    }
    if (!isComplete() && !isFailed() && canNotifyStatusChanged()) {
        target.onLoadStarted(getPlaceholderDrawable());
    }
    if (Log.isLoggable(TAG, Log.VERBOSE)) {
        logV("finished run method in " + LogTime.getElapsedMillis(startTime));
    }
}

首先我们的知道所谓的model其实就是URL，如果穿入的是为空的话我们就调用onException(null);方法，我们接着查看这个方法：

private void setErrorPlaceholder(Exception e) {
    if (!canNotifyStatusChanged()) {
        return;
    }
    Drawable error = model == null ? getFallbackDrawable() : null;
    if (error == null) {
      error = getErrorDrawable();
    }
    if (error == null) {
        error = getPlaceholderDrawable();
    }
    target.onLoadFailed(e, error);
}

可以看见我们会去尝试获得一个的错误图片，如果没有就是占位的图片，最后调用target.onLoadFailed方法：

ImageViewTarget#onLoadFailed：


@Override
    public void onLoadStarted(Drawable placeholder) {
        view.setImageDrawable(placeholder);
    }

    @Override
    public void onLoadFailed(Exception e, Drawable errorDrawable) {
        view.setImageDrawable(errorDrawable);
    }

可以错误就直接设定错误图了。然后看见开始的时候我们会调用占位图，这个在我们的begin方法时候就调用onLoadStarted方法了，这里错误就算差不多讲完了，我们回到begin，为了很清楚我还贴了下来：

@Override
public void begin() {
    startTime = LogTime.getLogTime();
    if (model == null) {
        onException(null);
        return;
    }
    status = Status.WAITING_FOR_SIZE;
    if (Util.isValidDimensions(overrideWidth, overrideHeight)) {
        onSizeReady(overrideWidth, overrideHeight);
    } else {
        target.getSize(this);
    }
    if (!isComplete() && !isFailed() && canNotifyStatusChanged()) {
        target.onLoadStarted(getPlaceholderDrawable());
    }
    if (Log.isLoggable(TAG, Log.VERBOSE)) {
        logV("finished run method in " + LogTime.getElapsedMillis(startTime));
    }
}

如果我们调用API指定了宽高那么onSizeReady(overrideWidth, overrideHeight);方法来给图片设定一个固定的宽高
如果没有我们就会调用target.getSize(this);方法来根据空间的宽高指定图片应有的宽高，这里不做具体分析我们知道最后调用的是onSizeReady方法就行了。

GenericRequest #onSizeReady：

  @Override
    public void onSizeReady(int width, int height) {
        if (Log.isLoggable(TAG, Log.VERBOSE)) {
            logV("Got onSizeReady in " + LogTime.getElapsedMillis(startTime));
        }
        if (status != Status.WAITING_FOR_SIZE) {
            return;
        }
        status = Status.RUNNING;

        width = Math.round(sizeMultiplier * width);
        height = Math.round(sizeMultiplier * height);

        ModelLoader<A, T> modelLoader = loadProvider.getModelLoader();
        final DataFetcher<T> dataFetcher = modelLoader.getResourceFetcher(model, width, height);

        if (dataFetcher == null) {
            onException(new Exception("Failed to load model: \'" + model + "\'"));
            return;
        }
        ResourceTranscoder<Z, R> transcoder = loadProvider.getTranscoder();
        if (Log.isLoggable(TAG, Log.VERBOSE)) {
            logV("finished setup for calling load in " + LogTime.getElapsedMillis(startTime));
        }
        loadedFromMemoryCache = true;
        loadStatus = engine.load(signature, width, height, dataFetcher, loadProvider, transformation, transcoder,
                priority, isMemoryCacheable, diskCacheStrategy, this);
        loadedFromMemoryCache = resource != null;
        if (Log.isLoggable(TAG, Log.VERBOSE)) {
            logV("finished onSizeReady in " + LogTime.getElapsedMillis(startTime));
        }
    }

我们查看loadProvider.getModelLoader();是不是之前貌似在哪遇到的？没错就是在loadGeneric方法中构建的两个下载者，那loadProvider又是什么呢？其实在我们创建DrawableTypeRequest的时候我们忽略了一点~

DrawableTypeRequest ：

public class DrawableTypeRequest<ModelType> extends DrawableRequestBuilder<ModelType> implements DownloadOptions {

    private final ModelLoader<ModelType, InputStream> streamModelLoader;
    private final ModelLoader<ModelType, ParcelFileDescriptor> fileDescriptorModelLoader;
    private final RequestManager.OptionsApplier optionsApplier;

    private static <A, Z, R> FixedLoadProvider<A, ImageVideoWrapper, Z, R> buildProvider(Glide glide,
            ModelLoader<A, InputStream> streamModelLoader,
            ModelLoader<A, ParcelFileDescriptor> fileDescriptorModelLoader, Class<Z> resourceClass,
            Class<R> transcodedClass,
            ResourceTranscoder<Z, R> transcoder) {
        if (streamModelLoader == null && fileDescriptorModelLoader == null) {
            return null;
        }
        if (transcoder == null) {
            transcoder = glide.buildTranscoder(resourceClass, transcodedClass);
        }
        DataLoadProvider<ImageVideoWrapper, Z> dataLoadProvider = glide.buildDataProvider(ImageVideoWrapper.class,
                resourceClass);
        ImageVideoModelLoader<A> modelLoader = new ImageVideoModelLoader<A>(streamModelLoader,
                fileDescriptorModelLoader);
        return new FixedLoadProvider<A, ImageVideoWrapper, Z, R>(modelLoader, transcoder, dataLoadProvider);
    }

    DrawableTypeRequest(Class<ModelType> modelClass, ModelLoader<ModelType, InputStream> streamModelLoader,
            ModelLoader<ModelType, ParcelFileDescriptor> fileDescriptorModelLoader, Context context, Glide glide,
            RequestTracker requestTracker, Lifecycle lifecycle, RequestManager.OptionsApplier optionsApplier) {
        super(context, modelClass,
                buildProvider(glide, streamModelLoader, fileDescriptorModelLoader, GifBitmapWrapper.class,
                        GlideDrawable.class, null),
                glide, requestTracker, lifecycle);
        this.streamModelLoader = streamModelLoader;
        this.fileDescriptorModelLoader = fileDescriptorModelLoader;
        this.optionsApplier = optionsApplier;
    }
    
    
    private static <A, Z, R> FixedLoadProvider<A, ImageVideoWrapper, Z, R> buildProvider(Glide glide,
            ModelLoader<A, InputStream> streamModelLoader,
            ModelLoader<A, ParcelFileDescriptor> fileDescriptorModelLoader, Class<Z> resourceClass,
            Class<R> transcodedClass,
            ResourceTranscoder<Z, R> transcoder) {
        if (streamModelLoader == null && fileDescriptorModelLoader == null) {
            return null;
        }
        if (transcoder == null) {
            transcoder = glide.buildTranscoder(resourceClass, transcodedClass);
        }
        DataLoadProvider<ImageVideoWrapper, Z> dataLoadProvider = glide.buildDataProvider(ImageVideoWrapper.class,
                resourceClass);
        ImageVideoModelLoader<A> modelLoader = new ImageVideoModelLoader<A>(streamModelLoader,
                fileDescriptorModelLoader);
        return new FixedLoadProvider<A, ImageVideoWrapper, Z, R>(modelLoader, transcoder, dataLoadProvider);
    }


    ...
}

查看构造器，我们是调用了buildProvider方法，传入了两个下载器的引用，在buildProvider方法中我们看见在调用了glide.buildTranscoder()方法来构建一个ResourceTranscoder，看见trans？？？，所以他是对图像进行转码的，这里实际会构造出这里实际会构建出一个GifBitmapWrapperDrawableTranscoder对象。
接下来调用了glide.buildDataProvider()方法来构建一个DataLoadProvider，它是用于对图片进行编解码的，由于DataLoadProvider是一个接口，这里实际会构建出一个ImageVideoGifDrawableLoadProvider对象。
new了一个ImageVideoModelLoader的实例，并把之前loadGeneric()方法中构建的两个ModelLoader封装到了ImageVideoModelLoader当中。
到最后把上面三放到FixedLoadProvider中，而激动人心的是FixedLoadProvider就是我们上文onSizeReady()方法中的loadProvider了。

回到onSizeReady方法中，我们拿到了ResourceTranscoder和ImageVideoModelLoader然后调用ResourceTranscoder得到ResourceFetcher()比较陌生那他什么呢？我们打开源码瞧一瞧：

public class ImageVideoModelLoader<A> implements ModelLoader<A, ImageVideoWrapper> {
    private static final String TAG = "IVML";

    private final ModelLoader<A, InputStream> streamLoader;
    private final ModelLoader<A, ParcelFileDescriptor> fileDescriptorLoader;

    public ImageVideoModelLoader(ModelLoader<A, InputStream> streamLoader,
            ModelLoader<A, ParcelFileDescriptor> fileDescriptorLoader) {
        if (streamLoader == null && fileDescriptorLoader == null) {
            throw new NullPointerException("At least one of streamLoader and fileDescriptorLoader must be non null");
        }
          this.streamLoader = streamLoader;
        this.fileDescriptorLoader = fileDescriptorLoader;
    }

    @Override
    public DataFetcher<ImageVideoWrapper> getResourceFetcher(A model, int width, int height) {
        DataFetcher<InputStream> streamFetcher = null;
        if (streamLoader != null) {
              // 1~
            streamFetcher = streamLoader.getResourceFetcher(model, width, height);
        }
        DataFetcher<ParcelFileDescriptor> fileDescriptorFetcher = null;
        if (fileDescriptorLoader != null) {
            fileDescriptorFetcher = fileDescriptorLoader.getResourceFetcher(model, width, height);
        }

        if (streamFetcher != null || fileDescriptorFetcher != null) {
        // 2~  
            return new ImageVideoFetcher(streamFetcher, fileDescriptorFetcher);
        } else {
            return null;
        }
    }

    static class ImageVideoFetcher implements DataFetcher<ImageVideoWrapper> {
        private final DataFetcher<InputStream> streamFetcher;
        private final DataFetcher<ParcelFileDescriptor> fileDescriptorFetcher;

        public ImageVideoFetcher(DataFetcher<InputStream> streamFetcher,
                DataFetcher<ParcelFileDescriptor> fileDescriptorFetcher) {
            this.streamFetcher = streamFetcher;
            this.fileDescriptorFetcher = fileDescriptorFetcher;
        }

        ...
    }
}

我们看见一那里 1 那里获得一个DataFetcher，这个streamLoader是在loadGeneric()方法中构建出的StreamStringLoader对象，的getResourceFetcher()方法会得到一个HttpUrlFetcher对象，在 2~ 那里会返回一个ImageVideoFetcher对象，现在我们返回onSizeReady方法中；

GenericRequest #onSizeReady：

@Override
public void onSizeReady(int width, int height) {
    if (Log.isLoggable(TAG, Log.VERBOSE)) {
        logV("Got onSizeReady in " + LogTime.getElapsedMillis(startTime));
    }
    if (status != Status.WAITING_FOR_SIZE) {
        return;
    }
    status = Status.RUNNING;
    width = Math.round(sizeMultiplier * width);
    height = Math.round(sizeMultiplier * height);
    ModelLoader<A, T> modelLoader = loadProvider.getModelLoader();
    final DataFetcher<T> dataFetcher = modelLoader.getResourceFetcher(model, width, height);
    if (dataFetcher == null) {
        onException(new Exception("Failed to load model: \'" + model + "\'"));
        return;
    }
    ResourceTranscoder<Z, R> transcoder = loadProvider.getTranscoder();
    if (Log.isLoggable(TAG, Log.VERBOSE)) {
        logV("finished setup for calling load in " + LogTime.getElapsedMillis(startTime));
    }
    loadedFromMemoryCache = true;
    / 3~
    loadStatus = engine.load(signature, width, height, dataFetcher, loadProvider, transformation, transcoder,
            priority, isMemoryCacheable, diskCacheStrategy, this);
    loadedFromMemoryCache = resource != null;
    if (Log.isLoggable(TAG, Log.VERBOSE)) {
        logV("finished onSizeReady in " + LogTime.getElapsedMillis(startTime));
    }
}

继续在 3~ 中我们调用了engine.load方法，并且传入上面的ImageVideoFetcher、GifBitmapWrapper.DrawableTranscoder等等一系列的值

engine#load


public class Engine implements EngineJobListener,
        MemoryCache.ResourceRemovedListener,
        EngineResource.ResourceListener {

    ...
    public <T, Z, R> LoadStatus load(Key signature, int width, int height, DataFetcher<T> fetcher,
            DataLoadProvider<T, Z> loadProvider, Transformation<Z> transformation, ResourceTranscoder<Z, R> transcoder,
            Priority priority, boolean isMemoryCacheable, DiskCacheStrategy diskCacheStrategy, ResourceCallback cb) {
        Util.assertMainThread();
        long startTime = LogTime.getLogTime();

        final String id = fetcher.getId();
        EngineKey key = keyFactory.buildKey(id, signature, width, height, loadProvider.getCacheDecoder(),
                loadProvider.getSourceDecoder(), transformation, loadProvider.getEncoder(),
                transcoder, loadProvider.getSourceEncoder());

        EngineResource<?> cached = loadFromCache(key, isMemoryCacheable);
        if (cached != null) {
            cb.onResourceReady(cached);
            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);
            if (Log.isLoggable(TAG, Log.VERBOSE)) {
                logWithTimeAndKey("Loaded resource from active resources", startTime, key);
            }
            return null;
        }
        
        //         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);
        }
        // 4~
        EngineJob engineJob = engineJobFactory.build(key, isMemoryCacheable);
        // 5~
        DecodeJob<T, Z, R> decodeJob = new DecodeJob<T, Z, R>(key, width, height, fetcher, loadProvider, transformation,
                transcoder, diskCacheProvider, diskCacheStrategy, priority);
        EngineRunnable runnable = new EngineRunnable(engineJob, decodeJob, priority);
        jobs.put(key, engineJob);
        engineJob.addCallback(cb);
        engineJob.start(runnable);

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

    ...
}

这些代码主要是处理缓存的，所以我们冲 4~ 开始看根据key创建EngineJob他主要的作用是用来开启线程的，后面异步加载需要，接着看 5~ 这里后面讲，继续看下面，EngineRunnable创建传入了上两个engine的引用，并在下面调用
engineJob.start(runnable);开始了运行run方法：

EngineRunnable#run()

@Override
public void run() {
    if (isCancelled) {
        return;
    }
    Exception exception = null;
    Resource<?> resource = null;
    try {
    //6~
        resource = decode();
    } catch (Exception e) {
        if (Log.isLoggable(TAG, Log.VERBOSE)) {
            Log.v(TAG, "Exception decoding", e);
        }
        exception = e;
    }
    if (isCancelled) {
        if (resource != null) {
            resource.recycle();
        }
        return;
    }
    if (resource == null) {
        onLoadFailed(exception);
    } else {
        onLoadComplete(resource);
    }
}

6~ 这里调用了decode ():

rivate Resource<?> decode() throws Exception {
    if (isDecodingFromCache()) {
        return decodeFromCache();
    } else {
        return decodeFromSource();
    }
}

这里看是不是决定用缓存还是根据资源获取，因为走线我们就只看decodeFromSource();：

private Resource<?> decodeFromSource() throws Exception {
    return decodeJob.decodeFromSource();
}

这里我们看见返回decodeJob调用decodeFromSource();的方法：

class DecodeJob<A, T, Z> {

    ...

    public Resource<Z> decodeFromSource() throws Exception {
        Resource<T> decoded = decodeSource();
        return transformEncodeAndTranscode(decoded);
    }

    private Resource<T> decodeSource() throws Exception {
        Resource<T> decoded = null;
        try {
            long startTime = LogTime.getLogTime();‘
            /6~
            final A data = fetcher.loadData(priority);
            if (Log.isLoggable(TAG, Log.VERBOSE)) {
                logWithTimeAndKey("Fetched data", startTime);
            }
            if (isCancelled) {
                return null;
            }
            //7~
            decoded = decodeFromSourceData(data);
        } finally {
            fetcher.cleanup();
        }
        return decoded;
    }

    ...
}

首先我们看见 6~ 中的 fetcher 其实是在onSizeReady()方法中得到的ImageVideoFetcher对象，接着看loadData方法：

ImageVideoFetcher# loadData：


@Override
public ImageVideoWrapper loadData(Priority priority) throws Exception {
    InputStream is = null;
    if (streamFetcher != null) {
        try {
        //7~
            is = streamFetcher.loadData(priority);
        } catch (Exception e) {
            if (Log.isLoggable(TAG, Log.VERBOSE)) {
                Log.v(TAG, "Exception fetching input stream, trying ParcelFileDescriptor", e);
            }
            if (fileDescriptorFetcher == null) {
                throw e;
            }
        }
    }
    ParcelFileDescriptor fileDescriptor = null;
    if (fileDescriptorFetcher != null) {
        try {
            fileDescriptor = fileDescriptorFetcher.loadData(priority);
        } catch (Exception e) {
            if (Log.isLoggable(TAG, Log.VERBOSE)) {
                Log.v(TAG, "Exception fetching ParcelFileDescriptor", e);
            }
            if (is == null) {
                throw e;
            }
        }
    }
    return new ImageVideoWrapper(is, fileDescriptor);
}

7~ 中我们发现他实际是调用streamFetcher.loadData(priority);方法，这个streamFetcher就是我们之前创建的HttpUrlFetcher，所以无聊的继续：

HttpUrlFetcher # loadData（）

public class HttpUrlFetcher implements DataFetcher<InputStream> {

    ...

    @Override
    public InputStream loadData(Priority priority) throws Exception {
        return loadDataWithRedirects(glideUrl.toURL(), 0 /*redirects*/, null /*lastUrl*/, glideUrl.getHeaders());
    }

    private InputStream loadDataWithRedirects(URL url, int redirects, URL lastUrl, Map<String, String> headers)
            throws IOException {
        if (redirects >= MAXIMUM_REDIRECTS) {
            throw new IOException("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 IOException("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(2500);
        urlConnection.setReadTimeout(2500);
        urlConnection.setUseCaches(false);
        urlConnection.setDoInput(true);

        // 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 IOException("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 IOException("Unable to retrieve response code from HttpUrlConnection.");
            }
            throw new IOException("Request failed " + statusCode + ": " + urlConnection.getResponseMessage());
        }
    }

    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;
    }

    ...
}

说实话看见urlConnection的时候我真的哭了，你终于开始请求数据了，不过这里主要是urlConnection相关的网络请求这里不多讲，但是我们需要知道理解的是返回了一个InputStream对象就行了。接着回到loadData()中我们可以看到他会将我们的InputStream和fileDescriptor打包成ImageVideoWrapper传回DecodeJob对象的decodeSource方法中,继续执行： 7~ 调用了decodeFromSourceData方法，将返回的ImageVideoWrapper作为参数穿进去：

decodeJob #decodeFromSourceData方法：

private Resource<T> decodeFromSourceData(A data) throws IOException {
    final Resource<T> decoded;
    if (diskCacheStrategy.cacheSource()) {
        decoded = cacheAndDecodeSourceData(data);
    } else {
        long startTime = LogTime.getLogTime();
        // 8~
        decoded = loadProvider.getSourceDecoder().decode(data, width, height);
        if (Log.isLoggable(TAG, Log.VERBOSE)) {
            logWithTimeAndKey("Decoded from source", startTime);
        } 
    }
    return decoded;
}

这里会执行 8~ 调用loadProvider.getSourceDecoder().decode(data, width, height);这里的loadProvider就是FixedLoadProvider也就是我们上文onSizeReady()方法中的loadProvider了这里再次出现，而.getSourceDecoder()得到的就是GifBitmapWrapperResourceDecoder对象，而就是这个对象调用了decode方法进行解码操作：

GifBitmapWrapperResourceDecoder#decode

public class GifBitmapWrapperResourceDecoder implements ResourceDecoder<ImageVideoWrapper, GifBitmapWrapper> {

    ...

    @SuppressWarnings("resource")
    // @see ResourceDecoder.decode
    @Override
    public Resource<GifBitmapWrapper> decode(ImageVideoWrapper source, int width, int height) throws IOException {
        ByteArrayPool pool = ByteArrayPool.get();
        byte[] tempBytes = pool.getBytes();
        GifBitmapWrapper wrapper = null;
        try {
        //9~
            wrapper = decode(source, width, height, tempBytes);
        } finally {
            pool.releaseBytes(tempBytes);
        }
        return wrapper != null ? new GifBitmapWrapperResource(wrapper) : null;
    }

    private GifBitmapWrapper decode(ImageVideoWrapper source, int width, int height, byte[] bytes) throws IOException {
        final GifBitmapWrapper result;
        if (source.getStream() != null) {
        //10~
            result = decodeStream(source, width, height, bytes);
        } else {
            result = decodeBitmapWrapper(source, width, height);
        }
        return result;
    }

    private GifBitmapWrapper decodeStream(ImageVideoWrapper source, int width, int height, byte[] bytes)
            throws IOException {
        InputStream bis = streamFactory.build(source.getStream(), bytes);
        bis.mark(MARK_LIMIT_BYTES);
        ImageHeaderParser.ImageType type = parser.parse(bis);
        bis.reset();
        GifBitmapWrapper result = null;
        if (type == ImageHeaderParser.ImageType.GIF) {
            result = decodeGifWrapper(bis, width, height);
        }
        // Decoding the gif may fail even if the type matches.
        if (result == null) {
            // We can only reset the buffered InputStream, so to start from the beginning of the stream, we need to
            // pass in a new source containing the buffered stream rather than the original stream.
            ImageVideoWrapper forBitmapDecoder = new ImageVideoWrapper(bis, source.getFileDescriptor());
            result = decodeBitmapWrapper(forBitmapDecoder, width, height);
        }
        return result;
    }

    private GifBitmapWrapper decodeBitmapWrapper(ImageVideoWrapper toDecode, int width, int height) throws IOException {
        GifBitmapWrapper result = null;
        //11~
        Resource<Bitmap> bitmapResource = bitmapDecoder.decode(toDecode, width, height);
        if (bitmapResource != null) {
            result = new GifBitmapWrapper(bitmapResource, null);
        }
        return result;
    }

    ...

9~中调用了 decode的重载方法接着在重载方法里 10~ 调用 decodeStream ()方法从服务器里读取数据流，来到decodeStream方法，会调用parser.parse(bis)，在.parse方法里从流中到了两个字节的数据，然后判断数据数据类型是GIF和还是静态图，从而decodeGifWrapper方法，还是decodeBitmapWrapper方法，由于decodeGifWrapper 超出本人能力范围，，我们就看看decodeBitmapWrapper，也就是静态图片的解决方案好了。
11~ 会调用了bitmapDecoder.decode()方法,这个bitmapDecoder是ImageVideoBitmapDecoder对象：

ImageVideoBitmapDecoder#decode


public class ImageVideoBitmapDecoder implements ResourceDecoder<ImageVideoWrapper, Bitmap> {
    private final ResourceDecoder<InputStream, Bitmap> streamDecoder;
    private final ResourceDecoder<ParcelFileDescriptor, Bitmap> fileDescriptorDecoder;

    public ImageVideoBitmapDecoder(ResourceDecoder<InputStream, Bitmap> streamDecoder,
            ResourceDecoder<ParcelFileDescriptor, Bitmap> fileDescriptorDecoder) {
        this.streamDecoder = streamDecoder;
        this.fileDescriptorDecoder = fileDescriptorDecoder;
    }

    @Override
    public Resource<Bitmap> decode(ImageVideoWrapper source, int width, int height) throws IOException {
        Resource<Bitmap> result = null;
        //12~
        InputStream is = source.getStream();
        if (is != null) {
            try {
                result = streamDecoder.decode(is, width, height);
            } catch (IOException e) {
                if (Log.isLoggable(TAG, Log.VERBOSE)) {
                    Log.v(TAG, "Failed to load image from stream, trying FileDescriptor", e);
                }
            }
        }
        if (result == null) {
            ParcelFileDescriptor fileDescriptor = source.getFileDescriptor();
            if (fileDescriptor != null) {
                result = fileDescriptorDecoder.decode(fileDescriptor, width, height);
            }
        }
        return result;
    }

    ...
}

12~调用source.getStream();方法，得到InputStream也就是服务器传回的流，接着用streamDecoder.decode()方法进行解码，这个streamDecoder就是StreamBitmapDecoder：

StreamBitmapDecoder#decode()：

public class StreamBitmapDecoder implements ResourceDecoder<InputStream, Bitmap> {

    ...

    private final Downsampler downsampler;
    private BitmapPool bitmapPool;
    private DecodeFormat decodeFormat;

    public StreamBitmapDecoder(Downsampler downsampler, BitmapPool bitmapPool, DecodeFormat decodeFormat) {
        this.downsampler = downsampler;
        this.bitmapPool = bitmapPool;
        this.decodeFormat = decodeFormat;
    }

    @Override
    public Resource<Bitmap> decode(InputStream source, int width, int height) {
    //13~
        Bitmap bitmap = downsampler.decode(source, bitmapPool, width, height, decodeFormat);
        return BitmapResource.obtain(bitmap, bitmapPool);
    }

    ...

13~ 看见这里让人欣喜传入的InputStream最终返回了Bitmap说明这行代码里包括了真真正正的解析代码：

Downsampler #decode

public abstract class Downsampler implements BitmapDecoder<InputStream> {

    ...

    @Override
    public Bitmap decode(InputStream is, BitmapPool pool, int outWidth, int outHeight, DecodeFormat decodeFormat) {
        final ByteArrayPool byteArrayPool = ByteArrayPool.get();
        final byte[] bytesForOptions = byteArrayPool.getBytes();
        final byte[] bytesForStream = byteArrayPool.getBytes();
        final BitmapFactory.Options options = getDefaultOptions();
        // Use to fix the mark limit to avoid allocating buffers that fit entire images.
        RecyclableBufferedInputStream bufferedStream = new RecyclableBufferedInputStream(
                is, bytesForStream);
        // Use to retrieve exceptions thrown while reading.
        // TODO(#126): when the framework no longer returns partially decoded Bitmaps or provides a way to determine
        // if a Bitmap is partially decoded, consider removing.
        ExceptionCatchingInputStream exceptionStream =
                ExceptionCatchingInputStream.obtain(bufferedStream);
        // Use to read data.
        // Ensures that we can always reset after reading an image header so that we can still attempt to decode the
        // full image even when the header decode fails and/or overflows our read buffer. See #283.
        MarkEnforcingInputStream invalidatingStream = new MarkEnforcingInputStream(exceptionStream);
        try {
            exceptionStream.mark(MARK_POSITION);
            int orientation = 0;
            try {
                orientation = new ImageHeaderParser(exceptionStream).getOrientation();
            } catch (IOException e) {
                if (Log.isLoggable(TAG, Log.WARN)) {
                    Log.w(TAG, "Cannot determine the image orientation from header", e);
                }
            } finally {
                try {
                    exceptionStream.reset();
                } catch (IOException e) {
                    if (Log.isLoggable(TAG, Log.WARN)) {
                        Log.w(TAG, "Cannot reset the input stream", e);
                    }
                }
            }
            options.inTempStorage = bytesForOptions;
            final int[] inDimens = getDimensions(invalidatingStream, bufferedStream, options);
            final int inWidth = inDimens[0];
            final int inHeight = inDimens[1];
            final int degreesToRotate = TransformationUtils.getExifOrientationDegrees(orientation);
            final int sampleSize = getRoundedSampleSize(degreesToRotate, inWidth, inHeight, outWidth, outHeight);
            final Bitmap downsampled =
                    downsampleWithSize(invalidatingStream, bufferedStream, options, pool, inWidth, inHeight, sampleSize,
                            decodeFormat);
            // BitmapFactory swallows exceptions during decodes and in some cases when inBitmap is non null, may catch
            // and log a stack trace but still return a non null bitmap. To avoid displaying partially decoded bitmaps,
            // we catch exceptions reading from the stream in our ExceptionCatchingInputStream and throw them here.
            final Exception streamException = exceptionStream.getException();
            if (streamException != null) {
                throw new RuntimeException(streamException);
            }
            Bitmap rotated = null;
            if (downsampled != null) {
                rotated = TransformationUtils.rotateImageExif(downsampled, pool, orientation);
                if (!downsampled.equals(rotated) && !pool.put(downsampled)) {
                    downsampled.recycle();
                }
            }
            return rotated;
        } finally {
            byteArrayPool.releaseBytes(bytesForOptions);
            byteArrayPool.releaseBytes(bytesForStream);
            exceptionStream.release();
            releaseOptions(options);
        }
    }

    private Bitmap downsampleWithSize(MarkEnforcingInputStream is, RecyclableBufferedInputStream  bufferedStream,
            BitmapFactory.Options options, BitmapPool pool, int inWidth, int inHeight, int sampleSize,
            DecodeFormat decodeFormat) {
        // Prior to KitKat, the inBitmap size must exactly match the size of the bitmap we're decoding.
        Bitmap.Config config = getConfig(is, decodeFormat);
        options.inSampleSize = sampleSize;
        options.inPreferredConfig = config;
        if ((options.inSampleSize == 1 || Build.VERSION_CODES.KITKAT <= Build.VERSION.SDK_INT) && shouldUsePool(is)) {
            int targetWidth = (int) Math.ceil(inWidth / (double) sampleSize);
            int targetHeight = (int) Math.ceil(inHeight / (double) sampleSize);
            // BitmapFactory will clear out the Bitmap before writing to it, so getDirty is safe.
            setInBitmap(options, pool.getDirty(targetWidth, targetHeight, config));
        }
        return decodeStream(is, bufferedStream, options);
    }

    /**
     * A method for getting the dimensions of an image from the given InputStream.
     *
     * @param is The InputStream representing the image.
     * @param options The options to pass to
     *          {@link BitmapFactory#decodeStream(InputStream, android.graphics.Rect,
     *              BitmapFactory.Options)}.
     * @return an array containing the dimensions of the image in the form {width, height}.
     */
    public int[] getDimensions(MarkEnforcingInputStream is, RecyclableBufferedInputStream bufferedStream,
            BitmapFactory.Options options) {
        options.inJustDecodeBounds = true;
        decodeStream(is, bufferedStream, options);
        options.inJustDecodeBounds = false;
        return new int[] { options.outWidth, options.outHeight };
    }

    private static Bitmap decodeStream(MarkEnforcingInputStream is, RecyclableBufferedInputStream bufferedStream,
            BitmapFactory.Options options) {
         if (options.inJustDecodeBounds) {
             // This is large, but jpeg headers are not size bounded so we need something large enough to minimize
             // the possibility of not being able to fit enough of the header in the buffer to get the image size so
             // that we don't fail to load images. The BufferedInputStream will create a new buffer of 2x the
             // original size each time we use up the buffer space without passing the mark so this is a maximum
             // bound on the buffer size, not a default. Most of the time we won't go past our pre-allocated 16kb.
             is.mark(MARK_POSITION);
         } else {
             // Once we've read the image header, we no longer need to allow the buffer to expand in size. To avoid
             // unnecessary allocations reading image data, we fix the mark limit so that it is no larger than our
             // current buffer size here. See issue #225.
             bufferedStream.fixMarkLimit();
         }
        final Bitmap result = BitmapFactory.decodeStream(is, null, options);
        try {
            if (options.inJustDecodeBounds) {
                is.reset();
            }
        } catch (IOException e) {
            if (Log.isLoggable(TAG, Log.ERROR)) {
                Log.e(TAG, "Exception loading inDecodeBounds=" + options.inJustDecodeBounds
                        + " sample=" + options.inSampleSize, e);
            }
        }

        return result;
    }

    ...
}

过长的篇幅确实让人充满睡意，Glide真是一场拉锯战。。
上面得篇幅不做解析，我们只需要知道他把流进行解析成Bitmap即可，当然其中还包括我们设定的对图片的压缩，甚至还有旋转、圆角等逻辑处理包扩在内。回到
StreamBitmapDecoder#decode()：中我们调用了BitmapResource.obtain(bitmap, bitmapPool);将Bitmap包装成Resource<Bitmap>对象：

public class BitmapResource implements Resource<Bitmap> {
    private final Bitmap bitmap;
    private final BitmapPool bitmapPool;

    /**
     * Returns a new {@link BitmapResource} wrapping the given {@link Bitmap} if the Bitmap is non-null or null if the
     * given Bitmap is null.
     *
     * @param bitmap A Bitmap.
     * @param bitmapPool A non-null {@link BitmapPool}.
     */
    public static BitmapResource obtain(Bitmap bitmap, BitmapPool bitmapPool) {
        if (bitmap == null) {
            return null;
        } else {
            return new BitmapResource(bitmap, bitmapPool);
        }
    }

    public BitmapResource(Bitmap bitmap, BitmapPool bitmapPool) {
        if (bitmap == null) {
            throw new NullPointerException("Bitmap must not be null");
        }
        if (bitmapPool == null) {
            throw new NullPointerException("BitmapPool must not be null");
        }
        this.bitmap = bitmap;
        this.bitmapPool = bitmapPool;
    }

    @Override
    public Bitmap get() {
        return bitmap;
    }

    @Override
    public int getSize() {
        return Util.getBitmapByteSize(bitmap);
    }

    @Override
    public void recycle() {
        if (!bitmapPool.put(bitmap)) {
            bitmap.recycle();
        }
    }
}

这个源码太简单我们知道是个Bitmap的包装类就行了，然后我们就一层一层的返回，StreamBitmapDecoder会将值返回到ImageVideoBitmapDecoder当中，而ImageVideoBitmapDecoder又会将值返回到GifBitmapWrapperResourceDecoder的decodeBitmapWrapper()方法当中。到这里停一停看看下面的工作：

GifBitmapWrapperResourceDecoder #decodeBitmapWrapper

private GifBitmapWrapper decodeBitmapWrapper(ImageVideoWrapper toDecode, int width, int height) throws IOException {
    GifBitmapWrapper result = null;
    Resource<Bitmap> bitmapResource = bitmapDecoder.decode(toDecode, width, height);
    if (bitmapResource != null) {
        result = new GifBitmapWrapper(bitmapResource, null);
    }
    return result;

他会把返回的Resource<Bitmap>又包装进GifBitmapWrapper中所以接下看看GifBitmapWrapper代码吧：

public class GifBitmapWrapper {
    private final Resource<GifDrawable> gifResource;
    private final Resource<Bitmap> bitmapResource;

    public GifBitmapWrapper(Resource<Bitmap> bitmapResource, Resource<GifDrawable> gifResource) {
        if (bitmapResource != null && gifResource != null) {
            throw new IllegalArgumentException("Can only contain either a bitmap resource or a gif resource, not both");
        }
        if (bitmapResource == null && gifResource == null) {
            throw new IllegalArgumentException("Must contain either a bitmap resource or a gif resource");
        }
        this.bitmapResource = bitmapResource;
        this.gifResource = gifResource;
    }

    /**
     * Returns the size of the wrapped resource.
     */
    public int getSize() {
        if (bitmapResource != null) {
            return bitmapResource.getSize();
        } else {
            return gifResource.getSize();
        }
    }

    /**
     * Returns the wrapped {@link Bitmap} resource if it exists, or null.
     */
    public Resource<Bitmap> getBitmapResource() {
        return bitmapResource;
    }

    /**
     * Returns the wrapped {@link GifDrawable} resource if it exists, or null.
     */
    public Resource<GifDrawable> getGifResource() {
        return gifResource;
    }
}

没什么技术含量只是做了进一步封装而已，然后又继续向上返回，，，最后返回到GifBitmapWrapperResourceDecoder对象中：

GifBitmapWrapperResourceDecoder# decode

@Override
public Resource<GifBitmapWrapper> decode(ImageVideoWrapper source, int width, int height) throws IOException {
    ByteArrayPool pool = ByteArrayPool.get();
    byte[] tempBytes = pool.getBytes();
    GifBitmapWrapper wrapper = null;
    try {
        wrapper = decode(source, width, height, tempBytes);
    } finally {
        pool.releaseBytes(tempBytes);
    }
    //14~
    return wrapper != null ? new GifBitmapWrapperResource(wrapper) : null;
}

14~中又会包装一次Resource<GifBitmapWrapper>对象返回，他和前文的BitmapResource很相似，都实现了Resource接口:

public class GifBitmapWrapperResource implements Resource<GifBitmapWrapper> {
    private final GifBitmapWrapper data;

    public GifBitmapWrapperResource(GifBitmapWrapper data) {
        if (data == null) {
            throw new NullPointerException("Data must not be null");
        }
        this.data = data;
    }

    @Override
    public GifBitmapWrapper get() {
        return data;
    }

    @Override
    public int getSize() {
        return data.getSize();
    }

    @Override
    public void recycle() {
        Resource<Bitmap> bitmapResource = data.getBitmapResource();
        if (bitmapResource != null) {
            bitmapResource.recycle();
        }
        Resource<GifDrawable> gifDataResource = data.getGifResource();
        if (gifDataResource != null) {
            gifDataResource.recycle();
        }
    }
}

这么多次的封装对象实际就是为了让他即实现Resource接口，同时又能包裹Bitmap图片和GIF图片。然后返回到DecodeJob的decodeFromSourceData()中，然后又回到decodeSource()方法中，然后回到decodeFromSource()方法中：

DecodeJob#decodeFromSource

    public Resource<Z> decodeFromSource() throws Exception {
        Resource<T> decoded = decodeSource();
        return transformEncodeAndTranscode(decoded);
    }

可以看见调用了 transformEncodeAndTranscode方法：

DecodeJob#transformEncodeAndTranscode：

private Resource<Z> transformEncodeAndTranscode(Resource<T> decoded) {
    long startTime = LogTime.getLogTime();
    Resource<T> transformed = transform(decoded);
    if (Log.isLoggable(TAG, Log.VERBOSE)) {
        logWithTimeAndKey("Transformed resource from source", startTime);
    }
    writeTransformedToCache(transformed);
    startTime = LogTime.getLogTime();
    //15~
    Resource<Z> result = transcode(transformed);
    if (Log.isLoggable(TAG, Log.VERBOSE)) {
        logWithTimeAndKey("Transcoded transformed from source", startTime);
    }
    return result;
}

private Resource<Z> transcode(Resource<T> transformed) {
    if (transformed == null) {
        return null;
    }
    return transcoder.transcode(transformed);
}

其他不用管都是关于缓存的，我们看 15~ Resource<T>对象转换成Resource<Z>对象了，而方法里调用transcoder.transcode方法，这个transcoder方法还记得吗？我们FixedLoadProvider里包裹的其中者之一，然后在onSizeReady()方法中传入了这个参数，并把它交给了Engine，Engine又给了DecodeJob中，所以他就是GifBitmapWrapperDrawableTranscoder对象：

GifBitmapWrapperDrawableTranscoder# transcode：

public class GifBitmapWrapperDrawableTranscoder implements ResourceTranscoder<GifBitmapWrapper, GlideDrawable> {
    private final ResourceTranscoder<Bitmap, GlideBitmapDrawable> bitmapDrawableResourceTranscoder;

    public GifBitmapWrapperDrawableTranscoder(
            ResourceTranscoder<Bitmap, GlideBitmapDrawable> bitmapDrawableResourceTranscoder) {
        this.bitmapDrawableResourceTranscoder = bitmapDrawableResourceTranscoder;
    }

    @Override
    public Resource<GlideDrawable> transcode(Resource<GifBitmapWrapper> toTranscode) {
        GifBitmapWrapper gifBitmap = toTranscode.get();
        Resource<Bitmap> bitmapResource = gifBitmap.getBitmapResource();
        final Resource<? extends GlideDrawable> result;
        if (bitmapResource != null) {
        //16~
            result = bitmapDrawableResourceTranscoder.transcode(bitmapResource);
        } else {
            result = gifBitmap.getGifResource();
        }
        return (Resource<GlideDrawable>) result;
    }

    ...
}

我们先说明一下transcode就是转码的意思，因为我们知道包装这么多GifBitmapWrapper是无法显示的，所以我们要做的事把包裹的“蚕茧”一层一层的撕掉，都出原有的样子然后放在所谓的Target显示出来，先得到GifBitmapWrapper-》Resource<Bitmap>，这时候我们判断Resource<Bitmap>对象是不是为空，如果是就代表他是GIF图，直接调用getGifResource()得到图片即可(因为GIF本身就是Drawable对象)，如果是Resource<Bitmap>我们得还做一次转码将Bitmap转换成Drawable对象才行:在16~中：

GlideBitmapDrawableTranscoder#transcode()

public class GlideBitmapDrawableTranscoder implements ResourceTranscoder<Bitmap, GlideBitmapDrawable> {
    private final Resources resources;
    private final BitmapPool bitmapPool;

    public GlideBitmapDrawableTranscoder(Context context) {
        this(context.getResources(), Glide.get(context).getBitmapPool());
    }

    public GlideBitmapDrawableTranscoder(Resources resources, BitmapPool bitmapPool) {
        this.resources = resources;
        this.bitmapPool = bitmapPool;
    }

    @Override
    public Resource<GlideBitmapDrawable> transcode(Resource<Bitmap> toTranscode) {
        GlideBitmapDrawable drawable = new GlideBitmapDrawable(resources, toTranscode.get());
        return new GlideBitmapDrawableResource(drawable, bitmapPool);
    }

    ...

这里new出了一个GlideBitmapDrawable对象并把Bitmap封装到里面。然后对GlideBitmapDrawable再进行一次封装

返回一个Resource<GlideBitmapDrawable>对象。因为GlideBitmapDrawable和GifDrawable他们的父类都是GlideDrawable对象，所以都OK，然后返回向上直接转型Resource<GlideDrawable>，而前文所谓的Resource<Z>就是他。接着回到DecodeJob#decodeFromSource方法继续返回 -> EngineRunnable#decodeFromCache-> EngineRunnable#decode()->EngineRunnable#run()中：

EngineRunnable#decode()


    @Override
    public void run() {
        if (isCancelled) {
            return;
        }

        Exception exception = null;
        Resource<?> resource = null;
        try {
            resource = decode();
        } catch (Exception e) {
            if (Log.isLoggable(TAG, Log.VERBOSE)) {
                Log.v(TAG, "Exception decoding", e);
            }
            exception = e;
        }

        if (isCancelled) {
            if (resource != null) {
                resource.recycle();
            }
            return;
        }

        if (resource == null) {
            onLoadFailed(exception);
        } else {
        //17~
            onLoadComplete(resource);
        }
    }

调用回调：17~：

EngineRunnable#onLoadComplete()：


    private void onLoadComplete(Resource resource) {
        manager.onResourceReady(resource);
    }

这个manager就是EngineJob对象：

EngineJob#onResourceReady()：

class EngineJob implements EngineRunnable.EngineRunnableManager {

    private static final Handler MAIN_THREAD_HANDLER = new Handler(Looper.getMainLooper(), new MainThreadCallback());

    private final List<ResourceCallback> cbs = new ArrayList<ResourceCallback>();

    ...

    public void addCallback(ResourceCallback cb) {
        Util.assertMainThread();
        if (hasResource) {
        //21~
            cb.onResourceReady(engineResource);
        } else if (hasException) {
            cb.onException(exception);
        } else {
            cbs.add(cb);
        }
    }

    @Override
    public void onResourceReady(final Resource<?> resource) {
        this.resource = resource;
        //18~
        MAIN_THREAD_HANDLER.obtainMessage(MSG_COMPLETE, this).sendToTarget();
    }

    private void handleResultOnMainThread() {
        if (isCancelled) {
            resource.recycle();
            return;
        } else if (cbs.isEmpty()) {
            throw new IllegalStateException("Received a resource without any callbacks to notify");
        }
        engineResource = engineResourceFactory.build(resource, isCacheable);
        hasResource = true;
        engineResource.acquire();
        listener.onEngineJobComplete(key, engineResource);
        for (ResourceCallback cb : cbs) {
            if (!isInIgnoredCallbacks(cb)) {
                engineResource.acquire();
                cb.onResourceReady(engineResource);
            }
        }
        engineResource.release();
    }

    @Override
    public void onException(final Exception e) {
        this.exception = e;
        MAIN_THREAD_HANDLER.obtainMessage(MSG_EXCEPTION, this).sendToTarget();
    }

    private void handleExceptionOnMainThread() {
        if (isCancelled) {
            return;
        } else if (cbs.isEmpty()) {
            throw new IllegalStateException("Received an exception without any callbacks to notify");
        }
        hasException = true;
        listener.onEngineJobComplete(key, null);
        for (ResourceCallback cb : cbs) {
            if (!isInIgnoredCallbacks(cb)) {
                cb.onException(exception);
            }
        }
    }
    //19~
    private static class MainThreadCallback implements Handler.Callback {

        @Override
        public boolean handleMessage(Message message) {
            if (MSG_COMPLETE == message.what || MSG_EXCEPTION == message.what) {
                EngineJob job = (EngineJob) message.obj;
                if (MSG_COMPLETE == message.what) {
                //20~
                    job.handleResultOnMainThread();
                } else {
                    job.handleExceptionOnMainThread();
                }
                return true;
            }
            return false;
        }
    }

    ...
}

18~中我们创建的是一个MSG发送的主线程，然后把自己导入进这个MSG中， 19~ 所以接下来的在MainThreadCallback中的handleMessage会在主线程中执行： 20~看见会调用job.handleResultOnMainThread();：我们看见这个方法只是在遍历寻找回调，而在之前Engine的load方法中我们早一注册了接口，这个回调真是GenericRequest自身，所以最终对调用到 21~哪一行也就是 cb.onResourceReady(engineResource);实际就是 GenericRequest的：

GenericRequest#onResourceReady

public void onResourceReady(Resource<?> resource) {
    if (resource == null) {
        onException(new Exception("Expected to receive a Resource<R> with an object of " + transcodeClass
                + " inside, but instead got null."));
        return;
    }
    //22~
    Object received = resource.get();
    if (received == null || !transcodeClass.isAssignableFrom(received.getClass())) {
        releaseResource(resource);
        onException(new Exception("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.")
        ));
        return;
    }
    if (!canSetResource()) {
        releaseResource(resource);
        // We can't set the status to complete before asking canSetResource().
        status = Status.COMPLETE;
        return;
    }
    onResourceReady(resource, (R) received);
}

private void onResourceReady(Resource<?> resource, R result) {
    // We must call isFirstReadyResource before setting status.
    boolean isFirstResource = isFirstReadyResource();
    status = Status.COMPLETE;
    this.resource = resource;
    if (requestListener == null || !requestListener.onResourceReady(result, model, target, loadedFromMemoryCache,
            isFirstResource)) {
        GlideAnimation<R> animation = animationFactory.build(loadedFromMemoryCache, isFirstResource);
        target.onResourceReady(result, animation);
    }
    notifyLoadSuccess();
    if (Log.isLoggable(TAG, Log.VERBOSE)) {
        logV("Resource ready in " + LogTime.getElapsedMillis(startTime) + " size: "
                + (resource.getSize() * TO_MEGABYTE) + " fromCache: " + loadedFromMemoryCache);
    }

这里有两个onResourceReady，其中只有一个参数的是我们调用的， 22~ 中我们得到的received不是GlideBitmapDrawable就是GifDrawable，然后调用含有两个参数的重载方法：onResourceReady：

设置标志完成
创建动画对象
调用target.onResourceReady

前文我们知道target就是GlideDrawableImageViewTarget

GlideDrawableImageViewTarget# onResourceReady

public class GlideDrawableImageViewTarget extends ImageViewTarget<GlideDrawable> {
    private static final float SQUARE_RATIO_MARGIN = 0.05f;
    private int maxLoopCount;
    private GlideDrawable resource;

    public GlideDrawableImageViewTarget(ImageView view) {
        this(view, GlideDrawable.LOOP_FOREVER);
    }

    public GlideDrawableImageViewTarget(ImageView view, int maxLoopCount) {
        super(view);
        this.maxLoopCount = maxLoopCount;
    }

    @Override
    public void onResourceReady(GlideDrawable resource, GlideAnimation<? super GlideDrawable> animation) {
        if (!resource.isAnimated()) {
            float viewRatio = view.getWidth() / (float) view.getHeight();
            float drawableRatio = resource.getIntrinsicWidth() / (float) resource.getIntrinsicHeight();
            if (Math.abs(viewRatio - 1f) <= SQUARE_RATIO_MARGIN
                    && Math.abs(drawableRatio - 1f) <= SQUARE_RATIO_MARGIN) {
                resource = new SquaringDrawable(resource, view.getWidth());
            }
        }
        super.onResourceReady(resource, animation);
        this.resource = resource;
        resource.setLoopCount(maxLoopCount);
        resource.start();
    }

    @Override
    protected void setResource(GlideDrawable resource) {
        view.setImageDrawable(resource);
    }

    @Override
    public void onStart() {
        if (resource != null) {
            resource.start();
        }
    }

    @Override
    public void onStop() {
        if (resource != null) {
            resource.stop();
        }
    }

这里除了动画以外貌似没有好处理的所以我们得不目光一道父类即super.onResourceReady(resource, animation);

ImageViewTarget#onResourceReady

 public abstract class ImageViewTarget<Z> extends ViewTarget<ImageView, Z> implements GlideAnimation.ViewAdapter {

    ...

    @Override
    public void onResourceReady(Z resource, GlideAnimation<? super Z> glideAnimation) {
        if (glideAnimation == null || !glideAnimation.animate(resource, this)) {
        //23~
            setResource(resource);
        }
    }

    protected abstract void setResource(Z resource);

23~ 我们可以看到调用了抽象方法setResource：所以他应该在子类也就是GlideDrawableImageViewTarget调用的setResource方法

GlideDrawableImageViewTarget#setResource

   @Override
    protected void setResource(GlideDrawable resource) {
        view.setImageDrawable(resource);
    }

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