Glide是一款由Bump Technologies开发的图片加载框架，使得我们可以在Android平台上以极度简单的方式来加载和展示图片。  使用Glide来加载网络图片非常简单，通过Glide.with(this).load(url).into(imageView)这样的一句代码就可以搞定，虽然很简单，但还是需要知其所以然。下面就来梳理一下Glide是如何加载网络图片（不涉及生命周期、缓存等功能点，仅加载网络图片的实现）。

Glide.with(context)

 由于本文不涉及Glide的生命周期，所以直接来看Glide构造方法。

  Glide(@NonNull Context context,@NonNull Engine engine,@NonNull MemoryCache memoryCache,@NonNull BitmapPool bitmapPool,@NonNull ArrayPool arrayPool,@NonNull RequestManagerRetriever requestManagerRetriever,@NonNull ConnectivityMonitorFactory connectivityMonitorFactory,int logLevel,@NonNull RequestOptions defaultRequestOptions,@NonNull Map<Class<?>, TransitionOptions<?, ?>> defaultTransitionOptions) {this.engine = engine;this.bitmapPool = bitmapPool;this.arrayPool = arrayPool;this.memoryCache = memoryCache;this.requestManagerRetriever = requestManagerRetriever;this.connectivityMonitorFactory = connectivityMonitorFactory;DecodeFormat decodeFormat = defaultRequestOptions.getOptions().get(Downsampler.DECODE_FORMAT);bitmapPreFiller = new BitmapPreFiller(memoryCache, bitmapPool, decodeFormat);final Resources resources = context.getResources();//管理组件注册以扩展或替换Glide的默认加载，解码和编码逻辑。registry = new Registry();// Right now we're only using this parser for HEIF images, which are only supported on OMR1+.// If we need this for other file types, we should consider removing this restriction.// Note that order here matters. We want to check the ExifInterface parser first for orientation// and then fall back to DefaultImageHeaderParser for other fields.if (Build.VERSION.SDK_INT >= Build.VERSION_CODES.O_MR1) {registry.register(new ExifInterfaceImageHeaderParser());}registry.register(new DefaultImageHeaderParser());Downsampler downsampler = new Downsampler(registry.getImageHeaderParsers(),resources.getDisplayMetrics(), bitmapPool, arrayPool);ByteBufferGifDecoder byteBufferGifDecoder =new ByteBufferGifDecoder(context, registry.getImageHeaderParsers(), bitmapPool, arrayPool);ResourceDecoder<ParcelFileDescriptor, Bitmap> parcelFileDescriptorVideoDecoder =VideoDecoder.parcel(bitmapPool);ByteBufferBitmapDecoder byteBufferBitmapDecoder = new ByteBufferBitmapDecoder(downsampler);StreamBitmapDecoder streamBitmapDecoder = new StreamBitmapDecoder(downsampler, arrayPool);ResourceDrawableDecoder resourceDrawableDecoder =new ResourceDrawableDecoder(context);ResourceLoader.StreamFactory resourceLoaderStreamFactory =new ResourceLoader.StreamFactory(resources);ResourceLoader.UriFactory resourceLoaderUriFactory =new ResourceLoader.UriFactory(resources);ResourceLoader.FileDescriptorFactory resourceLoaderFileDescriptorFactory =new ResourceLoader.FileDescriptorFactory(resources);ResourceLoader.AssetFileDescriptorFactory resourceLoaderAssetFileDescriptorFactory =new ResourceLoader.AssetFileDescriptorFactory(resources);BitmapEncoder bitmapEncoder = new BitmapEncoder(arrayPool);BitmapBytesTranscoder bitmapBytesTranscoder = new BitmapBytesTranscoder();GifDrawableBytesTranscoder gifDrawableBytesTranscoder = new GifDrawableBytesTranscoder();ContentResolver contentResolver = context.getContentResolver();registry.append(ByteBuffer.class, new ByteBufferEncoder())//编码器，当缓存数据到本地时会用到.append(InputStream.class, new StreamEncoder(arrayPool))/* Bitmaps */.append(Registry.BUCKET_BITMAP, ByteBuffer.class, Bitmap.class, byteBufferBitmapDecoder).append(Registry.BUCKET_BITMAP, InputStream.class, Bitmap.class, streamBitmapDecoder).append(Registry.BUCKET_BITMAP,ParcelFileDescriptor.class,Bitmap.class,parcelFileDescriptorVideoDecoder).append(Registry.BUCKET_BITMAP,AssetFileDescriptor.class,Bitmap.class,VideoDecoder.asset(bitmapPool)).append(Bitmap.class, Bitmap.class, UnitModelLoader.Factory.<Bitmap>getInstance()).append(Registry.BUCKET_BITMAP, Bitmap.class, Bitmap.class, new UnitBitmapDecoder()).append(Bitmap.class, bitmapEncoder)/* BitmapDrawables */.append(Registry.BUCKET_BITMAP_DRAWABLE,ByteBuffer.class,BitmapDrawable.class,new BitmapDrawableDecoder<>(resources, byteBufferBitmapDecoder)).append(Registry.BUCKET_BITMAP_DRAWABLE,InputStream.class,BitmapDrawable.class,new BitmapDrawableDecoder<>(resources, streamBitmapDecoder)).append(Registry.BUCKET_BITMAP_DRAWABLE,ParcelFileDescriptor.class,BitmapDrawable.class,new BitmapDrawableDecoder<>(resources, parcelFileDescriptorVideoDecoder)).append(BitmapDrawable.class, new BitmapDrawableEncoder(bitmapPool, bitmapEncoder))/* GIFs */.append(Registry.BUCKET_GIF,InputStream.class,GifDrawable.class,new StreamGifDecoder(registry.getImageHeaderParsers(), byteBufferGifDecoder, arrayPool)).append(Registry.BUCKET_GIF, ByteBuffer.class, GifDrawable.class, byteBufferGifDecoder).append(GifDrawable.class, new GifDrawableEncoder())/* GIF Frames */// Compilation with Gradle requires the type to be specified for UnitModelLoader here..append(GifDecoder.class, GifDecoder.class, UnitModelLoader.Factory.<GifDecoder>getInstance()).append(Registry.BUCKET_BITMAP,GifDecoder.class,Bitmap.class,new GifFrameResourceDecoder(bitmapPool))/* Drawables */.append(Uri.class, Drawable.class, resourceDrawableDecoder).append(Uri.class, Bitmap.class, new ResourceBitmapDecoder(resourceDrawableDecoder, bitmapPool))/* Files */.register(new ByteBufferRewinder.Factory()).append(File.class, ByteBuffer.class, new ByteBufferFileLoader.Factory()).append(File.class, InputStream.class, new FileLoader.StreamFactory()).append(File.class, File.class, new FileDecoder()).append(File.class, ParcelFileDescriptor.class, new FileLoader.FileDescriptorFactory())// Compilation with Gradle requires the type to be specified for UnitModelLoader here..append(File.class, File.class, UnitModelLoader.Factory.<File>getInstance())/* Models */.register(new InputStreamRewinder.Factory(arrayPool)).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>()).append(String.class, InputStream.class, new StringLoader.StreamFactory()).append(String.class, ParcelFileDescriptor.class, new StringLoader.FileDescriptorFactory()).append(String.class, AssetFileDescriptor.class, new StringLoader.AssetFileDescriptorFactory()).append(Uri.class, InputStream.class, new HttpUriLoader.Factory()).append(Uri.class, InputStream.class, new AssetUriLoader.StreamFactory(context.getAssets())).append(Uri.class,ParcelFileDescriptor.class,new AssetUriLoader.FileDescriptorFactory(context.getAssets())).append(Uri.class, InputStream.class, new MediaStoreImageThumbLoader.Factory(context)).append(Uri.class, InputStream.class, new MediaStoreVideoThumbLoader.Factory(context)).append(Uri.class,InputStream.class,new UriLoader.StreamFactory(contentResolver)).append(Uri.class,ParcelFileDescriptor.class,new UriLoader.FileDescriptorFactory(contentResolver)).append(Uri.class,AssetFileDescriptor.class,new UriLoader.AssetFileDescriptorFactory(contentResolver)).append(Uri.class, InputStream.class, new UrlUriLoader.StreamFactory()).append(URL.class, InputStream.class, new UrlLoader.StreamFactory()).append(Uri.class, File.class, new MediaStoreFileLoader.Factory(context))//进行网络下载.append(GlideUrl.class, InputStream.class, new HttpGlideUrlLoader.Factory()).append(byte[].class, ByteBuffer.class, new ByteArrayLoader.ByteBufferFactory()).append(byte[].class, InputStream.class, new ByteArrayLoader.StreamFactory()).append(Uri.class, Uri.class, UnitModelLoader.Factory.<Uri>getInstance()).append(Drawable.class, Drawable.class, UnitModelLoader.Factory.<Drawable>getInstance()).append(Drawable.class, Drawable.class, new UnitDrawableDecoder())/* Transcoders */.register(Bitmap.class,BitmapDrawable.class,new BitmapDrawableTranscoder(resources)).register(Bitmap.class, byte[].class, bitmapBytesTranscoder).register(Drawable.class,byte[].class,new DrawableBytesTranscoder(bitmapPool, bitmapBytesTranscoder, gifDrawableBytesTranscoder)).register(GifDrawable.class, byte[].class, gifDrawableBytesTranscoder);...}复制代码

 构造方法最重要的就是Register这个类，它主要是用于管理组件注册以扩展或替换Glide的默认加载，解码和编码逻辑，比如我们可以使用giflib替换Glide自带的GIF解码器，来提高性能，可以使用OKHttp来替换Glide默认的下载实现，也可以自己定义比Glide默认性能更好的编解码器等。构造方法里默认注册了HttpGlideUrlLoader这个类，为什么要说这个类尼，因为默认的下载实现就在这个类。  with是Glide的一组静态方法，它里面有关于Glide生命周期的实现。来看看这组静态方法的实现。

  public static RequestManager with(@NonNull FragmentActivity activity) {return getRetriever(activity).get(activity);}public static RequestManager with(@NonNull Activity activity) {return getRetriever(activity).get(activity);}...
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 从上面可以看出with都返回了一个RequestManager对象。

load(url)

 load是RequestManager里的一组方法，根据传入参数来不同的实现，这里就以传入一个字符串的url为例。

  public RequestBuilder<Drawable> load(@Nullable String string) {return asDrawable().load(string);}//这里代表返回一个Drawable类型的图片public RequestBuilder<Drawable> asDrawable() {return as(Drawable.class);}//创建一个RequestBuilder对象。public <ResourceType> RequestBuilder<ResourceType> as(@NonNull Class<ResourceType> resourceClass) {//resourceClass对应着RequestBuilder的transcodeClass变量return new RequestBuilder<>(glide, this, resourceClass, context);}
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 RequestBuilder对象创建成功后，在调用load方法将图片路径赋值给model这个变量。

into(imageView)

 前面快速介绍了Glide.with(context)与load(url)的实现，虽然它们也并不简单，但它们也没有实现图片的下载，那么图片的下载是从哪里开始的尼？就是通过into方法来实现的，来看一下into方法的实现。

  public ViewTarget<ImageView, TranscodeType> into(@NonNull ImageView view) {...return into(//transcodeClass就是前面传递过来的Drawable.classglideContext.buildImageViewTarget(view, transcodeClass),/*targetListener=*/ null,requestOptions);}
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 由于transcodeClass是一个Drawable类型，所以glideContext.buildImageViewTarget(view, transcodeClass)创建了一个DrawableImageViewTarget对象，来看看DrawableImageViewTarget的实现。

public class DrawableImageViewTarget extends ImageViewTarget<Drawable> {public DrawableImageViewTarget(ImageView view) {super(view);}/*** @deprecated Use {@link #waitForLayout()} instead.*/// Public API.@SuppressWarnings({"unused", "deprecation"})@Deprecatedpublic DrawableImageViewTarget(ImageView view, boolean waitForLayout) {super(view, waitForLayout);}//这里是不是很熟悉啊，就是展示图片@Overrideprotected void setResource(@Nullable Drawable resource) {view.setImageDrawable(resource);}
}
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 再来看into方法的实现。

  private <Y extends Target<TranscodeType>> Y into(//target就是前面创建的DrawableImageViewTarget对象@NonNull Y target,@Nullable RequestListener<TranscodeType> targetListener,@NonNull RequestOptions options) {...//创建一个Request对象，默认是SingleRequest对象Request request = buildRequest(target, targetListener, options);//target就是前面创建的DrawableImageViewTarget对象//拿到target中的Request对象Request previous = target.getRequest();if (request.isEquivalentTo(previous)&& !isSkipMemoryCacheWithCompletePreviousRequest(options, previous)) {//释放Request对象，因为target已存在同样的Request对象了request.recycle();if (!Preconditions.checkNotNull(previous).isRunning()) {//如果没有加载就开始加载previous.begin();}return target;}requestManager.clear(target);//设置target的Requesttarget.setRequest(request);//开始下载requestManager.track(target, request);return target;}
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 默认创建的Request对象是SingleRequest，由于本文分析的是第一次加载图片，所以我们来看RequestManager的track方法。

  void track(@NonNull Target<?> target, @NonNull Request request) {...requestTracker.runRequest(request);}//RequestTracker中public void runRequest(@NonNull Request request) {requests.add(request);if (!isPaused) {//当没有暂停时Request就开始执行request.begin();} else {//暂停执行request.clear();...pendingRequests.add(request);}}
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 由于这里Request的具体实现是SingleRequest，所以我们来看它的begin方法。

  @Overridepublic void begin() {...//传入的model为null，在本文中就是传入的URL为nullif (model == null) {if (Util.isValidDimensions(overrideWidth, overrideHeight)) {width = overrideWidth;height = overrideHeight;}...//图片加载失败onLoadFailed(new GlideException("Received null model"), logLevel);return;}//从缓存中拿数据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 {//根据View的宽高来计算出图片的宽高，最后回调的也是onSizeReady方法target.getSize(this);}if ((status == Status.RUNNING || status == Status.WAITING_FOR_SIZE)&& canNotifyStatusChanged()) {//图片开始加载时的默认显示target.onLoadStarted(getPlaceholderDrawable());}...}}public void onSizeReady(int width, int height) {stateVerifier.throwIfRecycled();...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);...}}
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 如果图片的宽高已经确定就直接调用onSizeReady，否先确定宽高再调用onSizeReady方法，该方法中最关键的是调用Engine的load方法，来看一下实现。

  public <R> LoadStatus load(GlideContext glideContext,Object model,Key signature,int width,int height,Class<?> resourceClass,Class<R> transcodeClass,Priority priority,DiskCacheStrategy diskCacheStrategy,Map<Class<?>, Transformation<?>> transformations,boolean isTransformationRequired,boolean isScaleOnlyOrNoTransform,Options options,boolean isMemoryCacheable,boolean useUnlimitedSourceExecutorPool,boolean useAnimationPool,boolean onlyRetrieveFromCache,ResourceCallback cb) {...//根据model计算出每张图片对应的keyEngineKey key = keyFactory.buildKey(model, signature, width, height, transformations,resourceClass, transcodeClass, options);//从ActiveResources中获取图片，ActiveResources是一个弱引用的HashMapEngineResource<?> active = loadFromActiveResources(key, isMemoryCacheable);if (active != null) {cb.onResourceReady(active, DataSource.MEMORY_CACHE);return null;}//从缓存中获取图片，采用了LrucacheEngineResource<?> cached = loadFromCache(key, isMemoryCacheable);if (cached != null) {cb.onResourceReady(cached, DataSource.MEMORY_CACHE);return null;}EngineJob<?> current = jobs.get(key, onlyRetrieveFromCache);if (current != null) {current.addCallback(cb);return new LoadStatus(cb, current);}EngineJob<R> engineJob =engineJobFactory.build(key,isMemoryCacheable,useUnlimitedSourceExecutorPool,useAnimationPool,onlyRetrieveFromCache);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);//从本地或者网络获取数据，会切换到子线程engineJob.start(decodeJob);return new LoadStatus(cb, engineJob);}
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 Glide会首先从缓存中获取数据，如果没有的话再从网络获取。EngineJob与DecodeJob两个类非常重要，EngineJob主要进行线程之间的切换，DecodeJob主要是从本地或者网络获取数据的实现，来看EngineJob的start的实现。

  public void start(DecodeJob<R> decodeJob) {this.decodeJob = decodeJob;GlideExecutor executor = decodeJob.willDecodeFromCache()? diskCacheExecutor: getActiveSourceExecutor();//向子线程添加任务executor.execute(decodeJob);}
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 在这里就切换到下载图片线程，由于DecodeJob实现了Runnable接口，所以就来看run的方法。

 public void run() {...try {if (isCancelled) {notifyFailed();return;}runWrapped();} catch (Throwable t) {...} finally {...}}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);}}
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 很明显这里的重点是runGenerators，来看看runGenerators的实现。

  private void runGenerators() {...boolean isStarted = false;while (!isCancelled && currentGenerator != null&& !(isStarted = currentGenerator.startNext())) {stage = getNextStage(stage);currentGenerator = getNextGenerator();//退出循环if (stage == Stage.SOURCE) {reschedule();return;}}// We've run out of stages and generators, give up.if ((stage == Stage.FINISHED || isCancelled) && !isStarted) {notifyFailed();}}
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 由于这里不涉及到缓存，所以调用SourceGenerator的startNext的方法，当网络返回数据时则dataToCache不为null，就会存储数据到本地。否则就从网络获取数据。

  public boolean startNext() {//当下载成功后，dataToCache 则不为null，需要写入缓存，后面会用到if (dataToCache != null) {Object data = dataToCache;dataToCache = null;cacheData(data);}//从缓存拿数据if (sourceCacheGenerator != null && sourceCacheGenerator.startNext()) {return true;}//从网络获取数据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;//加载数据，loadData的实现是MultiModelLoader，loadData.fetcher的实现是MultiFetcherloadData.fetcher.loadData(helper.getPriority(), this);}}return started;}
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 这里的loadData的实现是MultiModelLoader，fetcher的实现是MultiFetcher，然后调用loadData方法来加载数据。

    @Overridepublic void loadData(@NonNull Priority priority, @NonNull DataCallback<? super Data> callback) {...//这里的Fetcher是可以定制的，默认实现是HttpUrlFetcherfetchers.get(currentIndex).loadData(priority, this);}
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 由于我们没有任何定制fetcher，所以调用的是HttpUrlFetcher的load方法。

  @Overridepublic void loadData(@NonNull Priority priority,@NonNull DataCallback<? super InputStream> callback) {long startTime = LogTime.getLogTime();try {//请求网络并下载InputStream result = loadDataWithRedirects(glideUrl.toURL(), 0, null, glideUrl.getHeaders());//将值传递回去callback.onDataReady(result);} catch (IOException e) {...callback.onLoadFailed(e);} finally {...}}private InputStream loadDataWithRedirects(URL url, int redirects, URL lastUrl,Map<String, String> headers) throws IOException {if (redirects >= MAXIMUM_REDIRECTS) {throw new HttpException("Too many (> " + MAXIMUM_REDIRECTS + ") redirects!");} else {// Comparing the URLs using .equals performs additional network I/O and is generally broken.// See http://michaelscharf.blogspot.com/2006/11/javaneturlequals-and-hashcode-make.html.try {if (lastUrl != null && url.toURI().equals(lastUrl.toURI())) {throw new HttpException("In re-direct loop");}} catch (URISyntaxException e) {// Do nothing, this is best effort.}}urlConnection = connectionFactory.build(url);for (Map.Entry<String, String> headerEntry : headers.entrySet()) {urlConnection.addRequestProperty(headerEntry.getKey(), headerEntry.getValue());}urlConnection.setConnectTimeout(timeout);urlConnection.setReadTimeout(timeout);urlConnection.setUseCaches(false);urlConnection.setDoInput(true);// Stop the urlConnection instance of HttpUrlConnection from following redirects so that// redirects will be handled by recursive calls to this method, loadDataWithRedirects.urlConnection.setInstanceFollowRedirects(false);// Connect explicitly to avoid errors in decoders if connection fails.urlConnection.connect();// Set the stream so that it's closed in cleanup to avoid resource leaks. See #2352.stream = urlConnection.getInputStream();if (isCancelled) {return null;}final int statusCode = urlConnection.getResponseCode();if (isHttpOk(statusCode)) {return getStreamForSuccessfulRequest(urlConnection);} else if (isHttpRedirect(statusCode)) {String redirectUrlString = urlConnection.getHeaderField("Location");if (TextUtils.isEmpty(redirectUrlString)) {throw new HttpException("Received empty or null redirect url");}URL redirectUrl = new URL(url, redirectUrlString);// Closing the stream specifically is required to avoid leaking ResponseBodys in addition// to disconnecting the url connection below. See #2352.cleanup();return loadDataWithRedirects(redirectUrl, redirects + 1, url, headers);} else if (statusCode == INVALID_STATUS_CODE) {throw new HttpException(statusCode);} else {throw new HttpException(urlConnection.getResponseMessage(), statusCode);}}复制代码

 到这里想必大家就很熟悉了，就是连接网络并下载图片。将数据通过callback.onDataReady(result);返回，这个callback其实就是MultiFetcher。

    public void onDataReady(@Nullable Data data) {if (data != null) {callback.onDataReady(data);} else {startNextOrFail();}}
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 这个callback其实就是SourceGenerator。

  public void onDataReady(Object data) {DiskCacheStrategy diskCacheStrategy = helper.getDiskCacheStrategy();if (data != null && diskCacheStrategy.isDataCacheable(loadData.fetcher.getDataSource())) {dataToCache = data;...cb.reschedule();} else {...}}
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 这个cb其实就是DecodeJob。

  public void reschedule() {//切换runReason的值runReason = RunReason.SWITCH_TO_SOURCE_SERVICE;callback.reschedule(this);}
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 这个callback就是EngineJob，再来看它的reschedule方法。

  @Overridepublic void reschedule(DecodeJob<?> job) {//将任务提交给线程池getActiveSourceExecutor().execute(job);}
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 这里是切换到缓存数据线程，那么就会执行DecodeJob的run方法，前面介绍过，在该方法内执行的是runWrapped方法，由于前面将runReason的值修改为SWITCH_TO_SOURCE_SERVICE，所以就会直接执行runGenerators然后再次调用SourceGenerator的startNext方法，前面在介绍该方法时，说过如果有数据就写入缓存，这时候就会将数据写入缓存并调用DataCacheGenerator的startNext方法。

  @Overridepublic boolean startNext() {...loadData = null;boolean started = false;while (!started && hasNextModelLoader()) {ModelLoader<File, ?> modelLoader = modelLoaders.get(modelLoaderIndex++);loadData =modelLoader.buildLoadData(cacheFile, helper.getWidth(), helper.getHeight(),helper.getOptions());if (loadData != null && helper.hasLoadPath(loadData.fetcher.getDataClass())) {started = true;//加载从缓存中获取数据//loadData的实现类是ByteBufferFileLoader//loadData.fetcher的实现类是ByteBufferFetcherloadData.fetcher.loadData(helper.getPriority(), this);}}return started;}
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 这里就调用了ByteBufferFetcher的loadData方法。

    @Overridepublic void loadData(@NonNull Priority priority,@NonNull DataCallback<? super ByteBuffer> callback) {ByteBuffer result;try {//从缓存从获取数据result = ByteBufferUtil.fromFile(file);} catch (IOException e) {...//数据获取失败callback.onLoadFailed(e);return;}//数据获取成功callback.onDataReady(result);}
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 这里的callback就是DataCacheGenerator。

  @Overridepublic void onDataReady(Object data) {cb.onDataFetcherReady(sourceKey, data, loadData.fetcher, DataSource.DATA_DISK_CACHE, sourceKey);}
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 这里的cb就是SourceGenerator。

  @Overridepublic void onDataFetcherReady(Key sourceKey, Object data, DataFetcher<?> fetcher,DataSource dataSource, Key attemptedKey) {...cb.onDataFetcherReady(sourceKey, data, fetcher, loadData.fetcher.getDataSource(), sourceKey);}
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 前面说过SourceGenerator中的cb就是DecodeJob。

  @Overridepublic void onDataFetcherReady(Key sourceKey, Object data, DataFetcher<?> fetcher,DataSource dataSource, Key attemptedKey) {...if (Thread.currentThread() != currentThread) {runReason = RunReason.DECODE_DATA;callback.reschedule(this);} else {...try {decodeFromRetrievedData();} finally {...}}}private void decodeFromRetrievedData() {...Resource<R> resource = null;try {resource = decodeFromData(currentFetcher, currentData, currentDataSource);} catch (GlideException e) {...}if (resource != null) {notifyEncodeAndRelease(resource, currentDataSource);} else {runGenerators();}}private void notifyEncodeAndRelease(Resource<R> resource, DataSource dataSource) {...//该将图片显示在ImageView上了notifyComplete(result, dataSource);...}private void notifyComplete(Resource<R> resource, DataSource dataSource) {setNotifiedOrThrow();callback.onResourceReady(resource, dataSource);}
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 前面说过这个callback就是EngineJob。

  @Overridepublic void onResourceReady(Resource<R> resource, DataSource dataSource) {...//切换回主线程MAIN_THREAD_HANDLER.obtainMessage(MSG_COMPLETE, this).sendToTarget();}void handleResultOnMainThread() {...//noinspection ForLoopReplaceableByForEach to improve perffor (int i = 0, size = cbs.size(); i < size; i++) {ResourceCallback cb = cbs.get(i);if (!isInIgnoredCallbacks(cb)) {engineResource.acquire();cb.onResourceReady(engineResource, dataSource);}}...release(false /*isRemovedFromQueue*/);}
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 在EngineJob的onResourceReady中切换回主线程。并将数据传给cb的onResourceReady方法，这里的cb是SingleRequest。

  public void onResourceReady(Resource<?> resource, DataSource dataSource) {...onResourceReady((Resource<R>) resource, (R) received, dataSource);}private void onResourceReady(Resource<R> resource, R result, DataSource dataSource) {...try {...if (!anyListenerHandledUpdatingTarget) {Transition<? super R> animation =animationFactory.build(dataSource, isFirstResource);//展示图片target.onResourceReady(result, animation);}} finally {isCallingCallbacks = false;}notifyLoadSuccess();}
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 在前面说过，target的实现类是DrawableImageViewTarget。但在该类中并没有onResourceReady这个方法，于是去父类查找。

  @Overridepublic void onResourceReady(@NonNull Z resource, @Nullable Transition<? super Z> transition) {if (transition == null || !transition.transition(resource, this)) {setResourceInternal(resource);} else {maybeUpdateAnimatable(resource);}}private void setResourceInternal(@Nullable Z resource) {//setResource是个抽象方法setResource(resource);maybeUpdateAnimatable(resource);}
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 setResource是个抽象方法，在DrawableImageViewTarget中实现。

  @Overrideprotected void setResource(@Nullable Drawable resource) {//view是ImageViewview.setImageDrawable(resource);}
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 这里相必大家都很熟悉了吧。  到此Glide加载网络图片的流程就完结了，太复杂了，特别是into方法，由于很复杂，所以画了张时序图，如下：

 建议结合源码一起阅读。