深入解析Glide源码

Glide 是 Google的开源项目， Glide具有获取、解码和展示视频剧照、图片、动画等功能，它还有灵活的API，这些API使开发者能够将Glide应用在几乎任何网络协议栈里。创建Glide的主要目的有两个，一个是实现平滑的图片列表滚动效果，另一个是支持远程图片的获取、大小调整和展示。本篇博客，我们一起深入分析Glide的源码。

总体设计

with方法

首先我们来看一下glide的一般使用方法

Glide.with(this).load("http://image.baidu.com/search/detail?ct=503316480&z=0&ipn=d&word=%E5%9B%BE%E7%89%87&hs=0&pn=0&spn=0&di=157484237350&pi=0&rn=1&tn=baiduimagedetail&is=0%2C0&ie=utf-8&oe=utf-8&cl=2&lm=-1&cs=4271053251%2C2424464488&os=2375022793%2C1835605452&simid=4247939438%2C550168575&adpicid=0&lpn=0&ln=30&fr=ala&fm=&sme=&cg=&bdtype=0&oriquery=&objurl=http%3A%2F%2Fpic55.nipic.com%2Ffile%2F20141208%2F19462408_171130083000_2.jpg&fromurl=ippr_z2C%24qAzdH3FAzdH3Fooo_z%26e3Bgtrtv_z%26e3Bv54AzdH3Fzi7wgptAzdH3F8c8n9d9_d_z%26e3Bip4s&gsm=0").into(mImageView);

with方法主要就是用于创建RequestManager，这里是Glide通过Activity/Fragment生命周期管理Request原理所在，流程图如下：

那我们就从with方法入手，with方法传入的参数可以是Context，Activity，Fragment或FragmentActiviy，以FragmentActiviy为例子

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

获取了RequestManagerRetriever，并调用了其get方法

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

判断是否在主线程中调用，如果是子线程中，则调用get(activity.getApplicationContext())

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);
}

很明显传入的context是ApplicationContext,所以会调用getApplicationManager(context)

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.applicationManager = new RequestManager(context.getApplicationContext(),new ApplicationLifecycle(), new EmptyRequestManagerTreeNode());}}}return applicationManager;
}

创建了一个RequestManager，并将其返回，这里的ApplicationLifecycle很重要，下面会提到，再回到之前的get方法

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

如果在主线程中调用get方法，则会通过获取传入activity的FragmentManager，并将其传入到supportFragmentGet(activity, fm);方法中

RequestManager supportFragmentGet(Context context, FragmentManager fm) {SupportRequestManagerFragment current = getSupportRequestManagerFragment(fm);RequestManager requestManager = current.getRequestManager();if (requestManager == null) {requestManager = new RequestManager(context, current.getLifecycle(), current.getRequestManagerTreeNode());current.setRequestManager(requestManager);}return requestManager;
}

首先是调用getSupportRequestManagerFragment方法获取一个SupportRequestManagerFragment，SupportRequestManagerFragment 是一个无界面的Fragment类，起到把请求和Activity生命周期同步的作用。

SupportRequestManagerFragment getSupportRequestManagerFragment(final FragmentManager fm) {SupportRequestManagerFragment current = (SupportRequestManagerFragment) fm.findFragmentByTag(FRAGMENT_TAG);if (current == null) {current = pendingSupportRequestManagerFragments.get(fm);if (current == null) {current = new SupportRequestManagerFragment();pendingSupportRequestManagerFragments.put(fm, current);fm.beginTransaction().add(current, FRAGMENT_TAG).commitAllowingStateLoss();handler.obtainMessage(ID_REMOVE_SUPPORT_FRAGMENT_MANAGER, fm).sendToTarget();}}return current;
}

可以看到 getSupportRequestManagerFragment中，会先去判断传入的FragmentManager 中是否已经绑定了这个SupportRequestManagerFragment ，没有就将其创建，并添加到FragmentManager 中

我们可以看到SupportRequestManagerFragment 中，生命周期的方法，都会同步调用lifecycle的相应方法

@Override
public void onStart() {super.onStart();lifecycle.onStart();
}@Override
public void onStop() {super.onStop();lifecycle.onStop();
}

而这个lifecycle就是ActivityFragmentLifecycle，它将用了观察者模式，可以发现addListener就是添加观察者，然后每一个生命周期方法，同步通知给每一个listener

@Override
public void addListener(LifecycleListener listener) {lifecycleListeners.add(listener);if (isDestroyed) {listener.onDestroy();} else if (isStarted) {listener.onStart();} else {listener.onStop();}
}void onStart() {isStarted = true;for (LifecycleListener lifecycleListener : Util.getSnapshot(lifecycleListeners)) {lifecycleListener.onStart();}
}

回到supportFragmentGet方法中

RequestManager supportFragmentGet(Context context, FragmentManager fm) {SupportRequestManagerFragment current = getSupportRequestManagerFragment(fm);RequestManager requestManager = current.getRequestManager();if (requestManager == null) {requestManager = new RequestManager(context, current.getLifecycle(), current.getRequestManagerTreeNode());current.setRequestManager(requestManager);}return requestManager;
}

接下来就是获取SupportRequestManagerFragment 中的RequestManager，判断其是否存在，不在则进行创建，然后和SupportRequestManagerFragment 进行绑定，这里需要强调的是，构建RequestManager时，将SupportRequestManagerFragment 的lifecycle传入，我们看看RequestManager的构造函数里面

RequestManager(Context context, final Lifecycle lifecycle, RequestManagerTreeNode treeNode,RequestTracker requestTracker, ConnectivityMonitorFactory factory) {this.context = context.getApplicationContext();this.lifecycle = lifecycle;this.treeNode = treeNode;this.requestTracker = requestTracker;this.glide = Glide.get(context);this.optionsApplier = new OptionsApplier();ConnectivityMonitor connectivityMonitor = factory.build(context,new RequestManagerConnectivityListener(requestTracker));// If we're the application level request manager, we may be created on a background thread. In that case we// cannot risk synchronously pausing or resuming requests, so we hack around the issue by delaying adding// ourselves as a lifecycle listener by posting to the main thread. This should be entirely safe.if (Util.isOnBackgroundThread()) {new Handler(Looper.getMainLooper()).post(new Runnable() {@Overridepublic void run() {lifecycle.addListener(RequestManager.this);}});} else {lifecycle.addListener(this);}lifecycle.addListener(connectivityMonitor);
}

可以发现，lifecycle.addListener方法，将RequestManager自身添加到lifecycle中，这里的巧妙之处，在于，SupportRequestManagerFragment 的生命周期的方法，都会同步通知lifecycle的所有Listener，所以SupportRequestManagerFragment 的生命周期都可同步通知RequestManager，这就是glide的巧妙之处。

上面还做了判断，如果这个RequestManager是application级别的，将其post到主线程中，再调用 lifecycle.addListener(RequestManager.this);方法，保证生命周期的同步效果

到这里，with方法的逻辑就执行完毕了，那么总结一下这个方法的作用，通过with方法中的参数不同，调用RequestManagerRetriever的get方法，最终创建一个空的Fragment 名为SupportRequestManagerFragment 并绑定一个RequestManager

load方法

with方法返回的是一个RequestManager，那么load方法的逻辑显然就是在RequestManager中

load调用处理流程图：

public DrawableTypeRequest<String> load(String string) {return (DrawableTypeRequest<String>) fromString().load(string);
}

会先调用fromString再调用load方法

public DrawableTypeRequest<String> fromString() {return loadGeneric(String.class);
}

private <T> DrawableTypeRequest<T> loadGeneric(Class<T> modelClass) {ModelLoader<T, InputStream> streamModelLoader = Glide.buildStreamModelLoader(modelClass, context);ModelLoader<T, ParcelFileDescriptor> fileDescriptorModelLoader =Glide.buildFileDescriptorModelLoader(modelClass, context);if (modelClass != null && streamModelLoader == null && fileDescriptorModelLoader == null) {throw new IllegalArgumentException("Unknown type " + modelClass + ". You must provide a Model of a type for"+ " which there is a registered ModelLoader, if you are using a custom model, you must first call"+ " Glide#register with a ModelLoaderFactory for your custom model class");}return optionsApplier.apply(new DrawableTypeRequest<T>(modelClass, streamModelLoader, fileDescriptorModelLoader, context,glide, requestTracker, lifecycle, optionsApplier));
}

loadGeneric方法中创建了两个ModelLoader，分别是streamModelLoader 和fileDescriptorModelLoader，然后通过他们构造了一个DrawableTypeRequest

其实就是通过工厂模式创建相应的类型的ModelLoader，我们以Glide.buildStreamModelLoader为例

public static <T> ModelLoader<T, InputStream> buildStreamModelLoader(Class<T> modelClass, Context context) {return buildModelLoader(modelClass, InputStream.class, context);
}

这里传递了InputStream.class

public static <T, Y> ModelLoader<T, Y> buildModelLoader(Class<T> modelClass, Class<Y> resourceClass,Context context) {if (modelClass == null) {if (Log.isLoggable(TAG, Log.DEBUG)) {Log.d(TAG, "Unable to load null model, setting placeholder only");}return null;}return Glide.get(context).getLoaderFactory().buildModelLoader(modelClass, resourceClass);
}

通过Glide.get(context).getLoaderFactory()获取GenericLoaderFactory并调用其buildModelLoader创建InputStream.class对应的ModelLoader

public synchronized <T, Y> ModelLoader<T, Y> buildModelLoader(Class<T> modelClass, Class<Y> resourceClass) {ModelLoader<T, Y> result = getCachedLoader(modelClass, resourceClass);if (result != null) {// We've already tried to create a model loader and can't with the currently registered set of factories,// but we can't use null to demonstrate that failure because model loaders that haven't been requested// yet will be null in the cache. To avoid this, we use a special signal model loader.if (NULL_MODEL_LOADER.equals(result)) {return null;} else {return result;}}final ModelLoaderFactory<T, Y> factory = getFactory(modelClass, resourceClass);if (factory != null) {result = factory.build(context, this);cacheModelLoader(modelClass, resourceClass, result);} else {// We can't generate a model loader for the given arguments with the currently registered set of factories.cacheNullLoader(modelClass, resourceClass);}return result;
}

getCachedLoader方法判断InputStream.class对应的ModelLoader是否已经创建过，也就是是否存在内存缓存中，有则返回

然后调用了getFactory方法，根据InputStream.class，创建其对应的ModelLoaderFactory

private <T, Y> ModelLoaderFactory<T, Y> getFactory(Class<T> modelClass, Class<Y> resourceClass) {Map<Class/*Y*/, ModelLoaderFactory/*T, Y*/> resourceToFactories = modelClassToResourceFactories.get(modelClass);ModelLoaderFactory/*T, Y*/ result = null;if (resourceToFactories != null) {result = resourceToFactories.get(resourceClass);}if (result == null) {for (Class<? super T> registeredModelClass : modelClassToResourceFactories.keySet()) {// This accounts for model subclasses, our map only works for exact matches. We should however still// match a subclass of a model with a factory for a super class of that model if if there isn't a// factory for that particular subclass. Uris are a great example of when this happens, most uris// are actually subclasses for Uri, but we'd generally rather load them all with the same factory rather// than trying to register for each subclass individually.if (registeredModelClass.isAssignableFrom(modelClass)) {Map<Class/*Y*/, ModelLoaderFactory/*T, Y*/> currentResourceToFactories =modelClassToResourceFactories.get(registeredModelClass);if (currentResourceToFactories != null) {result = currentResourceToFactories.get(resourceClass);if (result != null) {break;}}}}}return result;
}

可以发现ModelLoaderFactory是保存在一个叫resourceToFactories的Map 中，而这个Map又是存储在名叫modelClassToResourceFactories的Map 中

然后是调用ModelLoaderFactory的build方法创建ModelLoader，然后通过cacheModelLoader方法将ModelLoader进行缓存起来

private <T, Y> void cacheModelLoader(Class<T> modelClass, Class<Y> resourceClass, ModelLoader<T, Y> modelLoader) {Map<Class/*Y*/, ModelLoader/*T, Y*/> resourceToLoaders = cachedModelLoaders.get(modelClass);if (resourceToLoaders == null) {resourceToLoaders = new HashMap<Class/*Y*/, ModelLoader/*T, Y*/>();cachedModelLoaders.put(modelClass, resourceToLoaders);}resourceToLoaders.put(resourceClass, modelLoader);
}

fromString的流程就分析完毕，然后回到RequestManager的load方法

public DrawableTypeRequest<String> load(String string) {return (DrawableTypeRequest<String>) fromString().load(string);
}

fromString返回DrawableTypeRequest，所以load方法在DrawableTypeRequest中

@Override
public DrawableRequestBuilder<ModelType> load(ModelType model) {super.load(model);return this;
}

public GenericRequestBuilder<ModelType, DataType, ResourceType, TranscodeType> load(ModelType model) {this.model = model;isModelSet = true;return this;
}

只是对我们传进来的图片的uri进行赋值

Into方法

into方法是加载图片的发起点，其逻辑在DrawableRequestBuilder类中

public Target<GlideDrawable> into(ImageView view) {return super.into(view);
}

调用了GenericRequestBuilder的into方法

ublic Target<TranscodeType> into(ImageView view) {Util.assertMainThread();if (view == null) {throw new IllegalArgumentException("You must pass in a non null View");}if (!isTransformationSet && view.getScaleType() != null) {switch (view.getScaleType()) {case CENTER_CROP:applyCenterCrop();break;case FIT_CENTER:case FIT_START:case FIT_END:applyFitCenter();break;//$CASES-OMITTED$default:// Do nothing.}}return into(glide.buildImageViewTarget(view, transcodeClass));
}

根据不同的ImageView 的ScaleType有不同的处理逻辑，applyCenterCrop()方法和 applyFitCenter()的实现都是在子类DrawableRequestBuilder中

public DrawableRequestBuilder<ModelType> fitCenter() {return transform(glide.getDrawableFitCenter());
}

然后调用了transform方法

public DrawableRequestBuilder<ModelType> transform(Transformation<GifBitmapWrapper>... transformation) {super.transform(transformation);return this;
}

public GenericRequestBuilder<ModelType, DataType, ResourceType, TranscodeType> transform(Transformation<ResourceType>... transformations) {isTransformationSet = true;if (transformations.length == 1) {transformation = transformations[0];} else {transformation = new MultiTransformation<ResourceType>(transformations);}return this;
}

其实就是对GenericRequestBuilder中的transformation 进行赋值，而这个transformation 是通过glide.getDrawableFitCenter()方法进行赋值

GifBitmapWrapperTransformation getDrawableFitCenter() {return drawableFitCenter;
}

而这个drawableFitCenter是在Glide的构造函数中进行赋值的

bitmapFitCenter = new FitCenter(bitmapPool);
drawableFitCenter = new GifBitmapWrapperTransformation(bitmapPool, bitmapFitCenter);

FitCenter是BitmapTransformation的子类，BitmapTransformation 实现了Transformation

接下来看一下GifBitmapWrapperTransformation的创建过程

public GifBitmapWrapperTransformation(BitmapPool bitmapPool, Transformation<Bitmap> bitmapTransformation) {this(bitmapTransformation, new GifDrawableTransformation(bitmapTransformation, bitmapPool));
}

传入了FitCenter和BitmapPool （此类很重要，后面会讲解），里面又创建了GifDrawableTransformation来包裹这两个参数

GifBitmapWrapperTransformation(Transformation<Bitmap> bitmapTransformation,Transformation<GifDrawable> gifDataTransformation) {this.bitmapTransformation = bitmapTransformation;this.gifDataTransformation = gifDataTransformation;
}

然后调用了glide.buildImageViewTarget(view, transcodeClass)

<R> Target<R> buildImageViewTarget(ImageView imageView, Class<R> transcodedClass) {return imageViewTargetFactory.buildTarget(imageView, transcodedClass);
}

这里的transcodedClass传入的是 GlideDrawable

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

根据clazz的类型不同，构建不同的ImageViewTarget

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

无论哪种类型的ImageViewTarget都调用了父类ImageViewTarget的构造方法

public ImageViewTarget(ImageView view) {super(view);
}public ViewTarget(T view) {if (view == null) {throw new NullPointerException("View must not be null!");}this.view = view;sizeDeterminer = new SizeDeterminer(view);
}
public SizeDeterminer(View view) {this.view = view;
}

然后这个GlideDrawableImageViewTarget会被传入到into方法中

public <Y extends Target<TranscodeType>> Y into(Y target) {Util.assertMainThread();if (target == null) {throw new IllegalArgumentException("You must pass in a non null Target");}if (!isModelSet) {throw new IllegalArgumentException("You must first set a model (try #load())");}Request previous = target.getRequest();if (previous != null) {previous.clear();requestTracker.removeRequest(previous);previous.recycle();}Request request = buildRequest(target);target.setRequest(request);lifecycle.addListener(target);requestTracker.runRequest(request);return target;
}

判断之前针对这个imageView创建的GlideDrawableImageViewTarget是否已经创建了Request ，如果已经存在则进行清空，并从requestTracker中移除，如果之前从创建过，就首先调用buildRequest方法创建Request

private Request buildRequest(Target<TranscodeType> target) {if (priority == null) {priority = Priority.NORMAL;}return buildRequestRecursive(target, null);
}

这里有Request 优先级的设置

private Request obtainRequest(Target<TranscodeType> target, float sizeMultiplier, Priority priority,RequestCoordinator requestCoordinator) {return GenericRequest.obtain(loadProvider,model,signature,context,priority,target,sizeMultiplier,placeholderDrawable,placeholderId,errorPlaceholder,errorId,fallbackDrawable,fallbackResource,requestListener,requestCoordinator,glide.getEngine(),transformation,transcodeClass,isCacheable,animationFactory,overrideWidth,overrideHeight,diskCacheStrategy);
}

进入到obtain方法中

GenericRequest<A, T, Z, R> request = (GenericRequest<A, T, Z, R>) REQUEST_POOL.poll();
if (request == null) {request = new GenericRequest<A, T, Z, R>();
}
request.init(loadProvider,model,signature,context,priority,target,sizeMultiplier,placeholderDrawable,placeholderResourceId,errorDrawable,errorResourceId,fallbackDrawable,fallbackResourceId,requestListener,requestCoordinator,engine,transformation,transcodeClass,isMemoryCacheable,animationFactory,overrideWidth,overrideHeight,diskCacheStrategy);
return request;

首先从REQUEST_POOL这个队列中拿出一个request，如果不存在，则新建一个，request的构造函数是空实现，其相关参数是在其init方法进行初始化的

{this.loadProvider = loadProvider;this.model = model;this.signature = signature;this.fallbackDrawable = fallbackDrawable;this.fallbackResourceId = fallbackResourceId;this.context = context.getApplicationContext();this.priority = priority;this.target = target;this.sizeMultiplier = sizeMultiplier;this.placeholderDrawable = placeholderDrawable;this.placeholderResourceId = placeholderResourceId;this.errorDrawable = errorDrawable;this.errorResourceId = errorResourceId;this.requestListener = requestListener;this.requestCoordinator = requestCoordinator;this.engine = engine;this.transformation = transformation;this.transcodeClass = transcodeClass;this.isMemoryCacheable = isMemoryCacheable;this.animationFactory = animationFactory;this.overrideWidth = overrideWidth;this.overrideHeight = overrideHeight;this.diskCacheStrategy = diskCacheStrategy;status = Status.PENDING;// We allow null models by just setting an error drawable. Null models will always have empty providers, we// simply skip our sanity checks in that unusual case.if (model != null) {check("ModelLoader", loadProvider.getModelLoader(), "try .using(ModelLoader)");check("Transcoder", loadProvider.getTranscoder(), "try .as*(Class).transcode(ResourceTranscoder)");check("Transformation", transformation, "try .transform(UnitTransformation.get())");if (diskCacheStrategy.cacheSource()) {check("SourceEncoder", loadProvider.getSourceEncoder(),"try .sourceEncoder(Encoder) or .diskCacheStrategy(NONE/RESULT)");} else {check("SourceDecoder", loadProvider.getSourceDecoder(),"try .decoder/.imageDecoder/.videoDecoder(ResourceDecoder) or .diskCacheStrategy(ALL/SOURCE)");}if (diskCacheStrategy.cacheSource() || diskCacheStrategy.cacheResult()) {// TODO if(resourceClass.isAssignableFrom(InputStream.class) it is possible to wrap sourceDecoder// and use it instead of cacheDecoder: new FileToStreamDecoder<Z>(sourceDecoder)// in that case this shouldn't throwcheck("CacheDecoder", loadProvider.getCacheDecoder(),"try .cacheDecoder(ResouceDecoder) or .diskCacheStrategy(NONE)");}if (diskCacheStrategy.cacheResult()) {check("Encoder", loadProvider.getEncoder(),"try .encode(ResourceEncoder) or .diskCacheStrategy(NONE/SOURCE)");}}
}

对request进行赋值，然后各种check校验，根据DiskCacheStrategy中策略的不同，一共4种策略，默认DiskCacheStrategy的策略是RESULT

public enum DiskCacheStrategy {/** Caches with both {@link #SOURCE} and {@link #RESULT}. */ALL(true, true),/** Saves no data to cache. */NONE(false, false),/** Saves just the original data to cache. */SOURCE(true, false),/** Saves the media item after all transformations to cache. */RESULT(false, true);private final boolean cacheSource;private final boolean cacheResult;DiskCacheStrategy(boolean cacheSource, boolean cacheResult) {this.cacheSource = cacheSource;this.cacheResult = cacheResult;}/*** Returns true if this request should cache the original unmodified data.*/public boolean cacheSource() {return cacheSource;}/*** Returns true if this request should cache the final transformed result.*/public boolean cacheResult() {return cacheResult;}
}

回到GenericRequestBuilder 的into方法中

Request request = buildRequest(target);
target.setRequest(request);
lifecycle.addListener(target);
requestTracker.runRequest(request);

将这个request 设置进GlideDrawableImageViewTarget中

public void setRequest(Request request) {setTag(request);
}private void setTag(Object tag) {if (tagId == null) {isTagUsedAtLeastOnce = true;view.setTag(tag);} else {view.setTag(tagId, tag);}
}

其实就是将imageView和request 通过setTag方法进行绑定

然后将GlideDrawableImageViewTarget添加到lifecycle中，接着调用requestTracker.runRequest(request)，执行这个request

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

首先将request添加到requests中

private final Set<Request> requests = Collections.newSetFromMap(new WeakHashMap<Request, Boolean>());

然后执行request的begin方法

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));}
}

onSizeReady方法中调用了Engine的load方法，首先创建了一个EngineKey

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

通过这个key去查找缓存是否存在

EngineResource<?> cached = loadFromCache(key, isMemoryCacheable);private EngineResource<?> loadFromCache(Key key, boolean isMemoryCacheable) {if (!isMemoryCacheable) {return null;}EngineResource<?> cached = getEngineResourceFromCache(key);if (cached != null) {cached.acquire();activeResources.put(key, new ResourceWeakReference(key, cached, getReferenceQueue()));}return cached;
}

通过getEngineResourceFromCache方法获取EngineResource

private EngineResource<?> getEngineResourceFromCache(Key key) {Resource<?> cached = cache.remove(key);final EngineResource result;if (cached == null) {result = null;} else if (cached instanceof EngineResource) {// Save an object allocation if we've cached an EngineResource (the typical case).result = (EngineResource) cached;} else {result = new EngineResource(cached, true /*isCacheable*/);}return result;
}

先从cache中寻找资源，如果找到则将其从cache中移除并放入activeResources中，，activeResources是一个Map

if (cached != null) {cb.onResourceReady(cached);if (Log.isLoggable(TAG, Log.VERBOSE)) {logWithTimeAndKey("Loaded resource from cache", startTime, key);}return null;
}

然后如果资源在缓存中找到后会放入cb.onResourceReady(cached)进行回调，cb是ResourceCallback

如果loadFromCache找不到则调用loadFromActiveResources

private EngineResource<?> loadFromActiveResources(Key key, boolean isMemoryCacheable) {if (!isMemoryCacheable) {return null;}EngineResource<?> active = null;WeakReference<EngineResource<?>> activeRef = activeResources.get(key);if (activeRef != null) {active = activeRef.get();if (active != null) {active.acquire();} else {activeResources.remove(key);}}return active;
}

其实就是从activeResources中获取EngineResource，如果能获取到EngineResource，则放入cb.onResourceReady进行回调

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;
}

cache是LruResourceCache对象，作为资源的LRU缓存；activeResources是以弱引用为值的Map，用于缓存使用中的资源。比一般内存缓存额外多一级缓存的意义在于，当内存不足时清理cache中的资源时，不会对使用中的Bitmap造成影响。

以上两个缓存中都找不到的话，就会从jobs通过key获取EngineJob，如果EngineJob存在的话，则用其构造LoadStatus进行返回

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);
}

否则，则创建EngineJob对象并调用其start方法，同时也创建了DecodeJob，将EngineJob和DecodeJob构造一个EngineRunnable

EngineJob engineJob = engineJobFactory.build(key, isMemoryCacheable);
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);

EngineJob的start方法执行EngineRunnable

public void start(EngineRunnable engineRunnable) {this.engineRunnable = engineRunnable;future = diskCacheService.submit(engineRunnable);
}

diskCacheService的类型是ThreadPoolExecutor的子类FifoPriorityThreadPoolExecutor，也就是说将engineRunnable放入线程池中执行。

然后自然会执行到EngineRunnable的run方法

@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 {onLoadComplete(resource);}
}

在EngineRunnable的run方法中进行编码，执行decode方法

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

根据缓存策略调用decodeFromCache或者decodeFromSource

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

先看decodeFromSource，调用DecodeJob的decodeFromSource方法。

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

先看decodeSource里面的逻辑

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

先通过DataFetcher访问网络获得文件流，接口DataFetcher的实现类根据配置而不同，我们看看HttpUrlFetcher的loadData方法的实现

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

其实内部就是建立网络链接，获取InputStream

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());}
}

之后会将InputStream 传入decodeFromSourceData方法中

private Resource<T> cacheAndDecodeSourceData(A data) throws IOException {long startTime = LogTime.getLogTime();SourceWriter<A> writer = new SourceWriter<A>(loadProvider.getSourceEncoder(), data);diskCacheProvider.getDiskCache().put(resultKey.getOriginalKey(), writer);if (Log.isLoggable(TAG, Log.VERBOSE)) {logWithTimeAndKey("Wrote source to cache", startTime);}startTime = LogTime.getLogTime();Resource<T> result = loadFromCache(resultKey.getOriginalKey());if (Log.isLoggable(TAG, Log.VERBOSE) && result != null) {logWithTimeAndKey("Decoded source from cache", startTime);}return result;
}

先将SourceWriter放入diskCacheProvider的DiskCache中，然后调用loadFromCache

private Resource<T> loadFromCache(Key key) throws IOException {File cacheFile = diskCacheProvider.getDiskCache().get(key);if (cacheFile == null) {return null;}Resource<T> result = null;try {result = loadProvider.getCacheDecoder().decode(cacheFile, width, height);} finally {if (result == null) {diskCacheProvider.getDiskCache().delete(key);}}return result;
}

根据需要将文件流写入磁盘缓存，再对文件流进行编码

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

ublic 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);}
}

编码过程中通过设置采样率缩放图片，降低内存占用，提高加载性能。

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);
}

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;
}

这样返回的InpuStream就通过downsampleWithSize方法变为了Bitmap，如果设置了degreesToRotate（旋转的角度），就调用rotateImageExif设置

Bitmap rotated = null;
if (downsampled != null) {rotated = TransformationUtils.rotateImageExif(downsampled, pool, orientation);if (!downsampled.equals(rotated) && !pool.put(downsampled)) {downsampled.recycle();}
}

public static Bitmap rotateImageExif(Bitmap toOrient, BitmapPool pool, int exifOrientation) {final Matrix matrix = new Matrix();initializeMatrixForRotation(exifOrientation, matrix);if (matrix.isIdentity()) {return toOrient;}// From Bitmap.createBitmap.final RectF newRect = new RectF(0, 0, toOrient.getWidth(), toOrient.getHeight());matrix.mapRect(newRect);final int newWidth = Math.round(newRect.width());final int newHeight = Math.round(newRect.height());Bitmap.Config config = getSafeConfig(toOrient);Bitmap result = pool.get(newWidth, newHeight, config);if (result == null) {result = Bitmap.createBitmap(newWidth, newHeight, config);}matrix.postTranslate(-newRect.left, -newRect.top);final Canvas canvas = new Canvas(result);final Paint paint = new Paint(PAINT_FLAGS);canvas.drawBitmap(toOrient, matrix, paint);return result;
}

回到decodeFromSource方法

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

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();Resource<Z> result = transcode(transformed);if (Log.isLoggable(TAG, Log.VERBOSE)) {logWithTimeAndKey("Transcoded transformed from source", startTime);}return result;
}

transform方法调用了Transformation的transform方法

private Resource<T> transform(Resource<T> decoded) {if (decoded == null) {return null;}Resource<T> transformed = transformation.transform(decoded, width, height);if (!decoded.equals(transformed)) {decoded.recycle();}return transformed;
}

Transformation是接口；BitmapTransformation实现了该接口但留下了另一个抽象方法transform；CenterCrop和FitCenter两个类继承了BitmapTransformation并实现了抽象方法transform。

public final Resource<Bitmap> transform(Resource<Bitmap> resource, int outWidth, int outHeight) {if (!Util.isValidDimensions(outWidth, outHeight)) {throw new IllegalArgumentException("Cannot apply transformation on width: " + outWidth + " or height: "+ outHeight + " less than or equal to zero and not Target.SIZE_ORIGINAL");}Bitmap toTransform = resource.get();int targetWidth = outWidth == Target.SIZE_ORIGINAL ? toTransform.getWidth() : outWidth;int targetHeight = outHeight == Target.SIZE_ORIGINAL ? toTransform.getHeight() : outHeight;Bitmap transformed = transform(bitmapPool, toTransform, targetWidth, targetHeight);final Resource<Bitmap> result;if (toTransform.equals(transformed)) {result = resource;} else {result = BitmapResource.obtain(transformed, bitmapPool);}return result;
}

接下来看一下CenterCrop的transform方法

protected Bitmap transform(BitmapPool pool, Bitmap toTransform, int outWidth, int outHeight) {final Bitmap toReuse = pool.get(outWidth, outHeight, toTransform.getConfig() != null? toTransform.getConfig() : Bitmap.Config.ARGB_8888);Bitmap transformed = TransformationUtils.centerCrop(toReuse, toTransform, outWidth, outHeight);if (toReuse != null && toReuse != transformed && !pool.put(toReuse)) {toReuse.recycle();}return transformed;
}

由BitmapPool提供一个Bitmap作为下一步的Canvas载体。BitmapPool的实现类是LruBitmapPool，顾名思义是一个基于LRU方式的Bitmap缓存池，用于Bitmap的复用。

接下来是TransformationUtils的centerCrop方法

public static Bitmap centerCrop(Bitmap recycled, Bitmap toCrop, int width, int height) {if (toCrop == null) {return null;} else if (toCrop.getWidth() == width && toCrop.getHeight() == height) {return toCrop;}// From ImageView/Bitmap.createScaledBitmap.final float scale;float dx = 0, dy = 0;Matrix m = new Matrix();if (toCrop.getWidth() * height > width * toCrop.getHeight()) {scale = (float) height / (float) toCrop.getHeight();dx = (width - toCrop.getWidth() * scale) * 0.5f;} else {scale = (float) width / (float) toCrop.getWidth();dy = (height - toCrop.getHeight() * scale) * 0.5f;}m.setScale(scale, scale);m.postTranslate((int) (dx + 0.5f), (int) (dy + 0.5f));final Bitmap result;if (recycled != null) {result = recycled;} else {result = Bitmap.createBitmap(width, height, getSafeConfig(toCrop));}// We don't add or remove alpha, so keep the alpha setting of the Bitmap we were given.TransformationUtils.setAlpha(toCrop, result);Canvas canvas = new Canvas(result);Paint paint = new Paint(PAINT_FLAGS);canvas.drawBitmap(toCrop, m, paint);return result;
}

在TransformationUtils的centerCrop方法中，根据目标尺寸调整矩阵并绘制结果。

FitCenter类的逻辑与centerCrop类相似，回到DecodeJob的transformEncodeAndTranscode方法
接下来调用writeTransformedToCache将转换结果写入磁盘缓存，

private void writeTransformedToCache(Resource<T> transformed) {if (transformed == null || !diskCacheStrategy.cacheResult()) {return;}long startTime = LogTime.getLogTime();SourceWriter<Resource<T>> writer = new SourceWriter<Resource<T>>(loadProvider.getEncoder(), transformed);diskCacheProvider.getDiskCache().put(resultKey, writer);if (Log.isLoggable(TAG, Log.VERBOSE)) {logWithTimeAndKey("Wrote transformed from source to cache", startTime);}
}

再调用transcode方法进行转码

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

transcode方法中的transcoder的实际类型是GlideBitmapDrawableTranscoder

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

GlideBitmapDrawableTranscoder的transcode方法将Bitmap资源进行封装

public GlideBitmapDrawableResource(GlideBitmapDrawable drawable, BitmapPool bitmapPool) {super(drawable);this.bitmapPool = bitmapPool;
}

到这里就结束了decodeFromSource的流程。

接下来看decodeFromCache

private Resource<?> decodeFromCache() throws Exception {Resource<?> result = null;try {result = decodeJob.decodeResultFromCache();} catch (Exception e) {if (Log.isLoggable(TAG, Log.DEBUG)) {Log.d(TAG, "Exception decoding result from cache: " + e);}}if (result == null) {result = decodeJob.decodeSourceFromCache();}return result;
}

先调用DecodeJob的decodeResultFromCache方法获取，获取失败则调用DecodeJob的decodeSourceFromCache方法

public Resource<Z> decodeResultFromCache() throws Exception {if (!diskCacheStrategy.cacheResult()) {return null;}long startTime = LogTime.getLogTime();Resource<T> transformed = loadFromCache(resultKey);if (Log.isLoggable(TAG, Log.VERBOSE)) {logWithTimeAndKey("Decoded transformed from cache", startTime);}startTime = LogTime.getLogTime();Resource<Z> result = transcode(transformed);if (Log.isLoggable(TAG, Log.VERBOSE)) {logWithTimeAndKey("Transcoded transformed from cache", startTime);}return result;
}

decodeResultFromCache方法从磁盘缓存中获取对应Bitmap并将其转码

public Resource<Z> decodeSourceFromCache() throws Exception {if (!diskCacheStrategy.cacheSource()) {return null;}long startTime = LogTime.getLogTime();Resource<T> decoded = loadFromCache(resultKey.getOriginalKey());if (Log.isLoggable(TAG, Log.VERBOSE)) {logWithTimeAndKey("Decoded source from cache", startTime);}return transformEncodeAndTranscode(decoded);
}

decodeSourceFromCache方法从磁盘缓存中获取对应Bitmap并将其转换（因为是原尺寸，需要调整大小）

EngineRunnable的decode是decodeFromCache和decodeFromSource二选一，decodeFromCache也获取失败的话会怎么样呢？

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 {onLoadComplete(resource);}
}

缓存中没有结果的情况下会调用onLoadFailed方法

private void onLoadFailed(Exception e) {if (isDecodingFromCache()) {stage = Stage.SOURCE;manager.submitForSource(this);} else {manager.onException(e);}
}public void submitForSource(EngineRunnable runnable) {future = sourceService.submit(runnable);
}

变更缓存策略重新放入线程池中执行，也就是从网络获取。这里的线程池是sourceService而不是上面的diskCacheService，

得到了处理结果，接下来调用EngineRunnable的onLoadComplete方法将结果传入

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

onLoadComplete方法调用了EngineJob的onResourceReady方法

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

private static class MainThreadCallback implements Handler.Callback {@Overridepublic boolean handleMessage(Message message) {if (MSG_COMPLETE == message.what || MSG_EXCEPTION == message.what) {EngineJob job = (EngineJob) message.obj;if (MSG_COMPLETE == message.what) {job.handleResultOnMainThread();} else {job.handleExceptionOnMainThread();}return true;}return false;}
}

关键在handleResultOnMainThread中

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;// Hold on to resource for duration of request so we don't recycle it in the middle of notifying if it// synchronously released by one of the callbacks.engineResource.acquire();listener.onEngineJobComplete(key, engineResource);for (ResourceCallback cb : cbs) {if (!isInIgnoredCallbacks(cb)) {engineResource.acquire();cb.onResourceReady(engineResource);}}// Our request is complete, so we can release the resource.engineResource.release();
}

onResourceReady方法中向Handler传递消息并由MainThreadCallback处理消息，也就切换到了主线程。handleResultOnMainThread方法会调用GenericRequest的onResourceReady方法

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;}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);}
}

onResourceReady方法调用GlideDrawableImageViewTarget的onResourceReady方法

public void onResourceReady(GlideDrawable resource, GlideAnimation<? super GlideDrawable> animation) {if (!resource.isAnimated()) {//TODO: Try to generalize this to other sizes/shapes.// This is a dirty hack that tries to make loading square thumbnails and then square full images less costly// by forcing both the smaller thumb and the larger version to have exactly the same intrinsic dimensions.// If a drawable is replaced in an ImageView by another drawable with different intrinsic dimensions,// the ImageView requests a layout. Scrolling rapidly while replacing thumbs with larger images triggers// lots of these calls and causes significant amounts of jank.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();
}

GlideDrawableImageViewTarget的onResourceReady方法调用其父类ImageViewTarget的onResourceReady方法

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

ImageViewTarget的onResourceReady方法中调用的抽象方法setResource在子类GlideDrawableImageViewTarget中实现，该方法中调用了ImageView的setImageDrawable方法设置图像。至此，整个加载流程就完成了。

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

最后简单归纳一下整个加载流程的图