1 缘起
近期新的项目中用到了广受好评的的图片加载库 Glide，着实提升了不少开发效率。以文档中最常见的代码为例
Glide.with(this).load("http://goo.gl/gEgYUd").into(imageView);
一行就能解决图片的自动加载，缓存，显示的问题。
2 核心类
Glide : 以单例模式提供了一种简单的静态接口，采用 RequestBuilder 来构建相关的请求。
GlideContext : Glide中所有负载（all loads）的全局上下文，包含以及对外提供多样化注册机制，以及加载资源所需的类类型。
Engine : 负载启动加载以及管理活动的，缓存的资源。
RequestManager : 管理以及发起请求（request）。
RequestTracker : 跟踪，取消，重启进行中的，已完成的，以及失败的请求。
SingleRequest : 为一个Target加载一种Resource的请求(request)。
3 分析
3.1 Glide的初始化
加载图片有着不同的使用场景，Glide利用 Facade 的模式做出了良好的封装。
让我们随即进入任一实现的代码中，最后发现了Glide的初始化逻辑：
private static volatile Glide glide;
public static Glide get(Context context) {
if (glide == null) {
synchronized (Glide.class) {
if (glide == null) {
initGlide(context);
}
}
}
return glide;
}
是的，“未能免俗的” Glide也采用了双重检查锁定(Double-Check Locking)实现了单例模式。显而易见的是，对于整个APP来说图片加载库有一个实例就够了。
3.2 加载相关的View dimension 的确定
如其官方文档所述 :
"Glide is smart enough to figure out the sizes of Views that use layout weights or match_parent as well as the sizes of those that have concrete dimensions."
它是如何做到的？让我们追踪下本文初始位置给出的第一行示例代码的具体调用过程：
Glide
.with(Fragment/Activity/View)
+ RequestManager -> LifecycleListener
+ load(model)
RequestBuilder<Drawable>.load(model)
+ loadGeneric(model)
+ into(Y target) // Y extends Target<TranscodeType>
+ into(ImageView view)
into (a DrawableImageViewTarget ) // Y target
buildRequest(target)
-buildRequestRecursive()
-obtainRequest()
-SingleRequest.obtain
target.setRequest(request); // set as Tag for ViewTarget
requestManager.track(target, request);
requestTracker.runRequest(request);
request.begin();
#SingleRequest.begin()
target.getSize(this);
// view.addPreDrawListener
// callback callled onSizeReady async
target.onLoadStarted() -> apply your placeholder
原来正在发起图片请求前，有一个获取target size的异步调用。继续来看相关的ViewTarget的getSize()方法:
ViewTarget.getSize(cb)
sizeDeterminer.getSize(cb);
ViewTreeObserver observer = view.getViewTreeObserver();
layoutListener = new SizeDeterminerLayoutListener(this);
observer.addOnPreDrawListener(layoutListener);
onPreDraw()
sizeDeterminer.checkCurrentDimens();
// consider padding
int currentWidth = getTargetWidth();
int currentHeight = getTargetHeight();
notifyCbs(currentWidth, currentHeight);
cb.onSizeReady(width, height);
此处可以发现，由于View自身会经历不同的阶段（measure - layout - draw），在当次调用时，其大小可能有不确定性。加载过程如果在view大小未确定前，会对目标View的PreDraw事件加入监听，以得到最终确定的大小。而这也为后续支持的scale type： center_crop 和 fit_center 做好了铺垫。
3.3 对视图层生命周期的智能处理
由于Android View层组件Fragment，Activity 有自身的生命周期，Glide如何封装生命周期的变化，动态管理请求。我们可以从 RequestManager 创建之初的设置看出一二：
private RequestManager supportFragmentGet(Context context, FragmentManager fm,
Fragment parentHint) {
SupportRequestManagerFragment current = getSupportRequestManagerFragment(fm, parentHint);
RequestManager requestManager = current.getRequestManager();
if (requestManager == null) {
// TODO(b/27524013): Factor out this Glide.get() call.
Glide glide = Glide.get(context);
requestManager =
factory.build(glide, current.getLifecycle(), current.getRequestManagerTreeNode());
current.setRequestManager(requestManager);
}
return requestManager;
}
其中 SupportRequestManagerFragment为UI 无关的Fragment，它被创建并且添加到当前的Activity或是主Fragment之下。它与RequestManager建立关联关系并能安全的存储它。
requestManager = factory.build(glide, current.getLifecycle(), current.getRequestManagerTreeNode());
即将Non-UI Fragment的生命周期关联到了当前的requestManager上。所对应的是，View层上的变化onStart，onStop，onDestroy最终会映射到对request的启动，暂停以及清除。
3.4 对于组件扩展的支持
让我们重新关注到Glide初始化时，所做的具体事宜。
initGlide()
// ...
manifestModules = new ManifestParser(applicationContext).parse();
GlideBuilder builder = new GlideBuilder()
.setRequestManagerFactory(factory);
module.applyOptions(applicationContext, builder);
annotationGeneratedModule.applyOptions(applicationContext, builder);
glide = builder.build(applicationContext);
// initialize executors, memorySizeCalculator, bitmapPool, caches
// Responsible for starting loads and managing active and cached resources
engine = new Engine()
requestManagerRetriever = new RequestManagerRetriever(
requestManagerFactory);
return new Glide(...);
// Glide Constructor
registry = new Registry();
registry.register(ByteBuffer.class, new ByteBufferEncoder())
.register(InputStream.class, new StreamEncoder(arrayPool))
/* Bitmaps */
.append(ByteBuffer.class, Bitmap.class,
new ByteBufferBitmapDecoder(downsampler))
.append(InputStream.class, Bitmap.class,
new StreamBitmapDecoder(downsampler, arrayPool))
.append(GlideUrl.class, InputStream.class, new HttpGlideUrlLoader.Factory())
//...
glideContext = new GlideContext(...)
// eg. OKHttp integration
// registry.replace(GlideUrl.class, InputStream.class, new OkHttpUrlLoader.Factory());
for (GlideModule module : manifestModules) {
module.applyOptions(applicationContext, builder);
}
module.registerComponents(applicationContext, glide.registry);
annotationGeneratedModule.registerComponents(applicationContext, glide.registry);
代码中的manifestModules包含了从manifest中解析得到的模块，对于模块非空的情况，这里也提供了替换的逻辑：
module.registerComponents(applicationContext, glide.registry);
以OKHttp为例，如果设置了对应的module，这将会替换Glide默认的网络模块，而采用配置的 OKHttp Client。
4 典型的调用栈
Glide
.with(Fragment/Activity/View)
+ RequestManager -> LifecycleListener
+ load(model)
RequestBuilder<Drawable>.load(model)
+ loadGeneric(model)
+ into(Y target) // Y extends Target<TranscodeType>
+ into(ImageView view)
into (a DrawableImageViewTarget ) // Y target
buildRequest(target)
-buildRequestRecursive()
-obtainRequest()
-SingleRequest.obtain
target.setRequest(request); // set as Tag for ViewTarget
requestManager.track(target, request);
requestTracker.runRequest(request);
request.begin();
# SingleRequest.begin()
target.getSize(this);
// view.addPreDrawListener
// callback callled onSizeReady async
target.onLoadStarted() -> apply your placeholder
onSizeReady() // req.status = Status.RUNNING;
Engine.load()
// check the cache
// check the active resources
EngineJob<R> engineJob = engineJobFactory.build(...)
DecodeJob<R> decodeJob = decodeJobFactory.build(..., engineJob)
engineJob.start(decodeJob);
decodeJob.run() // with runReason = RunReason.INITIALIZE;
runWrapped()
// state-machine
// INIT- {RES_CACHE} -{DATA_CACHE}-{SOURCE}|FINISHED
getNextGenerator()
runGenerators()
reschedule() // with Stage.SOURCE // reason = RunReason.SWITCH_TO_SOURCE_SERVICE
SourceGenerator.startNext()
loadData = helper.getLoadData().get() // ModelLoader registable ?
modelLoader.buildLoadData
loadData.fetcher.loadData(cb)// async
SourceGenerator.onDataReady()
decodeJob.onDataFetcherReady()
decodeFromRetrievedData
-decodeFromData(currentFetcher, currentData, currentDataSource);
decodeFromFetcher(data, dataSource)
runLoadPath()
LoadPath#Resource<Transcode> load(..., new DecodeCallback(...))
loadWithExceptionList()
DecodePath#decode()
decodeResource()
decodeResourceWithList
decoder.decode(data, width, height, options);
callback.onResourceDecoded
onResourceDecoded()
appliedTransformation.transform()
-notifyEncodeAndRelease
notifyComplete
callback.onResourceReady(resource, dataSource)
engineJob.onResourceReady()
MAIN_THREAD_HANDLER.obtainMessage(MSG_COMPLETE, this).sendToTarget();
handleResultOnMainThread()
cb.onResourceReady(engineResource, dataSource);
# SingleRequest.onResourceReady(Resource<?> resource, DataSource dataSource)
onResourceReady(Resource<R> resource, R result, DataSource dataSource)
if(!requestListener.onResourceReady())
target.onResourceReady(result, animation)
ImageViewTarget.onResourceReady()
setResourceInternal()
view.setImageDrawable(resource);
5 框架小结
封装组件的具体实现
支持组件的可配置化（registry）
对Application view layer的高效运用
