Glide是Android端开源图片加载库，能够帮助我们下载、缓存、展示多种格式图片。也是现在主流图片加载框架之一。源码内部究竟是如何实现的呢？讲解主流程，简略分析。

用法如下：

 Glide.with(context).load(url).into(imageView);

我这里拆分为三步分析：

一、with(context)

点击源码查看到是多个重载方法activity、fragment、view等等，下面用其中一个方法来展示

  @NonNullpublic static RequestManager with(@NonNull Activity activity) {return getRetriever(activity).get(activity);}

  @NonNullprivate static RequestManagerRetriever getRetriever(@Nullable Context context) {// Context could be null for other reasons (ie the user passes in null), but in practice it will// only occur due to errors with the Fragment lifecycle.Preconditions.checkNotNull(context,"You cannot start a load on a not yet attached View or a Fragment where getActivity() "+ "returns null (which usually occurs when getActivity() is called before the Fragment "+ "is attached or after the Fragment is destroyed).");return Glide.get(context).getRequestManagerRetriever();}

调用getRetriever方法获取RequestManagerRetriever对象。在创建该对象之前首先通过Glide.java中的get方法获得了Glide单例对象以及AppClideModule等配置。

@NonNullpublic static Glide get(@NonNull Context context) {if (glide == null) {GeneratedAppGlideModule annotationGeneratedModule =getAnnotationGeneratedGlideModules(context.getApplicationContext());synchronized (Glide.class) {if (glide == null) {checkAndInitializeGlide(context, annotationGeneratedModule);}}}return glide;}

 下面的get方法可知道，在子线程不会添加生命周期；主线程添加一个空白的fragment来处理生命周期。最后返回RequestManager对象

  @NonNullpublic RequestManager get(@NonNull 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// Only unwrap a ContextWrapper if the baseContext has a non-null application context.// Context#createPackageContext may return a Context without an Application instance,// in which case a ContextWrapper may be used to attach one.&& ((ContextWrapper) context).getBaseContext().getApplicationContext() != null) {return get(((ContextWrapper) context).getBaseContext());}}return getApplicationManager(context);}//调用get判断线程@NonNullpublic RequestManager get(@NonNull FragmentActivity activity) {if (Util.isOnBackgroundThread()) {//子线程return get(activity.getApplicationContext());} else {//主线程添加生命周期assertNotDestroyed(activity);FragmentManager fm = activity.getSupportFragmentManager();return supportFragmentGet(activity, fm, /*parentHint=*/ null, isActivityVisible(activity));}}

二、load(url)

 上面执行完成到这里已经拿到RequestManager对象，然后调用load(url)。看源码可知是多个重载方法，传不同类型的资源。最终拿到RequestBuilder对象

// RequestManager.java 的代码如下public RequestBuilder<Drawable> load(@Nullable Bitmap bitmap) {return asDrawable().load(bitmap);}public RequestBuilder<Drawable> load(@Nullable Drawable drawable) {return asDrawable().load(drawable);}public RequestBuilder<Drawable> load(@Nullable String string) {return asDrawable().load(string);}public RequestBuilder<Drawable> load(@Nullable Uri uri) {return asDrawable().load(uri);}public RequestBuilder<Drawable> load(@Nullable File file) {return asDrawable().load(file);}public RequestBuilder<Drawable> load(@RawRes @DrawableRes @Nullable Integer resourceId) {return asDrawable().load(resourceId);}public RequestBuilder<Drawable> load(@Nullable URL url) {return asDrawable().load(url);}public RequestBuilder<Drawable> load(@Nullable byte[] model) {return asDrawable().load(model);}public RequestBuilder<Drawable> load(@Nullable Object model) {return asDrawable().load(model);}

三、into(imageView)

  上一步拿到了RequestBuilder对象，调用into可知有2个重载方法。into的参数就是最终显示的控件。

  into方法内部代码分支很多，代码庞大，所以只需走主流程如何显示ImageView的实现即可。当into内部代码执行完成后回到 buildImageViewTarget方法，这个方法是显示使用的，通过Executors.mainThreadExecutor())来切主线程，最终显示控件。

    return into(glideContext.buildImageViewTarget(view, transcodeClass),/*targetListener=*/ null,requestOptions,Executors.mainThreadExecutor());

点击到into内部源码如下：

  private <Y extends Target<TranscodeType>> Y into(@NonNull Y target,@Nullable RequestListener<TranscodeType> targetListener,BaseRequestOptions<?> options,Executor callbackExecutor) {Preconditions.checkNotNull(target);if (!isModelSet) {throw new IllegalArgumentException("You must call #load() before calling #into()");}Request request = buildRequest(target, targetListener, options, callbackExecutor);Request previous = target.getRequest();if (request.isEquivalentTo(previous)&& !isSkipMemoryCacheWithCompletePreviousRequest(options, previous)) {// If the request is completed, beginning again will ensure the result is re-delivered,// triggering RequestListeners and Targets. If the request is failed, beginning again will// restart the request, giving it another chance to complete. If the request is already// running, we can let it continue running without interruption.if (!Preconditions.checkNotNull(previous).isRunning()) {// Use the previous request rather than the new one to allow for optimizations like skipping// setting placeholders, tracking and un-tracking Targets, and obtaining View dimensions// that are done in the individual Request.previous.begin();}return target;}

这里处理请求

 Request request = buildRequest(target, targetListener, options, callbackExecutor);

Request previous = target.getRequest();

将请求对象装到集合中，并且有加锁处理，运用于多线程的并发请求。

url请求走如下：

 

网络请求完成callback.onDataReady(result)，开始一步一步往回传数据。在这一系列过程中，进行了数据处理，比如：图片压缩等。 省略N步骤

//  HttpUrlFetcher.java 代码如下@Overridepublic void loadData(@NonNull Priority priority, @NonNull DataCallback<? super InputStream> callback) {long startTime = LogTime.getLogTime();try {InputStream result = loadDataWithRedirects(glideUrl.toURL(), 0, null, glideUrl.getHeaders());callback.onDataReady(result);} catch (IOException e) {if (Log.isLoggable(TAG, Log.DEBUG)) {Log.d(TAG, "Failed to load data for url", e);}callback.onLoadFailed(e);} finally {if (Log.isLoggable(TAG, Log.VERBOSE)) {Log.v(TAG, "Finished http url fetcher fetch in " + LogTime.getElapsedMillis(startTime));}}}

最后回到了ImageViewTarget类，显示控件。这就是整体简略主流程。

 @Overridepublic void setDrawable(Drawable drawable) {view.setImageDrawable(drawable);}