Android图片框架Glide原理深入探索

目录

• with
• load
• into
• 原理总结
• 缓存
• LruCache

首先引入依赖

下面一行代码，就是Glide最简单的使用方式了

Glide.with(this).load(url).into(imageView)

with

首先，我们来看with，其实with的功能就是根据传入的context来获取图片请求管理器RequestManager，用来启动和管理图片请求。

 public static RequestManager with(@NonNull FragmentActivity activity) { return getRetriever(activity).get(activity); } 

context可以传入app，activity和fragment，这关系着图片请求的生命周期。通常使用当前页面的context，这样当我们打开一个页面加载图片，然后退出页面时，图片请求会跟随页面销毁而被取消，而不是继续加载浪费资源。

当context是app时，获得的RequestManager是一个全局单例，图片请求的生命周期会跟随整个app。

注意：如果with发生在子线程，不管context是谁，都返回应用级别的RequestManager单例。

 private RequestManager getApplicationManager(@NonNull 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. // TODO(b/): Factor out this Glide.get() call. Glide glide = Glide.get(context.getApplicationContext()); applicationManager = factory.build( glide, new ApplicationLifecycle(), new EmptyRequestManagerTreeNode(), context.getApplicationContext()); } } } return applicationManager; }

当context是Activity时，会创建一个无界面的fragment添加到Activity，用于感知Activity的生命周期，同时创建RequestManager给该fragment持有。

 private RequestManager supportFragmentGet( @NonNull Context context, @NonNull FragmentManager fm, @Nullable Fragment parentHint, boolean isParentVisible) { SupportRequestManagerFragment current = getSupportRequestManagerFragment(fm, parentHint); RequestManager requestManager = current.getRequestManager(); if (requestManager == null) { // TODO(b/): Factor out this Glide.get() call. Glide glide = Glide.get(context); requestManager = factory.build( glide, current.getGlideLifecycle(), current.getRequestManagerTreeNode(), context); // This is a bit of hack, we're going to start the RequestManager, but not the // corresponding Lifecycle. It's safe to start the RequestManager, but starting the // Lifecycle might trigger memory leaks. See b/ if (isParentVisible) { requestManager.onStart(); } current.setRequestManager(requestManager); } return requestManager; } 

load

load方法会得到一个图片请求构建器RequestBuilder，用来创建图片请求。

 public RequestBuilder<Drawable> load(@Nullable String string) { return asDrawable().load(string); } 

into

首先是根据ImageView的ScaleType，来配置参数

 public ViewTarget<ImageView, TranscodeType> into(@NonNull ImageView view) { Util.assertMainThread(); Preconditions.checkNotNull(view); BaseRequestOptions<?> requestOptions = this; if (!requestOptions.isTransformationSet() && requestOptions.isTransformationAllowed() && view.getScaleType() != null) { // Clone in this method so that if we use this RequestBuilder to load into a View and then // into a different target, we don't retain the transformation applied based on the previous // View's scale type. switch (view.getScaleType()) { case CENTER_CROP: requestOptions = requestOptions.clone().optionalCenterCrop(); break; case CENTER_INSIDE: requestOptions = requestOptions.clone().optionalCenterInside(); break; case FIT_CENTER: case FIT_START: case FIT_END: requestOptions = requestOptions.clone().optionalFitCenter(); break; case FIT_XY: requestOptions = requestOptions.clone().optionalCenterInside(); break; case CENTER: case MATRIX: default: // Do nothing. } } return into( glideContext.buildImageViewTarget(view, transcodeClass), /*targetListener=*/ null, requestOptions, Executors.mainThreadExecutor()); }

继续跟进into，会创建图片请求，获取Target载体已有的请求，对比两个请求，如果等效，启动异步请求。然后，图片载体绑定图片请求，也就是imageView setTag为request

 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; } requestManager.clear(target); target.setRequest(request); requestManager.track(target, request); return target; }

继续跟进异步请求 requestManager.track(target, request)

 synchronized void track(@NonNull Target<?> target, @NonNull Request request) { targetTracker.track(target); requestTracker.runRequest(request); } 

 public void runRequest(@NonNull Request request) { requests.add(request); if (!isPaused) { request.begin();//开启图片请求 } else { request.clear(); if (Log.isLoggable(TAG, Log.VERBOSE)) { Log.v(TAG, "Paused, delaying request"); } pendingRequests.add(request);//如果是暂停状态，就把请求存起来 } } 

到这里就启动了图片请求了，我们继续跟进request.begin()

 public void begin() { synchronized (requestLock) { //...... if (Util.isValidDimensions(overrideWidth, overrideHeight)) { //如果有尺寸，开始加载 onSizeReady(overrideWidth, overrideHeight); } else { //如果无尺寸就先去获取 target.getSize(this); } //...... } } 

然后继续瞧瞧onSizeReady

 public void onSizeReady(int width, int height) { stateVerifier.throwIfRecycled(); synchronized (requestLock) { //...... 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, callbackExecutor); //...... } } 

跟进engine.load

 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, Executor callbackExecutor) { long startTime = VERBOSE_IS_LOGGABLE ? LogTime.getLogTime() : 0; EngineKey key = keyFactory.buildKey( model, signature, width, height, transformations, resourceClass, transcodeClass, options); EngineResource<?> memoryResource; synchronized (this) { //从内存加载 memoryResource = loadFromMemory(key, isMemoryCacheable, startTime); if (memoryResource == null) { //如果内存里没有 return waitForExistingOrStartNewJob( glideContext, model, signature, width, height, resourceClass, transcodeClass, priority, diskCacheStrategy, transformations, isTransformationRequired, isScaleOnlyOrNoTransform, options, isMemoryCacheable, useUnlimitedSourceExecutorPool, useAnimationPool, onlyRetrieveFromCache, cb, callbackExecutor, key, startTime); } } cb.onResourceReady( memoryResource, DataSource.MEMORY_CACHE, /* isLoadedFromAlternateCacheKey= */ false); return null; }

 private <R> LoadStatus waitForExistingOrStartNewJob( 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, Executor callbackExecutor, EngineKey key, long startTime) { EngineJob<?> current = jobs.get(key, onlyRetrieveFromCache); if (current != null) { current.addCallback(cb, callbackExecutor); if (VERBOSE_IS_LOGGABLE) { logWithTimeAndKey("Added to existing load", startTime, key); } 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, callbackExecutor); engineJob.start(decodeJob); if (VERBOSE_IS_LOGGABLE) { logWithTimeAndKey("Started new load", startTime, key); } return new LoadStatus(cb, engineJob); }

DecodeJob是一个Runnable，它通过一系列的调用，会来到HttpUrlFetcher的loadData方法

 public void loadData( @NonNull Priority priority, @NonNull DataCallback<? super InputStream> callback) { long startTime = LogTime.getLogTime(); try { //获取输入流，此处使用的是HttpURLConnection InputStream result = loadDataWithRedirects(glideUrl.toURL(), 0, null, glideUrl.getHeaders()); //回调出去 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)); } } } 

至此，网络请求结束，最后把图片设置上去就行了，在SingleRequest的onResourceReady方法，它会把结果回调给Target载体

target.onResourceReady(result, animation);

继续跟进它，最终会执行setResource，把图片设置上去

 protected void setResource(@Nullable Drawable resource) { view.setImageDrawable(resource); } 

原理总结

with根据传入的context来获取图片请求管理器RequestManager，当传入的context是App时，获得的RequestManager是一个全局单例，图片请求的生命周期会跟随这个应用，当传入的是Activity时，会创建一个无界面的空fragment添加到Activity，用来感知Activity的生命周期。load会得到了一个图片请求构建器RequestBuilder，用来创建图片请求。into开启加载，先会根据ImageView的ScaleType来配置参数，创建图片请求，图片载体绑定图片请求，然后开启图片请求，先从内存中加载，如果内存里没有，会创建一个Runnable，通过一系列的调用，使用HttpURLConnection获取网络输入流，把结果回调出去，最后把回调结果设置上去就OK了。

缓存

Glide三级缓存原理：读取一张图片时，顺序是： 弱引用缓存，LruCache，磁盘缓存。

用Glide加载某张图片时，先去弱引用缓存中寻找图片，如果有则直接取出来使用，如果没有，则去LruCache中寻找，如果LruCache中有，则从中取出图片使用，并将它放入弱引用缓存中，如果都没有图片，则从磁盘缓存或网络中加载图片。

 private EngineResource<?> loadFromMemory( EngineKey key, boolean isMemoryCacheable, long startTime) { if (!isMemoryCacheable) { return null; } EngineResource<?> active = loadFromActiveResources(key); //从弱引用获取图片 if (active != null) { if (VERBOSE_IS_LOGGABLE) { logWithTimeAndKey("Loaded resource from active resources", startTime, key); } return active; } EngineResource<?> cached = loadFromCache(key); //从LruCache获取缓存图片 if (cached != null) { if (VERBOSE_IS_LOGGABLE) { logWithTimeAndKey("Loaded resource from cache", startTime, key); } return cached; } return null; }

不过，这也会产生一个问题，就是Glide加载图片时，URL不变但是图片变了，这种情况会还是以前的旧图片。因为Glide加载图片会将图片缓存到本地，如果url不变则直接读取缓存不会再网络加载。

解决方案：

清除缓存让后台每次都更改图片的名字图片地址选用 ”url?key="+随机数这种格式

LruCache

LruCache就是维护一个缓存对象列表，其中对象列表的排列方式是按照访问顺序实现的，即一直没访问的对象，将放在队尾，即将被淘汰。而最近访问的对象将放在队头，最后被淘汰。其内部维护了一个集合LinkedHashMap，LinkHashMap继承HashMap，在HashMap的基础上，新增了双向链表结构，每次访问数据的时候，会更新被访问数据的链表指针，该LinkedHashMap是以访问顺序排序的，当调用put()方法时，就会在集合中添加元素，判断缓存是否已满，如果满了就用LinkedHashMap的迭代器删除队尾元素，即近期最少访问的元素。当调用get()方法访问缓存对象时，就会调用LinkedHashMap的get()方法获得对应集合元素，同时会更新该元素到队头。

那么，问题来了，如果把一个（100 * 100）的图片放到（800 * 800）的Imageview中会怎么样呢？由上可知，Glide会为每个不同尺寸的Imageview缓存一张图片，也就是说不管这张图片有没有加载过，只要Imageview的尺寸不一样，Glide就会重新加载一次，这时候，它会在加载的Imageview之前从网络上重新下载，然后再缓存。举个例子，如果一个页面的Imageview是100 * 100，另一个页Imageview是800 * 800，它俩展示同一张图片的话，Glide会下载两次图片，并且缓存两张图片，因为Glide缓存Key的生成条件之一就是控件的长宽。

除了缓存，Glide还有一点我觉得做的非常好，就是在图片加载中关闭页面，此页面也不会造成内存泄漏，因为Glide在加载资源的时候，如果是在 Activity，Fragment 这类有生命周期的组件上进行的话，会创建一个无界面的Fragment加入到FragmentManager之中，感知生命周期，当 Activity，Fragment进入不可见，或者已经销毁的时候，Glide会停止加载资源。但是如果，是在非生命周期的组件上进行时，会采用Application的生命周期贯穿整个应用，此时只有在应用程序关闭的时候才会停止加载。

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