Glide是一个图片加载框架，其他的图片加载框架还有UniversalImageLoader，Picasso，Fresco等，个人还是比较喜欢Glide这个框架的。Glide致力于打造更好的列表图片滑动体验，就如他的名字Glide（滑翔）一样顺畅，Glide还支持加载video，Gif，SVG格式。而且Glide会与你的Activity/Fragment绑定相关的生命周期，有自己的缓存策略，这样就让图片加载操作变得非常简单。本文的Glide源码是4.0的版本（我本来以为是3.7.0来着，后来一看有些代码对不上……)。
准备工作
如果你还没有使用过glide，可以通过下面两句话引入glide：
compile 'com.github.bumptech.glide:glide:3.7.0'
compile 'com.android.support:support-v4:19.1.0'
编译源码：
在阅读源码的时候，难免希望自己能加上几个log或者自己加点注释之类的来方便自己阅读。可以通过如下方式编译glide源码：
git clone git@github.com:bumptech/glide.git # use https://github.com/bumptech/glide.git if "Permission Denied"
cd glide
git submodule init && git submodule update
./gradlew jar
不过说实话，我按这个流程操作了几次才成功。编译成功之后你就可以运行sample和尽情的修改glide源码了。
如何打开Glide的log，这个官方仓库里有介绍，直接放上链接：Debugging and Error Handling
从最简流程切入读源码
我们用Glide加载一张图，最简单的代码是怎样的呢？
Glide.with(Activity).load(url).into(imageview);
因为流式api，整个调用流程显得非常简洁、简单，但是这短短的几个方法调用里，涉及到的东西却是非常之多的，所以看累了的话，可以起来走走，喝点水……
在正式看代码之前，先上一下Glide的设计图：
设计图
这图也不是我画的，我看到了几篇文都有这图，这图画得很好，文末会给出参考资料链接。
上面的东西，刚接触肯定是很陌生的。不过没事，大概知道有这么些个东西就好了。那么正式开始看代码吧，首先是Glide.with()，点进去看看：
public static RequestManager with(Context context) {
RequestManagerRetriever retriever = RequestManagerRetriever.get();
return retriever.get(context);
}
public static RequestManager with(Activity activity) {
RequestManagerRetriever retriever = RequestManagerRetriever.get();
return retriever.get(activity);
}
public static RequestManager with(FragmentActivity activity) {
RequestManagerRetriever retriever = RequestManagerRetriever.get();
return retriever.get(activity);
}
public static RequestManager with(android.app.Fragment fragment) {
RequestManagerRetriever retriever = RequestManagerRetriever.get();
return retriever.get(fragment);
}
public static RequestManager with(Fragment fragment) {
RequestManagerRetriever retriever = RequestManagerRetriever.get();
return retriever.get(fragment);
}
可以看到with有多个重载方法，适配了各种可能出现的情况。 继续追踪源码：
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);
}
public RequestManager get(FragmentActivity activity) {
if (Util.isOnBackgroundThread()) {
// 不在主线程
return get(activity.getApplicationContext());
} else {
assertNotDestroyed(activity);
FragmentManager fm = activity.getSupportFragmentManager();
return supportFragmentGet(activity, fm, null);
}
}
public RequestManager get(Fragment fragment) {
if (fragment.getActivity() == null) {
throw new IllegalArgumentException(
"You cannot start a load on a fragment before it is attached");
}
if (Util.isOnBackgroundThread()) {
// 不在主线程
return get(fragment.getActivity().getApplicationContext());
} else {
FragmentManager fm = fragment.getChildFragmentManager();
return supportFragmentGet(fragment.getActivity(), fm, fragment);
}
}
public RequestManager get(Activity activity) {
// 不在主线程
if (Util.isOnBackgroundThread() || Build.VERSION.SDK_INT < Build.VERSION_CODES.HONEYCOMB) {
return get(activity.getApplicationContext());
} else {
assertNotDestroyed(activity);
android.app.FragmentManager fm = activity.getFragmentManager();
return fragmentGet(activity, fm, null);
}
}
先不看参数是context的方法，看一下get(FragmentActivity activity)这个方法，首先判断是否在主线程，如果是的话调用supportFragmentGet方法，看下这个方法的代码：
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 =
new RequestManager(glide, current.getLifecycle(), current.getRequestManagerTreeNode());
current.setRequestManager(requestManager);
}
return requestManager;
}
这个方法里有一个SupportRequestManagerFragment（以下简称SRMFragment，如果参数是Activity或者app包中的Fragment，则是RequestManagerFragment）。这个SRMFragment是一个不可见的Fragment，用来和你的Fragment/Activity的生命周期关联起来。如何关联呢？通过ChildFragmentManager/FragmentManager，将这个Fragment添加到Fragment/Activity中，最终将生命周期传递到RequestManager中，让RequestManager对不同的状态做相应的处理。对于内存较低的情况，SRMFragment也有相应的周期。关于SRMFragment的简单介绍便到此为止。接着获取Glide和RequestManager对象，首先是glide：
public static Glide get(Context context) {
if (glide == null) {
synchronized (Glide.class) {
if (glide == null) {
Context applicationContext = context.getApplicationContext();
List<GlideModule> modules = new ManifestParser(applicationContext).parse();
GlideBuilder builder = new GlideBuilder(applicationContext);
for (GlideModule module : modules) {
module.applyOptions(applicationContext, builder);
}
glide = builder.createGlide();
for (GlideModule module : modules) {
module.registerComponents(applicationContext, glide.registry);
}
}
}
}
return glide;
}
很明显是单例的写法，采用双重校验锁的方式。接着是构造RequestManager对象。在Glide中，加载图片是以Request的形式交给Engine去处理的。而RequestManager是为Glide管理和开启请求的类，可以通过Activity/Fragment/Connectivity（网络连接监听）的生命周期方法进行stop，start，restart。
上面有一个参数为Context的get方法略过了，但是其实跟上面介绍的也差不多，只不过如果不是FragmentActivity、Activity和ContextWrapper的对象的引用，则会调用getApplicationManager方法，看一下这个方法：
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.
// TODO(b/27524013): Factor out this Glide.get() call.
Glide glide = Glide.get(context);
applicationManager =
new RequestManager(
glide, new ApplicationLifecycle(), new EmptyRequestManagerTreeNode());
}
}
}
return applicationManager;
}
可以看到也是获取了Glide和RequestManager对象，上面的注释比较简单就不翻译了，简单的解释了用这个context的生命周期的问题。至此简单的分析了Glide.with()方法，这个方法将Glide的生命周期与我们的程序相关联，让我们无需考虑各种复杂的情况。Glide.with()返回的是一个RequestManager方法，接下来就是分析Glide.with().load()了，很显然在RequestManager内：
public RequestBuilder<Drawable> load(@Nullable Object model) {
return asDrawable().load(model);
}
我就是在这发现我看的好像是假的源码，我从glide仓库clone编译的源码，发现跟我原来项目里引用的源码有些不一样，后来看了一下……果然版本不一样：
4.0
在3.7.0的源码中有好几个load的重载方法，但是在最新的代码中只有这一个load方法，更多的load的重载方法在RequestBuilder中了，大概是想让各个类的职责更加清晰一点吧。那么就进RequestBuilder看看几个load方法吧：
public RequestBuilder load(@Nullable Object model) {
return loadGeneric(model);
}
public RequestBuilder load(@Nullable String string) {
return loadGeneric(string);
}
public RequestBuilder load(@Nullable Uri uri) {
return loadGeneric(uri);
}
public RequestBuilder load(@Nullable File file) {
return loadGeneric(file);
}
public RequestBuilder load(@Nullable Integer resourceId) {
return loadGeneric(resourceId).apply(signatureOf(ApplicationVersionSignature.obtain(context)));
}
@Deprecated
public RequestBuilder load(@Nullable URL url) {
return loadGeneric(url);
}
public RequestBuilder load(@Nullable byte[] model) {
return loadGeneric(model).apply(signatureOf(new ObjectKey(UUID.randomUUID().toString()))
.diskCacheStrategy(DiskCacheStrategy.NONE).skipMemoryCache(true /*skipMemoryCache*/));
}
这么多重载方法，最终都调用了loadGeneric方法：
private RequestBuilder loadGeneric(@Nullable Object model) {
this.model = model;
isModelSet = true;
return this;
}
仅仅是设置了两个字段的值，那么继续看into方法，还是在RequestBuilder中：
public Target<TranscodeType> into(ImageView view) {
Util.assertMainThread();
Preconditions.checkNotNull(view);
if (!requestOptions.isTransformationSet()
&& requestOptions.isTransformationAllowed()
&& view.getScaleType() != null) {
if (requestOptions.isLocked()) {
requestOptions = requestOptions.clone();
}
// 根据ImageView的ScaleType配置requestOptions
switch (view.getScaleType()) {
case CENTER_CROP:
requestOptions.optionalCenterCrop(context);
break;
case CENTER_INSIDE:
requestOptions.optionalCenterInside(context);
break;
case FIT_CENTER:
case FIT_START:
case FIT_END:
requestOptions.optionalFitCenter(context);
break;
//$CASES-OMITTED$
default:
// Do nothing.
}
}
return into(context.buildImageViewTarget(view, transcodeClass));
}
根据ImageView的scaleType进行了一些配置，这里通过glideContext生成了一个ViewTarget。在Glide中Target是资源加载的目标，最后调用了into重载方法：
public <Y extends Target<TranscodeType>> Y into(@NonNull Y target) {
Util.assertMainThread();
Preconditions.checkNotNull(target);
if (!isModelSet) {
throw new IllegalArgumentException("You must call #load() before calling #into()");
}
Request previous = target.getRequest();
if (previous != null) {
requestManager.clear(target);
}
requestOptions.lock();
Request request = buildRequest(target);
target.setRequest(request);
requestManager.track(target, request);
return target;
}
调用了buildRequest方法，看下这个方法和这个方法调用的方法：
private Request buildRequest(Target<TranscodeType> target) {
return buildRequestRecursive(target, null, transitionOptions, requestOptions.getPriority(),
requestOptions.getOverrideWidth(), requestOptions.getOverrideHeight());
}
private Request buildRequestRecursive(Target<TranscodeType> target,
@Nullable ThumbnailRequestCoordinator parentCoordinator,
TransitionOptions<?, ? super TranscodeType> transitionOptions,
Priority priority, int overrideWidth, int overrideHeight) {
// 缩略图请求
if (thumbnailBuilder != null) {
// Recursive case: contains a potentially recursive thumbnail request builder.
if (isThumbnailBuilt) {
throw new IllegalStateException("You cannot use a request as both the main request and a "
+ "thumbnail, consider using clone() on the request(s) passed to thumbnail()");
}
TransitionOptions<?, ? super TranscodeType> thumbTransitionOptions =
thumbnailBuilder.transitionOptions;
if (DEFAULT_ANIMATION_OPTIONS.equals(thumbTransitionOptions)) {
thumbTransitionOptions = transitionOptions;
}
Priority thumbPriority = thumbnailBuilder.requestOptions.isPrioritySet()
? thumbnailBuilder.requestOptions.getPriority() : getThumbnailPriority(priority);
int thumbOverrideWidth = thumbnailBuilder.requestOptions.getOverrideWidth();
int thumbOverrideHeight = thumbnailBuilder.requestOptions.getOverrideHeight();
if (Util.isValidDimensions(overrideWidth, overrideHeight)
&& !thumbnailBuilder.requestOptions.isValidOverride()) {
thumbOverrideWidth = requestOptions.getOverrideWidth();
thumbOverrideHeight = requestOptions.getOverrideHeight();
}
ThumbnailRequestCoordinator coordinator = new ThumbnailRequestCoordinator(parentCoordinator);
Request fullRequest = obtainRequest(target, requestOptions, coordinator,
transitionOptions, priority, overrideWidth, overrideHeight);
isThumbnailBuilt = true;
// Recursively generate thumbnail requests.
Request thumbRequest = thumbnailBuilder.buildRequestRecursive(target, coordinator,
thumbTransitionOptions, thumbPriority, thumbOverrideWidth, thumbOverrideHeight);
isThumbnailBuilt = false;
coordinator.setRequests(fullRequest, thumbRequest);
return coordinator;
} else if (thumbSizeMultiplier != null) {
// Base case: thumbnail multiplier generates a thumbnail request, but cannot recurse.
ThumbnailRequestCoordinator coordinator = new ThumbnailRequestCoordinator(parentCoordinator);
Request fullRequest = obtainRequest(target, requestOptions, coordinator, transitionOptions,
priority, overrideWidth, overrideHeight);
BaseRequestOptions<?> thumbnailOptions = requestOptions.clone()
.sizeMultiplier(thumbSizeMultiplier);
Request thumbnailRequest = obtainRequest(target, thumbnailOptions, coordinator,
transitionOptions, getThumbnailPriority(priority), overrideWidth, overrideHeight);
coordinator.setRequests(fullRequest, thumbnailRequest);
return coordinator;
} else {
// Base case: no thumbnail.
return obtainRequest(target, requestOptions, parentCoordinator, transitionOptions, priority,
overrideWidth, overrideHeight);
}
}
buildRequest创建了请求，如果配置了缩略图请求，那么会生成一个ThumbnailRequestCoordinator请求。这个请求内部包含了一个FullRequest和ThumbnailRequest。如果没有配置，则通过obtainRequest方法生成一个SingleRequest对象返回。
private Request obtainRequest(Target<TranscodeType> target,
BaseRequestOptions<?> requestOptions, RequestCoordinator requestCoordinator,
TransitionOptions<?, ? super TranscodeType> transitionOptions, Priority priority,
int overrideWidth, int overrideHeight) {
requestOptions.lock();
return SingleRequest.obtain(
context,
model,
transcodeClass,
requestOptions,
overrideWidth,
overrideHeight,
priority,
target,
requestListener,
requestCoordinator,
context.getEngine(),
transitionOptions.getTransitionFactory());
}
看完了buildRequest方法，接着看requestManager.track(target,requst)，这个方法调用了RequestTrack的runRequest方法：
void track(Target<?> target, Request request) {
targetTracker.track(target);
requestTracker.runRequest(request);
}
public void runRequest(Request request) {
requests.add(request);
if (!isPaused) {
request.begin();
} else {
pendingRequests.add(request);
}
}
request.biegin调用的是SingleRequest的begin方法，begin调用了onSizeReady方法而onSizeReady又调用了engine.load()方法。至此，终于摸到了真正加载的门槛了：
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,
Options options,
boolean isMemoryCacheable,
boolean useUnlimitedSourceExecutorPool,
boolean onlyRetrieveFromCache,
ResourceCallback cb) {
Util.assertMainThread();
long startTime = LogTime.getLogTime();
// 创建key，资源的唯一标识
EngineKey key = keyFactory.buildKey(model, signature, width, height, transformations,
resourceClass, transcodeClass, options);
// 内存缓存中读取数据
EngineResource<?> cached = loadFromCache(key, isMemoryCacheable);
if (cached != null) {
cb.onResourceReady(cached, DataSource.MEMORY_CACHE);
if (Log.isLoggable(TAG, Log.VERBOSE)) {
logWithTimeAndKey("Loaded resource from cache", startTime, key);
}
return null;
}
EngineResource<?> active = loadFromActiveResources(key, isMemoryCacheable);
if (active != null) {
cb.onResourceReady(active, DataSource.MEMORY_CACHE);
if (Log.isLoggable(TAG, Log.VERBOSE)) {
logWithTimeAndKey("Loaded resource from active resources", startTime, key);
}
return null;
}
// 根据key获取缓存的job
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);
}
// 创建job
EngineJob<R> engineJob = engineJobFactory.build(key, isMemoryCacheable,
useUnlimitedSourceExecutorPool);
DecodeJob<R> decodeJob = decodeJobFactory.build(
glideContext,
model,
key,
signature,
width,
height,
resourceClass,
transcodeClass,
priority,
diskCacheStrategy,
transformations,
isTransformationRequired,
onlyRetrieveFromCache,
options,
engineJob);
jobs.put(key, engineJob);
// 放入线程池，执行
engineJob.addCallback(cb);
engineJob.start(decodeJob);
if (Log.isLoggable(TAG, Log.VERBOSE)) {
logWithTimeAndKey("Started new load", startTime, key);
}
return new LoadStatus(cb, engineJob);
}
可以看到首先从内存缓存中获取资源，其次从存活的资源中加载缓存，这么做可以提高命中率。内存缓存使用的是LruResourceCache继承自LruCache（lru，最近最少使用），当然了，这个LruCache也是glide内部自己实现的一个，现在还不必深入去看怎么实现的知道就行。如果loadFromCache命中，那么会将资源放入activeResources中，同时将资源从LruCache中移除。命中后会直接调用ResourceCallback回调方法onResourceReady，而最终又会调用target的onResourceReady方法，继续看下去就会发现最终的实现类调用了view.setImageBitmap(BitmapImageViewTarget)或者view.setImageDrawable(DrawableImageViewTarget)。另外一个内存缓存用弱引用缓存当前正在使用的资源，回调方式和之前的LruCache是一样的，不做更多的介绍了。
前面说的都是命中的情况，接着看未命中的。内存中读取数据都没有命中的话，则会生成EngineJob和DecodeJob。EngineJob的职责是调度DecodeJob，添加，移除资源回调，并notify回调。DecodeJob负责从缓存资源或者原始数据中读取资源，Glide中的脏累活基本都是这个DecodeJob干的。回到代码，最后调用了engineJob.start方法，看代码：
public void start(DecodeJob<R> decodeJob) {
this.decodeJob = decodeJob;
GlideExecutor executor = decodeJob.willDecodeFromCache()
? diskCacheExecutor
: getActiveSourceExecutor();
executor.execute(decodeJob);
}
这个GlideExecutor是一个线程池，而DecodeJob是实现了Runnable接口的类，所以看一下DecodeJob的run方法：
public void run() {
// This should be much more fine grained, but since Java's thread pool implementation silently
// swallows all otherwise fatal exceptions, this will at least make it obvious to developers
// that something is failing.
try {
if (isCancelled) {
notifyFailed();
return;
}
runWrapped();
} catch (RuntimeException e) {
if (Log.isLoggable(TAG, Log.DEBUG)) {
Log.d(TAG, "DecodeJob threw unexpectedly"
+ ", isCancelled: " + isCancelled
+ ", stage: " + stage, e);
}
// When we're encoding we've already notified our callback and it isn't safe to do so again.
if (stage != Stage.ENCODE) {
notifyFailed();
}
if (!isCancelled) {
throw e;
}
}
}
private void runWrapped() {
switch (runReason) {
// 初始化 获取下一个阶段状态
case INITIALIZE:
stage = getNextStage(Stage.INITIALIZE);
currentGenerator = getNextGenerator();
// 运行load数据
runGenerators();
break;
case SWITCH_TO_SOURCE_SERVICE:
runGenerators();
break;
case DECODE_DATA:
// 处理已经load到的数据
decodeFromRetrievedData();
break;
default:
throw new IllegalStateException("Unrecognized run reason: " + runReason);
}
private DataFetcherGenerator getNextGenerator() {
switch (stage) {
case RESOURCE_CACHE:
return new ResourceCacheGenerator(decodeHelper, this);
case DATA_CACHE:
return new DataCacheGenerator(decodeHelper, this);
case SOURCE:
return new SourceGenerator(decodeHelper, this);
case FINISHED:
return null;
default:
throw new IllegalStateException("Unrecognized stage: " + stage);
}
}
private Stage getNextStage(Stage current) {
switch (current) {
case INITIALIZE:
return diskCacheStrategy.decodeCachedResource()
? Stage.RESOURCE_CACHE : getNextStage(Stage.RESOURCE_CACHE);
case RESOURCE_CACHE:
return diskCacheStrategy.decodeCachedData()
? Stage.DATA_CACHE : getNextStage(Stage.DATA_CACHE);
case DATA_CACHE:
// Skip loading from source if the user opted to only retrieve the resource from cache.
return onlyRetrieveFromCache ? Stage.FINISHED : Stage.SOURCE;
case SOURCE:
case FINISHED:
return Stage.FINISHED;
default:
throw new IllegalArgumentException("Unrecognized stage: " + current);
}
}
主要的加载逻辑就在这几个方法里。这个runReason初始值就是INITIALIZE，进入这个case首先调用了getNextStage方法，获取到Stage然后根据Stage获取相应的Generator，最后执行Generator。一共有三种Generator：
- ResourceCacheGenerator：从处理过的缓存加载数据
- DataCacheGenerator：从原始缓存加载数据
- SourceGenerator：从数据源请求数据
上面提到执行Generator，会调用currentGenerator.startNext方法，这里主要看一下SourceGenerator的starNext方法：
public boolean startNext() {
if (dataToCache != null) {
Object data = dataToCache;
dataToCache = null;
cacheData(data);
}
if (sourceCacheGenerator != null && sourceCacheGenerator.startNext()) {
return true;
}
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;
loadData.fetcher.loadData(helper.getPriority(), this);
}
}
return started;
}
前面老长一段先不看了，第一次加载肯定是null，直接看后面，首先是getLoadData：
List<LoadData<?>> getLoadData() {
if (!isLoadDataSet) {
isLoadDataSet = true;
loadData.clear();
List<ModelLoader<Object, ?>> modelLoaders = glideContext.getRegistry().getModelLoaders(model);
int size = modelLoaders.size();
for (int i = 0; i < size; i++) {
ModelLoader<Object, ?> modelLoader = modelLoaders.get(i);
LoadData<?> current =
modelLoader.buildLoadData(model, width, height, options);
if (current != null) {
loadData.add(current);
}
}
}
return loadData;
}
通过glide的上下文获取到所有能处理Model类型的注册过的ModelLoader，遍历这些ModelLoader，通过buildLoadData生成LoadData，最终返回一个LoadData的列表。loadData咋来的弄清楚了，接着看重点了，loadData，这一看就是加载数据的方法了。由于Glide适配了多种网络请求框架，这基本都是用的接口来解耦，看起来真的挺蛋疼的……关于fetcher的实现类就不去分析了，直接看数据加载成功的回调就好。上面的代码是将SourceGenertor自身传递进去作为回调，直接看回调方法：
public void onDataReady(Object data) {
DiskCacheStrategy diskCacheStrategy = helper.getDiskCacheStrategy();
if (data != null && diskCacheStrategy.isDataCacheable(loadData.fetcher.getDataSource())) {
dataToCache = data;
// We might be being called back on someone else's thread. Before doing anything, we should
// reschedule to get back onto Glide's thread.
cb.reschedule();
} else {
cb.onDataFetcherReady(loadData.sourceKey, data, loadData.fetcher,
loadData.fetcher.getDataSource(), originalKey);
}
}
这里调用了reschedule，重新调度当前任务，这个时候我们前面忽略的代码就起作用了，会进行写缓存的操作。这之后会切换加载的策略，最终获取数据成功时会调用onDataFecherReady，这个方法又会调用decodeFromRetrieveData方法，这个方法又调用了decodeFromData，这个方法又调用了decodeFromFetcher（有完没完了……）
private <Data> Resource<R> decodeFromFetcher(Data data, DataSource dataSource)
throws GlideException {
LoadPath<Data, ?, R> path = decodeHelper.getLoadPath((Class<Data>) data.getClass());
return runLoadPath(data, dataSource, path);
}
private <Data, ResourceType> Resource<R> runLoadPath(Data data, DataSource dataSource,
LoadPath<Data, ResourceType, R> path) throws GlideException {
DataRewinder<Data> rewinder = glideContext.getRegistry().getRewinder(data);
try {
return path.load(rewinder, options, width, height,
new DecodeCallback<ResourceType>(dataSource));
} finally {
rewinder.cleanup();
}
}
看到这我真是无语了，又是一堆看不懂的……我能怎么办啊，我也很绝望啊！不过行百里者半九十（虽然可能连60都没……），咬咬牙就挺过去了~
里面涉及到的东西，一个一个的简介：
- LoadPath：根据给定的数据类型的DataFetcher尝试获取数据，然后尝试通过一个或多个decodePath进行decode
- DecodePath：根据指定的数据类型对resource进行decode和transcode
- DataRewinder：负责将流转换成数据类型
最终在run方法里开始了最终的加载，传入的参数是DecodeJob的内部类，实现了DecodePath.DecodeCallback，看一下最终的回调方法：
public Resource<Z> onResourceDecoded(Resource<Z> decoded) {
Class<Z> resourceSubClass = getResourceClass(decoded);
Transformation<Z> appliedTransformation = null;
Resource<Z> transformed = decoded;
if (dataSource != DataSource.RESOURCE_DISK_CACHE) {
appliedTransformation = decodeHelper.getTransformation(resourceSubClass);
// 资源转换
transformed = appliedTransformation.transform(decoded, width, height);
}
// TODO: Make this the responsibility of the Transformation.
if (!decoded.equals(transformed)) {
decoded.recycle();
}
final EncodeStrategy encodeStrategy;
final ResourceEncoder<Z> encoder;
if (decodeHelper.isResourceEncoderAvailable(transformed)) {
encoder = decodeHelper.getResultEncoder(transformed);
encodeStrategy = encoder.getEncodeStrategy(options);
} else {
encoder = null;
encodeStrategy = EncodeStrategy.NONE;
}
Resource<Z> result = transformed;
boolean isFromAlternateCacheKey = !decodeHelper.isSourceKey(currentSourceKey);
if (diskCacheStrategy.isResourceCacheable(isFromAlternateCacheKey, dataSource,
encodeStrategy)) {
if (encoder == null) {
throw new Registry.NoResultEncoderAvailableException(transformed.get().getClass());
}
final Key key;
if (encodeStrategy == EncodeStrategy.SOURCE) {
key = new DataCacheKey(currentSourceKey, signature);
} else if (encodeStrategy == EncodeStrategy.TRANSFORMED) {
key = new ResourceCacheKey(currentSourceKey, signature, width, height,
appliedTransformation, resourceSubClass, options);
} else {
throw new IllegalArgumentException("Unknown strategy: " + encodeStrategy);
}
LockedResource<Z> lockedResult = LockedResource.obtain(transformed);
// 根据缓存策略初始化
deferredEncodeManager.init(key, encoder, lockedResult);
result = lockedResult;
}
return result;
}
最后还是回到了decodeFromRetrievedData：
private void decodeFromRetrievedData() {
if (Log.isLoggable(TAG, Log.VERBOSE)) {
logWithTimeAndKey("Retrieved data", startFetchTime,
"data: " + currentData
+ ", cache key: " + currentSourceKey
+ ", fetcher: " + currentFetcher);
}
Resource<R> resource = null;
try {
resource = decodeFromData(currentFetcher, currentData, currentDataSource);
} catch (GlideException e) {
e.setLoggingDetails(currentAttemptingKey, currentDataSource);
exceptions.add(e);
}
if (resource != null) {
notifyEncodeAndRelease(resource, currentDataSource);
} else {
runGenerators();
}
}
notifyEncodeAndRelease中处理了对处理过的图片的缓存操作。当缓存完成后（如果有需要的话）就通过回调告诉外面加载完成了。至此，整个加载过程完成。
后记
这里只是简单的过了一遍最简的调用所经历的过程，但是这中间涉及到的东西真的很多。Glide用起来还是挺方便的，由于需要考虑各种情况和适配不同的网络请求框架，内部采用了各种接口解耦，读的时候这也算是比较痛苦的一点吧，往往得往前追溯几个类你才能知道这个实现类是什么。不过Glide还是很值得我们去学习一下，阅读一下的，以后我也会更加深入的去阅读Glide。
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