Android图片框架Glide原理深入探索
作者：明智的健哥
首先引入依赖
implementation 'com.github.bumptech.glide:glide:4.12.0'
annotationProcessor 'com.github.bumptech.glide:compiler:4.12.0'
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/27524013): 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/27524013): 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/154405040
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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