图片加载框架Glide解析
对比Picasso内存占用
Glide是一个高效、开源、 Android设备上的媒体管理框架，它遵循BSD、MIT以及Apache 2.0协议发布。Glide具有获取、解码和展示视频剧照、图片、动画等功能，它还有灵活的API，这些API使开发者能够将Glide应用在几乎任何网络协议栈里。创建Glide的主要目的有两个，一个是实现平滑的图片列表滚动效果，另一个是支持远程图片的获取、大小调整和展示。近日，Glide 3.0发布，现已提供jar包下载，同时还支持使用Gradle以及Maven进行构建。该版本包括很多值得关注的新功能，如支持Gif 动画和视频剧照解码、智能的暂停和重新开始请求、支持缩略图等，具体新增功能如下如下：GIF动画的解码：通过调用Glide.with(context).load(“图片路径“)方法，GIF动画图片可以自动显示为动画效果。如果想有更多的控制，还可以使用Glide.with(context).load(“图片路径“).asBitmap()方法加载静态图片，使用Glide.with(context).load(“图片路径“).asGif()方法加载动画图片
1.本地视频剧照的解码：通过调用Glide.with(context).load(“图片路径“)方法，Glide能够支持Android设备中的所有视频剧照的加载和展示
2.缩略图的支持：为了减少在同一个view组件里同时加载多张图片的时间，可以调用Glide.with(context).load(“图片路径“).thumbnail(“缩略比例“).into(“view组件“)方法加载一个缩略图，还可以控制thumbnail()中的参数的大小，以控制显示不同比例大小的缩略图
3.Activity生命周期的集成：当Activity暂停和重启时，Glide能够做到智能的暂停和重新开始请求，并且当Android设备的连接状态变化时，所有失败的请求能够自动重新请求4.转码的支持：Glide的toBytes() 和transcode() 两个方法可以用来获取、解码和变换背景图片，并且transcode() 方法还能够改变图片的样式
5.动画的支持：新增支持图片的淡入淡出动画效果（调用crossFade()方法）和查看动画的属性的功能
6.OkHttp和Volley的支持：默认选择HttpUrlConnection作为网络协议栈，还可以选择OkHttp和Volley作为网络协议
7.其他功能：如在图片加载过程中，使用Drawables对象作为占位符、图片请求的优化、图片的宽度和高度可重新设定、缩略图和原图的缓存等功能
Glide.class
利用这个方法可以获得一个RequestManager对象.
RequestManager.class
这个类主要是用来处理低内存的时候的处理；管理请求，比如请求的开始，暂停，结束；还有通过加载方法获得一个DrawableTypeRequest对象。
低内存处理方法：
管理请求的方法：与activity或者fragment的生命周期同步。
加载的方法：
load方法都是设置了一个泛型，最后都调用了：
DrawableRequestBuilder.class
@Override
public DrawableRequestBuilder load(ModelType model) {
super.load(model);
return this;
} GenericRequestBuilder.class
public GenericRequestBuilder load(ModelType model) {
this.model = model;
isModelSet = true;
return this;
} 由GenericRequestBuilder得四种泛型参数可以看出， 这个类提供了加载各种资源的方法，是各种资源请求构建类的父类：
通过以上方法来加载缩略图。
第一个是对原数据的解码。
第二个是对磁盘缓存的数据进行解码。
第三个是对请求的数据进行编码，然后存到缓存里，之后从缓存中拿出的时候要通过解码。
设置磁盘缓存的内容，DiskCacheStrategy类是个枚举类，他提供了四个参数：
/** Caches with both {@link #SOURCE} and {@link #RESULT}. */
ALL(true, true),
/** Saves no data to cache. */
NONE(false, false),
/** Saves just the original data to cache. */
SOURCE(true, false),
/** Saves the media item after all transformations to cache. */
RESULT(false, true);none:不作任何磁盘缓存
source:缓存源资源
result：缓存转换后的资源
priority(Priority priority)transform(Transformation... transformations) 
对于加载一般的图片到Imageview中，默认提供了CenterCrop和FitCenter两种实现。
dontTransform()transcoder(ResourceTranscoder transcoder) 
设置资源加载完成后的动画，不包括从内存缓存中获取。
dontAnimate()placeholder(int resourceId)
placeholder(Drawable drawable)error(int resourceId)
error(Drawable drawable)listener(RequestListener requestListener)这个监听接口中有两个回调方法：
skipMemoryCache(boolean skip)override(int width, int height)signature(Key signature)load(ModelType model)
通过上面的方法开启加载，中间的方法直接加载到view上，前后的方法用于预加载。
预加载，真正调用的是上面的两个into方法。
接着看子类DrawableRequestBuilder里面特有的方法：
DrawableRequestBuilder类不仅继承了父类GenericRequestBuilder里面的方法，还显示了BitmapOptions和DrawableOptions两个接口：
一个是控制bitmap的显示的位置或者大小(就是bitmap的转换)，一个是控制图片显示出来时候的动画效果(不包括从内存中获取的图片资源)。接着看这些方法的具体实现：
public DrawableRequestBuilder centerCrop() {
return transform(glide.getDrawableCenterCrop());
} public DrawableRequestBuilder fitCenter() {
return transform(glide.getDrawableFitCenter());
} public final DrawableRequestBuilder crossFade() {
super.animate(new DrawableCrossFadeFactory());
return this;
} 而设置淡入淡出效果其实是设置了默认动画实现的DrawableCrossFadeFactory类到父类animationFactory属性中。
public > Y into(Y target) {
Util.assertMainThread();
if (target == null) {
throw new IllegalArgumentException("You must pass in a non null Target");
}
if (!isModelSet) {
throw new IllegalArgumentException("You must first set a model (try #load())");
}
Request previous = target.getRequest();
if (previous != null) {
previous.clear();
requestTracker.removeRequest(previous);
previous.recycle();
}
Request request = buildRequest(target);
target.setRequest(request);
lifecycle.addListener(target);
requestTracker.runRequest(request);
return target;
} @Override
public void begin() {
startTime = LogTime.getLogTime();
if (model == null) {
onException(null);
return;
}
status = Status.WAITING_FOR_SIZE;
if (Util.isValidDimensions(overrideWidth, overrideHeight)) {
onSizeReady(overrideWidth, overrideHeight);
} else {
target.getSize(this);
}
if (!isComplete() && !isFailed() && canNotifyStatusChanged()) {
target.onLoadStarted(getPlaceholderDrawable());
}
if (Log.isLoggable(TAG, Log.VERBOSE)) {
logV("finished run method in " + LogTime.getElapsedMillis(startTime));
}
} public void onSizeReady(int width, int height) {
if (Log.isLoggable(TAG, Log.VERBOSE)) {
logV("Got onSizeReady in " + LogTime.getElapsedMillis(startTime));
}
if (status != Status.WAITING_FOR_SIZE) {
return;
}
status = Status.RUNNING;
width = Math.round(sizeMultiplier * width);
height = Math.round(sizeMultiplier * height);
ModelLoader modelLoader = loadProvider.getModelLoader();
final DataFetcher dataFetcher = modelLoader.getResourceFetcher(model, width, height);
if (dataFetcher == null) {
onException(new Exception("Got null fetcher from model loader"));
return;
}
ResourceTranscoder transcoder = loadProvider.getTranscoder();
if (Log.isLoggable(TAG, Log.VERBOSE)) {
logV("finished setup for calling load in " + LogTime.getElapsedMillis(startTime));
}
loadedFromMemoryCache = true;
loadStatus = engine.load(signature, width, height, dataFetcher, loadProvider, transformation, transcoder,
priority, isMemoryCacheable, diskCacheStrategy, this);
loadedFromMemoryCache = resource != null;
if (Log.isLoggable(TAG, Log.VERBOSE)) {
logV("finished onSizeReady in " + LogTime.getElapsedMillis(startTime));
}
} public LoadStatus load(Key signature, int width, int height, DataFetcher fetcher,
DataLoadProvider loadProvider, Transformation transformation, ResourceTranscoder transcoder,
Priority priority, boolean isMemoryCacheable, DiskCacheStrategy diskCacheStrategy, ResourceCallback cb) {
Util.assertMainThread();
long startTime = LogTime.getLogTime();
final String id = fetcher.getId();
EngineKey key = keyFactory.buildKey(id, signature, width, height, loadProvider.getCacheDecoder(),
loadProvider.getSourceDecoder(), transformation, loadProvider.getEncoder(),
transcoder, loadProvider.getSourceEncoder());
EngineResource cached = loadFromCache(key, isMemoryCacheable);
if (cached != null) {
cb.onResourceReady(cached);
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);
if (Log.isLoggable(TAG, Log.VERBOSE)) {
logWithTimeAndKey("Loaded resource from active resources", startTime, key);
}
return null;
}
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);
}
EngineJob engineJob = engineJobFactory.build(key, isMemoryCacheable);
DecodeJob decodeJob = new DecodeJob(key, width, height, fetcher, loadProvider, transformation,
transcoder, diskCacheProvider, diskCacheStrategy, priority);
EngineRunnable runnable = new EngineRunnable(engineJob, decodeJob, priority);
jobs.put(key, engineJob);
engineJob.addCallback(cb);
engineJob.start(runnable);
if (Log.isLoggable(TAG, Log.VERBOSE)) {
logWithTimeAndKey("Started new load", startTime, key);
}
return new LoadStatus(cb, engineJob);
} public void run() {
if (isCancelled) {
return;
}
Exception exception = null;
Resource resource = null;
try {
resource = decode();
} catch (Exception e) {
if (Log.isLoggable(TAG, Log.VERBOSE)) {
Log.v(TAG, "Exception decoding", e);
}
exception = e;
}
if (isCancelled) {
if (resource != null) {
resource.recycle();
}
return;
}
if (resource == null) {
onLoadFailed(exception);
} else {
onLoadComplete(resource);
}
} private Resource decode() throws Exception {
if (isDecodingFromCache()) {
return decodeFromCache();
} else {
return decodeFromSource();
}
} private Resource decodeFromCache() throws Exception {
Resource result = null;
try {
result = decodeJob.decodeResultFromCache();
} catch (Exception e) {
if (Log.isLoggable(TAG, Log.DEBUG)) {
Log.d(TAG, "Exception decoding result from cache: " + e);
}
}
if (result == null) {
result = decodeJob.decodeSourceFromCache();
}
return result;
} public Resource decodeResultFromCache() throws Exception {
if (!diskCacheStrategy.cacheResult()) {
return null;
}
long startTime = LogTime.getLogTime();
Resource transformed = loadFromCache(resultKey);
if (Log.isLoggable(TAG, Log.VERBOSE)) {
logWithTimeAndKey("Decoded transformed from cache", startTime);
}
startTime = LogTime.getLogTime();
Resource result = transcode(transformed);
if (Log.isLoggable(TAG, Log.VERBOSE)) {
logWithTimeAndKey("Transcoded transformed from cache", startTime);
}
return result;
} 第二个，decodeJob.decodeSourceFromCache()，如果设置缓存的是DiskCacheStrategy.SOURCE,那么磁盘缓存中缓存的是原始数据，所以就会调用该方法：
public Resource decodeSourceFromCache() throws Exception {
if (!diskCacheStrategy.cacheSource()) {
return null;
}
long startTime = LogTime.getLogTime();
Resource decoded = loadFromCache(resultKey.getOriginalKey());
if (Log.isLoggable(TAG, Log.VERBOSE)) {
logWithTimeAndKey("Decoded source from cache", startTime);
}
return transformEncodeAndTranscode(decoded);
} private Resource transformEncodeAndTranscode(Resource decoded) {
long startTime = LogTime.getLogTime();
Resource transformed = transform(decoded);
if (Log.isLoggable(TAG, Log.VERBOSE)) {
logWithTimeAndKey("Transformed resource from source", startTime);
}
writeTransformedToCache(transformed);
startTime = LogTime.getLogTime();
Resource result = transcode(transformed);
if (Log.isLoggable(TAG, Log.VERBOSE)) {
logWithTimeAndKey("Transcoded transformed from source", startTime);
}
return result;
} private Resource decodeFromSource() throws Exception {
return decodeJob.decodeFromSource();
} public Resource decodeFromSource() throws Exception {
Resource decoded = decodeSource();
return transformEncodeAndTranscode(decoded);
} private Resource decodeSource() throws Exception {
Resource decoded = null;
try {
long startTime = LogTime.getLogTime();
final A data = fetcher.loadData(priority);
if (Log.isLoggable(TAG, Log.VERBOSE)) {
logWithTimeAndKey("Fetched data", startTime);
}
if (isCancelled) {
return null;
}
decoded = decodeFromSourceData(data);
} finally {
fetcher.cleanup();
}
return decoded;
} private Resource decodeFromSourceData(A data) throws IOException {
final Resource decoded;
if (diskCacheStrategy.cacheSource()) {
decoded = cacheAndDecodeSourceData(data);
} else {
long startTime = LogTime.getLogTime();
decoded = loadProvider.getSourceDecoder().decode(data, width, height);
if (Log.isLoggable(TAG, Log.VERBOSE)) {
logWithTimeAndKey("Decoded from source", startTime);
}
}
return decoded;
} private Resource cacheAndDecodeSourceData(A data) throws IOException {
long startTime = LogTime.getLogTime();
SourceWriter writer = new SourceWriter(loadProvider.getSourceEncoder(), data);
diskCacheProvider.getDiskCache().put(resultKey.getOriginalKey(), writer);
if (Log.isLoggable(TAG, Log.VERBOSE)) {
logWithTimeAndKey("Wrote source to cache", startTime);
}
startTime = LogTime.getLogTime();
Resource result = loadFromCache(resultKey.getOriginalKey());
if (Log.isLoggable(TAG, Log.VERBOSE) && result != null) {
logWithTimeAndKey("Decoded source from cache", startTime);
}
return result;
} 但是如果不需要保存源数据，就直接decode源数据：
private Resource decodeFromSourceData(A data) throws IOException {
final Resource decoded;
if (diskCacheStrategy.cacheSource()) {
decoded = cacheAndDecodeSourceData(data);
} else {
long startTime = LogTime.getLogTime();
decoded = loadProvider.getSourceDecoder().decode(data, width, height);
if (Log.isLoggable(TAG, Log.VERBOSE)) {
logWithTimeAndKey("Decoded from source", startTime);
}
}
return decoded;
} private Resource transformEncodeAndTranscode(Resource decoded) {
long startTime = LogTime.getLogTime();
Resource transformed = transform(decoded);
if (Log.isLoggable(TAG, Log.VERBOSE)) {
logWithTimeAndKey("Transformed resource from source", startTime);
}
writeTransformedToCache(transformed);
startTime = LogTime.getLogTime();
Resource result = transcode(transformed);
if (Log.isLoggable(TAG, Log.VERBOSE)) {
logWithTimeAndKey("Transcoded transformed from source", startTime);
}
return result;
} 这里是将直接从源数据decode出来资源后，经过transform的转换，然后将transform后的资源存进磁盘缓存，然后在经过transcode方法来进行资源类型的转换（默认是使用UnitTranscoder，没有做任何操作）。还是提供逻辑图，一目了然吧：
使用方法：
1.设置的不是imageview，可以用SimpleTarget来接受，然后在回调里面设置view的现实（但是缺少目标view的设置，似乎在列表中会出现错乱）
Glide.with(StaggeredGridViewDemo.this).load(mImages.get(arg0).getPic()).asBitmap().override(250, 250).into(new SimpleTarget() {
@Override
public void onResourceReady(Bitmap arg0, GlideAnimation arg1) {
Log.i("llj", "arg0.getWidth()+arg0.getHeight()" + arg0.getWidth() + arg0.getHeight());
holder.textView.setBackgroundDrawable(new BitmapDrawable(arg0));
}
}); 
也可以通过SimpleTarget的构造函数传参数：
Glide.with(StaggeredGridViewDemo.this).load(mImages.get(arg0).getPic()).asBitmap().into(new SimpleTarget(250, 250) {
@Override
public void onResourceReady(Bitmap arg0, GlideAnimation arg1) {
Log.i("llj", "arg0.getWidth()+arg0.getHeight()" + arg0.getWidth() + ";" + arg0.getHeight());
// holder.textView.setBackgroundDrawable(new BitmapDrawable(arg0));
}
}); 但是SimpleTarget的构造函数穿进去的参数也没有起作用：
2.设置预加载：（可以提前将资源加载到磁盘缓存中，默认的缓存目录是data\data下的250M空间，自己可以根据需求更改）
final PreloadTarget target = PreloadTarget.obtain(250, 250);
Glide.with(StaggeredGridViewDemo.this).load(mImages.get(arg0).getPic()).asBitmap().listener(new RequestListener() {
@Override
public boolean onException(Exception e, String model, Target target, boolean isFirstResource) {
return false;
}
@Override
public boolean onResourceReady(Bitmap resource, String model, Target target, boolean isFromMemoryCache, boolean isFirstResource) {
Log.i("llj", "arg0.getWidth()+arg0.getHeight()" + resource.getWidth() + resource.getHeight());
return false;
}
}).into(target);
final PreloadTarget target = PreloadTarget.obtain(0, 0);
Glide.with(StaggeredGridViewDemo.this).load(mImages.get(arg0).getPic()).asBitmap().override(250, 250).listener(new RequestListener() {
@Override
public boolean onException(Exception e, String model, Target target, boolean isFirstResource) {
return false;
}
@Override
public boolean onResourceReady(Bitmap resource, String model, Target target, boolean isFromMemoryCache, boolean isFirstResource) {
Log.i("llj", "arg0.getWidth()+arg0.getHeight()" + resource.getWidth() + resource.getHeight());
return false;
}
}).into(target);
3.加载到imageview上
Glide.with(StaggeredGridViewDemo.this).load(mImages.get(arg0).getPic()).asBitmap().override(250, 250).listener(new RequestListener() {
@Override
public boolean onException(Exception e, String model, Target target, boolean isFirstResource) {
return false;
}
@Override
public boolean onResourceReady(Bitmap resource, String model, Target target, boolean isFromMemoryCache, boolean isFirstResource) {
Log.i("llj", "arg0.getWidth()+arg0.getHeight()" + resource.getWidth() + resource.getHeight());
return false;
}
}).into(holder.imageview); 
但是如果设置了centerCrop()：
而用Picasso转换后的图片是250：250：
Picasso.with(StaggeredGridViewDemo.this).load(mImages.get(arg0).getPic()).resize(250, 250).into(new com.squareup.picasso.Target() {
@Override
public void onPrepareLoad(Drawable arg0) {
}
@Override
public void onBitmapLoaded(Bitmap arg0, LoadedFrom arg1) {
Log.i("llj", "arg0.getWidth()+arg0.getHeight()" + arg0.getWidth() + ";" + arg0.getHeight());
}
@Override
public void onBitmapFailed(Drawable arg0) {
}
});说明两者的按比例缩放的方法不一样！