0使用手册
http://square.github.io/picasso/
1使用方法简介
Picasso.with(context)
.load(url)
.resize(50, 50)
.centerCrop()
.into(imageView)
2 基本原理
- 先使用Picasso.Builder生成一个单例Picasso，
- load的时候生成一个RequestCreator，再设置不同的Reqeust参数，into的时候生成一个Request，
- 再将Request和Target组合成Action，
- 由Picasso交给Dispatcher后台执行，
- Dispatcher用Action生成BitmapHunter，交于PicassoExecutorService执行，BitmapHunter下载完成，执行相应的变换后，
- 用Handler通知Dispatcher缓存在内存中，
- Dispatcher再用Handler通知Picasso调用Action.complete()方法
3数据结构分析
RequestHandler: 根据Request获取相应的内容。FileRequestHandler使用ContentResolver从文件中获取。NetworkRequestHandler使用Downloader从网络获取。ResourceRequestHandler从apk资源中获取。ContactsPhotoRequestHandler使用ContentResolver从通讯录中获取。ContentStreamRequestHandler使用ContentResolver获取。
Downloader： 从网络获取时需要下载。UrlConnectionDownloader使用UrlConnection从网络下载，HttpResponseCache可以让UrlConnection使用文件缓存数据。OkHttpDownloader使用OkHttp库来下载图片，OkHttpClient.setCache可以让OkHttp使用文件缓存
Stats：统计数据，缓存的命中率等等
Action：包含Request，Target和PlaceHolder，和异步加载后的回调，complete，error，cancel。
BitmapHunter： 异步Runnable任务。
4源代码分析
- Picasso.Builder.build()
/**
2. 构造Picasso,使用Picasso.with会构造默认的方法，如果要自定义Picasso，则需要使用Builder，并在构造好了
3. 之后使用Picasso.setSingletonInstance.
*/
/** Create the {@link Picasso} instance. */
public Picasso build() {
Context context = this.context;
if (downloader == null) {
downloader = Utils.createDefaultDownloader(context);
}
if (cache == null) {
cache = new LruCache(context);
}
if (service == null) {
service = new PicassoExecutorService();
}
if (transformer == null) {
transformer = RequestTransformer.IDENTITY;
}
Stats stats = new Stats(cache);
Dispatcher dispatcher = new Dispatcher(context, service, HANDLER, downloader, cache, stats);
return new Picasso(context, dispatcher, cache, listener, transformer, requestHandlers, stats,
defaultBitmapConfig, indicatorsEnabled, loggingEnabled);
}- RequestCreator.into()
/**
* 调用into方法时，使用Request.Builder创建一个Request，然后交给Dispatcher调度执行。
* @param target
*/
public void into(Target target) {
long started = System.nanoTime();
checkMain();
if (target == null) {
throw new IllegalArgumentException("Target must not be null.");
}
//当target是ImageView时，需要deferred，生成DeferredRequestCreator,利用ImageView的addOnPreDrawListener()当需要绘制的时候再来进行加载
if (deferred) {
throw new IllegalStateException("Fit cannot be used with a Target.");
}
//url为空或者resourceid为空，直接Cancel
if (!data.hasImage()) {
picasso.cancelRequest(target);
target.onPrepareLoad(setPlaceholder ? getPlaceholderDrawable() : null);
return;
}
Request request = createRequest(started);
String requestKey = createKey(request);
if (shouldReadFromMemoryCache(memoryPolicy)) {
Bitmap bitmap = picasso.quickMemoryCacheCheck(requestKey);
if (bitmap != null) {
picasso.cancelRequest(target);
target.onBitmapLoaded(bitmap, MEMORY);
return;
}
}
target.onPrepareLoad(setPlaceholder ? getPlaceholderDrawable() : null);
Action action =
new TargetAction(picasso, target, request, memoryPolicy, networkPolicy, errorDrawable,
requestKey, tag, errorResId);
picasso.enqueueAndSubmit(action);
}- BitmapHunter.hunt()
Bitmap hunt() throws IOException {
Bitmap bitmap = null;
//判断是否从内存加载
if (shouldReadFromMemoryCache(memoryPolicy)) {
bitmap = cache.get(key);
if (bitmap != null) {
stats.dispatchCacheHit();
loadedFrom = MEMORY;
if (picasso.loggingEnabled) {
log(OWNER_HUNTER, VERB_DECODED, data.logId(), "from cache");
}
return bitmap;
}
}
//使用RequestHandler加载图片
data.networkPolicy = retryCount == 0 ? NetworkPolicy.OFFLINE.index : networkPolicy;
RequestHandler.Result result = requestHandler.load(data, networkPolicy);
if (result != null) {
loadedFrom = result.getLoadedFrom();
exifRotation = result.getExifOrientation();
bitmap = result.getBitmap();
//如果得到的不是图片而是InputStream，则转换成Bitmap
if (bitmap == null) {
InputStream is = result.getStream();
try {
bitmap = decodeStream(is, data);
} finally {
Utils.closeQuietly(is);
}
}
}
if (bitmap != null) {
if (picasso.loggingEnabled) {
log(OWNER_HUNTER, VERB_DECODED, data.logId());
}
stats.dispatchBitmapDecoded(bitmap);
//进行裁剪，旋转等变换
if (data.needsTransformation() || exifRotation != 0) {
synchronized (DECODE_LOCK) {
if (data.needsMatrixTransform() || exifRotation != 0) {
bitmap = transformResult(data, bitmap, exifRotation);
if (picasso.loggingEnabled) {
log(OWNER_HUNTER, VERB_TRANSFORMED, data.logId());
}
}
if (data.hasCustomTransformations()) {
bitmap = applyCustomTransformations(data.transformations, bitmap);
if (picasso.loggingEnabled) {
log(OWNER_HUNTER, VERB_TRANSFORMED, data.logId(), "from custom transformations");
}
}
}
if (bitmap != null) {
stats.dispatchBitmapTransformed(bitmap);
}
}
}
return bitmap;
}- Dispatcher.performComplete()
//先判断是否要写内存缓存，然后将当前的hunter批量。
//批量的原因是在Picasso的Handler当前可能还有没有处理的。再Dispatcher这一级别再缓冲一下
void performComplete(BitmapHunter hunter) {
if (shouldWriteToMemoryCache(hunter.getMemoryPolicy())) {
cache.set(hunter.getKey(), hunter.getResult());
}
hunterMap.remove(hunter.getKey());
batch(hunter);
if (hunter.getPicasso().loggingEnabled) {
log(OWNER_DISPATCHER, VERB_BATCHED, getLogIdsForHunter(hunter), "for completion");
}
}- Picasso.comple()
//从Hunter中取出结果，调用Action.complete，or Action.error
void complete(BitmapHunter hunter) {
Action single = hunter.getAction();
List<Action> joined = hunter.getActions();
boolean hasMultiple = joined != null && !joined.isEmpty();
boolean shouldDeliver = single != null || hasMultiple;
if (!shouldDeliver) {
return;
}
Uri uri = hunter.getData().uri;
Exception exception = hunter.getException();
Bitmap result = hunter.getResult();
LoadedFrom from = hunter.getLoadedFrom();
if (single != null) {
deliverAction(result, from, single);
}
if (hasMultiple) {
//noinspection ForLoopReplaceableByForEach
for (int i = 0, n = joined.size(); i < n; i++) {
Action join = joined.get(i);
deliverAction(result, from, join);
}
}
if (listener != null && exception != null) {
listener.onImageLoadFailed(this, uri, exception);
}
}