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picasso是Square公司开源的一个Android图形缓存库，地址http://square.github.io/picasso/，可以实现图片下载和缓存功能。仅仅只需要一行代码就能完全实现图片的异步加载：
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Api看起来非常独特，是吧。
Picasso不仅实现了图片异步加载的功能，还解决了android中加载图片时需要解决的一些常见问题：
1.在adapter中需要取消已经不在视野范围的ImageView图片资源的加载，否则会导致图片错位，Picasso已经解决了这个问题。
2.使用复杂的图片压缩转换来尽可能的减少内存消耗
3.自带内存和硬盘二级缓存功能
特性以及示例代码：
ADAPTER 中的下载：Adapter的重用会被自动检测到，Picasso会取消上次的加载
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@Override public void getView(int position, View convertView, ViewGroup parent) { SquaredImageView view = (SquaredImageView) convertView; if (view == null) { view = new SquaredImageView(context); } String url = getItem(position); Picasso.with(context).load(url).into(view);} |
图片转换：转换图片以适应布局大小并减少内存占用
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Picasso.with(context) .load(url) .resize(50, 50) .centerCrop() .into(imageView); |
你还可以自定义转换：
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public class CropSquareTransformation implements Transformation { @Override public Bitmap transform(Bitmap source) { int size = Math.min(source.getWidth(), source.getHeight()); int x = (source.getWidth() - size) / 2; int y = (source.getHeight() - size) / 2; Bitmap result = Bitmap.createBitmap(source, x, y, size, size); if (result != source) { source.recycle(); } return result; } @Override public String key() { return "square()"; }} |
将CropSquareTransformation 的对象传递给transform 方法即可。
Place holders-空白或者错误占位图片：picasso提供了两种占位图片，未加载完成或者加载发生错误的时需要一张图片作为提示。
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Picasso.with(context) .load(url) .placeholder(R.drawable.user_placeholder) .error(R.drawable.user_placeholder_error).into(imageView); |
如果加载发生错误会重复三次请求，三次都失败才会显示erro Place holder
资源文件的加载：除了加载网络图片picasso还支持加载Resources, assets, files, content providers中的资源文件。
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Picasso.with(context).load(R.drawable.landing_screen).into(imageView1);Picasso.with(context).load(new File(...)).into(imageView2); |
下面是picasso源码的解析（不看不影响使用）
Cache，缓存类
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this.map = new LinkedHashMap<String, Bitmap>(0, 0.75f, true); |
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private void trimToSize(int maxSize) { while (true) { String key; Bitmap value; synchronized (this) { if (size < 0 || (map.isEmpty() && size != 0)) { throw new IllegalStateException(getClass().getName() + ".sizeOf() is reporting inconsistent results!"); } if (size <= maxSize || map.isEmpty()) { break; } Map.Entry<String, Bitmap> toEvict = map.entrySet().iterator() .next(); key = toEvict.getKey(); value = toEvict.getValue(); map.remove(key); size -= Utils.getBitmapBytes(value); evictionCount++; } }} |
Request，操作封装类
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public interface Transformation { /** * Transform the source bitmap into a new bitmap. If you create a new bitmap instance, you must * call {@link android.graphics.Bitmap#recycle()} on {@code source}. You may return the original * if no transformation is required. */ Bitmap transform(Bitmap source); /** * Returns a unique key for the transformation, used for caching purposes. If the transformation * has parameters (e.g. size, scale factor, etc) then these should be part of the key. */ String key();} |
当操作封装好以后，会将Request传到另一个结构中Action。
Action
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@Override public void complete(Bitmap result, Picasso.LoadedFrom from) { if (result == null) { throw new AssertionError(String.format( "Attempted to complete action with no result!\n%s", this)); } ImageView target = this.target.get(); if (target == null) { return; } Context context = picasso.context; boolean debugging = picasso.debugging; PicassoDrawable.setBitmap(target, context, result, from, noFade, debugging); if (callback != null) { callback.onSuccess(); } } |
有了加载任务，具体的图片下载与解析是在哪里呢？这些都是耗时的操作，应该放在异步线程中进行，就是下面的BitmapHunter。
BitmapHunter
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@Override public void run() { try { Thread.currentThread() .setName(Utils.THREAD_PREFIX + data.getName()); result = hunt(); if (result == null) { dispatcher.dispatchFailed(this); } else { dispatcher.dispatchComplete(this); } } catch (IOException e) { exception = e; dispatcher.dispatchRetry(this); } catch (Exception e) { exception = e; dispatcher.dispatchFailed(this); } finally { Thread.currentThread().setName(Utils.THREAD_IDLE_NAME); } } abstract Bitmap decode(Request data) throws IOException; Bitmap hunt() throws IOException { Bitmap bitmap; if (!skipMemoryCache) { bitmap = cache.get(key); if (bitmap != null) { stats.dispatchCacheHit(); loadedFrom = MEMORY; return bitmap; } } bitmap = decode(data); if (bitmap != null) { stats.dispatchBitmapDecoded(bitmap); if (data.needsTransformation() || exifRotation != 0) { synchronized (DECODE_LOCK) { if (data.needsMatrixTransform() || exifRotation != 0) { bitmap = transformResult(data, bitmap, exifRotation); } if (data.hasCustomTransformations()) { bitmap = applyCustomTransformations( data.transformations, bitmap); } } stats.dispatchBitmapTransformed(bitmap); } } return bitmap; } |
Dispatcher任务调度器
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void dispatchSubmit(Action action) { handler.sendMessage(handler.obtainMessage(REQUEST_SUBMIT, action)); } |
handler接到消息后转换到performSubmit方法
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void performSubmit(Action action) { BitmapHunter hunter = hunterMap.get(action.getKey()); if (hunter != null) { hunter.attach(action); return; } if (service.isShutdown()) { return; } hunter = forRequest(context, action.getPicasso(), this, cache, stats, action, downloader); hunter.future = service.submit(hunter); hunterMap.put(action.getKey(), hunter); } |
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public static Picasso with(Context context) { if (singleton == null) { singleton = new Builder(context).build(); } return singleton; } 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, stats, debugging); } |
在Picasso.with()的时候会将执行所需的所有必备元素创建出来，如缓存cache、执行executorService、调度dispatch等，在load()时创建Request，在into()中创建action、bitmapHunter，并最终交给dispatcher执行。
转：http://blog.csdn.net/xu_fu/article/details/17043231
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原文地址：http://www.cnblogs.com/spring87/p/4263916.html
