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：

1. Picasso.with(context).load("http://i.imgur.com/DvpvklR.png").into(imageView);

  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

1. @Override public void getView(int position, View convertView, ViewGroup parent) {
2. SquaredImageView view = (SquaredImageView) convertView;
3. if (view == null) {
4. view = new SquaredImageView(context);
5. }
6. String url = getItem(position);
7. Picasso.with(context).load(url).into(view);
8. }

   图片转换：转换图片以适应布局大小并减少内存占用

1. Picasso.with(context)
2. .load(url)
3. .resize(50, 50)
4. .centerCrop()
5. .into(imageView);

   你还可以自定义转换：

1. public class CropSquareTransformation implements Transformation {
2. @Override public Bitmap transform(Bitmap source) {
3. int size = Math.min(source.getWidth(), source.getHeight());
4. int x = (source.getWidth() - size) / 2;
5. int y = (source.getHeight() - size) / 2;
6. Bitmap result = Bitmap.createBitmap(source, x, y, size, size);
7. if (result != source) {
8. source.recycle();
9. }
10. return result;
11. }
12. @Override public String key() { return "square()"; }
13. }

   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。

1. Picasso.with(context)
2. .load(url)
3. .placeholder(R.drawable.user_placeholder)
4. .error(R.drawable.user_placeholder_error)
5. .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。

1. Picasso.with(context).load(R.drawable.landing_screen).into(imageView1);
2. 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

1. this.map = new LinkedHashMap<String, Bitmap>(0, 0.75f, true);

1. private void trimToSize(int maxSize) {
2. while (true) {
3. String key;
4. Bitmap value;
5. synchronized (this) {
6. if (size < 0 || (map.isEmpty() && size != 0)) {
7. throw new IllegalStateException(getClass().getName()
8. + ".sizeOf() is reporting inconsistent results!");
9. }
10.  
11. if (size <= maxSize || map.isEmpty()) {
12. break;
13. }
14.  
15. Map.Entry<String, Bitmap> toEvict = map.entrySet().iterator()
16. .next();
17. key = toEvict.getKey();
18. value = toEvict.getValue();
19. map.remove(key);
20. size -= Utils.getBitmapBytes(value);
21. evictionCount++;
22. }
23. }
24. }

Request，操作封装类

1. public interface Transformation {
2. /**
3. * Transform the source bitmap into a new bitmap. If you create a new bitmap instance, you must
4. * call {@link android.graphics.Bitmap#recycle()} on {@code source}. You may return the original
5. * if no transformation is required.
6. */
7. Bitmap transform(Bitmap source);
8.  
9. /**
10. * Returns a unique key for the transformation, used for caching purposes. If the transformation
11. * has parameters (e.g. size, scale factor, etc) then these should be part of the key.
12. */
13. String key();
14. }

当操作封装好以后，会将Request传到另一个结构中Action。

Action

1. @Override
2. public void complete(Bitmap result, Picasso.LoadedFrom from) {
3. if (result == null) {
4. throw new AssertionError(String.format(
5. "Attempted to complete action with no result!\n%s", this));
6. }
7.  
8. ImageView target = this.target.get();
9. if (target == null) {
10. return;
11. }
12.  
13. Context context = picasso.context;
14. boolean debugging = picasso.debugging;
15. PicassoDrawable.setBitmap(target, context, result, from, noFade,
16. debugging);
17.  
18. if (callback != null) {
19. callback.onSuccess();
20. }
21. }

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

1. @Override
2. public void run() {
3. try {
4. Thread.currentThread()
5. .setName(Utils.THREAD_PREFIX + data.getName());
6.  
7. result = hunt();
8.  
9. if (result == null) {
10. dispatcher.dispatchFailed(this);
11. } else {
12. dispatcher.dispatchComplete(this);
13. }
14. } catch (IOException e) {
15. exception = e;
16. dispatcher.dispatchRetry(this);
17. } catch (Exception e) {
18. exception = e;
19. dispatcher.dispatchFailed(this);
20. } finally {
21. Thread.currentThread().setName(Utils.THREAD_IDLE_NAME);
22. }
23. }
24.  
25. abstract Bitmap decode(Request data) throws IOException;
26.  
27. Bitmap hunt() throws IOException {
28. Bitmap bitmap;
29.  
30. if (!skipMemoryCache) {
31. bitmap = cache.get(key);
32. if (bitmap != null) {
33. stats.dispatchCacheHit();
34. loadedFrom = MEMORY;
35. return bitmap;
36. }
37. }
38.  
39. bitmap = decode(data);
40.  
41. if (bitmap != null) {
42. stats.dispatchBitmapDecoded(bitmap);
43. if (data.needsTransformation() || exifRotation != 0) {
44. synchronized (DECODE_LOCK) {
45. if (data.needsMatrixTransform() || exifRotation != 0) {
46. bitmap = transformResult(data, bitmap, exifRotation);
47. }
48. if (data.hasCustomTransformations()) {
49. bitmap = applyCustomTransformations(
50. data.transformations, bitmap);
51. }
52. }
53. stats.dispatchBitmapTransformed(bitmap);
54. }
55. }
56.  
57. return bitmap;
58. }

Dispatcher任务调度器

1. void dispatchSubmit(Action action) {
2. handler.sendMessage(handler.obtainMessage(REQUEST_SUBMIT, action));
3. }

handler接到消息后转换到performSubmit方法

1. void performSubmit(Action action) {
2. BitmapHunter hunter = hunterMap.get(action.getKey());
3. if (hunter != null) {
4. hunter.attach(action);
5. return;
6. }
7.  
8. if (service.isShutdown()) {
9. return;
10. }
11.  
12. hunter = forRequest(context, action.getPicasso(), this, cache, stats,
13. action, downloader);
14. hunter.future = service.submit(hunter);
15. hunterMap.put(action.getKey(), hunter);
16. }

1. public static Picasso with(Context context) {
2. if (singleton == null) {
3. singleton = new Builder(context).build();
4. }
5. return singleton;
6. }
7.  
8. public Picasso build() {
9. Context context = this.context;
10.  
11. if (downloader == null) {
12. downloader = Utils.createDefaultDownloader(context);
13. }
14. if (cache == null) {
15. cache = new LruCache(context);
16. }
17. if (service == null) {
18. service = new PicassoExecutorService();
19. }
20. if (transformer == null) {
21. transformer = RequestTransformer.IDENTITY;
22. }
23.  
24. Stats stats = new Stats(cache);
25.  
26. Dispatcher dispatcher = new Dispatcher(context, service, HANDLER,
27. downloader, cache, stats);
28.  
29. return new Picasso(context, dispatcher, cache, listener,
30. transformer, stats, debugging);
31. }

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。