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

Dispatcher内有一个HandlerThread，所有的请求都会通过这个thread转换，也就是请求也是异步的，这样应该是为了Ui线程更加流畅，同时保证请求的顺序，因为handler的消息队列。
外部调用的是dispatchXXX方法，然后通过handler将请求转换到对应的performXXX方法。
例如生成Action以后就会调用dispather的dispatchSubmit()来请求执行，

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

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);  
    }  

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);  
        }