图片加载框架Picasso解析

picasso是Square公司开源的一个Android图形缓存库

主要有以下一些特性：

在adapter中回收和取消当前的下载；

使用最少的内存完成复杂的图形转换操作；

自动的内存和硬盘缓存；

图形转换操作，如变换大小，旋转等，提供了接口来让用户可以自定义转换操作；

加载载网络或本地资源；

Picasso.class

他有一个内部类，一般是通过他来创建实例的：

downloader(Downloader downloader) ：

容许使用自定义的下载器，可以用okhttp或者volley，必须实现Downloader接口。

executor(ExecutorService executorService):

容许使用自己的线程池来进行下载

memoryCache(Cache memoryCache)：

容许使用自己的缓存类，必须实现Cache接口。

requestTransformer(RequestTransformer transformer)：

listener(Listener listener)：

addRequestHandler(RequestHandler requestHandler)：

indicatorsEnabled(boolean enabled)：设置图片来源的指示器。

loggingEnabled(boolean enabled)：

再来看build()方法：

 

/** 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, indicatorsEnabled, loggingEnabled);
   }

这里会使用默认的下载器，缓存类，线程池，并将这些对象封装到了分发器Dispatcher里。然后在返回一个Picasso对象。

 

一般情况下，如果不需要自定义bulid里的这些方法，可以使用Picasso.with(context)默认方法来获得单例对象：

 

public static Picasso with(Context context) {
    if (singleton == null) {
      synchronized (Picasso.class) {
        if (singleton == null) {
          singleton = new Builder(context).build();
        }
      }
    }
    return singleton;
  }

如果需要自定义一些对象：

 

 

public class SamplePicassoFactory {

  private static Picasso sPicasso;

  public static Picasso getPicasso(Context context) {
    if (sPicasso == null) {
        sPicasso = new Picasso.Builder(context)
            .downloader(new OkHttpDownloader(context, ConfigConstants.MAX_DISK_CACHE_SIZE))
            .memoryCache(new LruCache(ConfigConstants.MAX_MEMORY_CACHE_SIZE))
            .build();
    }
    return sPicasso;
  }
}

通过上面的方法，获得sPicasso对象后就设置成了单例，但是最好设置成双重校验锁模式。

 

如果通过以上方法获得对象后，还可以通过Picasso.setSingletonInstance(Picasso picasso)方法设置对象到Picasso中，这样以后还是通过Picasso.with(context)来调用。

接着就可以通过以下方式设置加载源：

可以是uri地址，file文件，res资源drawable。

最终都是通过以下方法来创建一个RequestCreator对象：

 

public RequestCreator load(Uri uri) {
    return new RequestCreator(this, uri, 0);
  }

RequestCreator(Picasso picasso, Uri uri, int resourceId) {
    if (picasso.shutdown) {
      throw new IllegalStateException(
          "Picasso instance already shut down. Cannot submit new requests.");
    }
    this.picasso = picasso;
    this.data = new Request.Builder(uri, resourceId);
  }

RequestCreator对象是用来设置一系列属性的，如：

 

noPlaceholder()：设置没有加载等待图片

placeholder(int placeholderResId)：设置加载等待图片

placeholder(Drawable placeholderDrawable)：设置加载等待图片

error(int errorResId)：设置加载出错的图片。

error(Drawable errorDrawable)：设置加载出错的图片。

tag(Object tag)：设置标记

fit():自适应，下载的图片有多少像素就显示多少像素

resizeDimen(int targetWidthResId, int targetHeightResId)：设置图片显示的像素。

resize(int targetWidth, int targetHeight)：设置图片显示的像素。

centerCrop()：设置ImageView的ScaleType属性.

centerInside():设置ImageView的ScaleType属性.

rotate(float degrees):设置旋转角度。

rotate(float degrees, float pivotX, float pivotY):设置以某个中心点设置某个旋转角度。

config(Bitmap.Config config)：设置Bitmap的Config属性

priority(Priority priority)：设置请求的优先级。

transform(Transformation transformation)：

skipMemoryCache()：跳过内存缓存

memoryPolicy(MemoryPolicy policy, MemoryPolicy... additional)：

networkPolicy(NetworkPolicy policy, NetworkPolicy... additional)：

noFade()：没有淡入淡出效果

get()：获得bitmap对象

fetch()：

设置完以上一系列属性之后，最关键的就是into方法，现在以into(ImageView target)举例：

 

public void into(ImageView target) {
    into(target, null);
  }

他实际调用的是：

 

 

public void into(ImageView target, Callback callback) {
   long started = System.nanoTime();
   checkMain();

   if (target == null) {
     throw new IllegalArgumentException("Target must not be null.");
   }

   if (!data.hasImage()) {
     picasso.cancelRequest(target);
     if (setPlaceholder) {
       setPlaceholder(target, getPlaceholderDrawable());
     }
     return;
   }

   if (deferred) {
     if (data.hasSize()) {
       throw new IllegalStateException("Fit cannot be used with resize.");
     }
     int width = target.getWidth();
     int height = target.getHeight();
     if (width == 0 || height == 0) {
       if (setPlaceholder) {
         setPlaceholder(target, getPlaceholderDrawable());
       }
       picasso.defer(target, new DeferredRequestCreator(this, target, callback));
       return;
     }
     data.resize(width, height);
   }

   Request request = createRequest(started);
   String requestKey = createKey(request);

   if (shouldReadFromMemoryCache(memoryPolicy)) {
     Bitmap bitmap = picasso.quickMemoryCacheCheck(requestKey);
     if (bitmap != null) {
       picasso.cancelRequest(target);
       setBitmap(target, picasso.context, bitmap, MEMORY, noFade, picasso.indicatorsEnabled);
       if (picasso.loggingEnabled) {
         log(OWNER_MAIN, VERB_COMPLETED, request.plainId(), "from " + MEMORY);
       }
       if (callback != null) {
         callback.onSuccess();
       }
       return;
     }
   }

   if (setPlaceholder) {
     setPlaceholder(target, getPlaceholderDrawable());
   }

   Action action =
       new ImageViewAction(picasso, target, request, memoryPolicy, networkPolicy, errorResId,
           errorDrawable, requestKey, tag, callback, noFade);

   picasso.enqueueAndSubmit(action);
 }

checkMain();

首先检查是否是主线程

 

if (target == null) {
      throw new IllegalArgumentException("Target must not be null.");
    }

检查目标view是否存在

 

 

if (!data.hasImage()) {
      picasso.cancelRequest(target);
      if (setPlaceholder) {
        setPlaceholder(target, getPlaceholderDrawable());
      }
      return;
    }

如果没有设置uri或者resDrawable,就停止请求，如果设置了加载等待的图片就设置，然后就return

 

 

if (deferred) {
      if (data.hasSize()) {
        throw new IllegalStateException("Fit cannot be used with resize.");
      }
      int width = target.getWidth();
      int height = target.getHeight();
      if (width == 0 || height == 0) {
        if (setPlaceholder) {
          setPlaceholder(target, getPlaceholderDrawable());
        }
        picasso.defer(target, new DeferredRequestCreator(this, target, callback));
        return;
      }
      data.resize(width, height);
    }

如果设置fit自适应：如果已经设置了图片像素大小就抛异常，如果目标view的长宽等于0，就在设置等待图片后延期处理，如果不等于0就设置size到Request.Builder的data里。

 

 

Request request = createRequest(started);
 String requestKey = createKey(request);

接着就创建Request，并生成一个String类型的requestKey。

 

 

/** Create the request optionally passing it through the request transformer. */
  private Request createRequest(long started) {
    int id = nextId.getAndIncrement();

    Request request = data.build();
    request.id = id;
    request.started = started;

    boolean loggingEnabled = picasso.loggingEnabled;
    if (loggingEnabled) {
      log(OWNER_MAIN, VERB_CREATED, request.plainId(), request.toString());
    }

    Request transformed = picasso.transformRequest(request);
    if (transformed != request) {
      // If the request was changed, copy over the id and timestamp from the original.
      transformed.id = id;
      transformed.started = started;

      if (loggingEnabled) {
        log(OWNER_MAIN, VERB_CHANGED, transformed.logId(), "into " + transformed);
      }
    }

    return transformed;
  }

在以上代码可以看出，这里调用picasso.transformRequest(request);来给使用者提供一个可以更改request的机会，他最终调用的是Picasso.Build里面的通过requestTransformer(RequestTransformer transformer)方法传进去的RequestTransformer 接口：

 

 

public interface RequestTransformer {
   /**
    * Transform a request before it is submitted to be processed.
    *
    * @return The original request or a new request to replace it. Must not be null.
    */
   Request transformRequest(Request request);

   /** A {@link RequestTransformer} which returns the original request. */
   RequestTransformer IDENTITY = new RequestTransformer() {
     @Override public Request transformRequest(Request request) {
       return request;
     }
   };
 }

默认使用的是IDENTITY。这里没有做任何的修改，如果有需要可以自己设置接口以达到修改Request的目的。

 

 

if (shouldReadFromMemoryCache(memoryPolicy)) {
      Bitmap bitmap = picasso.quickMemoryCacheCheck(requestKey);
      if (bitmap != null) {
        picasso.cancelRequest(target);
        setBitmap(target, picasso.context, bitmap, MEMORY, noFade, picasso.indicatorsEnabled);
        if (picasso.loggingEnabled) {
          log(OWNER_MAIN, VERB_COMPLETED, request.plainId(), "from " + MEMORY);
        }
        if (callback != null) {
          callback.onSuccess();
        }
        return;
      }
    }

接着要判断是否需要从缓存中读取图片，如果需要，就要根据requestKey从缓存中读取：

 

 

Bitmap quickMemoryCacheCheck(String key) {
    Bitmap cached = cache.get(key);
    if (cached != null) {
      stats.dispatchCacheHit();
    } else {
      stats.dispatchCacheMiss();
    }
    return cached;
  }

这里的cache用的是LruCache内存缓存类，这个内存缓存类，实现了Cache接口，最后调用的是：

 

 

@Override public Bitmap get(String key) {
   if (key == null) {
     throw new NullPointerException("key == null");
   }

   Bitmap mapValue;
   synchronized (this) {
     mapValue = map.get(key);
     if (mapValue != null) {
       hitCount++;
       return mapValue;
     }
     missCount++;
   }

   return null;
 }

这里的map是LinkedHashMap<String, Bitmap>：

 

 

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

缓存的策略稍后分析，现在先回到前面，如果从缓存中拿到的bitmap不等于null,就调用picasso.cancelRequest(target)来删除请求，然后通过

 

 

setBitmap(target, picasso.context, bitmap, MEMORY, noFade, picasso.indicatorsEnabled);

设置图片，然后如果设置了callback回调，再回掉callback.onSuccess();方法，然后就return。

 

 

如果没有设置内存缓存，或者缓存中的图片已经不存在：

 

if (setPlaceholder) {
     setPlaceholder(target, getPlaceholderDrawable());
   }

   Action action =
       new ImageViewAction(picasso, target, request, memoryPolicy, networkPolicy, errorResId,
           errorDrawable, requestKey, tag, callback, noFade);

   picasso.enqueueAndSubmit(action);

接着就设置等待加载图片，然后封装一个action,然后将action加入到分发器中：

 

 

void enqueueAndSubmit(Action action) {
    Object target = action.getTarget();
    if (target != null && targetToAction.get(target) != action) {
      // This will also check we are on the main thread.
      cancelExistingRequest(target);
      targetToAction.put(target, action);
    }
    submit(action);
  }

  void submit(Action action) {
    dispatcher.dispatchSubmit(action);
  }

然后调用了dispatcher的performSubmit()方法：

 

 

void performSubmit(Action action) {
   performSubmit(action, true);
 }

 void performSubmit(Action action, boolean dismissFailed) {
   if (pausedTags.contains(action.getTag())) {
     pausedActions.put(action.getTarget(), action);
     if (action.getPicasso().loggingEnabled) {
       log(OWNER_DISPATCHER, VERB_PAUSED, action.request.logId(),
           "because tag '" + action.getTag() + "' is paused");
     }
     return;
   }

   BitmapHunter hunter = hunterMap.get(action.getKey());
   if (hunter != null) {
     hunter.attach(action);
     return;
   }

   if (service.isShutdown()) {
     if (action.getPicasso().loggingEnabled) {
       log(OWNER_DISPATCHER, VERB_IGNORED, action.request.logId(), "because shut down");
     }
     return;
   }

   hunter = forRequest(action.getPicasso(), this, cache, stats, action);
   hunter.future = service.submit(hunter);
   hunterMap.put(action.getKey(), hunter);
   if (dismissFailed) {
     failedActions.remove(action.getTarget());
   }

   if (action.getPicasso().loggingEnabled) {
     log(OWNER_DISPATCHER, VERB_ENQUEUED, action.request.logId());
   }
 }

这里通过 hunter = forRequest(action.getPicasso(), this, cache, stats, action);获得一个BitmapHunter对象，接着就提交到线程池中 hunter.future = service.submit(hunter);：

 

接着线程池就会调用hunter 的run方法，因为这实现了Runnable对象：

@Override public void run() {
   try {
     updateThreadName(data);

     if (picasso.loggingEnabled) {
       log(OWNER_HUNTER, VERB_EXECUTING, getLogIdsForHunter(this));
     }

     result = hunt();

     if (result == null) {
       dispatcher.dispatchFailed(this);
     } else {
       dispatcher.dispatchComplete(this);
     }
   } catch (Downloader.ResponseException e) {
     if (!e.localCacheOnly || e.responseCode != 504) {
       exception = e;
     }
     dispatcher.dispatchFailed(this);
   } catch (NetworkRequestHandler.ContentLengthException e) {
     exception = e;
     dispatcher.dispatchRetry(this);
   } catch (IOException e) {
     exception = e;
     dispatcher.dispatchRetry(this);
   } catch (OutOfMemoryError e) {
     StringWriter writer = new StringWriter();
     stats.createSnapshot().dump(new PrintWriter(writer));
     exception = new RuntimeException(writer.toString(), e);
     dispatcher.dispatchFailed(this);
   } catch (Exception e) {
     exception = e;
     dispatcher.dispatchFailed(this);
   } finally {
     Thread.currentThread().setName(Utils.THREAD_IDLE_NAME);
   }
 }

接着调用hunt()方法：

 

 

Bitmap hunt() throws IOException {
    Bitmap bitmap = null;

    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;
      }
    }

    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();

      // If there was no Bitmap then we need to decode it from the stream.
      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);
      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;
  }

这里如果从内存缓存中渠道bitmap对象，就直接返回了；否则通过requestHandler.load(data, networkPolicy);来发起网络请求(这个是NetworkRequestHandler类型的)：

 

 

@Override public Result load(Request request, int networkPolicy) throws IOException {
    Response response = downloader.load(request.uri, request.networkPolicy);
    if (response == null) {
      return null;
    }

    Picasso.LoadedFrom loadedFrom = response.cached ? DISK : NETWORK;

    Bitmap bitmap = response.getBitmap();
    if (bitmap != null) {
      return new Result(bitmap, loadedFrom);
    }

    InputStream is = response.getInputStream();
    if (is == null) {
      return null;
    }
    // Sometimes response content length is zero when requests are being replayed. Haven't found
    // root cause to this but retrying the request seems safe to do so.
    if (loadedFrom == DISK && response.getContentLength() == 0) {
      Utils.closeQuietly(is);
      throw new ContentLengthException("Received response with 0 content-length header.");
    }
    if (loadedFrom == NETWORK && response.getContentLength() > 0) {
      stats.dispatchDownloadFinished(response.getContentLength());
    }
    return new Result(is, loadedFrom);
  }

这里调用了downloader.load(request.uri, request.networkPolicy)方法，这是一个UrlConnectionDownloader类型的对象，调用的是UrlConnectionDownloader的load()方法：

 

 

@Override public Response load(Uri uri, int networkPolicy) throws IOException {
   if (Build.VERSION.SDK_INT >= Build.VERSION_CODES.ICE_CREAM_SANDWICH) {
     installCacheIfNeeded(context);
   }

   HttpURLConnection connection = openConnection(uri);
   connection.setUseCaches(true);

   if (networkPolicy != 0) {
     String headerValue;

     if (NetworkPolicy.isOfflineOnly(networkPolicy)) {
       headerValue = FORCE_CACHE;
     } else {
       StringBuilder builder = CACHE_HEADER_BUILDER.get();
       builder.setLength(0);

       if (!NetworkPolicy.shouldReadFromDiskCache(networkPolicy)) {
         builder.append("no-cache");
       }
       if (!NetworkPolicy.shouldWriteToDiskCache(networkPolicy)) {
         if (builder.length() > 0) {
           builder.append(',');
         }
         builder.append("no-store");
       }

       headerValue = builder.toString();
     }

     connection.setRequestProperty("Cache-Control", headerValue);
   }

   int responseCode = connection.getResponseCode();
   if (responseCode >= 300) {
     connection.disconnect();
     throw new ResponseException(responseCode + " " + connection.getResponseMessage(),
         networkPolicy, responseCode);
   }

   long contentLength = connection.getHeaderFieldInt("Content-Length", -1);
   boolean fromCache = parseResponseSourceHeader(connection.getHeaderField(RESPONSE_SOURCE));

   return new Response(connection.getInputStream(), fromCache, contentLength);
 }

如果系统版本是4.0以上，就使用installCacheIfNeeded(context)来启动data\data\包名\cacha下的某个目录的磁盘缓存：

 

 

private static void installCacheIfNeeded(Context context) {
   // DCL + volatile should be safe after Java 5.
   if (cache == null) {
     try {
       synchronized (lock) {
         if (cache == null) {
           cache = ResponseCacheIcs.install(context);
         }
       }
     } catch (IOException ignored) {
     }
   }
 }

 private static class ResponseCacheIcs {
   static Object install(Context context) throws IOException {
     File cacheDir = Utils.createDefaultCacheDir(context);
     HttpResponseCache cache = HttpResponseCache.getInstalled();
     if (cache == null) {
       long maxSize = Utils.calculateDiskCacheSize(cacheDir);
       cache = HttpResponseCache.install(cacheDir, maxSize);
     }
     return cache;
   }

但是如果要是用这个磁盘缓存，就要在HttpURLConnection的响应头上加上缓存的控制头"Cache-Control"！

 

最后返回new Response(connection.getInputStream(), fromCache, contentLength)请求结果。

接着回到上面的hunt()方法的流程继续：

 

if (result != null) {
     loadedFrom = result.getLoadedFrom();
     exifRotation = result.getExifOrientation();

     bitmap = result.getBitmap();

     // If there was no Bitmap then we need to decode it from the stream.
     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);
     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;

然后bitmap返回到run()方法里面的result应用上：

 

 

if (result == null) {
       dispatcher.dispatchFailed(this);
     } else {
       dispatcher.dispatchComplete(this);
     }

如果result有结果就分发完成消息，最后将会调用到Picasso里面：

 

 

static final Handler HANDLER = new Handler(Looper.getMainLooper()) {
   @Override public void handleMessage(Message msg) {
     switch (msg.what) {
       case HUNTER_BATCH_COMPLETE: {
         @SuppressWarnings("unchecked") List<BitmapHunter> batch = (List<BitmapHunter>) msg.obj;
         //noinspection ForLoopReplaceableByForEach
         for (int i = 0, n = batch.size(); i < n; i++) {
           BitmapHunter hunter = batch.get(i);
           hunter.picasso.complete(hunter);
         }
         break;
       }

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);
   }
 }

接着分发action:

 

 

private void deliverAction(Bitmap result, LoadedFrom from, Action action) {
    if (action.isCancelled()) {
      return;
    }
    if (!action.willReplay()) {
      targetToAction.remove(action.getTarget());
    }
    if (result != null) {
      if (from == null) {
        throw new AssertionError("LoadedFrom cannot be null.");
      }
      action.complete(result, from);
      if (loggingEnabled) {
        log(OWNER_MAIN, VERB_COMPLETED, action.request.logId(), "from " + from);
      }
    } else {
      action.error();
      if (loggingEnabled) {
        log(OWNER_MAIN, VERB_ERRORED, action.request.logId());
      }
    }
  }

然后调用的是ImageViewAction里面的action.complete(result, from)方法：

 

 

@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 indicatorsEnabled = picasso.indicatorsEnabled;
   PicassoDrawable.setBitmap(target, context, result, from, noFade, indicatorsEnabled);

   if (callback != null) {
     callback.onSuccess();
   }
 }

最后一步通过PicassoDrawable.setBitmap来设置，就算大功完成了。

附上部分流程图：