Picasso 是 Square 公司的大作，叫「畢加索」还棱，意為加載圖片就像畫畫一樣载慈，是一門藝術(shù)。他的組成結(jié)構(gòu)也很分明 ：任務(wù)分發(fā)珍手，對象處理办铡，圖片的加載，緩存等琳要。
Picasso結(jié)構(gòu)圖

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picasso有著非常強大的功能寡具，下面是picasso的流程圖

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優(yōu)雅的picasso的調(diào)用形式：

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1 with()：單例創(chuàng)建picasso對象，實力化的默認的dowmloder焙蹭，LruCache晒杈，PicassoExecutorService，RequestTransformer孔厉，以及分發(fā)器Dispatcher拯钻，其中dispatcher創(chuàng)建了子線程和對應(yīng)的handler.

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picasso主線程自己也有handler這兩個不一樣帖努，流程是，Dispatcher,接受到圖片加載完畢以后的消息后粪般，回調(diào)給picasso內(nèi)主handler提示圖片加載完畢

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2 .load(imageUrl)：返回RequestCreator的實例拼余。

  public RequestCreator load(String path) {
    if (path == null) {
      return new RequestCreator(this, null, 0);
    }
    if (path.trim().length() == 0) {
      throw new IllegalArgumentException("Path must not be empty.");
    }
    return load(Uri.parse(path));
  }

內(nèi)部又創(chuàng)建了 Request.Builder

  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, picasso.defaultBitmapConfig);
  }

3 .into(imageView);創(chuàng)建了ImageViewAction，并將action提交給diapatecher,
performSubmit()方法中實例了BitmapHunter（runnable）并提交到線程池中亩歹，

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bitmaphunter的run方法：

 @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);
      }

hunt內(nèi)部：通過 requestHandler.load()方法拿到bitmap

 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;
  }

拿到bitmap之后回調(diào)到diapatcher中

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dispatcher中List<BitmapHunter>回調(diào)到Picasso主線程的handler中

  void performBatchComplete() {
    List<BitmapHunter> copy = new ArrayList<BitmapHunter>(batch);
    batch.clear();
    mainThreadHandler.sendMessage(mainThreadHandler.obtainMessage(HUNTER_BATCH_COMPLETE, copy));
    logBatch(copy);
  }

主線程handler的方法

 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;
        }

deliverAction方法中的 action.complete(result, from)實現(xiàn)了為ImageView賦值的操作

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());
      }
    }
  }

上述流程的圖

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