【Android】Fresco圖片加載框架（二） - Producer

  private Producer<CloseableReference<CloseableImage>> getBasicDecodedImageSequence(
      ImageRequest imageRequest) {
    Preconditions.checkNotNull(imageRequest);
    Uri uri = imageRequest.getSourceUri();
    Preconditions.checkNotNull(uri, "Uri is null.");
    if (UriUtil.isNetworkUri(uri)) {
      return getNetworkFetchSequence();
    } else if (UriUtil.isLocalFileUri(uri)) {
      if (MediaUtils.isVideo(MediaUtils.extractMime(uri.getPath()))) {
        return getLocalVideoFileFetchSequence();
      } else {
        return getLocalImageFileFetchSequence();
      }
    } else if (UriUtil.isLocalContentUri(uri)) {
      return getLocalContentUriFetchSequence();
    } else if (UriUtil.isLocalAssetUri(uri)) {
      return getLocalAssetFetchSequence();
    } else if (UriUtil.isLocalResourceUri(uri)) {
      return getLocalResourceFetchSequence();
    } else if (UriUtil.isDataUri(uri)) {
      return getDataFetchSequence();
    } else {
      String uriString = uri.toString();
      if (uriString.length() > 30) {
        uriString = uriString.substring(0, 30) + "...";
      }
      throw new RuntimeException("Unsupported uri scheme! Uri is: " + uriString);
    }
  }

看代碼可知道，就是根據 ImageRequest 的 Uri ，選擇一個sequence Producer，我們這里假設是網絡請求圖片，所以選擇的是圖中紅色方法 getNetworkFetchSequence ， 它也是返回一個Producer （可粗略看） ：

  /**


swallow result if prefetch -> bitmap cache get ->
background thread hand-off -> multiplex -> bitmap cache -> decode -> multiplex ->
encoded cache -> disk cache -> (webp transcode) -> network fetch.
*/
private synchronized Producer<CloseableReference<CloseableImage>> getNetworkFetchSequence() {
if (mNetworkFetchSequence == null) {
 mNetworkFetchSequence =
 newBitmapCacheGetToDecodeSequence(getCommonNetworkFetchToEncodedMemorySequence());

}
return mNetworkFetchSequence;
}</pre> 
先看紅色代碼， getCommonNetworkFetchToEncodedMemorySequence ， 它也是返回一個Producer （可粗略看） ： 
  /**
multiplex -> encoded cache -> disk cache -> (webp transcode) -> network fetch.
*/
private synchronized Producer<EncodedImage> getCommonNetworkFetchToEncodedMemorySequence() {
if (mCommonNetworkFetchToEncodedMemorySequence == null) {
 Producer<EncodedImage> inputProducer =
 newEncodedCacheMultiplexToTranscodeSequence(
     mProducerFactory.newNetworkFetchProducer(mNetworkFetcher));

 mCommonNetworkFetchToEncodedMemorySequence =
 ProducerFactory.newAddImageTransformMetaDataProducer(inputProducer);

 if (mResizeAndRotateEnabledForNetwork && !mDownsampleEnabled) {
   mCommonNetworkFetchToEncodedMemorySequence =
   mProducerFactory.newResizeAndRotateProducer(
       mCommonNetworkFetchToEncodedMemorySequence);

 }
}
return mCommonNetworkFetchToEncodedMemorySequence;
}</pre> 
再看， newEncodedCacheMultiplexToTranscodeSequence ， 它也是返回一個Producer （可粗略看） ： 
</div>
來自： http://www.cnblogs.com/chiefhsing/p/5125950.html