@Override
        public @Nullable Uri apply(@Nullable ImageRequest arg) {
          return arg != null ? arg.getSourceUri() : null;
        }
This is a utility method which returns the type of Uri

@param uri The Uri to test
@return The type of the given Uri if available or SOURCE_TYPE_UNKNOWN if not
imagepipeline/src/main/java/com/facebook/imagepipeline/request/ImageRequest.java