Added option to use the cached image instead of the placeholder for U…

…IImageView. Replaces #541. Fixes #599

SDWebImage/UIImageView+WebCache.h

@@ -145,6 +145,23 @@
  */
 - (void)sd_setImageWithURL:(NSURL *)url placeholderImage:(UIImage *)placeholder options:(SDWebImageOptions)options progress:(SDWebImageDownloaderProgressBlock)progressBlock completed:(SDWebImageCompletionBlock)completedBlock;
 
+/**
+ * Set the imageView `image` with an `url` and a optionaly placeholder image.
+ *
+ * The download is asynchronous and cached.
+ *
+ * @param url            The url for the image.
+ * @param placeholder    The image to be set initially, until the image request finishes.
+ * @param options        The options to use when downloading the image. @see SDWebImageOptions for the possible values.
+ * @param progressBlock  A block called while image is downloading
+ * @param completedBlock A block called when operation has been completed. This block has no return value
+ *                       and takes the requested UIImage as first parameter. In case of error the image parameter
+ *                       is nil and the second parameter may contain an NSError. The third parameter is a Boolean
+ *                       indicating if the image was retrived from the local cache of from the network.
+ *                       The forth parameter is the original image url.
+ */
+- (void)sd_setImageWithPreviousCachedImageWithURL:(NSURL *)url andPlaceholderImage:(UIImage *)placeholder options:(SDWebImageOptions)options progress:(SDWebImageDownloaderProgressBlock)progressBlock completed:(SDWebImageCompletionBlock)completedBlock;
+
 /**
  * Download an array of images and starts them in an animation loop
  *

SDWebImage/UIImageView+WebCache.m

@@ -77,6 +77,13 @@ - (void)sd_setImageWithURL:(NSURL *)url placeholderImage:(UIImage *)placeholder
     }
 }
 
+- (void)sd_setImageWithPreviousCachedImageWithURL:(NSURL *)url andPlaceholderImage:(UIImage *)placeholder options:(SDWebImageOptions)options progress:(SDWebImageDownloaderProgressBlock)progressBlock completed:(SDWebImageCompletionBlock)completedBlock {
+    NSString *key = [[SDWebImageManager sharedManager] cacheKeyForURL:url];
+    UIImage *lastPreviousCachedImage = [[SDImageCache sharedImageCache] imageFromDiskCacheForKey:key];
+
+    [self sd_setImageWithURL:url placeholderImage:lastPreviousCachedImage ?: placeholder options:options progress:progressBlock completed:completedBlock];    
+}
+
 - (NSURL *)sd_imageURL {
     return objc_getAssociatedObject(self, &imageURLKey);
 }