Merge pull request #701 from billburgess/delayplaceholder

Delay the loading of the placeholder image until after load

SDWebImage/SDWebImageManager.h

@@ -68,7 +68,13 @@ typedef NS_OPTIONS(NSUInteger, SDWebImageOptions) {
      * the front of the queue and is loaded immediately instead of waiting for the current queue to be loaded (which 
      * could take a while).
      */
-    SDWebImageHighPriority = 1 << 8
+    SDWebImageHighPriority = 1 << 8,
+    
+    /**
+     * By default, placeholder images are loaded while the image is loading. This flag will delay the loading
+     * of the placeholder image until after the image has finished loading.
+     */
+    SDWebImageDelayPlaceholder = 1 << 9
 };
 
 typedef void(^SDWebImageCompletedBlock)(UIImage *image, NSError *error, SDImageCacheType cacheType);

SDWebImage/UIImageView+WebCache.h

@@ -120,7 +120,7 @@
 /**
  * Set the imageView `image` with an `url`, placeholder and custom options.
  *
- * The downloand is asynchronous and cached.
+ * 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.

SDWebImage/UIImageView+WebCache.m

@@ -40,9 +40,11 @@ static char operationArrayKey;
 
 - (void)setImageWithURL:(NSURL *)url placeholderImage:(UIImage *)placeholder options:(SDWebImageOptions)options progress:(SDWebImageDownloaderProgressBlock)progressBlock completed:(SDWebImageCompletedBlock)completedBlock {
     [self cancelCurrentImageLoad];
-
-    self.image = placeholder;
-
+    
+    if (!(options & SDWebImageDelayPlaceholder)) {
+        self.image = placeholder;
+    }
+    
     if (url) {
         __weak UIImageView *wself = self;
         id <SDWebImageOperation> operation = [SDWebImageManager.sharedManager downloadWithURL:url options:options progress:progressBlock completed:^(UIImage *image, NSError *error, SDImageCacheType cacheType, BOOL finished) {
@@ -52,6 +54,11 @@ static char operationArrayKey;
                 if (image) {
                     wself.image = image;
                     [wself setNeedsLayout];
+                } else {
+                    if ((options & SDWebImageDelayPlaceholder)) {
+                        wself.image = placeholder;
+                        [wself setNeedsLayout];
+                    }
                 }
                 if (completedBlock && finished) {
                     completedBlock(image, error, cacheType);