Loading Indicator view with 2 lines of code

added support for UIActivityIndicatorView while the image is
downloaded, without adding extra ‘setImageWithUrl’ methods to the
UIImageView+WebCache category.
I used [this pull
request](https://github.com/mythodeia/UIActivityIndicator-for-SDWebImage
), original author is [JJSaccolo](https://github.com/JJSaccolo), and
modded it a bit to support loading indicator while the image is
downloading.

the usage is basically this right before the usual sd_setImage...
methods:
```
        // show activity indicator
        [cell.imageView setShowActivityIndicatorView:YES];
        // choose indicator style
        [cell.imageView
setIndicatorStyle:UIActivityIndicatorViewStyleGray];
```

@@ -377,6 +377,10 @@
     if (cell == nil)
     {
         cell = [[UITableViewCell alloc] initWithStyle:UITableViewCellStyleDefault reuseIdentifier:CellIdentifier];
+        // show activity indicator
+        [cell.imageView setShowActivityIndicatorView:YES];
+        // choose indicator style
+        [cell.imageView setIndicatorStyle:UIActivityIndicatorViewStyleGray];
     }
 
     cell.textLabel.text = [NSString stringWithFormat:@"Image #%ld", (long)indexPath.row];

@@ -176,4 +176,16 @@
 
 - (void)sd_cancelCurrentAnimationImagesLoad;
 
+/**
+ *  Show activity UIActivityIndicatorView
+ */
+- (void)setShowActivityIndicatorView:(BOOL)show;
+
+/**
+ *  set desired UIActivityIndicatorViewStyle
+ *
+ *  @param style The style of the UIActivityIndicatorView
+ */
+- (void)setIndicatorStyle:(UIActivityIndicatorViewStyle)style;
+
 @end

@@ -11,6 +11,9 @@
 #import "UIView+WebCacheOperation.h"
 
 static char imageURLKey;
+static char TAG_ACTIVITY_INDICATOR;
+static char TAG_ACTIVITY_STYLE;
+static char TAG_ACTIVITY_SHOW;
 
 @implementation UIImageView (WebCache)
 
@@ -47,13 +50,19 @@ static char imageURLKey;
             self.image = placeholder;
         });
     }
-    
+
+    // check if activityView is enabled or not
+    if ([self showActivityIndicatorView]) {
+        [self addActivityIndicator];
+    }
+
     if (url) {
         __weak UIImageView *wself = self;
         id <SDWebImageOperation> operation = [SDWebImageManager.sharedManager downloadImageWithURL:url options:options progress:progressBlock completed:^(UIImage *image, NSError *error, SDImageCacheType cacheType, BOOL finished, NSURL *imageURL) {
             if (!wself) return;
             dispatch_main_sync_safe(^{
                 if (!wself) return;
+                [wself removeActivityIndicator];
                 if (image) {
                     wself.image = image;
                     [wself setNeedsLayout];
@@ -71,6 +80,7 @@ static char imageURLKey;
         [self sd_setImageLoadOperation:operation forKey:@"UIImageViewImageLoad"];
     } else {
         dispatch_main_async_safe(^{
+            [self removeActivityIndicator];
             NSError *error = [NSError errorWithDomain:@"SDWebImageErrorDomain" code:-1 userInfo:@{NSLocalizedDescriptionKey : @"Trying to load a nil url"}];
             if (completedBlock) {
                 completedBlock(nil, error, SDImageCacheTypeNone, url);
@@ -82,8 +92,8 @@ static char imageURLKey;
 - (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];    
+
+    [self sd_setImageWithURL:url placeholderImage:lastPreviousCachedImage ?: placeholder options:options progress:progressBlock completed:completedBlock];
 }
 
 - (NSURL *)sd_imageURL {
@@ -129,4 +139,60 @@ static char imageURLKey;
     [self sd_cancelImageLoadOperationWithKey:@"UIImageViewAnimationImages"];
 }
 
+- (UIActivityIndicatorView *)activityIndicator {
+    return (UIActivityIndicatorView *)objc_getAssociatedObject(self, &TAG_ACTIVITY_INDICATOR);
+}
+- (void)setActivityIndicator:(UIActivityIndicatorView *)activityIndicator {
+    objc_setAssociatedObject(self, &TAG_ACTIVITY_INDICATOR, activityIndicator, OBJC_ASSOCIATION_RETAIN);
+}
+
+- (void)setShowActivityIndicatorView:(BOOL)show{
+    objc_setAssociatedObject(self, &TAG_ACTIVITY_SHOW, [NSNumber numberWithBool:show], OBJC_ASSOCIATION_RETAIN);
+}
+- (BOOL)showActivityIndicatorView{
+    return [objc_getAssociatedObject(self, &TAG_ACTIVITY_SHOW) boolValue];
+}
+
+- (void)setIndicatorStyle:(UIActivityIndicatorViewStyle)style{
+    objc_setAssociatedObject(self, &TAG_ACTIVITY_STYLE, [NSNumber numberWithInt:style], OBJC_ASSOCIATION_RETAIN);
+}
+- (int)getIndicatorStyle{
+    return [objc_getAssociatedObject(self, &TAG_ACTIVITY_STYLE) intValue];
+}
+
+- (void)addActivityIndicator {
+    if (!self.activityIndicator) {
+        self.activityIndicator = [[UIActivityIndicatorView alloc] initWithActivityIndicatorStyle:[self getIndicatorStyle]];
+        self.activityIndicator.autoresizingMask = UIViewAutoresizingNone;
+
+        dispatch_async(dispatch_get_main_queue(), ^(void) {
+            [self addSubview:self.activityIndicator];
+            [self updateActivityIndicatorFrame];
+        });
+    }
+
+    dispatch_async(dispatch_get_main_queue(), ^(void) {
+        [self.activityIndicator startAnimating];
+    });
+
+}
+- (void)updateActivityIndicatorFrame {
+    if (self.activityIndicator) {
+        CGRect activityIndicatorBounds = self.activityIndicator.bounds;
+        float x = (self.frame.size.width - activityIndicatorBounds.size.width) / 2.0;
+        float y = (self.frame.size.height - activityIndicatorBounds.size.height) / 2.0;
+        self.activityIndicator.frame = CGRectMake(x, y, activityIndicatorBounds.size.width, activityIndicatorBounds.size.height);
+    }
+}
+- (void)removeActivityIndicator {
+    if (self.activityIndicator) {
+        [self.activityIndicator removeFromSuperview];
+        self.activityIndicator = nil;
+    }
+}
+- (void)layoutSubviews {
+    [super layoutSubviews];
+    [self updateActivityIndicatorFrame];
+}
+
 @end