ios通过SDWebImage实现图片加载时的渐变效果

- (void)sd_setImageWithURL:(NSURL *)url placeholderImage:(UIImage *)placeholder options:(SDWebImageOptions)options progress:(SDWebImageDownloaderProgressBlock)progressBlock completed:(SDWebImageCompletionBlock)completedBlock {
 [self sd_cancelCurrentImageLoad];
 objc_setAssociatedObject(self, &imageURLKey, url, OBJC_ASSOCIATION_RETAIN_NONATOMIC);

 if (!(options & SDWebImageDelayPlaceholder)) {
  dispatch_main_async_safe(^{
   self.image = placeholder;
  });
 }

 if (url) {

  // check if activityView is enabled or not
  if ([self showActivityIndicatorView]) {
   [self addActivityIndicator];
  }

  __weak __typeof(self)wself = self;
  id <SDWebImageOperation> operation = [SDWebImageManager.sharedManager downloadImageWithURL:url options:options progress:progressBlock completed:^(UIImage *image, NSError *error, SDImageCacheType cacheType, BOOL finished, NSURL *imageURL) {
   [wself removeActivityIndicator];
   if (!wself) return;
   dispatch_main_sync_safe(^{
    if (!wself) return;
    if (image && (options & SDWebImageAvoidAutoSetImage) && completedBlock)
    {
     completedBlock(image, error, cacheType, url);
     return;
    }
    else if (image) {
     CATransition *animation = [CATransition animation];
     animation.duration = .85f;
     animation.type = kCATransitionFade;
     animation.removedOnCompletion = YES;
     [wself.layer addAnimation:animation forKey:@"transition"];
     wself.image = image;
     [wself setNeedsLayout];
    } else {
     if ((options & SDWebImageDelayPlaceholder)) {
      wself.image = placeholder;
      [wself setNeedsLayout];
     }
    }
    if (completedBlock && finished) {
     completedBlock(image, error, cacheType, url);
    }
   });
  }];
  [self.layer removeAnimationForKey:@"transition"];
  [self sd_setImageLoadOperation:operation forKey:@"UIImageViewImageLoad"];
 } else {
  dispatch_main_async_safe(^{
   [self removeActivityIndicator];
   if (completedBlock) {
    NSError *error = [NSError errorWithDomain:SDWebImageErrorDomain code:-1 userInfo:@{NSLocalizedDescriptionKey : @"Trying to load a nil url"}];
    completedBlock(nil, error, SDImageCacheTypeNone, url);
   }
  });
 }
}

     CATransition *animation = [CATransition animation];
     animation.duration = .85f;
     animation.type = kCATransitionFade;
     animation.removedOnCompletion = YES;
     [wself.layer addAnimation:animation forKey:@"transition"];