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"];