iOS sdwebimage 图片淡入效果

[self.img sd_setImageWithURL:[NSURL URLWithString:[NSString stringWithFormat:@"%@%@",YUMINGIMG,imgUrl]]placeholderImage:[UIImage imageNamed:@"zhanweitu1"]completed:^(UIImage *image,NSError *error,SDImageCacheType cacheType,NSURL *imageURL){

            self.img.alpha = 0.0;

            [UIView transitionWithView:self.img duration:1.0                         options:UIViewAnimationOptionTransitionCrossDissolve animations:^{

             [self.img setImage:image];

              self.img.alpha = 1.0;

               } completion:NULL];

        }];