UIImageview 显示网络图片及问题

    //同步加载

//    [imageview setImage:[UIImage imageWithData:[NSData dataWithContentsOfURL:[NSURL URLWithString:annotation.subtitle]]]];

    //异步加载

//    [imageview sd_setImageWithURL:[NSURL URLWithString:annotation.subtitle] placeholderImage:[UIImage imageNamed:@"placehodeLoading.png"]];

//异步加载图片不显示问题

     [imageview sd_setImageWithURL:[NSURL URLWithString:annotation.subtitle] placeholderImage:[UIImage imageNamed:@"placehodeLoading.png"] options:SDWebImageLowPriority | SDWebImageRetryFailed];