SDWebImage

//    [cell.imageView sd_setImageWithURL:[NSURL URLWithString:app.icon] placeholderImage:[UIImage imageNamed:@"placeholder"]];

    [cell.imageView sd_setImageWithURL:[NSURL URLWithString:app.icon] placeholderImage:[UIImage imageNamed:@"placeholder"] options:0 progress:^(NSInteger receivedSize, NSInteger expectedSize) {
        // expectedSize: 图片的总字节数
        // receivedSize: 已经接收的图片字节数

        NSLog(@"下载进度：%f", 1.0 * receivedSize / expectedSize);
    } completed:^(UIImage *image, NSError *error, SDImageCacheType cacheType, NSURL *imageURL) {
        NSLog(@"下载完图片");
    }];

    // SDWebImage的图片缓存周期是多长:1个星期