添加图片到scrollveiw不成功

为什么我用下面的代码添加图片到UIscrollView不成功？显示的是一个空白屏幕。

UIImageView *imageView = [[UIImageView alloc] initWithFrame:CGRectMake(self.scrollView.frame.size.width * 1, 0, self.scrollView.frame.size.width, self.scrollView.frame.size.height)];
    imageView.backgroundColor = [UIColor redColor];
    [imageView setImage:[UIImage imageNamed:@"placeholder.png"]];
    [self.scrollView addSubview:imageView];
    self.scrollView.contentSize = CGSizeMake(self.scrollView.frame.size.width * self.urlArray.count, self.scrollView.frame.size.height);

UIImageView *imageView = [[UIImageView alloc] initWithFrame:CGRectMake(0, 0, self.scrollView.frame.size.width, self.scrollView.frame.size.height)];

要给 UIScrollView 添加合适的 contentSize ，还有给 UIView 添加 UIScrollView