在UIScrollView上绘制上边框

import UIKit

class ViewController: UIViewController {
    override func viewDidLoad() {
        super.viewDidLoad()
        
        // 创建UIScrollView
        let scrollView = UIScrollView(frame: CGRect(x: 0, y: 0, width: 200, height: 200))
        scrollView.backgroundColor = UIColor.white
        
        // 创建CALayer边框视图
        let borderLayer = CALayer()
        borderLayer.frame = CGRect(x: 0, y: 0, width: scrollView.frame.width, height: 1)
        borderLayer.backgroundColor = UIColor.black.cgColor
        
        // 设置CALayer边框样式、颜色和宽度
        borderLayer.borderStyle = .solid
        borderLayer.borderColor = UIColor.black.cgColor
        borderLayer.borderWidth = 1.0
        
        // 将CALayer添加到UIScrollView的layer图层中
        scrollView.layer.addSublayer(borderLayer)
        
        // 将UIScrollView添加到视图层次结构中
        self.view.addSubview(scrollView)
    }
}