UILabel的内阴影

import UIKit

class CustomUILabel: UILabel {
    var shadowLayer: CALayer?
    
    override func layoutSubviews() {
        super.layoutSubviews()
        
        if shadowLayer == nil {
            let shadowLayer = CALayer()
            shadowLayer.frame = bounds
            shadowLayer.shadowColor = UIColor.black.cgColor
            shadowLayer.shadowOffset = CGSize(width: 0, height: 2)
            shadowLayer.shadowOpacity = 0.5
            shadowLayer.shadowRadius = 2
            layer.addSublayer(shadowLayer)
            self.shadowLayer = shadowLayer
        }
    }
    
    override func drawText(in rect: CGRect) {
        let insetRect = rect.insetBy(dx: 2, dy: 2)
        super.drawText(in: insetRect)
    }
}

let label = CustomUILabel()
label.text = "Hello, world!"
label.font = UIFont.systemFont(ofSize: 24)
label.textColor = UIColor.white
label.textAlignment = .center
label.frame = CGRect(x: 0, y: 0, width: 200, height: 50)