Swift 3自定义UIButton子类设置按钮圆角和阴影

定义UIButton的子类CustomButton，在CustomButton设置按钮的圆角和阴影。

Swift 3

import UIKit
class CustomButton: UIButton {
    var shadowLayer: CAShapeLayer!
    override func layoutSubviews() {
        super.layoutSubviews()
        if shadowLayer == nil {
            shadowLayer = CAShapeLayer()
            shadowLayer.path = UIBezierPath(roundedRect: bounds, cornerRadius: 12).cgPath
            shadowLayer.fillColor = UIColor.white.cgColor
            shadowLayer.shadowColor = UIColor.darkGray.cgColor
            shadowLayer.shadowPath = shadowLayer.path
            shadowLayer.shadowOffset = CGSize(width: 2.0, height: 2.0)
            shadowLayer.shadowOpacity = 0.8
            shadowLayer.shadowRadius = 2
            layer.insertSublayer(shadowLayer, at: 0)
            //layer.insertSublayer(shadowLayer, below: nil) // also works
        }        
    }
}

在UIViewController里使用CustomButton：

import UIKit
class ViewController: UIViewController {
    override func viewDidLoad() {
        super.viewDidLoad()
        let button = CustomButton(type: .system)
        button.setTitle("Button", for: .normal)
        view.addSubview(button)
        // 必要时设置witdh和height constraint
        button.translatesAutoresizingMaskIntoConstraints = false
        let horizontalConstraint = button.centerXAnchor.constraint(equalTo: view.centerXAnchor)
        let verticalConstraint = button.centerYAnchor.constraint(equalTo: view.centerYAnchor)        
        let widthConstraint = button.widthAnchor.constraint(equalToConstant: 100)
        let heightConstraint = button.heightAnchor.constraint(equalToConstant: 100)
        NSLayoutConstraint.activate([horizontalConstraint, verticalConstraint, widthConstraint, heightConstraint])
    }
}

from：https://stackoverflow.com/questions/24644802/swift-problems-with-corner-radius-and-drop-shadow/25475536#25475536