CODE SNIPPET

extension UIView {

  // OUTPUT 1
  func dropShadow(scale: Bool = true) {
    layer.masksToBounds = false
    layer.shadowColor = UIColor.black.cgColor
    layer.shadowOpacity = 0.5
    layer.shadowOffset = CGSize(width: -1, height: 1)
    layer.shadowRadius = 1

    layer.shadowPath = UIBezierPath(rect: bounds).cgPath
    layer.shouldRasterize = true
    layer.rasterizationScale = scale ? UIScreen.main.scale : 1
  }

  // OUTPUT 2
  func dropShadow(color: UIColor, opacity: Float = 0.5, offSet: CGSize, radius: CGFloat = 1, scale: Bool = true) {
    layer.masksToBounds = false
    layer.shadowColor = color.cgColor
    layer.shadowOpacity = opacity
    layer.shadowOffset = offSet
    layer.shadowRadius = radius

    layer.shadowPath = UIBezierPath(rect: self.bounds).cgPath
    layer.shouldRasterize = true
    layer.rasterizationScale = scale ? UIScreen.main.scale : 1
  }
}

注意：如果未将任何参数传递给该函数，则默认情况下scale参数将为true。您可以通过在该参数的类型之后为参数赋值来为函数中的任何参数定义默认值。如果定义了默认值，则可以在调用函数时省略该参数。

NOTE: If you don't pass any parameter to that function, then the scale argument will be true by default. You can define a default value for any parameter in a function by assigning a value to the parameter after that parameter’s type. If a default value is defined, you can omit that parameter when calling the function.

输出1：

shadowView.dropShadow()

输出2：

shadowView.dropShadow(color: .red, opacity: 1, offSet: CGSize(width: -1, height: 1), radius: 3, scale: true)