import UIKit
let a : Int = 10
//报错——当一个字段定义为常量时是不可以修改的
// a = 20
var b : Int = 20
// 因为b定义为变量,因此是可以修改的
b = 30
//定义一个UIView
let view : UIView = UIView()
//定义一个普通UIView
let redView : UIView = UIView(frame: CGRect(x: 0, y: 0, width: 100, height: 100))
redView.backgroundColor = UIColor.red
view.addSubview(redView)
// 枚举类型的用法:类型.枚举的值
let btn : UIButton = UIButton(type: UIButtonType.custom)
btn.backgroundColor = UIColor.blue
btn.setTitle("按钮", for: .normal)
btn.frame = CGRect(x: 20, y: 20, width: 60, height: 30)
view.addSubview(btn)