在手写代码的时候，常常会用到一些懒加载的方式来书写控件，下面是个Then协议，不会产生循环引用的问题，代码看起来比较的紧凑，更加的直观。
public protocol Then {}
extension Then where Self: AnyObject {
public func then( block: (Self) -> Void) -> Self {
block(self)
return self
}
/*
let _ = UILabel().then { (label) in
label.backgroundColor = .blue
label.font = UIFont.systemFont(ofSize: 18)
label.textAlignment = .center
label.text = "Then协议库"
label.frame = CGRect.init(x: 20, y: 200, width: 150, height: 40)
view.addSubview(label)
}
*/
/*
// 2.1 (推荐)无参数，无需命名，用$取参数，可自动联想属性
let lable = UILabel().then {
$0.backgroundColor = .blue
$0.font = UIFont.systemFont(ofSize: 18)
$0.textAlignment = .center
$0.text = "Then库写法_2.1"
$0.frame = CGRect.init(x: 200, y: 260, width: 150, height: 40)
view.addSubview($0)
}
lable.backgroundColor = UIColor.red
*/
}
extension UIView: Then {}
