常规写法：
let subview = UIView(frame: CGRect(x: 0, y: 0, width: 100, height: 100))
subview.backgroundColor = .red
subview.layer.masksToBounds = true
subview.layer.cornerRadius = 8
subview.isUserInteractionEnabled = true
self.addSubview(subview)
链式调用：
self.view.addSubview(
UIView()
.frame(CGRect(x: 0, y: 0, width: 100, height: 100))
.backgroundColor(.red)
.masksToBounds(true)
.cornerRadius(8)
.isUserInteractionEnabled(true)
)
实现原理：
调用方法之后返回自己，以实现链式调用
具体代码：
UIViewExtension.swift
extension UIView {
public func frame(_ frame: CGRect) -> Self {
self.frame = frame
return self
}
public func isUserInteractionEnabled(_ isUserInteractionEnabled: Bool) -> Self {
self.isUserInteractionEnabled = isUserInteractionEnabled
return self
}
public func backgroundColor(_ backgroundColor: UIColor) -> Self {
self.backgroundColor = backgroundColor
return self
}
}
