Swift：通过Closure设置UIButton的action

通常情况下，我们需要响应UIButton中的点击事件，一般的做法是：

let button = UIButton()
button.addTarget(self, action: #selector(onClicked), for: .touchUpInside)

@objc func onClicked() {
}

上面的实现给人的感觉不是很符合Swift所提倡的编程范式，所以这里提供一个通过Closure的方式来设置UIButton的action响应事件：

import UIKit

typealias UIButtonClickedListener = (UIButton) -> ()

extension UIButton {

    private struct Keys {
        static var clickedListener = "clickedListener"
    }

    private var clickedListener: UIButtonClickedListener? {
        get {
            return objc_getAssociatedObject(self, &Keys.clickedListener) as? UIButtonClickedListener
        }
        set(newValue) {
            objc_setAssociatedObject(self, &Keys.clickedListener, newValue, objc_AssociationPolicy.OBJC_ASSOCIATION_RETAIN_NONATOMIC)
        }
    }

    func setOnClickedListener(listener: @escaping UIButtonClickedListener) {
        clickedListener = listener
        addTarget(self, action: #selector(onButtonClicked), for: .touchUpInside)
    }

    @objc func onButtonClicked() {
        if let listener = self.clickedListener {
            listener(self)
        }
    }
}

let button = UIButton()
button.setOnClickedListener { print("hello world") }