Swift-y Target-Action (UIKit version)

swifty-targetaction.swift

// Swifty Target/Action
// Friday Q&A 2015-12-25
// https://www.mikeash.com/pyblog/friday-qa-2015-12-25-swifty-targetaction.html

import UIKit

class ActionTrampoline<T>: NSObject {
    var action: T -> Void
    init(action: T -> Void) {
        self.action = action
    }
    @objc func action(sender: UIControl) {
        action(sender as! T)
    }
}

// this code just allocates a bit of memory and uses that address,
// since Swift won't guarantee `&` to point to same pointer
let NSControlActionFunctionProtocolAssociatedObjectKey = UnsafeMutablePointer<Int8>.alloc(1)

protocol NSControlActionFunctionProtocol {}

extension NSControlActionFunctionProtocol where Self: UIControl {
    func addAction(events: UIControlEvents, _ action: Self -> Void) {
        let trampoline = ActionTrampoline(action: action)
        self.addTarget(trampoline, action: "action:", forControlEvents: events)
        objc_setAssociatedObject(self, NSControlActionFunctionProtocolAssociatedObjectKey,
            trampoline, .OBJC_ASSOCIATION_RETAIN)
    }
}

extension UIControl: NSControlActionFunctionProtocol {}


// usage

let button = UIButton()
button.setTitle("Hello Button", forState: .Normal)
button.addAction([.TouchUpInside], { (button: UIButton) in
    // use `[unowned self]` or `[weak self]` to avoid retain cycles
    print("Action from \(button.titleLabel?.text)")
})

// makig test call
button.sendActionsForControlEvents([.TouchUpInside])