UIButton の動作を Callback で設定する方法 ref: http://qiita.com/lovee/items/65b1f10313454fca4a05

file0.swift

import UIKit

class CallbackButton: UIButton {

    private var action: (() -> Void)?

    init(frame: CGRect, action: (() -> Void)? = nil) {

        self.action = action

        super.init(frame: frame)

        self.addTarget(self, action: "tapped:", forControlEvents: .TouchUpInside)

    }

    required init?(coder aDecoder: NSCoder) {
        fatalError("init(coder:) has not been implemented")
    }

    func setAction(action: () -> Void) {
        self.action = action
    }

    func tapped(sender: CallbackButton) {
        self.action?()
    }

}

file1.swift

class SomeView: UIView {

    var a = 1

    override init(frame: CGRect) {

        super.init(frame: frame)

        let button = UIButton(frame: CGRectZero)
        button.addTarget(self, action: "tapped:", forControlEvents: .TouchUpInside)
        self.addSubview(button)

    }

    required init?(coder aDecoder: NSCoder) {
        fatalError("init(coder:) has not been implemented")
    }

    func tapped(sender: UIButton) {
        self.a *= 2
    }

}

file2.swift

class SomeView: UIView {

    var a = 1

    override init(frame: CGRect) {

        super.init(frame: frame)

        let button = CallbackButton(frame: CGRectZero) { () -> Void in
            self.a *= 2
        }
        self.addSubview(button)

    }

    required init?(coder aDecoder: NSCoder) {
        fatalError("init(coder:) has not been implemented")
    }

}