swift – 简单的tvOS UIButton无效

>在模拟器中,我可以直观地看到按下按钮.

我的ViewController.swift代码是：

import UIKit

class ViewController: UIViewController {

    override func viewDidLoad() {
        super.viewDidLoad()

        let button = UIButton(type: UIButtonType.System)
        button.frame = CGRectMake(100,100,400,150)
        button.BACkgroundColor = UIColor.greenColor()
        button.settitle("Test",forState: UIControlState.Normal)
        button.addTarget(self,action: "buttonAction:",forControlEvents: UIControlEvents.TouchDown)

        self.view.addSubview(button)

        self.view.userInteractionEnabled = true    
    }

    func buttonAction(sender:UIButton!){
        NSLog("Clicked")
    }
}

为了检测按钮点击,我需要做什么？