如何使用DX和DY变量进行UIImageView移动？

Question

目前，这是我的代码：

import UIKit


class ViewController: UIViewController {


var dx : CGFloat = 0
var dy : CGFloat = 0

@IBOutlet var sprite: UIImageView!


@IBAction func A(sender: AnyObject) {

    dx = -1;
}
@IBAction func W(sender: AnyObject) {


    dy = -1;
}
@IBAction func D(sender: AnyObject) {


    dx = 1;
}
@IBAction func S(sender: AnyObject) {


    dy = 1;
}
override func viewDidLoad() {
    super.viewDidLoad()

    backgroundThread(delay: 10
        , background: {
        while(true){

            self.sprite.frame.origin.x += self.dx;

            self.sprite.frame.origin.x += self.dy;


        }

    })


}

override func didReceiveMemoryWarning() {
    super.didReceiveMemoryWarning()
    // Dispose of any resources that can be recreated.
}


}

这是backgroundThread函数（它确实有效。我用简单的println（）测试过）：

func backgroundThread(delay: Double = 0.0, background: (() -> Void)? = nil, completion: (() -> Void)? = nil) {
dispatch_async(dispatch_get_global_queue(Int(QOS_CLASS_USER_INITIATED.value), 0)) {
    if(background != nil){ background!(); }

    let popTime = dispatch_time(DISPATCH_TIME_NOW, Int64(delay * Double(NSEC_PER_SEC)))
    dispatch_after(popTime, dispatch_get_main_queue()) {
        if(completion != nil){ completion!(); }
    }
}
}

出于某种原因，当我按下任何按钮时，它不会开始移动。有没有人有任何建议？