Call other class function before its own class in iOS

I created a checkbox button with class CheckBox. I want to enable/disable ResultTextField according to the value of checkbox ischecked. So i linked a button with @IBAction func retestRequestedCheckbox_onChange(). The proble is the func retestRequestedCheckbox_onChange() is getting called before it updated value in CheckBox class. How can i update the checkbox value before calling retestRequestedCheckbox_onChange() function. Here is my code:

class Checkbox: UIButton {

let checkedImage = UIImage(named: "checked_checkBox") as UIImage?
let unCheckedImage = UIImage(named: "unchecked_checkBox") as UIImage?
var isChecked:Bool = false{
    didSet
    {
        if isChecked == true{
            self.setImage(checkedImage, forState: .Normal)
        }else {
            self.setImage(unCheckedImage, forState: .Normal)
        }
    }
}

override func awakeFromNib() {
    self.addTarget(self, action: "buttonClicked:", forControlEvents: UIControlEvents.TouchUpInside)
self.isChecked = false
}

func buttonClicked(sender: UIButton){
    if(sender == self){
        if isChecked == true {
            isChecked = false
        } else {
            isChecked = true
        }
    }
}

}

 @IBAction func Checkbox_onChange(){


        if CheckboxButton.isChecked ?? false {
           ResultTextField.enabled = true

        } else {
          ResultTextField = false    
        }

}