Swift Xcode attach an action to a checkbox using a checkbox subclass

I’m new to swift so I am stumped on something that is probably very simple.

I want to have checkboxes for each day of the week and be able to toggle their states a bit like the standard Apple clock app alarm repeat page. I am using a subclass borrowed from http://ift.tt/1MwLmW2 to create a set of checkboxes. All that works like a charm in the storyboard but now I am struggling with how to refer to the subclass checkbox ‘state’ in my view controller to actually do something.

So here is the subclass:

import UIKit

class checkBox: UIButton {

//images
let checkedImage = UIImage(named: "checkBoxChecked")
let unCheckedImage = UIImage(named: "checkBoxUnchecked")

//bool property
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
        }
    }
}

}

Here is an outlet example:

@IBOutlet weak var sun: checkBox!

Here is my empty array ready to be appended (working):

var daysArray:[String] = []

Here is an example action:

@IBAction func setSunday(sender: checkBox) {
    if (****what do I do here to retrieve the button state?****) {
        daysArray.append("0")
        print(daysArray) // Check the array is working then delete
    } else {
        daysArray.removeAtIndex(0)
        print(daysArray) // Check the array is working then delete
    }
}

If I can get this working then I can apply to all days of the week and make the state of the array persistent using NSUserDefaults. Hope you guys can help.