将字符串传递给unwind segue和/或dismissViewController SWIFT

Question

VC1 has 1 button F1 which >segues> to VC2 which has 2 buttons Qd and Qs

Qd is connected to dismissViewController

Qs is connected to unwindSegue

My Objective : learn to change F1 's image to "Qd" image through dismissViewController, and learn to change F1 's image to "Qs" image through unwindSegue.

Question1 : Why is Qd (VC2) NOT setting F1String(VC1) = F1Card(VC2)?

Question2 : How do I apply this to unwindSegue for Qs ?

VC1 (Table)

import UIKit

class Table: UIViewController {
@IBOutlet weak var F1: UIButton!
var F1String:String = "Default"

override func viewDidLoad() {
    super.viewDidLoad()

    let F1Image = UIImage(named: "\(F1String)") as UIImage!
    F1.setImage(F1Image, forState: UIControlState.Normal)
}

@IBAction func unwindFromFlop(unwindSegue:UIStoryboardSegue) {

}
}

VC2 (FlopPop)

import UIKit

class FlopPop: UIViewController {
var F1Card = String()

override func prepareForSegue(segue: UIStoryboardSegue, sender: AnyObject?) {
    var DVC : Table = segue.destinationViewController as Table
    DVC.F1String = F1Card
}
@IBAction func Qd(sender: AnyObject)
{
    let F1Card = "Qd"
    self.dismissViewControllerAnimated(true, completion: nil)
}
}

THANKS DDD

Answer 1

This solved the unwind part

VC1

import UIKit

class Table: UIViewController {
@IBOutlet weak var F1: UIButton!
var message : NSString = "Default"

@IBAction func unwindFromFlop(unwindSegue:UIStoryboardSegue) {
    let F1Image = UIImage(named: "\(message)") as UIImage!
    F1.setImage(F1Image, forState: UIControlState.Normal)
    checkLabel.text = message
}
}

VC2

import UIKit

class FlopPop: UIViewController {
var F1Card = String()

override func prepareForSegue(segue: UIStoryboardSegue, sender: AnyObject?) {
    F1Card = segue.identifier!

    let DVC = segue.destinationViewController as Table
    DVC.message = F1Card

}
}

I think it had to do with using NSString to pass the data instead of just a string.

If anyone can help to explain why this is I'd greatly appreciate it and I'm still hoping to learn the work around for dismissViewController.