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Let's say you've control click dragged from a UIButton to another UIViewController in your storyboard. You've now created a segue.

Click on it, and in the attributes inspector tab, name its identifier segue1.

In your view controller from which you're leaving for the new one, add this:

override func prepare(for segue: UIStoryboardSegue, sender: Any?) {
    if (segue.identifier == "segue1") {
        let detailVC = segue.destination as! SomeViewController;
        detailVC.someValue = "JOHN COLTRANE"
    }
}

This says for the identifier segue1 which we named above, cast the UIViewController to SomeViewController, the name of our destination view controller.

Then set a property on that view controller, named simply via var someValue = "blar" for example in your destination controller.

Note, as soon as you have detailVC in the example above, the view controller will be initialised and visible on the screen.