Q:

Placing common methods in a separate file using Swift

I like to place often used methods in a separate file. I found this answer Use function from one class in another class in Swift but I get errors using it the way I want. 
Say i want to create a method called msgBox that pops up an alert box. 
I created a separate empty Swift file and place this code in it.
import UIKit

class Utils: UIViewController {

    class func msgBox (titleStr:String = "Untitled", messageStr:String = "Alert text", buttonStr:String = "Ok") {
        var alert = UIAlertController(title: titleStr, message: messageStr, preferredStyle: UIAlertControllerStyle.Alert)
        alert.addAction(UIAlertAction(title: buttonStr, style: .Default, handler: { (action) -> Void in
            self.dismissViewControllerAnimated(true, completion: nil)
        }))
        self.presentViewController(alert, animated: true, completion: nil)
    }

}

I want to call it like this, but I get errors doing so. Does anyone know what I'm doing wrong?
Utils.msgBox(titleStr: "Hello!", messageStr: "Are you sure?")

The error looks like this:

A:

The error is because you are using self in a class method. There is no self instance, in this case.
One thing you could do in this situation is make a class extension. In the following example, you would be able to call the alert method from any UIViewController instance:
extension UIViewController {

    func alert(title: String?, message: String?, buttonTitle: String = "OK") {
        let alert = UIAlertController(title: title, message: message, preferredStyle: .Alert)
        alert.addAction(UIAlertAction(title: buttonTitle, style: .Default, handler: { action in
            self.dismissViewControllerAnimated(true, completion: nil)
        }))
        self.presentViewController(alert, animated: true, completion: nil)
    }

}

Notice that I changed a couple names and types, but you can use what you like.

