Loading Screen from any Class in Swift

I am trying to implement a loading screen in swift, that I can reuse from any class in the project (I have a few classes that will handle 'long running' activities)

The public function is, from another answer:

public class LoadingOverlay{

var overlayView = UIView()
var activityIndicator = UIActivityIndicatorView()

class var shared: LoadingOverlay {
    struct Static {
        static let instance: LoadingOverlay = LoadingOverlay()
    }
    return Static.instance
}

public func showOverlay(view: UIView) {

    overlayView.frame = CGRectMake(0, 0, 80, 80)
    overlayView.center = view.center
    overlayView.backgroundColor = MyGlobalVariables.UICOLORGREEN
    overlayView.clipsToBounds = true
    overlayView.layer.cornerRadius = 10

    activityIndicator.frame = CGRectMake(0, 0, 40, 40)
    activityIndicator.activityIndicatorViewStyle = .WhiteLarge
    activityIndicator.center = CGPointMake(overlayView.bounds.width / 2, overlayView.bounds.height / 2)

    overlayView.addSubview(activityIndicator)
    view.addSubview(overlayView)

    activityIndicator.startAnimating()
}

public func hideOverlayView() {
    activityIndicator.stopAnimating()
    overlayView.removeFromSuperview()
}

}

Which works fine in FirstViewController.swift (Where it is located) by using:

LoadingOverlay.shared.showOverlay(self.view)

My question is, how do I use it in XYZ.swift? As self.view may not be referring to a view generated by that class. Is it possible to call and find the current super view, then add the loading screen on top of it?

You need to add your overlay to your main UIWindow class. In that case you will overlay all views.

public func showOverlay() {
    if  let appDelegate = UIApplication.sharedApplication().delegate as? AppDelegate,
        let window = appDelegate.window {
            overlayView.frame = CGRectMake(0, 0, 80, 80)
            overlayView.center = CGPointMake(window.frame.width / 2.0, window.frame.height / 2.0)
            overlayView.backgroundColor = MyGlobalVariables.UICOLORGREEN
            overlayView.clipsToBounds = true
            overlayView.layer.cornerRadius = 10

            activityIndicator.frame = CGRectMake(0, 0, 40, 40)
            activityIndicator.activityIndicatorViewStyle = .WhiteLarge
            activityIndicator.center = CGPointMake(overlayView.bounds.width / 2, overlayView.bounds.height / 2)

            overlayView.addSubview(activityIndicator)
            window.addSubview(overlayView)

            activityIndicator.startAnimating()
    }
}

PS: you can move most of overlayView customization code to init function, because you don't need to update backgroundColor or other things every time on display.

PPS: There is great libraries for loading overlay like

https://github.com/TransitApp/SVProgressHUD

https://github.com/jdg/MBProgressHUD

and others. Just google it