How to use an NSAlert with storyboards

Question

I'm teaching myself Swift (currently using Xcode 7.3) and I'm working with storyboards for the first time. I'm writing an OS X-based app and I want to display an alert when the user attempts to load data when data already exists. I've read the following thread, Add completion handler to presentViewControllerAsSheet but I'm having trouble wrapping my head around closures/completion handlers. I understand them "in theory" but not yet well enough to write one.

In the thread above, a Struct is being returned. I just need to return an Int or Bool to indicate whether the user wants to overwrite the data or not.

Answer 1

You don't need to create a second view controller. Just configure and display an NSAlert object:

@IBAction func loadData(sender : AnyObject) {
    let dataAlreadyExists = true // assume this is always true

    if dataAlreadyExists {
        let alert = NSAlert()
        alert.messageText = "Do you want to reload data?"
        alert.addButtonWithTitle("Reload")
        alert.addButtonWithTitle("Do not reload")

        alert.beginSheetModalForWindow(self.view.window!) { response in
            if response == NSAlertFirstButtonReturn {
                // reload data
            }
        }
    }
}