Swift controller for a modal window / sheet; see http://acapio.de/xcode/2014/07/12/swift_set_modal_window.html

MyModalWindowController.swift

import Cocoa

class MyModalWindowController: NSWindowController {

    var mainW: NSWindow = NSWindow()
    
    override init() {
        super.init()
    }
    
    override init(window: NSWindow!) {
        super.init(window: window)
        //Initialization code here.
    }
    
    required init?(coder: (NSCoder!)){
        super.init(coder: coder);
    }
    
    override func windowDidLoad() {
        super.windowDidLoad()
        // Implement this method to handle any initialization after your window controller's window has been loaded from its nib file.
    }
    
    //method called to display the modal window
    func beginSheet(mainWindow: NSWindow){
        self.mainW = mainWindow
        NSApp.beginSheet(self.window!, modalForWindow: mainWindow, modalDelegate: self, didEndSelector:nil, contextInfo: nil)
        
    }
    
    //method called, when "Close" - Button clicked
    @IBAction func btnClicked(sender: AnyObject) {
        self.endSheet();
    }
    
    //method called to slide out the modal window
    func endSheet(){
        NSApp.endSheet(self.window!)
        self.window!.orderOut(mainW)
    }
    
}

Nice. You should update it to use NSWindow beginsheet.