frontmost_window.swift

import Cocoa

let workspace = NSWorkspace.sharedWorkspace()
let activeApps = workspace.runningApplications
for app in activeApps {
    if app.active {
        let listOptions = CGWindowListOption(arrayLiteral: CGWindowListOption.OptionOnScreenOnly)
        let windowList : NSArray = CGWindowListCopyWindowInfo(listOptions, 0)!
        for window in windowList {
            if  (window["kCGWindowOwnerName"] as! String) == app.localizedName {
                let topWindow = window as! NSDictionary
                break
            }
        }
    }
}