How to retrieve the OSX application that currently receives key events - xcode

Question

I am following the cocoa documentation to determine the current front most application in OSX - aka the application which receives key events.
However, when I am executing the following swift the API always returns me the same value - XCode, but it never changes to chrome or any other application when I switch to them. I also tried to execute the compiled program but instead of constantly showing XCode it now shows whichever terminal app I am running.
What is the correct way of determining the application that receives the key events from OSX? Is my code in this regard broken?
import Cocoa
func getActiveApplication() -> String{
// Return the localized name of the currently active application
let ws = NSWorkspace.sharedWorkspace()
let frontApp = ws.frontmostApplication
return frontApp.localizedName
}
var frontMostApp : String
while true {
frontMostApp = getActiveApplication();
NSLog("front app: %#", frontMostApp)
sleep(1);
}

Answer 1

This thread is a bit old but was very helpful. I did some research based on Marco's post and uchuugaka's answer. The following is the result.
// swift 3.0
// compile: xcrun -sdk macosx swiftc -framework Cocoa foo.swift -o foo
// run: ./foo
import Cocoa
class MyObserver: NSObject {
override init() {
super.init()
NSWorkspace.shared().notificationCenter.addObserver(self,
selector: #selector(printMe(_:)),
name: NSNotification.Name.NSWorkspaceDidActivateApplication,
object:nil)
}
dynamic private func printMe(_ notification: NSNotification) {
let app = notification.userInfo!["NSWorkspaceApplicationKey"] as! NSRunningApplication
print(app.localizedName!)
}
}
let observer = MyObserver()
RunLoop.main.run()
I am a newbie in both Cocoa and Swift. I don't know if the above is efficient, but it works for me. I got help from How to create a minimal daemon process in a swift 2 command line tool? and Swift 3 NSNotificationCenter Keyboardwillshow/hide among numerous others.
Swift 4:
NSWorkspace.shared.notificationCenter.addObserver(self, // HERE, shared
selector: #selector(printMe(_:)),
name: NSWorkspace.didActivateApplicationNotification, // HERE
object:nil)
Edit (Swift 4)
The compiler says the printMe function must be #objc. (I don't know the meaning, but it worked when I prepend #objc at the beginning of the line. Here is the full code for easy copy-paste.
// swift 4.0
// compile: xcrun -sdk macosx swiftc -framework Cocoa foo.swift -o foo
// run: ./foo
import Cocoa
class MyObserver: NSObject {
override init() {
super.init()
NSWorkspace.shared.notificationCenter.addObserver(self,
selector: #selector(printMe(_:)),
name: NSWorkspace.didActivateApplicationNotification,
object:nil)
}
#objc dynamic private func printMe(_ notification: NSNotification) {
let app = notification.userInfo!["NSWorkspaceApplicationKey"] as! NSRunningApplication
print(app.localizedName!)
}
}
let observer = MyObserver()
RunLoop.main.run()

Answer 2

You you should do one thing differently, that is follow the NSWorkSpace notifications that tell you the applications resigned active or became active.
This is key especially as you are running in debug mode.
In debug mode Xcode is spawning your app as a child process.
In release mode it is basically doing the same as calling the open command in terminal.
In debug mode if you call this code too early and only once, you're not catching changes. Remember it's dynamic.