Object X of class Y does not implement methodSignatureForSelector in Swift

I have a class Person which is instantiated multiple times.Each person get's their own timer. Upon in my init for Person I call startTimer().

class Person {
 var timer = NSTimer()
 func startTimer() {
    timer = NSTimer.scheduledTimerWithTimeInterval(1, target: self, selector: Selector("timerTick"), userInfo: nil, repeats: true)
 }

 func timerTick() {
    angerLevel++
    println("Angry! \(angerLevel)")
 }
...
...
}

So I may have 3 instances of Person in an array of Person[]. I am getting an error:

2014-06-25 13:57:14.956 ThisProgram[3842:148856] *** NSForwarding: warning: object 0x113760048 of class '_TtC11ThisProgram6Person' does not implement methodSignatureForSelector: -- trouble ahead

I read elsewhere that I should inherit from NSObject but this is in Swift not Obj-C. The function is within the class so I am not sure what to do.

Don't think of NSObject as an Objective-C class, think of it as a Cocoa/Foundation class. Even though you're using Swift instead of Objective-C, you're still using all the same frameworks.

Two options: (1) add the dynamic attribute to the function you want to reference as a selector:

    dynamic func timerTick() {
        self.angerLevel++
        print("Angry! \(self.angerLevel)")
    }

Or (2) declare Person as a subclass of NSObject, then just call super.init() at the beginning of your initializer:

class Person: NSObject {
    var timer = NSTimer()
    var angerLevel = 0

    func startTimer() {
        print("starting timer")
        timer = NSTimer.scheduledTimerWithTimeInterval(1, target: self, selector: "timerTick", userInfo: nil, repeats: true)
    }

    func timerTick() {
        self.angerLevel++
        print("Angry! \(self.angerLevel)")
    }

    override init() {
        super.init()
        self.startTimer()
    }
}

Since XCode6 beta 6, you can use 'dynamic' func

dynamic func timerTick() { .... }

I had a similar error trying to use let encodedArchive = NSKeyedArchiver.archivedDataWithRootObject(archive) as NSData where archive was an array of a custom class. I found that declaring that custom class as a subclass of NSObject and NSCoding did the trick. It'll require a few more lines to conform to the protocol of NSCoding so it'll look something like this to start with:

class Person: NSObject, NSCoding {
  init() {
    super.init()
  }

  func encodeWithCoder(_aCoder: NSCoder) {   }
}