Request for a simple example for GKStateMachine?

Question

I've been researching Apple's state machine for Swift and found several examples, but none of them really dead simple.

Could someone whip up a super simple GKStateMachine, perhaps with two states, in Swift, preferably all in one Swift file? Thanks!

Answer 1

Here is an example of a state machine for a traffic light as they work in the USA. A traffic light moves from green -> yellow -> red -> green.

In an app, you might have the didEnter(from:) routines update an onscreen graphic, or allow another actor to move.

import UIKit
import GameKit

class Green: GKState {

    override func isValidNextState(_ stateClass: AnyClass) -> Bool {
        return stateClass is Yellow.Type
    }

    override func didEnter(from previousState: GKState?) {
        print("Traffic light is green")
    }
}

class Yellow: GKState {
    override func isValidNextState(_ stateClass: AnyClass) -> Bool {
        return stateClass is Red.Type
    }

    override func didEnter(from previousState: GKState?) {
        print("Traffic light is yellow")
    }

}

class Red: GKState {
    override func isValidNextState(_ stateClass: AnyClass) -> Bool {
        return stateClass is Green.Type
    }

    override func didEnter(from previousState: GKState?) {
        print("Traffic light is red")
    }
}

class ViewController: UIViewController {

    var stateMachine: GKStateMachine?

    override func viewDidLoad() {
        super.viewDidLoad()

        // Create the states            
        let green = Green()
        let yellow = Yellow()
        let red = Red()

        // Initialize the state machine            
        stateMachine = GKStateMachine(states: [green, yellow, red])

        // Try entering various states...            
        if stateMachine?.enter(Green.self) == false {
            print("failed to move to green")
        }
        if stateMachine?.enter(Red.self) == false {
            print("failed to move to red")
        }
        if stateMachine?.enter(Yellow.self) == false {
            print("failed to move to yellow")
        }
        if stateMachine?.enter(Green.self) == false {
            print("failed to move to green")
        }
        if stateMachine?.enter(Red.self) == false {
            print("failed to move to red")
        }

    }

}

Output:

Traffic light is green
failed to move to red
Traffic light is yellow
failed to move to green
Traffic light is red