State Machine In Swift 3 - ios

Question

Does anyone have an example of a simple state machine that can be implemented in Swift? I've been googling around, but have only seen a lot of third party libraries which are pretty opaque to me. I've also seen a lot of high level discussions where people talk about state machines, but haven't shown any code. Is it possible for someone to do a simple counter example (like add 1 / subtract 1)? Or point me to one? I know it's a big ask, but my google-fu is failing me. Thank you.

Answer 1

Apple's GamePlayKit already has state machine implementation as GKStateMachine. But GamePlayKit is available only from iOS 9 so I had to clone GKStateMachine (and GKState) to support iOS 8 in one of my apps. Hope it will be helpful.
EHStateMachine.swift
import Foundation
class EHStateMachine {
private(set) var currentState: EHState?
private let states = NSMapTable<AnyObject, EHState>(keyOptions: [.objectPointerPersonality],
valueOptions: [.strongMemory])
// MARK:
init(states: [EHState]) {
guard states.count > 0 else {
fatalError("Can't create state machine with zero states.")
}
let tempStates = NSHashTable<AnyObject>(options: [.objectPointerPersonality])
for state in states {
guard !tempStates.contains(type(of: state)) else {
fatalError("Duplicate instances of \(type(of: state)) found.")
}
tempStates.add(type(of: state))
}
for state in states {
state.stateMachine = self
self.states.setObject(state, forKey: type(of: state))
}
}
// MARK:
func canEnterState(_ stateClass: AnyClass) -> Bool {
if (states.object(forKey: stateClass) == nil) {
return false
}
if currentState == nil {
return true
}
return currentState!.isValidNextState(stateClass)
}
func enter(_ stateClass: AnyClass) -> Bool {
if !canEnterState(stateClass) {
return false
}
let previousState = currentState
let nextState = states.object(forKey: stateClass)
previousState?.willExit(to: nextState!)
currentState = nextState
currentState!.didEnter(from: previousState)
return true
}
func update(deltaTime seconds: TimeInterval) {
currentState?.update(deltaTime: seconds)
}
func state<T: EHState>(forClass stateClass: T.Type) -> T? {
return states.object(forKey: stateClass) as? T
}
}
EHState.swift
import Foundation
class EHState {
weak var stateMachine: EHStateMachine?
// MARK:
func isValidNextState(_ stateClass: AnyClass) -> Bool {
return true
}
func didEnter(from previousState: EHState?) {
}
func willExit(to nextState: EHState) {
}
func update(deltaTime seconds: TimeInterval) {
}
}
If you target iOS 9 you should probably use GKStateMachine and GKState instead of these.