Swift state machine example

state_machine.swift

protocol State {
    func nextState() -> Self
}

class StateMachine<T: State> {
    private(set) var currentState: T

    init(_ initialState: T) {
        currentState = initialState
    }

    func nextState() -> T {
        currentState = currentState.nextState()
        return currentState
    }
}

enum PlayerState: State {
    case idle, walking

    func nextState() -> PlayerState {
        switch self {
        case .idle:
            return .walking
        case .walking:
            return .idle
        }
    }
}

let playerStateMachine = StateMachine<PlayerState>(.idle)
print(playerStateMachine.currentState) // idle
print(playerStateMachine.nextState()) // walking