SimpleFsm.cs

public class BetterSimpleStateMachine<TEnum> : Component, IUpdatable
    where TEnum : struct, IComparable, IFormattable
{
    class StateMethodCache
    {
        public Action EnterState;
        public Func<TEnum> Tick;
        public Action ExitState;
    }

    protected float elapsedTimeInState = 0f;
    protected TEnum previousState;
    Dictionary<TEnum, StateMethodCache> _stateCache;
    StateMethodCache _stateMethods;

    TEnum _currentState;

    public TEnum CurrentState
    {
        get => _currentState;
        set
        {
            // dont change to the current state
            if (_stateCache.Comparer.Equals(_currentState, value))
                return;

            // swap previous/current
            previousState = _currentState;
            _currentState = value;

            // exit the state, fetch the next cached state methods then enter that state
            if (_stateMethods.ExitState != null)
                _stateMethods.ExitState();

            elapsedTimeInState = 0f;
            _stateMethods = _stateCache[_currentState];

            if (_stateMethods.EnterState != null)
                _stateMethods.EnterState();
        }
    }

    public TEnum InitialState
    {
        set
        {
            _currentState = value;
            _stateMethods = _stateCache[_currentState];

            if (_stateMethods.EnterState != null)
                _stateMethods.EnterState();
        }
    }

    public BetterSimpleStateMachine(IEqualityComparer<TEnum> customComparer = null)
    {
        _stateCache = new Dictionary<TEnum, StateMethodCache>(customComparer);

        // cache all of our state methods
        var enumValues = (TEnum[]) Enum.GetValues(typeof(TEnum));
    }

    public virtual void Update()
    {
        elapsedTimeInState += Time.DeltaTime;

        if (_stateMethods.Tick != null)
            CurrentState = _stateMethods.Tick();
    }

    public void AddCallbacks(TEnum stateEnum, Action enterState, Func<TEnum> tick, Action exitState)
    {
        var state = new StateMethodCache
        {
            EnterState = enterState,
            Tick = tick,
            ExitState = exitState,
        };

        _stateCache[stateEnum] = state;
    }
}

TrivialExample.cs

enum MoveState {
    Normal,
    Dashing,
}

class Player {
// ...
    MoveState NormalUpdate() {
        if (ShouldDash && TryDash()) {
            return MoveState.Dashing;
        }

        // do normal movement stuff
    }

    bool TryDash() {
        // do some preliminary logic to see if they can dash
        return canDash;
    }

    void EnterDash() {
        // begin dashing
    }

    MoveState Dashing() {
        // update loop or dashing state
        if (DashOver) {
            return MoveState.Normal;
        }
    }

    void ExitDash() {
        // after dash is over
    }
// ...
}