UnityStackBasedFSM.cs

using System.Collections.Generic;
using UnityEngine;

public class FSMState : MonoBehaviour
{
    private void Awake()
    {
        Setup();
    }

    private void OnDestroy()
    {
        Teardown();
    }

    private void OnEnable()
    {
        GainControl();
    }

    private void OnDisable()
    {
        LoseControl();
    }

    protected virtual void Setup() { }
    protected virtual void Teardown() { }
    protected virtual void GainControl() { }
    protected virtual void LoseControl() { }
}

public class TestState : FSMState
{
    protected override void Setup()
    {
        Debug.Log("Initialize state");
    }
    
    protected override void Teardown()
    {
        Debug.Log("Destroy state");
    }

    protected override void GainControl()
    {
        Debug.Log("State gained control");
    }

    protected override void LoseControl()
    {
        Debug.Log("State lost control");
    }
}

public class FSM : MonoBehaviour
{
    private static FSM instance;

    private Stack<FSMState> states = new Stack<FSMState>();

    private bool IsEmpty { get { return states.Count == 0; } }

    private void Awake()
    {
        instance = this;
    }

    public static void Push<T>() where T : FSMState
    {
        instance.PushState<T>();
    }

    public static void Pop()
    {
        instance.PopState();
    }

    private void PushState<T>() where T : FSMState
    {
        Debug.Assert(states != null);

        if (!IsEmpty)
        {
            var prev = states.Peek();
            prev.enabled = false;
        }

        var cur = gameObject.AddComponent<T>();
        states.Push(cur);
    }

    private void PopState()
    {
        Debug.Assert(states != null);
        
        if (!IsEmpty)
        {
            var cur = states.Pop();
            Destroy(cur);

            if (!IsEmpty)
            {
                var prev = states.Peek();
                prev.enabled = true;
            }
        }
    }
}