MenuManager.cs

using System;
using System.Collections;
using System.Collections.Generic;
using System.Reflection;
using UnityEngine;

public class MenuManager : MonoBehaviour
{
	public MainMenu MainMenuPrefab;
	public GameMenu GameMenuPrefab;
	public PauseMenu PauseMenuPrefab;
	public OptionsMenu OptionsMenuPrefab;
	public AwesomeMenu AwesomeMenuPrefab;

    private Stack<Menu> menuStack = new Stack<Menu>();

    public static MenuManager Instance { get; set; }

    private void Awake()
    {
        Instance = this;

		MainMenu.Show(); 
    }

    private void OnDestroy()
    {
        Instance = null;
    }

	public void CreateInstance<T>() where T : Menu
	{
		var prefab = GetPrefab<T>();

		Instantiate(prefab, transform);
	}

	public void OpenMenu(Menu instance, float animInTime)
    {
        if (menuStack.Count > 0)
        {
            var topCanvas = instance.GetComponent<Canvas>();
            var previousCanvas = menuStack.Peek().GetComponent<Canvas>();
            topCanvas.sortingOrder = previousCanvas.sortingOrder + 1;
        }
        menuStack.Push(instance);
        StartCoroutine(DeactivateMenusUnder(instance, animInTime));
    }

    private IEnumerator DeactivateMenusUnder(Menu instance, float waitTime)
    {
        yield return new WaitForSeconds(waitTime);

        //De-activate previous top menu
        if (instance.DisableMenusUnderneath)
        {
            foreach (var menu in menuStack)
            {
                if (menu == instance)
                    continue;

                menu.gameObject.SetActive(false);

                if (menu.DisableMenusUnderneath)
                    break;
            }
        }
    }

    private T GetPrefab<T>() where T : Menu
    {
        // Get prefab dynamically, based on public fields set from Unity
		// You can use private fields with SerializeField attribute too
        var fields = this.GetType().GetFields(BindingFlags.Public | BindingFlags.Instance | BindingFlags.DeclaredOnly);
        foreach (var field in fields)
        {
            var prefab = field.GetValue(this) as T;
            if (prefab != null)
            {
                return prefab;
            }
        }

        throw new MissingReferenceException("Prefab not found for type " + typeof(T));
    }
	
	public void CloseMenu(Menu topMenu, float animOutTime)
    {
        if (menuStack.Count == 0)
        {
            Debug.LogErrorFormat(topMenu, "{0} cannot be closed because menu stack is empty", topMenu.GetType());
            return;
        }

        if (menuStack.Peek() != topMenu)
        {
            Debug.LogErrorFormat(topMenu, "{0} cannot be closed because it is not on top of stack", topMenu.GetType());
            return;
        }

        menuStack.Pop();

        StartCoroutine(CloseTopMenu(topMenu, animOutTime));

        ReactivateOldMenu();
    }

    private IEnumerator CloseTopMenu(Menu instance, float waitTime)
    {
        yield return new WaitForSeconds(waitTime);

        if (instance == null)
            yield break;

        if (instance.DestroyWhenClosed)
            Destroy(instance.gameObject);
        else
            instance.gameObject.SetActive(false);
    }

    private void ReactivateOldMenu()
    {
        // If a re-activated menu is an overlay we need to activate the menu under it
        foreach (var menu in menuStack)
        {
            menu.gameObject.SetActive(true);

            if (menu.DisableMenusUnderneath)
                break;
        }
    }

    private void Update()
    {
        // On Android the back button is sent as Esc
        if (Input.GetKeyDown(KeyCode.Escape) && menuStack.Count > 0)
        {
            menuStack.Peek().OnBackPressed();
        }
    }
}