UIScreenManager.cs

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using UnityEngine;
using System.Collections.Generic;

namespace HKI.Core.UI
{
    [RequireComponent(typeof(Canvas))]
    [DisallowMultipleComponent]
    public class UIScreenManager : MonoBehaviour
    {
        // Public variable
        public static UIScreenManager Instance { get; private set; }
        [SerializeField] UIScreen StartScreen = null;

        // Private variables
        Canvas rootCanvas = null;
        Transform rootCanvasTransform = null;

        Dictionary<int, GameObject> istanciatedScreens = new Dictionary<int, GameObject>();

        List<UIScreen> screens = new List<UIScreen>();
        List<GameObject> screenGOs = new List<GameObject>();
        
        // Awake function
        void Awake()
        {
            if(Instance == null)
                Instance = this;
            else
                Destroy(gameObject);

            rootCanvas = GetComponent<Canvas>();
            rootCanvasTransform = transform;
        }

        // Start function
        void Start()
        {
            if(StartScreen != null)
                StartScreen.Open();
        }

        // OnDestroy
        void OnDestroy()
        {
            if(Instance == this)
                Instance = null;
        }
        
        // Interface function
        public void OpenScreen(UIScreen screen)
        {
            if(screen == null)
            {
                Debug.LogError("(UIScreenManager)" + name + " you are trying to open a screen that is null!");
                return;
            }

            if(screen.GetScreenPrefab == null)
            {
                Debug.LogError("(UIScreenManager)" + name + " you are trying to open a (UIScreen)" + screen.name + " but the screen has no prefab attached to it!");
                return;
            }

            GameObject screenGO = istanciatedScreens.ContainsKey(screen.GetScreenId) ? istanciatedScreens[screen.GetScreenId] : null;

            if (screenGO == null)
            {
                screenGO = GameObject.Instantiate<GameObject>(screen.GetScreenPrefab, rootCanvasTransform);
                istanciatedScreens.Add(screen.GetScreenId, screenGO);
            }
            else
                screenGO.SetActive(true);
            
            if(screens.Count > 0)
            {
                if(screen.GetDisableScreensUnderneath)
                {
                    for(int i = screens.Count - 1; i >= 0; i--)
                    {
                        screenGOs[i].SetActive(false);

                        if(screens[i].GetDisableScreensUnderneath)
                            break;
                    }
                }

                Canvas topCanvas = screenGO.GetComponent<Canvas>();
                Canvas previousCanvas = screenGOs[screenGOs.Count - 1].GetComponent<Canvas>();
                topCanvas.sortingOrder = previousCanvas.sortingOrder + 1;
            }
            else
            {
                Canvas topCanvas = screenGO.GetComponent<Canvas>();
                topCanvas.sortingOrder = rootCanvas.sortingOrder + 1;
            }

            screens.Add(screen);
            screenGOs.Add(screenGO);
        }

        public void CloseScreen(UIScreen screen)
        {
            if(screen == null)
            {
                Debug.LogError("(UIScreenManager)" + name + " you are trying to close a screen that is null!");
                return;
            }

            if(screens.Count == 0)
            {
                Debug.LogError("(UIScreenManager)" + name + " you are trying to open a (UIScreen)" + screen.name + " but the screens stack is empty!");
                return;
            }

            if(screens[screens.Count - 1] != screen)
            {
                Debug.LogError("(UIScreenManager)" + name + " you are trying to open a (UIScreen)" + screen.name + " but it is not on top of screens stack!");
                return;
            }

            CloseTopMenu();
        }

        public void CloseTopMenu()
        {
            int lastIndex = screens.Count - 1;
            UIScreen screen = screens[lastIndex];
            screens.RemoveAt(lastIndex);

            GameObject screenGO = screenGOs[lastIndex];
            screenGOs.RemoveAt(lastIndex);

            if (screen.GetDestroyWhenClosed)
                Destroy(screenGO);
            else
                screenGO.SetActive(false);
                        
            for (int i = screens.Count - 1; i >= 0; i--)
            {
                screenGOs[i].SetActive(true);

                if(screens[i].GetDisableScreensUnderneath)
                    break;
            }
        }
    }
}