using System;
using System.Collections;
using System.Collections.Generic;
using System.Threading;
using System.Threading.Tasks;
using UnityEngine;
using UnityEngine.EventSystems;
using UnityEngine.SceneManagement;
using BaseGames.Core;
using BaseGames.Core.Events;

namespace BaseGames.UI
{
    /// <summary>
    /// <see cref="IUINavigator"/> 的实现：统一 UI 导航栈。常驻 Persistent 场景，
    /// 经 ServiceLocator 暴露，主菜单与游戏内共用同一套栈语义。
    ///
    /// <para>压栈：记录压栈前焦点；据新面板 <see cref="PushMode"/> 处理下方面板
    /// （Replace→停用；Modal→保留可见但 <see cref="UIPanelBase.SetInteractableLayer"/>(false)
    /// 使其退出导航图）；激活新面板并延后一帧聚焦其首项。</para>
    /// <para>出栈：关闭栈顶，按其压栈时的 mode 还原下方面板，恢复焦点到压栈前的选中项。</para>
    /// <para>取消：本类是 EVT_UICancelPressed 的唯一消费者，仅在栈顶 <see cref="UIPanelBase.CanCancel"/>
    /// 时出栈一层（逐层回退）。</para>
    /// </summary>
    [DefaultExecutionOrder(+40)]   // 早于 UIManager(+50)，确保委托方解析得到本服务
    public class UINavigator : MonoBehaviour, IUINavigator
    {
        [Tooltip("UI 取消操作（ESC / 手柄 B·Circle）。本导航器为唯一订阅者，按下时关闭栈顶一层。对应 EVT_UICancelPressed。")]
        [SerializeField] private VoidEventChannelSO _onUICancelPressed;

        private readonly Stack<UIStackEntry> _stack = new();
        private readonly CompositeDisposable _subs  = new();
        private Coroutine _focusRoutine;

        public UIPanelBase Top   => _stack.Count > 0 ? _stack.Peek().Panel : null;
        public int         Depth => _stack.Count;

        public event Action StackChanged;

        // ── 生命周期 ──────────────────────────────────────────────────────────
        private void OnEnable()
        {
            ServiceLocator.Register<IUINavigator>(this);
            _onUICancelPressed?.Subscribe(HandleCancel).AddTo(_subs);
            SceneManager.sceneUnloaded += OnSceneUnloaded;
        }

        private void OnDisable()
        {
            SceneManager.sceneUnloaded -= OnSceneUnloaded;
            _subs.Clear();
            ServiceLocator.Unregister<IUINavigator>(this);
        }

        // ── 压栈 ──────────────────────────────────────────────────────────────
        public void Push(UIPanelBase panel, PushMode? mode = null)
        {
            if (panel == null) return;
            PurgeDead();

            PushMode m = mode ?? panel.DefaultMode;

            // 处理下方面板：Replace 停用、Modal 屏蔽交互（退出导航图，杜绝上下键穿透）。
            if (_stack.Count > 0)
            {
                var below = _stack.Peek().Panel;
                if (below != null)
                {
                    below.OnFocusLost();
                    if (m == PushMode.Replace) below.gameObject.SetActive(false);
                    else                       below.SetInteractableLayer(false);
                }
            }

            var entry = new UIStackEntry
            {
                Panel          = panel,
                Mode           = m,
                FocusToRestore = EventSystem.current != null ? EventSystem.current.currentSelectedGameObject : null,
                OwningScene    = panel.gameObject.scene,
            };
            _stack.Push(entry);

            panel.gameObject.SetActive(true);
            panel.SetInteractableLayer(true);
            FocusNextFrame(panel.FirstSelectableGO);

            StackChanged?.Invoke();
        }

        // ── 出栈 ──────────────────────────────────────────────────────────────
        public void Pop()
        {
            PurgeDead();
            if (_stack.Count == 0) return;

            var top = _stack.Pop();
            if (top.Panel != null) top.Panel.gameObject.SetActive(false);

            UIPanelBase below = _stack.Count > 0 ? _stack.Peek().Panel : null;
            if (below != null)
            {
                // 按 top 压栈时的 mode 还原下方面板。
                if (top.Mode == PushMode.Replace) below.gameObject.SetActive(true);
                else                              below.SetInteractableLayer(true);
                below.OnFocusGained();
            }

            // 恢复焦点到压栈前的选中项（失效则回落到下层首项）。
            GameObject restore = top.FocusToRestore != null && top.FocusToRestore.activeInHierarchy
                ? top.FocusToRestore
                : below != null ? below.FirstSelectableGO : null;
            FocusNextFrame(restore);

            StackChanged?.Invoke();
        }

        public void PopToRoot()
        {
            while (_stack.Count > 0) Pop();
        }

        // ── 取消（ESC / 手柄 B）──────────────────────────────────────────────
        private void HandleCancel()
        {
            PurgeDead();
            if (_stack.Count == 0) return;          // 栈空（如主菜单根）：无操作
            if (Top != null && !Top.CanCancel) return;
            Pop();                                   // 仅关栈顶一层
        }

        // ── 结果面板 ──────────────────────────────────────────────────────────
        public Task<T> PushForResultAsync<T>(UIResultPanel<T> panel, CancellationToken ct = default)
        {
            if (panel == null) return Task.FromResult<T>(default);
            Task<T> task = panel.BeginResult(ct);
            Push(panel, PushMode.Modal);   // 结果对话框天然模态：下层保留可见但屏蔽交互
            return task;
        }

        // ── 场景卸载清理 ──────────────────────────────────────────────────────
        private void OnSceneUnloaded(Scene s)
        {
            if (_stack.Count == 0) return;

            // Stack 无法删中间项：过滤后按原序重建（保留非本场景且未销毁的层）。
            var kept = new List<UIStackEntry>();
            foreach (var e in _stack)                // 枚举顺序：栈顶→栈底
                if (e.Panel != null && e.OwningScene != s) kept.Add(e);

            if (kept.Count == _stack.Count) return;  // 无变化

            _stack.Clear();
            for (int i = kept.Count - 1; i >= 0; i--) _stack.Push(kept[i]);
            StackChanged?.Invoke();
        }

        private void PurgeDead()
        {
            bool changed = false;
            while (_stack.Count > 0 && _stack.Peek().Panel == null) { _stack.Pop(); changed = true; }
            if (changed) StackChanged?.Invoke();
        }

        // ── 焦点（延后一帧，避开 OnEnable / UISelectionRestorer 同帧竞争）────────
        private void FocusNextFrame(GameObject target)
        {
            if (_focusRoutine != null) StopCoroutine(_focusRoutine);
            if (!isActiveAndEnabled) return;
            _focusRoutine = StartCoroutine(FocusRoutine(target));
        }

        private IEnumerator FocusRoutine(GameObject target)
        {
            yield return null;
            _focusRoutine = null;
            if (target == null || !target.activeInHierarchy) yield break;
            if (EventSystem.current != null)
                EventSystem.current.SetSelectedGameObject(target);
        }
    }
}