using System;
using System.Collections;
using System.Collections.Generic;
using UnityEngine;
using UnityEngine.SceneManagement;
using UnityEngine.UI;

namespace A2W
{
    public class SceneLoader : Singleton<SceneLoader>
    {
        ITransition currentTransition; // 当前的过场实例
        Canvas canvas;

        public bool isPreLoading { get; set; } = false; // 预加载中

        public Action OnFinished; // 转场结束

        bool isLoading = false;// 现在是否可以转场
        WaitForSeconds leastTime;


        public void Init()
        {
            GameObject go = new GameObject();
            go.name = "LoadingCanvas";
            go.transform.SetParent(transform, false);

            canvas = go.AddComponent<Canvas>();
            canvas.renderMode = RenderMode.ScreenSpaceOverlay;
            canvas.sortingOrder = 1000;

            CanvasScaler canvasScaler = go.AddComponent<CanvasScaler>();
            canvasScaler.uiScaleMode = CanvasScaler.ScaleMode.ScaleWithScreenSize;
            canvasScaler.referenceResolution = new Vector2(Screen.width, Screen.height);
        }


        /// <summary>
        /// 使用转场加载场景
        /// </summary>
        /// <param name="SceneName">场景名称</param>
        /// <param name="transition">转场方式</param>
        /// <param name="hasPreload">是否有预加载</param>
        /// <param name="leastTime">至少等待一定时间</param>
        public void LoadScene(string SceneName, ITransition transition = null, bool hasPreload = false, float leastSecond = 0.2f)
        {
            // 如果现在不能转换场景直接返回
            if (isLoading) return;

            if (canvas is null)
            {
                Init();
            }

            isPreLoading = hasPreload;
            leastTime = new WaitForSeconds(leastSecond);

            currentTransition = transition;

            // 如果有设置过场就设置过场
            if (transition is not null)
            {
                currentTransition.Init(canvas.transform);
            }
                
            // 开始转换场景
            StartCoroutine(LoadLevel(SceneName));
        }

        /// <summary>
        /// 转换场景并且使用过场动画
        /// </summary>
        /// <param name="levelName">场景名称</param>
        /// <returns></returns>
        private IEnumerator LoadLevel(string levelName)
        {
            // 异步加载场景
            AsyncOperation loading = SceneManager.LoadSceneAsync(levelName);

            // 不允许场景加载完后直接转换
            loading.allowSceneActivation = false;

            // 现在不再能转换场景
            isLoading = true;


            if (currentTransition is not null)
            {
                // 开始过场
                currentTransition.Begin();

                // 等待一帧
                // 理由再下面有解释，但其实这里本来不需要，因为检查动画前还夹着一个检查加载的过程。基本不会在一帧内就加载完
                // 但是保险起见还是在播放动画后延迟一帧
                yield return null;

                // 至少等待一定时间
                yield return leastTime;


                // 等待动画播放完成
                while (!currentTransition.IsDone())
                    yield return null;
            }

            // 等待场景加载几乎完成
            while (loading.progress < 0.899)
                yield return null;

            // 允许场景加载完成
            loading.allowSceneActivation = true;

            // 等待场景加载彻底完成
            while (loading.progress != 1)
                yield return null;

            //等待预加载完毕
            while (isPreLoading)
                yield return null;

            if (currentTransition is not null)
            {
                // 结束过场
                currentTransition.Finish();

                // 等待一帧
                // 因为我发现如果在开始动画后不等待一帧的话，第二个动画其实还没开始播放，
                // 后面检测动画完成检测的就是第一个动画，就起不到检测第二个动画的作用。
                yield return null;

                // 至少等待一定时间
                yield return leastTime;

                // 等待动画播放完成
                while (!currentTransition.IsDone())
                    yield return null;
            }

            // 可以继续转换场景
            isLoading = false;
            OnFinished?.Invoke();
        }
    }
}