using System;
using System.Collections;
using UnityEngine;
using UnityEngine.EventSystems;
using UnityEngine.UI;

/// <summary>
/// 滑动列表扩展(吸附功能)
/// </summary>
public sealed class ScrollRect_Adsorb : MonoBehaviour, IBeginDragHandler, IEndDragHandler
{
    /// <summary>
    /// 吸附的速度阈值
    /// </summary>
    private const float SNAP_VELOCITY_THRESHOLD = 50f;
    /// <summary>
    /// 吸附的平滑速度
    /// </summary>
    private const float SNAP_SPEED = 50f;

    /// <summary>
    /// 吸附目标
    /// </summary>
    public Transform TsSnapTarget;
    private ScrollRect scrollRect;
    private RectTransform rtsContentPanel;
    /// <summary>
    /// 当前拖动状态
    /// </summary>
    private bool isDragging = false;
    /// <summary>
    /// 是否正在吸附
    /// </summary>
    private bool isSnapping = false;
    private bool isSnap = false;
    private int curIndex;

    /// <summary>
    /// 索引变化
    /// </summary>
    public Action<int> IndexChange;

    private void Awake()
    {
        scrollRect = GetComponent<ScrollRect>();
        rtsContentPanel = scrollRect.content;

        isDragging = false;
        curIndex = 0;
    }

    private void Update()
    {
        CheckChange();

        // 如果用户没有在拖动，且滚动速度已低于阈值，则开始吸附
        if (isSnap &&!isDragging && !isSnapping && scrollRect.velocity != Vector2.zero && Mathf.Abs(scrollRect.velocity.y) < SNAP_VELOCITY_THRESHOLD)
        {
            isSnap = false;
            if (Vector2.zero != scrollRect.velocity)
                scrollRect.velocity = Vector2.zero; // 立即停止惯性滚动

            StartCoroutine(SnapToPosition());
        }
        // 如果用户开始拖动，则停止当前的吸附过程
        else if (isDragging && isSnapping)
        {
            StopAllCoroutines();
            isSnapping = false;
        }
    }

    #region API
    /// <summary>
    /// 将指定索引的 cell 移动到 TsSnapTarget 的位置（平滑过渡）
    /// </summary>
    /// <param name="index">cell 的索引</param>
    public void SnapToIndex(int index)
    {
        if (null == rtsContentPanel)
            return;

        if (null == TsSnapTarget)
            return;

        if (index < 0 || index >= rtsContentPanel.childCount)
            return;

        if (isSnapping)
        {
            StopAllCoroutines();
            isSnapping = false;
        }

        StartCoroutine(SnapToIndexCoroutine(index));
    }
    #endregion

    private void CheckChange()
    {
        if (Vector2.zero == scrollRect.velocity)
            return;

        if (null == TsSnapTarget)
            return;

        Transform tsSelect = null;
        float minYDis = float.MaxValue;
        int index = -1;

        float targetLocalY = scrollRect.viewport.InverseTransformPoint(TsSnapTarget.position).y;
        for (int i = 0; i < rtsContentPanel.childCount; ++i)
        {
            Transform tsCell = rtsContentPanel.GetChild(i);
            float cellLocalY = scrollRect.viewport.InverseTransformPoint(tsCell.position).y;
            float dis = cellLocalY - targetLocalY;
            if (Mathf.Abs(dis) < Mathf.Abs(minYDis))
            {
                minYDis = dis;
                tsSelect = tsCell;
                index = i;
            }
        }

        if (null != tsSelect)
        {
            curIndex = index;
            IndexChange?.Invoke(curIndex);
        }
    }

    private IEnumerator SnapToIndexCoroutine(int index)
    {
        isSnapping = true;

        Transform tsCell = rtsContentPanel.GetChild(index);
        float targetLocalY = scrollRect.viewport.InverseTransformPoint(TsSnapTarget.position).y;
        float cellLocalY = scrollRect.viewport.InverseTransformPoint(tsCell.position).y;
        float dis = cellLocalY - targetLocalY;
        float dt = 0f;
        Vector2 targetPosition = rtsContentPanel.anchoredPosition + new Vector2(0f, -dis);
        while (dt < 0.2f && Vector2.Distance(rtsContentPanel.anchoredPosition, targetPosition) > 0.1f)
        {
            dt += Time.deltaTime;
            rtsContentPanel.anchoredPosition = Vector2.Lerp(rtsContentPanel.anchoredPosition, targetPosition, Time.deltaTime * SNAP_SPEED);
            yield return null;
        }
        rtsContentPanel.anchoredPosition = targetPosition;

        isSnapping = false;
    }

    void IBeginDragHandler.OnBeginDrag(PointerEventData eventData)
    {
        isDragging = true;
        isSnap = true;
    }

    void IEndDragHandler.OnEndDrag(PointerEventData eventData)
    {
        isDragging = false;
    }

    IEnumerator SnapToPosition()
    {
        isSnapping = true;

        if (null == TsSnapTarget)
        {
            isSnapping = false;
            yield break;
        }

        Transform tsSelect = null;
        float minYDis = float.MaxValue;
        float targetLocalY = scrollRect.viewport.InverseTransformPoint(TsSnapTarget.position).y;
        for (int i = 0; i < rtsContentPanel.childCount; ++i)
        {
            Transform tsCell = rtsContentPanel.GetChild(i);
            float cellLocalY = scrollRect.viewport.InverseTransformPoint(tsCell.position).y;
            float dis = cellLocalY - targetLocalY;
            if (Mathf.Abs(dis) < Mathf.Abs(minYDis))
            {
                minYDis = dis;
                tsSelect = tsCell;
            }
        }

        if (null != tsSelect)
        {
            float dt = 0f;
            Vector2 targetPosition = rtsContentPanel.anchoredPosition + new Vector2(0f, -minYDis);
            while (dt < 0.2f && Vector2.Distance(rtsContentPanel.anchoredPosition, targetPosition) > 0.1f)
            {
                dt += Time.deltaTime;
                rtsContentPanel.anchoredPosition = Vector2.Lerp(rtsContentPanel.anchoredPosition, targetPosition, Time.deltaTime * SNAP_SPEED);
                yield return null;
            }
            rtsContentPanel.anchoredPosition = targetPosition;
        }

        isSnapping = false;
    }
}