using UnityEngine;
using UnityEngine.EventSystems;

namespace DIG.UIExpansion
{
    /// <summary>
    /// Used for draging UI object inside the bounds of a canvas
    /// </summary>
    public class SnapWindowDrag : MonoBehaviour, IBeginDragHandler, IDragHandler
    {
        private RectTransform canvasRectTransform;
        private Vector2 pointerOffset;
        private RectTransform panelRectTransform;
        private bool clampedToLeft;
        private bool clampedToRight;
        private bool clampedToTop;
        private bool clampedToBottom;

        private void Awake()
        {
            Canvas canvas = GetComponentInParent<Canvas>();
            if (canvas != null)
            {
                canvasRectTransform = canvas.transform as RectTransform;
                panelRectTransform = transform as RectTransform;
            }
            clampedToLeft = false;
            clampedToRight = false;
            clampedToTop = false;
            clampedToBottom = false;
        }

        public void OnBeginDrag(PointerEventData eventData)
        {
            RectTransformUtility.ScreenPointToLocalPointInRectangle(panelRectTransform, eventData.position, eventData.pressEventCamera, out pointerOffset);
        }

        public void OnDrag(PointerEventData eventData)
        {
            if (panelRectTransform == null)
            {
                return;
            }
            Vector2 localPointerPosition;
            if (RectTransformUtility.ScreenPointToLocalPointInRectangle(canvasRectTransform, eventData.position, eventData.pressEventCamera, out localPointerPosition))
            {
                panelRectTransform.localPosition = localPointerPosition - pointerOffset;
                ClampToWindow();
                Vector2 clampedPosition = panelRectTransform.localPosition;
                if (clampedToRight)
                {
                    clampedPosition.x = (canvasRectTransform.rect.width * 0.5f) - (panelRectTransform.rect.width * (1 - panelRectTransform.pivot.x));
                }
                else if (clampedToLeft)
                {
                    clampedPosition.x = (-canvasRectTransform.rect.width * 0.5f) + (panelRectTransform.rect.width * panelRectTransform.pivot.x);
                }

                if (clampedToTop)
                {
                    clampedPosition.y = (canvasRectTransform.rect.height * 0.5f) - (panelRectTransform.rect.height * (1 - panelRectTransform.pivot.y));
                }
                else if (clampedToBottom)
                {
                    clampedPosition.y = (-canvasRectTransform.rect.height * 0.5f) + (panelRectTransform.rect.height * panelRectTransform.pivot.y);
                }
                panelRectTransform.localPosition = clampedPosition;
            }
        }


        void ClampToWindow()
        {
            Vector3[] canvasCorners = new Vector3[4];
            Vector3[] panelRectCorners = new Vector3[4];
            canvasRectTransform.GetWorldCorners(canvasCorners);
            panelRectTransform.GetWorldCorners(panelRectCorners);

            if (panelRectCorners[2].x > canvasCorners[2].x)
            {
                if (!clampedToRight)
                {
                    clampedToRight = true;
                }
            }
            else if (clampedToRight)
            {
                clampedToRight = false;
            }
            else if (panelRectCorners[0].x < canvasCorners[0].x)
            {
                if (!clampedToLeft)
                {
                    clampedToLeft = true;
                }
            }
            else if (clampedToLeft)
            {
                clampedToLeft = false;
            }

            if (panelRectCorners[2].y > canvasCorners[2].y)
            {
                if (!clampedToTop)
                {
                    clampedToTop = true;
                }
            }
            else if (clampedToTop)
            {
                clampedToTop = false;
            }
            else if (panelRectCorners[0].y < canvasCorners[0].y)
            {
                if (!clampedToBottom)
                {
                    clampedToBottom = true;
                }
            }
            else if (clampedToBottom)
            {
                clampedToBottom = false;
            }
        }
    }
}