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

/* Snap a scroll rect to its children items. All self contained.
 * Note: Supports 1 direction only.
 */

namespace eToile_example
{
    [RequireComponent(typeof(ScrollRect))]

    public class DragSnapper : UIBehaviour, IEndDragHandler, IBeginDragHandler
    {
        public SnapDirection direction;                                                     // The scroll direction.
        public AnimationCurve decelerationCurve = AnimationCurve.Linear(0f, 0f, 1f, 1f);    // Transition curve (easing).
        public float speed = 0.5f;                                                          // The speed in which we snap.
        public float snapPercentage = 0.2f;                                                 // Percent to snal to next streen.
        public int startItem = 0;                                                           // First item to show.

        ScrollRect scrollRect;
        int itemCount;                                                                      // Item count in our scroll rect (2 minimum)
        float value;                                                                        // Last valid snapped position.
        int target;                                                                         // Last target snap position.

        new void Start()
        {
            itemCount = transform.Find("Container").childCount;
            // Set first item view:
            scrollRect = gameObject.GetComponent<ScrollRect>();
            target = startItem;
            scrollRect.normalizedPosition = new Vector2(startItem / (float)itemCount, 0f);
            value = scrollRect.normalizedPosition.x;
            // Set ScrollRect scroll direction:
            if (direction == SnapDirection.Horizontal)
            {
                scrollRect.horizontal = true;
                scrollRect.vertical = false;
            }
            else if (direction == SnapDirection.Vertical)
            {
                scrollRect.horizontal = false;
                scrollRect.vertical = true;
            }
        }

        // The direction we are snapping in:
        public enum SnapDirection
        {
            Horizontal,
            Vertical,
        }

        public void OnBeginDrag(PointerEventData eventData)
        {
            StopCoroutine(SnapRect());                                                      // If we are snapping, stop for the next input.
        }

        public void OnEndDrag(PointerEventData eventData)
        {
            StartCoroutine(SnapRect());
        }

        private IEnumerator SnapRect()
        {
            if (itemCount < 2) print("Item count must be 2 or more");

            float delta = 1f / (itemCount - 1);                                             // Item percentage (just in case they change dynamically).
            float startNormal = direction == SnapDirection.Horizontal ? scrollRect.horizontalNormalizedPosition : scrollRect.verticalNormalizedPosition;
            float tempDelta = (startNormal - value) / delta;                                // Displacement percentage.

            // Closest item depending on the current position:
            target = Mathf.RoundToInt(value / delta);
            if (tempDelta > snapPercentage) target++;
            else if (tempDelta < -snapPercentage) target--;
            target = Mathf.Clamp(target, 0, itemCount - 1);

            float endNormal = delta * target;                                               // Normalized value for the target item.
            float duration = Mathf.Abs((endNormal - startNormal) / speed);                  // Time needed to get the target depending on speed.

            float timer = 0f;
            while (timer < 1f)                                                              // Animate deceleration.
            {
                timer = Mathf.Min(1f, timer + Time.deltaTime / duration);                   // Calculate our timer based on our speed.
                value = Mathf.Lerp(startNormal, endNormal, decelerationCurve.Evaluate(timer));  // Our value based on the animation curve.

                if (direction == SnapDirection.Horizontal)                                  // Depending on direction we set our horizontal or vertical position.
                    scrollRect.horizontalNormalizedPosition = value;
                else
                    scrollRect.verticalNormalizedPosition = value;

                yield return new WaitForEndOfFrame();                                       // Wait until next frame.
            }
        }
    }
}