using System;
using UnityEngine;
using UnityEngine.EventSystems;

/// <summary>
/// Turns a pointer press/drag on a UI graphic into a normalized (x, y) position in
/// [0,1] within its RectTransform (origin bottom-left). Added in code to the colour
/// picker's saturation/value field and hue bar so they react to both clicking and
/// dragging. The graphic on the same object must be a raycast target.
/// </summary>
[RequireComponent(typeof(RectTransform))]
public class UiPointerSurface : MonoBehaviour, IPointerDownHandler, IDragHandler
{
    /// <summary>Fired on press and while dragging with the normalized (x, y) in [0,1].</summary>
    public event Action<Vector2> Picked;

    private RectTransform _rect;

    private void Awake() => _rect = (RectTransform)transform;

    public void OnPointerDown(PointerEventData eventData) => Report(eventData);
    public void OnDrag(PointerEventData eventData) => Report(eventData);

    private void Report(PointerEventData eventData)
    {
        // pressEventCamera is null for a Screen Space - Overlay canvas, which is the
        // correct camera argument there.
        if (!RectTransformUtility.ScreenPointToLocalPointInRectangle(
                _rect, eventData.position, eventData.pressEventCamera, out Vector2 local))
            return;

        Rect r = _rect.rect;
        if (r.width <= 0f || r.height <= 0f) return;

        float x = Mathf.Clamp01((local.x - r.xMin) / r.width);
        float y = Mathf.Clamp01((local.y - r.yMin) / r.height);
        Picked?.Invoke(new Vector2(x, y));
    }
}