using UnityEngine;
using UnityEngine.UI;
using UnityEngine.EventSystems;

/// <summary>
/// Makes a UI panel draggable. Attach to the panel itself: it follows the pointer from any of the panel's
/// non-interactive surface (title bar, padding, labels, background) but ignores presses that begin on a
/// widget - a <see cref="Selectable"/> (toggle, slider, ...) or a <see cref="UiPointerSurface"/> (the
/// colour-picker field) - so those keep their own click/drag behaviour. It keeps a slice of the panel on
/// screen and raises it above its siblings while interacting.
///
/// Works under any canvas scale (it tracks the pointer in the drag area's local space and applies the
/// movement delta) and under any anchor/pivot (the on-screen clamp is derived from the panel's real
/// corners in its parent, not from a fixed top-left assumption).
/// </summary>
[RequireComponent(typeof(RectTransform))]
public class DraggablePanel : MonoBehaviour, IPointerDownHandler, IBeginDragHandler, IDragHandler
{
    [Tooltip("Panel moved by the drag. Empty = this object's RectTransform.")]
    public RectTransform target;

    [Tooltip("Panel pixels (canvas units) kept on screen on every edge so it can never be lost.")]
    public float keepVisible = 48f;

    private RectTransform _area; // the panel's parent; pointer positions are tracked in its local space
    private Vector2 _grabOffset;
    private bool _dragging;

    private void Awake() => Resolve();

    private void Resolve()
    {
        if (!target) target = transform as RectTransform;
        if (target) _area = target.parent as RectTransform;
    }

    public void OnPointerDown(PointerEventData eventData)
    {
        // Raise the panel when pressing its own surface. (A press that lands on a widget is consumed by
        // that widget's own handler and never reaches us, so this rarely needs the guard - but it is cheap.)
        if (!StartedOnWidget(eventData)) BringToFront();
    }

    public void OnBeginDrag(PointerEventData eventData)
    {
        _dragging = false;
        if (!target) Resolve();
        if (!target || !_area) return;
        if (StartedOnWidget(eventData)) return; // a toggle/slider/picker owns this gesture, not the panel

        _dragging = true;
        BringToFront();
        // Capture grab offset; the constant frame difference between the area pivot and the panel anchor
        // cancels out because we only ever apply the pointer delta on top of this offset.
        if (RectTransformUtility.ScreenPointToLocalPointInRectangle(
                _area, eventData.position, eventData.pressEventCamera, out var local))
            _grabOffset = target.anchoredPosition - local;
    }

    public void OnDrag(PointerEventData eventData)
    {
        if (!_dragging || !target || !_area) return;
        if (!RectTransformUtility.ScreenPointToLocalPointInRectangle(
                _area, eventData.position, eventData.pressEventCamera, out var local))
            return;
        target.anchoredPosition = Clamp(local + _grabOffset);
    }

    // True when the press started on an interactive widget inside the panel (which then owns the gesture).
    private bool StartedOnWidget(PointerEventData eventData)
    {
        var hit = eventData.pointerPressRaycast.gameObject;
        for (var t = hit ? hit.transform : null; t && t != target; t = t.parent)
            if (t.GetComponent<Selectable>() || t.GetComponent<UiPointerSurface>())
                return true;
        return false;
    }

    private void BringToFront()
    {
        if (target) target.SetAsLastSibling();
    }

    // Anchor/pivot-agnostic clamp: keep at least `keepVisible` px of the panel inside the parent on every
    // edge, computed from the panel's real corners so it is correct wherever the panel is anchored.
    private Vector2 Clamp(Vector2 pos)
    {
        if (!_area) return pos;
        Rect area = _area.rect;
        Rect size = target.rect;
        Vector2 pivot = target.pivot;

        // Anchor reference point in the area's local space (panels are point-anchored: anchorMin == anchorMax).
        Vector2 anchorRef = new Vector2(
            area.xMin + target.anchorMin.x * area.width,
            area.yMin + target.anchorMin.y * area.height);
        Vector2 pivotToCorner = new Vector2(pivot.x * size.width, pivot.y * size.height);

        // Bottom-left corner of the panel for this anchoredPosition.
        Vector2 corner = anchorRef + pos - pivotToCorner;

        float keepX = Mathf.Min(keepVisible, size.width);
        float keepY = Mathf.Min(keepVisible, size.height);
        corner.x = Mathf.Clamp(corner.x, area.xMin + keepX - size.width, area.xMax - keepX);
        corner.y = Mathf.Clamp(corner.y, area.yMin + keepY - size.height, area.yMax - keepY);

        return corner + pivotToCorner - anchorRef;
    }
}