DraggableUIState.cs

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using Microsoft.Xna.Framework;
using Microsoft.Xna.Framework.Graphics;
using System.Collections.Generic;
using Terraria;
using Terraria.UI;

namespace Loot.UI.Common.Core
{
    internal abstract class DraggableUIState : VisibilityUI
    {
        private int _dragging;

        private readonly List<(UIElement, UIElement)> _draggingList = new List<(UIElement, UIElement)>();
        private readonly Dictionary<UIElement, Vector2> _offsets = new Dictionary<UIElement, Vector2>();

        protected void AddDragging(UIElement dragger, UIElement draggable = null)
        {
            draggable = draggable ?? dragger;
            _draggingList.Add((dragger, draggable));
            if (!_offsets.ContainsKey(dragger))
            {
                _offsets.Add(dragger, Vector2.Zero);
            }

            dragger.OnMouseDown += (evt, element) =>
            {
                //start
                _offsets[dragger] = new Vector2(evt.MousePosition.X - draggable.Left.Pixels, evt.MousePosition.Y - draggable.Top.Pixels);
                _dragging++;
            };

            dragger.OnMouseUp += (evt, element) =>
            {
                //end
                Vector2 end = evt.MousePosition;
                _dragging--;

                draggable.Left.Set(end.X - _offsets[dragger].X, 0f);
                draggable.Top.Set(end.Y - _offsets[dragger].Y, 0f);

                dragger.Recalculate();
                draggable.Recalculate();
            };
        }

        protected override void DrawSelf(SpriteBatch spriteBatch)
        {
            Main.LocalPlayer.mouseInterface = IsMouseHovering;

            if (_dragging <= 0)
            {
                return;
            }

            foreach (var (dragger, draggable) in _draggingList)
            {
                draggable.Left.Set(Main.MouseScreen.X - _offsets[dragger].X, 0f);
                draggable.Top.Set(Main.MouseScreen.Y - _offsets[dragger].Y, 0f);

                dragger.Recalculate();
                draggable.Recalculate();
            }
        }
    }
}