Rijam_RijamsMod/UI/DragableUIPanel.cs
using Microsoft.Xna.Framework;
using Terraria;
using Terraria.GameContent.UI.Elements;
using Terraria.UI;

namespace RijamsMod.UI
{
	// This DragableUIPanel class inherits from UIPanel. 
	// Inheriting is a great tool for UI design. By inheriting, we get the background drawing for free from UIPanel
	// We've added some code to allow the panel to be dragged around. 
	// We've also added some code to ensure that the panel will bounce back into bounds if it is dragged outside or the screen resizes.
	// UIPanel does not prevent the player from using items when the mouse is clicked, so we've added that as well.
	internal class DragableUIPanel : UIPanel
	{
		// Stores the offset from the top left of the UIPanel while dragging.
		private Vector2 offset;
		public bool dragging;

		public override void LeftMouseDown(UIMouseEvent evt)
		{
			base.LeftMouseDown(evt);
			DragStart(evt);
		}

		public override void LeftMouseUp(UIMouseEvent evt)
		{
			base.LeftMouseUp(evt);
			DragEnd(evt);
		}

		private void DragStart(UIMouseEvent evt)
		{
			offset = new Vector2(evt.MousePosition.X - Left.Pixels, evt.MousePosition.Y - Top.Pixels);
			dragging = true;
		}

		private void DragEnd(UIMouseEvent evt)
		{
			Vector2 end = evt.MousePosition;
			dragging = false;

			Left.Set(end.X - offset.X, 0f);
			Top.Set(end.Y - offset.Y, 0f);

			Recalculate();
		}

		public override void Update(GameTime gameTime) {
			base.Update(gameTime); // don't remove.

			// Checking ContainsPoint and then setting mouseInterface to true is very common. This causes clicks on this UIElement to not cause the player to use current items. 
			if (ContainsPoint(Main.MouseScreen))
			{
				Main.LocalPlayer.mouseInterface = true;
			}

			if (dragging)
			{
				Left.Set(Main.mouseX - offset.X, 0f); // Main.MouseScreen.X and Main.mouseX are the same.
				Top.Set(Main.mouseY - offset.Y, 0f);
				Recalculate();
			}

			// Here we check if the DragableUIPanel is outside the Parent UIElement rectangle. 
			// (In our example, the parent would be ExampleUI, a UIState. This means that we are checking that the DragableUIPanel is outside the whole screen)
			// By doing this and some simple math, we can snap the panel back on screen if the user resizes his window or otherwise changes resolution.
			var parentSpace = Parent.GetDimensions().ToRectangle();
			if (!GetDimensions().ToRectangle().Intersects(parentSpace))
			{
				Left.Pixels = Utils.Clamp(Left.Pixels, 0, parentSpace.Right - Width.Pixels);
				Top.Pixels = Utils.Clamp(Top.Pixels, 0, parentSpace.Bottom - Height.Pixels);
				// Recalculate forces the UI system to do the positioning math again.
				Recalculate();
			}
		}
		public Vector2 GetOffset()
		{
			return offset;
		}
	}
}
