ImGui up and running

project_name/ImGui/ImGuiFinalRenderDelegate.cs

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+using System;
+using System.Reflection;
+using ImGuiNET;
+using ImGuiNET.SampleProgram.XNA;
+using Microsoft.Xna.Framework;
+using Microsoft.Xna.Framework.Graphics;
+
+namespace Nez
+{
+	public class ImGuiFinalRenderDelegate : IFinalRenderDelegate
+	{
+		public Scene scene { get; set; }
+
+        ImGuiRenderer _imGuiRenderer;
+        RenderTarget2D _lastRenderTarget;
+        IntPtr _renderTargetId;
+
+        public ImGuiFinalRenderDelegate()
+        {
+            var core = typeof(Core).GetField("_instance", BindingFlags.Static | BindingFlags.NonPublic).GetValue(null) as Core;
+            _imGuiRenderer = new ImGuiRenderer(core);
+            _imGuiRenderer.RebuildFontAtlas();
+        }
+
+		public void handleFinalRender( Color letterboxColor, RenderTarget2D source, Rectangle finalRenderDestinationRect, SamplerState samplerState )
+		{
+            if(_lastRenderTarget != source)
+            {
+                // unbind the old texture if we had one
+                if(_lastRenderTarget != null)
+                    _imGuiRenderer.UnbindTexture(_renderTargetId);
+
+                // bind the new texture
+                _lastRenderTarget = source;
+                _renderTargetId = _imGuiRenderer.BindTexture(source);
+            }
+
+            Core.graphicsDevice.setRenderTarget( null );
+            Core.graphicsDevice.Clear( letterboxColor );
+
+
+            _imGuiRenderer.BeforeLayout(new GameTime(TimeSpan.FromDays(0), TimeSpan.FromMilliseconds(Time.deltaTime * 1000)));
+            layoutGui();
+            _imGuiRenderer.AfterLayout();
+		}
+
+        void layoutGui()
+        {
+            ImGui.ShowDemoWindow();
+
+            var maxSize = new System.Numerics.Vector2(_lastRenderTarget.Width, _lastRenderTarget.Height);
+            var minSize = maxSize / 4;
+            unsafe
+            {
+                ImGui.SetNextWindowSizeConstraints(minSize, maxSize, data =>
+                {
+                    var size = (*data).CurrentSize;
+                    var ratio = size.X / _lastRenderTarget.Width;
+                    (*data).DesiredSize.Y = ratio * _lastRenderTarget.Height;
+                });
+            }
+
+            ImGui.SetNextWindowPos(new System.Numerics.Vector2(0, 0), ImGuiCond.FirstUseEver);
+            ImGui.Begin("Game Window");
+            ImGui.Image(_renderTargetId, ImGui.GetContentRegionAvail());
+            ImGui.End();
+        }
+
+		public void onAddedToScene()
+		{}
+
+		public void onSceneBackBufferSizeChanged( int newWidth, int newHeight )
+		{}
+
+		public void unload()
+		{}
+	}
+}