fungus/Assets/Fungus/Scripts/Editor/EditorZoomArea.cs

// This code is part of the Fungus library (http://fungusgames.com) maintained by Chris Gregan (http://twitter.com/gofungus).
// It is released for free under the MIT open source license (https://github.com/snozbot/fungus/blob/master/LICENSE)

// Original code by Martin Ecker (http://martinecker.com)

using UnityEngine;

namespace Fungus.EditorUtils
{
    // Helper Rect extension methods
    public static class RectExtensions
    {
        public static Vector2 TopLeft(this Rect rect)
        {
            return new Vector2(rect.xMin, rect.yMin);
        }

        public static Rect ScaleSizeBy(this Rect rect, float scale)
        {
            return rect.ScaleSizeBy(scale, rect.center);
        }

        public static Rect ScaleSizeBy(this Rect rect, float scale, Vector2 pivotPoint)
        {
            Rect result = rect;
            result.x -= pivotPoint.x;
            result.y -= pivotPoint.y;
            result.xMin *= scale;
            result.xMax *= scale;
            result.yMin *= scale;
            result.yMax *= scale;
            result.x += pivotPoint.x;
            result.y += pivotPoint.y;
            return result;
        }

        public static Rect ScaleSizeBy(this Rect rect, Vector2 scale)
        {
            return rect.ScaleSizeBy(scale, rect.center);
        }

        public static Rect ScaleSizeBy(this Rect rect, Vector2 scale, Vector2 pivotPoint)
        {
            Rect result = rect;
            result.x -= pivotPoint.x;
            result.y -= pivotPoint.y;
            result.xMin *= scale.x;
            result.xMax *= scale.x;
            result.yMin *= scale.y;
            result.yMax *= scale.y;
            result.x += pivotPoint.x;
            result.y += pivotPoint.y;
            return result;
        }
    }

    public class EditorZoomArea
    {
        private const float kEditorWindowTabHeight = 21.0f;
        private static Matrix4x4 _prevGuiMatrix;
        
        public static Rect Begin(float zoomScale, Rect screenCoordsArea)
        {
            GUI.EndGroup();        // End the group Unity begins automatically for an EditorWindow to clip out the window tab. This allows us to draw outside of the size of the EditorWindow.
            
            Rect clippedArea = screenCoordsArea.ScaleSizeBy(1.0f / zoomScale, screenCoordsArea.TopLeft());
            clippedArea.y += kEditorWindowTabHeight;
            GUI.BeginGroup(clippedArea);
            
            _prevGuiMatrix = GUI.matrix;

            Matrix4x4 translation = Matrix4x4.TRS(clippedArea.TopLeft(), Quaternion.identity, Vector3.one);
            Matrix4x4 scale = Matrix4x4.Scale(new Vector3(zoomScale, zoomScale, 1.0f));
            GUI.matrix = translation * scale * translation.inverse * GUI.matrix;
            

            return clippedArea;
        }
        
        public static void End()
        {
            GUI.matrix = _prevGuiMatrix;
            GUI.EndGroup();
            GUI.BeginGroup(new Rect(0.0f, kEditorWindowTabHeight, Screen.width, Screen.height));
        }
    }
}