Adds custom IMGUI to existing host views (such as Unity's built in windows)

IMGUIInjectionProofOfConcept.cs

// MIT License
// 
// Copyright (c) 2022 Sabresaurus
// 
// Permission is hereby granted, free of charge, to any person obtaining a copy
// of this software and associated documentation files (the "Software"), to deal
// in the Software without restriction, including without limitation the rights
// to use, copy, modify, merge, publish, distribute, sublicense, and/or sell
// copies of the Software, and to permit persons to whom the Software is
// furnished to do so, subject to the following conditions:
// 
//  The above copyright notice and this permission notice shall be included in all
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// 
//  THE SOFTWARE IS PROVIDED "AS IS", WITHOUT WARRANTY OF ANY KIND, EXPRESS OR
//  IMPLIED, INCLUDING BUT NOT LIMITED TO THE WARRANTIES OF MERCHANTABILITY,
//  FITNESS FOR A PARTICULAR PURPOSE AND NONINFRINGEMENT. IN NO EVENT SHALL THE
//  AUTHORS OR COPYRIGHT HOLDERS BE LIABLE FOR ANY CLAIM, DAMAGES OR OTHER
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//  OUT OF OR IN CONNECTION WITH THE SOFTWARE OR THE USE OR OTHER DEALINGS IN THE
//  SOFTWARE.

using System;
using System.Reflection;
using UnityEditor;
using UnityEngine;
using Object = UnityEngine.Object;

public class IMGUIInjectionProofOfConcept
{
    private static readonly Assembly editorAssembly = typeof(Editor).Assembly;
    private static readonly Type hostViewType = editorAssembly.GetType("UnityEditor.HostView");
    private static readonly Type delegateType = editorAssembly.GetType("UnityEditor.HostView+EditorWindowDelegate");
    private static MethodInfo invokeOriginalMethod = delegateType.GetMethod("Invoke");

    private static object oldDelegate;

    [MenuItem("test/Attach")]
    static void Attach()
    {
        EditorWindow.GetWindow<BuildPlayerWindow>().Show();
        foreach (Object hostView in Resources.FindObjectsOfTypeAll(hostViewType))
        {
            if (hostView.ToString() == "BuildPlayerWindow (UnityEditor.DockArea)")
            {
                FieldInfo fieldInfo = hostView.GetType().GetField("m_OnGUI", BindingFlags.Instance | BindingFlags.NonPublic);

                oldDelegate = fieldInfo.GetValue(hostView);

                Delegate newDelegate = Delegate.CreateDelegate(delegateType, typeof(IMGUIInjectionProofOfConcept).GetMethod(nameof(OnGUI), BindingFlags.Static | BindingFlags.NonPublic));
                fieldInfo.SetValue(hostView, newDelegate);
            }
        }
    }

    static void OnGUI()
    {
        EditorGUILayout.HelpBox("hello", MessageType.Info);

        invokeOriginalMethod.Invoke(oldDelegate, null);

        EditorGUILayout.HelpBox("world", MessageType.Info);
    }
}