ISerializationCallbackReceiverを使用したバージョン

EditorWindowTest.cs

using UnityEngine;
using UnityEditor;

public class EditorWindowTest : EditorWindow ,ISerializationCallbackReceiver
{
	public static string message;

	[MenuItem("Window/Show")]
	static void Init()
	{
		var window = EditorWindow.CreateInstance<EditorWindowTest> ();
		EditorWindowTest.message = "C";
		window.Show ();
	}
	 
	void OnGUI()
	{
		GUILayout.Label (EditorWindowTest.message);
		if (GUILayout.Button ("PUSH")) { EditorWindowTest.message += "#"; }
	}

	public void OnBeforeSerialize ()
	{
		using (System.IO.StreamWriter writer = new System.IO.StreamWriter ("Temp/EditorWindowTest")) {
			writer.WriteLine (EditorWindowTest.message);
		}
	}

	public void OnAfterDeserialize ()
	{
		using (System.IO.StreamReader reader = new System.IO.StreamReader ("Temp/EditorWindowTest")) {
			EditorWindowTest.message = reader.ReadLine ();
		}
	}
}