Undo Code

http://answers.unity3d.com/questions/50954/how-to-correctly-register-undos-in-custom-inspecto.html

	private bool listeningForGuiChanges;
	private bool guiChanged;
	public TargetType thisTarget;

	void OnEnable () {
		thisTarget = target as TargetType;
		// The rest of the OnEnable code...
	}

	public override void OnInspectorGUI () {
		CheckUndo();
		// The rest of the EditorGUI code...
		if (GUILayout.Button( "SomeButton" ) ) {
			guiChanged = true;
			DoSomething();
		}
		if ( GUI.changed ) {
			guiChanged = true;
		}
	}

	private void CheckUndo() {
		//	Modified from code found here:
		//	http://answers.unity3d.com/questions/50954/how-to-correctly-register-undos-in-custom-inspecto.html
		Event e = Event.current;

		if (e.type == EventType.MouseDown && e.button == 0 || e.type == EventType.KeyUp && (e.keyCode == KeyCode.Tab)) {
			// When the LMB is pressed or the TAB key is released,
			// store a snapshot, but don't register it as an undo
			// (so that if nothing changes we avoid storing a useless undo)
//			Debug.Log("PREPARE UNDO SNAPSHOT");
			Undo.SetSnapshotTarget(thisTarget, "Message Displays");
			Undo.CreateSnapshot();
			Undo.ClearSnapshotTarget();
			listeningForGuiChanges = true;
			guiChanged = false;
		}

		if (listeningForGuiChanges && guiChanged) {
			// Some GUI value changed after pressing the mouse.
			// Register the previous snapshot as a valid undo.
//			Debug.Log("REGISTER UNDO");
			Undo.SetSnapshotTarget(thisTarget, "Message Displays");
			Undo.RegisterSnapshot();
			Undo.ClearSnapshotTarget();
			listeningForGuiChanges = false;
		}
	}


NOTES:

Add this to the end of a custom editor.

Make sure there is a member variable for thisTarget..

Make sure target code is in OnEnable()

Make sure CheckUndo(); is at the top of the OnInspectorGUI() method

Make sure you have: guiChange = true; as part of a button.

Make sure you have if (Gui.Changed) {guiChanged = true;} at the end of OnInspectorGUI;

Paste CheckUndo method at the end of the custom editor script.