The OnUndo field is an event handler that is triggered whenever an undo operation is executed within the UndoSystem. This field is of type System.Action<Sandbox.Helpers.UndoSystem.Entry>, allowing you to attach custom logic that should be executed when an undo action occurs.

To use the OnUndo field, you can subscribe to it with a method that matches the signature of System.Action<Sandbox.Helpers.UndoSystem.Entry>. This method will be called with the Entry object that represents the undo action being performed.

Example usage:

UndoSystem undoSystem = new UndoSystem();
undoSystem.OnUndo += entry => {
    // Custom logic to execute when an undo is performed
    Console.WriteLine($"Undo action: {entry.Title}");
};