The OnItemAdded property in the SelectionSystem class is an event handler that is triggered whenever an item is added to the selection system. It is a public instance property of type System.Action<System.Object>, allowing you to assign a delegate that will be called with the added item as its parameter.

To use the OnItemAdded property, assign a method that matches the Action<Object> delegate signature. This method will be invoked whenever an item is added to the selection system.

Example usage:

SelectionSystem selectionSystem = new SelectionSystem();
selectionSystem.OnItemAdded = item => {
    // Handle the item added event
    Console.WriteLine($"Item added: {item}");
};

// Add an item to trigger the event
selectionSystem.Add(new GameObject());