public void ToggleSpinnerState(bool toggle_on)
        {
            if (MainToolbar.VisibleItems[1].Identifier == "Spinner")
            {
                (MainToolbar.VisibleItems[1].View as NSProgressIndicator).StopAnimation(this);
                MainToolbar.RemoveItem(1);
            }

            if (toggle_on)
            {
                MainToolbar.InsertItem("Spinner", 1);
                (MainToolbar.VisibleItems[1].View as NSProgressIndicator).StartAnimation(this);
            }
        }
        void ToggleSpinnerState(bool toggle_on)
        {
            const int spinner_position = 2;

            if (MainToolbar.VisibleItems[spinner_position].Identifier == "Spinner")
            {
                (MainToolbar.VisibleItems[spinner_position].View as NSProgressIndicator).StopAnimation(this);
                MainToolbar.RemoveItem(spinner_position);
            }

            if (toggle_on)
            {
                MainToolbar.InsertItem("Spinner", spinner_position);
                (MainToolbar.VisibleItems[spinner_position].View as NSProgressIndicator).StartAnimation(this);
            }
        }