public MainPage()
    {
        this.InitializeComponent();
        this.BindingContext = this.mainViewModel = new MainViewModel();


        CollectionView courseCollectionView = new CollectionView
        {
            ItemSizingStrategy = ItemSizingStrategy.MeasureFirstItem,
            ItemTemplate = new DataTemplate(typeof(ItemView))
        };


     courseCollectionView.SetBinding(CollectionView.ItemsSourceProperty,
     new Binding() { Source = this.mainViewModel.CountryNames, Mode =
     BindingMode.TwoWay });

     courseCollectionView.ItemsLayout = new GridItemsLayout(4,
      ItemsLayoutOrientation.Vertical);

        this.CoursesStack.Children.Clear();
        this.CoursesStack.Children.Add(courseCollectionView);

    }

ObservableCollection<Country> countryNames;

    public ObservableCollection<Country> CountryNames
    {
        get => this.countryNames;
        set
        {
            this.countryNames = value;
            RaisePropertyChanged("CountryNames");
        }
    }