Xamarin 新的可绑定选取器控件

Question

https://blog.xamarin.com/new-bindable-picker-control-for-xamarin-forms/

我对 xamarin 和 c# 很陌生不知何故它对我不起作用...

 public class RegistrationPageViewModel : INotifyPropertyChanged
    {
        List<string> _countries = new List<string>
{
    "Afghanistan",
    "Albania",
    "Algeria",
    "Andorra",
    "Angola",
};
        public List<string> Countries => _countries;
        public event PropertyChangedEventHandler PropertyChanged;


    }
    public partial class RegistrationPage : ContentPage
    {
        RegistrationPageViewModel vm;
        public RegistrationPage()
        {
            InitializeComponent();
            this.BindingContext = vm = new RegistrationPageViewModel();
        }
        private void InitializeComponent()
        {
            throw new NotImplementedException();
        }
    }

    int countriesSelectedIndex;
            public int CountriesSelectedIndex
            {
                get
                {
                    return countriesSelectedIndex;
                }
                set
                {
                    if (countriesSelectedIndex != value)
                    {
                        countriesSelectedIndex = value;
                        // trigger some action to take such as updating other labels or fields
                        OnPropertyChanged(nameof(CountriesSelectedIndex));
                        SelectedCountry = Countries[countriesSelectedIndex];
                    }
                }
            }
        public string SelectedCountry { get; private set; }
    }

我收到错误："国家"名称在当前上下文中不存在

我做错了什么？有人有工作的例子吗

Answer 1

以下是我的写法(使用 Fody 而不是打扰INotifyPropertyChanged(：

public class Country
    {
        public string Name { get; }
        public Country(string name) => Name = name;
        public static Country Afghanistan = new Country("Afghanistan");
        public static Country Albania = new Country("Albania");
        public static Country Algeria = new Country("Algeria");
        public static Country Andorra = new Country("Andorra");
        public static Country Angola = new Country("Angola");
        public static IList<Country> Countries = new List<Country> { Afghanistan, Albania, Algeria, Andorra, Andorra, Angola };
    }
    [ImplementPropertyChanged]
    public class RegistrationPageViewModel
    {
        public Country SelectedCountry { get; set; }
    }
    public class RegistrationPage : ContentPage
    {
        public RegistrationPage()
        {
            var vm = new RegistrationPageViewModel();
            BindingContext = vm;
            var picker = new Picker();
            foreach (var country in Country.Countries) picker.Items.Add(country.Name);
            picker.SetBinding<RegistrationPageViewModel>(Picker.SelectedIndexProperty, x => x.SelectedCountry, converter: new PickerItemsToTypeConverter<Country>(Country.Countries));
            Content = picker;
        }
    }
    public class PickerItemsToTypeConverter<T> : IValueConverter
    {
        private readonly IList<T> _pickerItemsList;
        public PickerItemsToTypeConverter(IList<T> pickerItemsList) => _pickerItemsList = pickerItemsList;
        public object Convert(object value, Type targetType, object parameter, CultureInfo culture)
        {
            var itemType = (T)value;
            return _pickerItemsList.IndexOf(itemType);
        }
        public object ConvertBack(object value, Type targetType, object parameter, CultureInfo culture)
        {
            var selectedIndex = value as int?;
            if (selectedIndex == null) throw new Exception("No selection");
            return _pickerItemsList[selectedIndex.Value];
        }
    }