MvvmCross：将 nint 绑定到枚举并将其双向转换

Question

我正在尝试将 UISegmentedControl.SelectedSegment 的本机 nint 值绑定到视图模型中的 enum 属性。它需要自定义值转换器，因为无法将 enum 直接转换为 nint，反之亦然。

Convert 方法工作正常，因此 UISegmentedControl 从视图模型中选取值。但是当我单击控件的段时，会引发以下异常：

(MvvmCross.Converters.MvxValueConverter) Failed to ConvertBack from System.nint to MyApp.Core.ViewModels.MyViewModel+Selection with Exception: System.InvalidCastException: Specified cast is not valid. at MvvmCross.Converters.MvxValueConverter`2[TFrom,TTo].ConvertBack (System.Object value, System.Type targetType, System.Object parameter, System.Globalization.CultureInfo culture)

好像是MvxValueConvertercasts on its own before calling the virtual ConvertBack方法。除了使视图模型使用 const nint 值而不是单个 enum.

之外，我不确定有什么解决方法

public partial class MyView : MvxTableViewController<MyViewModel>
{
    // SegmentedControl is iOS UISegmentedControl in .designer.cs

    public MyView() : base("MyView", null) {}

    public override void ViewDidLoad()
    {
        base.ViewDidLoad();

        var set = this.CreateBindingSet<MyView, MyViewModel>();
        set.Bind(SegmentedControl).To(vm => vm.SelectedSegment).WithConversion(new SelectionValueConverter());
        set.Apply();
    }

    public class SelectionValueConverter : MvxValueConverter<MyViewModel.Selection, nint>
    {
        protected override nint Convert(MyViewModel.Selection value, Type targetType, object parameter, CultureInfo culture)
        {
            return (int)value; // called
        }

        protected override MyViewModel.Selection ConvertBack(nint value, Type targetType, object parameter, CultureInfo culture)
        {
            var intVal = (int)value; // never called, exception in MvxValueConverter[TFrom,TTo].ConvertBack
            return (MyViewModel.Selection)intVal;
        }
    }
}

public class MyViewModel : MvxViewModel
{
    public enum Selection
    {
        One = 0,
        Two = 1,
        Three = 2
    }

    Selection _selectedSegment;
    public Selection SelectedSegment
    {
        get => _selectedSegment;
        set => SetProperty(ref _selectedSegment, value);
    }

    public MyViewModel() {}
}

Answer 1

如您所见，here and here MvxUISegmentedControlSelectedSegmentTargetBinding 已经进行了转换 from/to nint。所以你的转换器应该直接 int 而不是 nint

public class SelectionValueConverter : MvxValueConverter<MyViewModel.Selection, int>
{
    protected override int Convert(MyViewModel.Selection value, Type targetType, object parameter, CultureInfo culture)
    {
        return (int)value;
    }

    protected override MyViewModel.Selection ConvertBack(int value, Type targetType, object parameter, CultureInfo culture)
    {
        return (MyViewModel.Selection)value;
    }
}

高

MvvmCross：将 nint 绑定到枚举并将其双向转换

MvvmCross: Binding nint to enum and converting it two-way

mvvmcross

xamarin