Question

以下代码通过mvvmcross消息传递协议即时更新属性。

我现在面临的问题是，当用户清除age或category textviews时，我会收到systemformat异常，我猜当textview变为空白时，它不会解析它到一个数字？

System.FormatException：输入字符串的格式不正确。

MainViewModel

private readonly IMvxMessenger _messenger;
private readonly MvxSubscriptionToken _token;

public MainViewModel(IMvxMessenger messenger) {

    _messenger = messenger;
    _token = messenger.Subscribe<SelectedItemMessage>(OnMessageReceived);;
}

private void OnMessageReceived(SelectedItemMessage obj)
{
    Age = obj.Age;
    Category= obj.Category;
}

DetailViewModel

private readonly IMvxMessenger _messenger;

public DetailViewModel(IMvxMessenger messenger) {
    _messenger = messenger;
}

public void UpdateMethod() {
    var message = new SelectedItemMessage(this,  age, category); 
    _messenger.Publish(message, typeof(SelectedItemMessage));
}

SelectedItemMessage

public SelectedItemMessage(object sender, double age, int category) : base(sender)
    {
        Age = age;
        Category = category;
    }

    public double Age { get; set; }
    public int Category{ get; set; }
}

Answer 1

您需要一个将空字符串转换为null的值转换器，并且您需要使Age属性为空。之后，您需要在绑定中指定convert：

public class NullableValueConverter : MvxValueConverter
{
    public override object Convert(object value, Type targetType, object parameter, System.Globalization.CultureInfo culture)
    {
        return value;
    }

    public override object ConvertBack(object value, Type targetType, object parameter, System.Globalization.CultureInfo culture)
    {
        if (value == null || string.IsNullOrEmpty(value.ToString()))
        {
            return null;
        }

        return value;
    }
}

并在您的视图中：

local:MvxBind="Text Age,Converter=Nullable;"

您可以在Value Converters Wiki

了解有关值转换器的更多信息

MVVMCross中的System.FormatException

1 个答案: