MVVMCross中的System.FormatException [英] System.FormatException in MVVMCross

问题描述

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文本视图时，我得到了systemformat异常，我猜想textview为空时，它不会将其解析为数字吗? /p>

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; }
}

解决方案

您需要一个将空字符串转换为null的值转换器，并且需要将Age属性设置为可空.之后，您将需要在绑定中指定转换后的内容:

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;
    }
}

并在您的视图内:

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

您可以在价值转换器Wiki

The following code updates properties on-the-fly via mvvmcross messaging protocol.

The problem that I am facing now, when user clears either age or category textviews, then I am getting systemformat exception, I guess when textview gets blank, it does not parse it to a number?

System.FormatException: Input string was not in a correct format.

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; }
}

解决方案

You need a value converter which converts empty string to null and you need to make Age property nullable. After that you will need to specify the converted in the binding:

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;
    }
}

and inside your view:

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

You can read more about value converters at Value Converters Wiki

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