c# - 当我从 EditText 中删除所有内容时，MvvmCross 不更新值

Question

我正在使用 Xamarin 和 MvvmCross 开发适用于 Android 的应用程序。在我的 axml 中，我有一个 EditText定义:

<EditText
    local:MvxBind="Text PorcentagemDesconto"
    android:inputType="number"
    android:layout_width="1dp"
    android:layout_weight="0.3"
    android:layout_height="wrap_content"
    android:hint="%" />

PorcentagemDesconto 字段是这样声明的:

private decimal? _porcentagemDesconto;
public decimal? PorcentagemDesconto
{
    get { return _porcentagemDesconto; }
    set
    {
        _porcentagemDesconto = value;
    }
}

假设我在 EditText 上插入值 256。然后，我在属性的 set 方法上插入一个断点，然后，删除所有数字(按退格键 3 次)，断点只会被命 2 次 ，使私有(private)变量具有不需要的值 2。

有一些解决方法，还是我在这里做错了什么？

Answer 1

这是因为值转换失败。如果您连接了调试器，您将在调试输出中看到类似这样的内容:

MvxBind:Error: 47,35 SetValue failed with exception - ArgumentException: Object of type 'System.String' cannot be converted to type 'System.Nullable`1[System.Decimal]'.
03-15 22:11:40.939 I/mono-stdout(25671): MvxBind:Error: 47,35 SetValue failed with exception - ArgumentException: Object of type 'System.String' cannot be converted to type 'System.Nullable`1[System.Decimal]'.
      at System.RuntimeType.CheckValue (System.Object value, System.Reflection.Binder binder, System.Globalization.CultureInfo culture, BindingFlags invokeAttr) [0x00062] in /Users/builder/data/lanes/1196/e79c13cd/source/mono/mcs/class/corlib/ReferenceSources/RuntimeType.cs:131 
  at System.Reflection.MonoMethod.ConvertValues (System.Reflection.Binder binder, System.Object[] args, System.Reflection.ParameterInfo[] pinfo, System.Globalization.CultureInfo culture, BindingFlags invokeAttr) [0x0007f] in /Users/builder/data/lanes/1196/e79c13cd/source/mono/mcs/class/corlib/System.Reflection/MonoMethod.cs:335 
  at System.Reflection.MonoMethod.Invoke (System.Object obj, BindingFlags invokeAttr, System.Reflection.Binder binder, System.Object[] parameters, System.Globalization.CultureInfo culture) [0x00014] in /Users/builder/data/lanes/1196/e79c13cd/source/mono/mcs/class/corlib/System.Reflection/MonoMethod.cs:283 
  at System.Reflection.MonoProperty.SetValue (System.Object obj, System.Object value, BindingFlags invokeAttr, System.Reflection.Binder binder, System.Object[] index, System.Globalization.CultureInfo culture) [0x0006a] in /Users/builder/data/lanes/1196/e79c13cd/source/mono/mcs/class/corlib/System.Reflection/MonoProperty.cs:445 
  at System.Reflection.PropertyInfo.SetValue (System.Object obj, System.Object value, System.Object[] index) [0x00000] in /Users/builder/data/lanes/1196/e79c13cd/source/mono/mcs/class/corlib/System.Reflection/PropertyInfo.cs:111 

消息的重要部分是

Object of type 'System.String' cannot be converted to type 'System.Nullable`1[System.Decimal]'.

您可以通过在核心库中使用值转换器来解决这个问题

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 (string.IsNullOrEmpty(value?.ToString()))
            return null;

        return value;
    }
}

然后绑定(bind)你的值

<EditText
    local:MvxBind="Text PorcentagemDesconto, Converter=Nullable"
    android:inputType="number"
    android:layout_width="1dp"
    android:layout_weight="0.3"
    android:layout_height="wrap_content"
    android:hint="%" />