Xamarin Forms IValueConverter：处理转换错误

Question

我有一个 IValueConverter 可以将 byte[] 转换为 string，反之亦然。当字符串格式正确时，转换器只能将其从用户给定的 string 转换为 byte。现在我只是 return 转换失败时的原始对象，这会给我一个

Binding: '4' can not be converted to type 'System.Byte[]'.

日志中有错误。这没关系，但我想通过在编辑器周围显示红色边框并禁用“发送”按钮来通知用户他写的字符串格式不正确。

是否可以通过 MVVM 模式 (PRISM) 告诉 UI 转换失败？在 WPF 中有一个可以使用的 ValidationRule，我还没有找到 Xamarin 的类似内容。

转换器：

public class ByteArrayConverter : IValueConverter
{
    public object Convert(object value, Type targetType, object parameter, System.Globalization.CultureInfo culture)
    {
        if (value is byte[] b)
            return BitConverter.ToString(b);//Success
        return value;//Failed
    }

    public object ConvertBack(object value, Type targetType, object parameter, System.Globalization.CultureInfo culture)
    {
        if (value is string str && (str.Length - 2) % 3 == 0)
        {
            int len = (str.Length + 1) / 3;
            byte[] byteArray = new byte[len];

            for (int i = 0; i < len; i++)
                byteArray[i] = System.Convert.ToByte(str.Substring(3 * i, 2), 16);
            return byteArray;//Success
        }
        return value;//Failed
    }
}

XAML:

<?xml version="1.0" encoding="utf-8" ?>
<ContentPage xmlns="http://xamarin.com/schemas/2014/forms"
             xmlns:x="http://schemas.microsoft.com/winfx/2009/xaml"
             xmlns:prism="http://prismlibrary.com"
             xmlns:vc="clr-namespace:XXX.ValueConverter"
             prism:ViewModelLocator.AutowireViewModel="True"
             x:Class="XXX.Views.Device.SendMessagePage">
    <ContentPage.Resources>
        <vc:ByteArrayConverter x:Key="byteArrayConverter"/>
    </ContentPage.Resources>
    <Editor Text="{Binding Payload, Converter={StaticResource byteArrayConverter}}"></Editor>
    
</ContentPage>

Answer 1

对于 Xamarin.Forms，您可以使用 IValidationRule 来执行此操作。

首先，创建一个派生自 IValidationRule 接口的 class 来指定验证规则。

 public interface IValidationRule<T>
{
    string ValidationMessage { get; set; }
    bool Check(T value);
}

 public class IsNotNullOrEmptyRule<T> : IValidationRule<T>
{
    public string ValidationMessage { get; set; }

    public bool Check(T value)
    {
        if (value == null)
        {
            return false;
        }

        var str = $"{value }";
        return !string.IsNullOrWhiteSpace(str);
    }
}

public class HasValidAgeRule<T> : IValidationRule<T>
{
    public string ValidationMessage { get; set; }

    public bool Check(T value)
    {
        if (value is DateTime bday)
        {
            DateTime today = DateTime.Today;
            int age = today.Year - bday.Year;
            return (age >= 18);
        }

        return false;
    }
}

其次，将验证规则添加到属性。

public interface IValidatable<T> : INotifyPropertyChanged
{
    List<IValidationRule<T>> Validations { get; }

    List<string> Errors { get; set; }

    bool Validate();

    bool IsValid { get; set; }
}

 public class ValidatableObject<T> : IValidatable<T>
{
    public event PropertyChangedEventHandler PropertyChanged;

    public List<IValidationRule<T>> Validations { get; } = new List<IValidationRule<T>>();

    public List<string> Errors { get; set; } = new List<string>();

    public bool CleanOnChange { get; set; } = true;

    T _value;
    public T Value
    {
        get => _value;
        set
        {
            _value = value;

            if (CleanOnChange)
                IsValid = true;
        }
    }

    public bool IsValid { get; set; } = true;

    public virtual bool Validate()
    {
        Errors.Clear();

        IEnumerable<string> errors = Validations.Where(v => !v.Check(Value))
            .Select(v => v.ValidationMessage);

        Errors = errors.ToList();
        IsValid = !Errors.Any();

        return this.IsValid;
    }
    public override string ToString()
    {
        return $"{Value}";
    }
}

 public class validationmodel: INotifyPropertyChanged
{
    public ValidatableObject<string> FirstName { get; set; } = new ValidatableObject<string>();
    public ValidatableObject<string> LastName { get; set; } = new ValidatableObject<string>();
    public ValidatableObject<DateTime> BirthDay { get; set; } = new ValidatableObject<DateTime>() { Value = DateTime.Now };
    public validationmodel()
    {

        FirstName.Value = null;
        
        
        AddValidationRules();
        AreFieldsValid();
    }

    public event PropertyChangedEventHandler PropertyChanged;

    public void AddValidationRules()
    {
        FirstName.Validations.Add(new IsNotNullOrEmptyRule<string> { ValidationMessage = "First Name Required" });
        LastName.Validations.Add(new IsNotNullOrEmptyRule<string> { ValidationMessage = "Last Name Required" });
        BirthDay.Validations.Add(new HasValidAgeRule<DateTime> { ValidationMessage = "You must be 18 years of age or older" });
    }

    bool AreFieldsValid()
    {
        bool isFirstNameValid = FirstName.Validate();
        bool isLastNameValid = LastName.Validate();
        bool isBirthDayValid = BirthDay.Validate();
        return isFirstNameValid && isLastNameValid && isBirthDayValid;
    }



 }

突出显示包含无效数据的控件：

public class FirstValidationErrorConverter : IValueConverter
{
    public object Convert(object value, Type targetType, object parameter, CultureInfo culture)
    {
        ICollection<string> errors = value as ICollection<string>;
        return errors != null && errors.Count > 0 ? errors.ElementAt(0) : null;
    }

   

    public object ConvertBack(object value, Type targetType, object parameter, CultureInfo culture)
    {
        throw new NotImplementedException();
    }
}

 public class InverseBoolConverter : IValueConverter
{
    public object Convert(object value, Type targetType, object parameter, CultureInfo culture)
    {
        if (!(value is bool))
        {
            throw new InvalidOperationException("The target must be a boolean");
        }

        return !(bool)value;
    }

    public object ConvertBack(object value, Type targetType, object parameter, CultureInfo culture)
    {
        return null;
    }
}

public class BehaviorBase<T> : Behavior<T>
where T : BindableObject
{
    #region Properties
    public T AssociatedObject
    {
        get;
        private set;
    }
    #endregion
    #region NormalMethods
    private void OnBindingContextChanged(object sender, EventArgs e)
    {
        OnBindingContextChanged();
    }
    #endregion
    #region Overrides
    protected override void OnAttachedTo(T bindable)
    {
        base.OnAttachedTo(bindable);
        AssociatedObject = bindable;
        if (bindable.BindingContext != null)
        {
            BindingContext = bindable.BindingContext;
        }

        bindable.BindingContextChanged += OnBindingContextChanged;
    }
    protected override void OnDetachingFrom(T bindable)
    {
        base.OnDetachingFrom(bindable);
        bindable.BindingContextChanged -= OnBindingContextChanged;
        AssociatedObject = null;
    }
    protected override void OnBindingContextChanged()
    {
        base.OnBindingContextChanged();
        BindingContext = AssociatedObject.BindingContext;
    }
    #endregion
}

 public class EntryLineValidationBehaviour : BehaviorBase<Entry>
{
    #region StaticFields
    public static readonly BindableProperty IsValidProperty = BindableProperty.Create(nameof(IsValid), typeof(bool), typeof(EntryLineValidationBehaviour), true, BindingMode.Default, null, (bindable, oldValue, newValue) => OnIsValidChanged(bindable, newValue));
    #endregion
    #region Properties
    public bool IsValid
    {
        get
        {
            return (bool)GetValue(IsValidProperty);
        }
        set
        {
            SetValue(IsValidProperty, value);
        }
    }
    #endregion
    #region StaticMethods
    private static void OnIsValidChanged(BindableObject bindable, object newValue)
    {
        if (bindable is EntryLineValidationBehaviour IsValidBehavior &&
             newValue is bool IsValid)
        {
            IsValidBehavior.AssociatedObject.PlaceholderColor = IsValid ? Color.Default : Color.Red;
        }
    }

    #endregion
}

MainPage.xaml:

<ContentPage
x:Class="validationapp.MainPage"
xmlns="http://xamarin.com/schemas/2014/forms"
xmlns:x="http://schemas.microsoft.com/winfx/2009/xaml"
xmlns:behaviour="clr-namespace:validationapp.Behaviors"
xmlns:converter="clr-namespace:validationapp.converters"
xmlns:local="clr-namespace:validationapp">
<ContentPage.Resources>
   
    <converter:InverseBoolConverter x:Key="InverseBoolConverter" />
    <converter:FirstValidationErrorConverter x:Key="FirstValidationErrorConverter" />
    <Style x:Key="ErrorTextStyle" TargetType="Label">
        <Setter Property="TextColor" Value="Red" />
        <Setter Property="FontSize" Value="12" />
    </Style>
</ContentPage.Resources>
<StackLayout>
    <!--  First Name  -->
    <Entry Placeholder="First Name" Text="{Binding FirstName.Value}">
        <Entry.Behaviors>
            <behaviour:EntryLineValidationBehaviour IsValid="{Binding FirstName.IsValid}" />
        </Entry.Behaviors>
    </Entry>

    <Label
        IsVisible="{Binding FirstName.IsValid, Converter={StaticResource InverseBoolConverter}}"
        Style="{StaticResource ErrorTextStyle}"
        Text="{Binding FirstName.Errors, Converter={StaticResource FirstValidationErrorConverter}}" />
    <!--  /First Name  -->

    <!--  Last Name  -->
    <Entry Placeholder="Last Name" Text="{Binding LastName.Value}">
        <Entry.Behaviors>
            <behaviour:EntryLineValidationBehaviour IsValid="{Binding LastName.IsValid}" />
        </Entry.Behaviors>
    </Entry>

    <Label
        IsVisible="{Binding LastName.IsValid, Converter={StaticResource InverseBoolConverter}}"
        Style="{StaticResource ErrorTextStyle}"
        Text="{Binding LastName.Errors, Converter={StaticResource FirstValidationErrorConverter}}" />
    <!--  /Last Name  -->


    <!--  Birthday  -->
    <DatePicker Date="{Binding BirthDay.Value}" />
    <Label
        IsVisible="{Binding BirthDay.IsValid, Converter={StaticResource InverseBoolConverter}}"
        Style="{StaticResource ErrorTextStyle}"
        Text="{Binding BirthDay.Errors, Converter={StaticResource FirstValidationErrorConverter}}" />
    <!--  Birthday  -->
</StackLayout>

您还可以在条目命令的属性更改时触发验证。

更详细的验证信息，请看：

https://docs.microsoft.com/en-us/xamarin/xamarin-forms/enterprise-application-patterns/validation

Xamarin Forms IValueConverter：处理转换错误

Xamarin Forms IValueConverter: Handle converting error

c#

prism

ivalueconverter

xamarin

xamarin.forms