Xamarin表单iOS验证Texbox并显示错误

Question

我为Android定制了Xamarin Forms Entry控件，

我正在使用MVVM Validation通过MVVM验证我的输入，并为每个表单元素生成一条消息。

 Validator.AddRequiredRule(() => UserName, Messages.FieldCannotBlank);
 Validator.AddRequiredRule(() => Password, Messages.FieldCannotBlank);

我使用的自定义条目是Xlabs的扩展条目的扩展版本。这是条目的代码

 public class ExtendedEntry : Entry
{
    public ExtendedEntry()
    {
        BorderColor = Color.Default;
        BGColorForRounded = Color.Default;
    }

    #region XAlign
    /// <summary>
    /// The XAlign property
    /// </summary>
    public static readonly BindableProperty XAlignProperty =
        BindableProperty.Create(nameof(XAlign), typeof(TextAlignment), typeof(ExtendedEntry),
        TextAlignment.Start);

    /// <summary>
    /// Gets or sets the X alignment of the text
    /// </summary>
    public TextAlignment XAlign
    {
        get { return (TextAlignment)GetValue(XAlignProperty); }
        set { SetValue(XAlignProperty, value); }
    }
    #endregion

    #region BGColorForRounded
    public static BindableProperty BGColorForRoundedProperty =
        BindableProperty.Create(nameof(BGColorForRounded), typeof(Color), typeof(ExtendedEntry), default(Color));

    public Color BGColorForRounded
    {
        get { return (Color)GetValue(BGColorForRoundedProperty); }
        set { SetValue(BGColorForRoundedProperty, value); }
    }
    #endregion

    #region Padding

    public static BindableProperty PaddingProperty =
        BindableProperty.Create(nameof(Padding), typeof(Thickness), typeof(ExtendedEntry), default(Thickness), defaultBindingMode: BindingMode.OneWay);

    public Thickness Padding
    {
        get { return (Thickness)GetValue(PaddingProperty); }
        set { SetValue(PaddingProperty, value); }
    }

    #endregion Padding

    #region IsReadOnly

    public static readonly BindableProperty IsReadOnlyProperty =
        BindableProperty.Create("IsReadOnly", typeof(bool), typeof(ExtendedEntry), false);

    public bool IsReadOnly
    {
        get { return (bool)GetValue(IsReadOnlyProperty); }
        set { SetValue(IsReadOnlyProperty, value); }
    }
    #endregion

    #region IsValid
    public static readonly BindableProperty IsValidProperty =
        BindableProperty.Create("IsValid", typeof(bool), typeof(ExtendedEntry), true);

    public bool IsValid
    {
        get { return (bool)GetValue(IsValidProperty); }
        set { SetValue(IsValidProperty, value); }
    }
    #endregion

    #region ErrorHint
    public static readonly BindableProperty ErrorHintProperty =
        BindableProperty.Create("ErrorHint", typeof(string), typeof(ExtendedEntry), "");

    public string ErrorHint
    {
        get { return (string)GetValue(ErrorHintProperty); }
        set { SetValue(ErrorHintProperty, value); }
    }

    #endregion

    #region PlaceholderTextColor
    /// <summary>
    /// The PlaceholderTextColor property
    /// </summary>
    public static readonly BindableProperty PlaceholderTextColorProperty =
        BindableProperty.Create("PlaceholderTextColor", typeof(Color), typeof(ExtendedEntry), Color.Default);
    /// <summary>
    /// Sets color for placeholder text
    /// </summary>
    public Color PlaceholderTextColor
    {
        get { return (Color)GetValue(PlaceholderTextColorProperty); }
        set { SetValue(PlaceholderTextColorProperty, value); }
    }
    #endregion

    #region HasBorder
    /// <summary>
    /// The HasBorder property
    /// </summary>
    public static readonly BindableProperty HasBorderProperty =
        BindableProperty.Create("HasBorder", typeof(bool), typeof(ExtendedEntry), true);




    /// <summary>
    /// Gets or sets if the border should be shown or not
    /// </summary>
    public bool HasBorder
    {
        get { return (bool)GetValue(HasBorderProperty); }
        set { SetValue(HasBorderProperty, value); }
    }
    #endregion

    #region BorderRadius
    public int BorderRadius
    {
        get { return (int)GetValue(BorderRadiusProperty); }
        set { SetValue(BorderRadiusProperty, value); }
    }

    // Using a DependencyProperty as the backing store for BorderRadius.  This enables animation, styling, binding, etc...
    public static readonly BindableProperty BorderRadiusProperty =
       BindableProperty.Create("BorderRadius", typeof(int), typeof(ExtendedEntry), 0);
    #endregion

    #region BorderColor
    /// <summary>
    /// The Border Color Property
    /// </summary>
    public static readonly BindableProperty BorderColorProperty =
        BindableProperty.Create("BorderColor", typeof(Color), typeof(ExtendedEntry), default(Color));

    /// <summary>
    /// Gets or sets the border color
    /// </summary>
    public Color BorderColor
    {
        get { return (Color)GetValue(BorderColorProperty); }
        set { SetValue(BorderColorProperty, value); }
    }
    #endregion
}

每当按下一个按钮时，我都会调用Validators ValidateAll方法来验证我的所有字段

 var result = Validator.ValidateAll();
        if (!result.IsValid)
        {
            App.Current.MainPage.DisplayAlert("Error", "The forms has errors, please review the errors and fix them.", "Ok");
        }
        Errors = result.AsDictionary();

错误是Viewmodel的另一个属性，它基本上是一个字典，它将在验证后保存属性名称和错误的键值对列表。我将此值映射到Xaml中的ExtendedEntry的ErrorHint属性

 <controls:ExtendedEntry ErrorHint="{Binding Errors[UserName]}"
       IsValid="{Binding Errors[UserName],                                                                           Converter={StaticResource DictBool},
                                                  Mode=TwoWay}"
                                Placeholder="sallysmith@gmail.com"
                                Style="{DynamicResource WhiteEntryStandard}"
                                Text="{Binding UserName}" />

最后，这是Android的实现

 private void SetValidityAndHint(ExtendedEntry view)
    {

        if (!view.IsValid)
        {
            if (!string.IsNullOrEmpty(view.ErrorHint))
            {
                Control.Error = view.ErrorHint;
            }
        }
    }

这适用于Android。但我一直无法找到iOS的解决方案。看起来iOS根本没有任何类型的错误属性。有人指点什么？