使用 DataAnnotation 在 Xamarin 中进行验证

这种方法看起来很棒，并且为改进提供了许多可能性。

只是为了不让它没有答案，我认为您可以尝试创建一个组件来包装您想要处理的视图并公开您需要在外部使用的事件和属性。 它将是可重复使用的，它可以解决问题。

因此，逐步将是：

1. 创建包装器组件;
2. 将此控件定位到您的行为;
3. 公开/处理您打算使用的属性和事件;
4. 在代码中将 simple 替换为 this。EntryCheckableEntryView

下面是组件的 XAML 代码：

<ContentView xmlns="http://xamarin.com/schemas/2014/forms" 
         xmlns:x="http://schemas.microsoft.com/winfx/2009/xaml"
         x:Class="MyApp.CheckableEntryView">
<ContentView.Content>
    <StackLayout>
        <Label x:Name="lblContraintText" 
               Text="This is not valid"
               TextColor="Red"
               AnchorX="0"
               AnchorY="0"
               IsVisible="False"/>
        <Entry x:Name="txtEntry"
               Text="Value"/>
    </StackLayout>
</ContentView.Content>

它是代码隐藏的：

[XamlCompilation(XamlCompilationOptions.Compile)]
public partial class CheckableEntryView : ContentView
{
    public event EventHandler<TextChangedEventArgs> TextChanged;

    private BindableProperty TextProperty = BindableProperty.Create(nameof(Text), typeof(string), typeof(CheckableEntryView), string.Empty);
    public string Text
    {
        get { return (string)GetValue(TextProperty); }
        set { SetValue( TextProperty, value); }
    }

    public CheckableEntryView ()
    {
        InitializeComponent();

        txtEntry.TextChanged += OnTextChanged;
        txtEntry.SetBinding(Entry.TextProperty, new Binding(nameof(Text), BindingMode.Default, null, null, null, this));
    }

    protected virtual void OnTextChanged(object sender, TextChangedEventArgs args)
    {
        TextChanged?.Invoke(this, args);
    }

    public Task ShowValidationMessage()
    {
        Task.Yield();
        lblContraintText.IsVisible = true;
        return lblContraintText.ScaleTo(1, 250, Easing.SinInOut);
    }

    public Task HideValidationMessage()
    {
        Task.Yield();
        return lblContraintText.ScaleTo(0, 250, Easing.SinInOut)
            .ContinueWith(t => 
                Device.BeginInvokeOnMainThread(() => lblContraintText.IsVisible = false));
    }
}

我更改了行为的事件逻辑，使其更简单。仅供参考，它是：

void _associatedObject_TextChanged(object sender, TextChangedEventArgs e)
{
    if(e.NewTextValue == "test")
        ((CheckableEntryView)sender).ShowValidationMessage();
    else
        ((CheckableEntryView)sender).HideValidationMessage();
}

要使用它，您基本上可以执行与以前相同的操作：

<local:CheckableEntryView HorizontalOptions="FillAndExpand">
    <local:CheckableEntryView.Behaviors>
        <local:EntryValidationBehavior PropertyName="Test"/><!-- this property is not being used on this example -->
    </local:CheckableEntryView.Behaviors>
</local:CheckableEntryView>

这是它的样子：

我没有在此示例代码上绑定验证消息，但您可以保持相同的想法。

我希望它对你有所帮助。

经过一段时间，我想出了所有建议的混合体。 由于您调用了更改属性，因此 Your 被多次触发。请改用 Dictionary 作为支持字段。ViewModelBase 的样子如下：FirstErrorConverterErrorsList_errors

public ViewModelBase()
{
    PropertyInfo[] properties = GetType().GetProperties();
    foreach (PropertyInfo property in properties)
    {
        var attrs = property.GetCustomAttributes(true);
        if (attrs?.Length > 0)
        {
            Errors[property.Name] = new SmartCollection<ValidationResult>();
        }
    }
}

private Dictionary<string, SmartCollection<ValidationResult>> _errors = new Dictionary<string, SmartCollection<ValidationResult>>();
public Dictionary<string, SmartCollection<ValidationResult>> Errors
{
    get => _errors;
    set => SetProperty(ref _errors, value);
}

protected void Validate(string propertyName, string propertyValue)
{
    var validationContext = new ValidationContext(this, null)
    {
        MemberName = propertyName
    };

    var validationResults = new List<ValidationResult>();
    var isValid = Validator.TryValidateProperty(propertyValue, validationContext, validationResults);

    if (!isValid)
    {
        Errors[propertyName].Reset(validationResults);
    }
    else
    {
        Errors[propertyName].Clear();
    }
}

由于每个项目添加时都会发生火灾事件，因此我使用了 SmartCollection，并添加了一个名为ObservableCollectionCollectionChangedFirstItem

public class SmartCollection<T> : ObservableCollection<T>
{
    public T FirstItem => Items.Count > 0 ? Items[0] : default(T);

    public SmartCollection()
        : base()
    {
    }

    public SmartCollection(IEnumerable<T> collection)
        : base(collection)
    {
    }

    public SmartCollection(List<T> list)
        : base(list)
    {
    }

    public void AddRange(IEnumerable<T> range)
    {
        foreach (var item in range)
        {
            Items.Add(item);
        }

        this.OnPropertyChanged(new PropertyChangedEventArgs("FirstItem"));
        this.OnPropertyChanged(new PropertyChangedEventArgs("Count"));
        this.OnPropertyChanged(new PropertyChangedEventArgs("Item[]"));
        this.OnCollectionChanged(new NotifyCollectionChangedEventArgs(NotifyCollectionChangedAction.Reset));
    }

    public void Reset(IEnumerable<T> range)
    {
        this.Items.Clear();

        AddRange(range);
    }
}

这是我的 xaml 的样子：

<StackLayout Orientation="Vertical">
    <Entry Placeholder="Email" Text="{Binding Email}">
        <Entry.Behaviors>
            <behaviors:EntryValidatorBehavior PropertyName="Email" />
        </Entry.Behaviors>
    </Entry>
    <Label Text="{Binding Errors[Email].FirstItem, Converter={StaticResource firstErrorToTextConverter}}"
           IsVisible="{Binding Errors[Email].Count, Converter={StaticResource errorToBoolConverter}}" />

    <Entry Placeholder="Password" Text="{Binding Password}">
        <Entry.Behaviors>
            <behaviors:EntryValidatorBehavior PropertyName="Password" />
        </Entry.Behaviors>
    </Entry>
    <Label Text="{Binding Errors[Password].FirstItem, Converter={StaticResource firstErrorToTextConverter}}"
           IsVisible="{Binding Errors[Password].Count, Converter={StaticResource errorToBoolConverter}}" />
</StackLayout>

其他一切都是一样的！

使用 Xamarin.FormsEnterprise 应用程序模式电子书中的企业应用中的验证和以下组件，XAML 可能如下所示：EntryLabelView

xmlns:local="clr-namespace:View"
...
<local:EntryLabelView ValidatableObject="{Binding MyValue, Mode=TwoWay}"
                      ValidateCommand="{Binding ValidateValueCommand}" />

视图模型：

private ValidatableObject<string> _myValue;

public ViewModel()
{
  _myValue = new ValidatableObject<string>();

  _myValue.Validations.Add(new IsNotNullOrEmptyRule<string> { ValidationMessage = "A value is required." });
}

public ValidatableObject<string> MyValue
{
  get { return _myValue; }
  set
  {
      _myValue = value;
      OnPropertyChanged(nameof(MyValue));
  }
}

public ICommand ValidateValueCommand => new Command(() => ValidateValue());

private bool ValidateValue()
{
  return _myValue.Validate(); //updates ValidatableObject.Errors
}

引用的类的实现（包括 、 、 和）可在 eShopOnContainers 示例中找到。ValidatableObjectIsNotNullOrEmptyRuleEventToCommandBehaviorFirstValidationErrorConverter

EntryLabelView.xaml：（请注意使用Source={x:Reference view})

<ContentView xmlns="http://xamarin.com/schemas/2014/forms"
         xmlns:x="http://schemas.microsoft.com/winfx/2009/xaml"
         xmlns:converters="clr-namespace:Toolkit.Converters;assembly=Toolkit"
         xmlns:behaviors="clr-namespace:Toolkit.Behaviors;assembly=Toolkit"
         x:Name="view"
         x:Class="View.EntryLabelView">
  <ContentView.Resources>
    <converters:FirstValidationErrorConverter x:Key="FirstValidationErrorConverter" />
  </ContentView.Resources>
  <ContentView.Content>
    <StackLayout>
      <Entry Text="{Binding ValidatableObject.Value, Mode=TwoWay, Source={x:Reference view}}">
        <Entry.Behaviors>
          <behaviors:EventToCommandBehavior 
                            EventName="TextChanged"
                            Command="{Binding ValidateCommand, Source={x:Reference view}}" />
        </Entry.Behaviors>
      </Entry>
      <Label Text="{Binding ValidatableObject.Errors, Source={x:Reference view},
                        Converter={StaticResource FirstValidationErrorConverter}}" />
    </StackLayout>
  </ContentView.Content>
</ContentView>

EntryLabelView.xaml.cs：（请注意 的使用 ）。OnPropertyChanged

[XamlCompilation(XamlCompilationOptions.Compile)]
public partial class EntryLabelView : ContentView
{
    public EntryLabelView ()
    {
        InitializeComponent ();
    }

    public static readonly BindableProperty ValidatableObjectProperty = BindableProperty.Create(
        nameof(ValidatableObject), typeof(ValidatableObject<string>), typeof(EntryLabelView), default(ValidatableObject<string>),
        BindingMode.TwoWay,
        propertyChanged: (b, o, n) => ((EntryLabelView)b).ValidatableObjectChanged(o, n));

    public ValidatableObject<string> ValidatableObject
    {
        get { return (ValidatableObject<string>)GetValue(ValidatableObjectProperty); }
        set { SetValue(ValidatableObjectProperty, value); }
    }

    void ValidatableObjectChanged(object o, object n)
    {
        ValidatableObject = (ValidatableObject<string>)n;
        OnPropertyChanged(nameof(ValidatableObject));
    }

    public static readonly BindableProperty ValidateCommandProperty = BindableProperty.Create(
        nameof(Command), typeof(ICommand), typeof(EntryLabelView), null,
        propertyChanged: (b, o, n) => ((EntryLabelView)b).CommandChanged(o, n));

    public ICommand ValidateCommand
    {
        get { return (ICommand)GetValue(ValidateCommandProperty); }
        set { SetValue(ValidateCommandProperty, value); }
    }

    void CommandChanged(object o, object n)
    {
        ValidateCommand = (ICommand)n;
        OnPropertyChanged(nameof(ValidateCommand));
    }
}

我可能有点晚了，但对于将来偶然发现这篇文章的任何人。

也许可以尝试一下这个库：https://www.nuget.org/packages/Xamarin.AttributeValidation/

它允许您通过简单地将 Attributes 放在 ViewModel 中的属性上方来验证 UI。就是这样。没什么可做的了。就像你在 ASP.NET Core 中所做的那样。验证消息自动以小浮动文本气泡的形式显示，悬停在条目上。与原生 Android 验证非常相似。

有关示例或详细说明，请查看 GitHub 存储库：https://github.com/kevin-mueller/Xamarin.AttributeValidation