在 WPF 中，如何在 LostFocus 上进行绑定但在 PropertyChanged 上进行验证？

Question

我希望能够绑定一个将 UpdateSourceTrigger 设置为 LostFocus（默认值）的 TextBox，但在用户键入文本时执行验证。我能想到的最好办法是处理 TextChanged 事件并在视图模型上调用验证方法。我想知道是否有更好的解决方案。

我的视图模型侦听模型中的属性更改以更新自身（包括格式化）。我不想将 UpdateSourceTrigger 设置为 PropertyChanged 进行绑定，因为这会导致在用户键入后立即格式化文本（例如，用户可能希望在键入“1”后立即键入“1.2” " 由于视图模型的自动格式化，文本变为 "1.0"）。

Answer 1

详细说明我留下的评论，这是一个如何完成的示例。

仅供参考，我使用 nuget 包 MvvmLight 进行管道。

MainWindow.xaml

<StackPanel>
    <TextBox x:Name="myTextBox" Text="{Binding SomeNumberViewModel.IntermediateText, UpdateSourceTrigger=PropertyChanged, ValidatesOnDataErrors=True}" Width="100" Margin="5"/>
    <Button Content="Hi" HorizontalAlignment="Center" Padding="5,15" Margin="5"/>
</StackPanel>

MainWindow.xaml.cs

public MainViewModel ViewModel
{
    get
    {
        return this.DataContext as MainViewModel;
    }
}
public MainWindow()
{
    InitializeComponent();
    this.myTextBox.LostFocus += MyTextBox_LostFocus;
}

private void MyTextBox_LostFocus(object sender, RoutedEventArgs e)
{
    // If the IntermediateText has no validation errors, then update your model.
    if (string.IsNullOrEmpty(this.ViewModel.SomeNumberViewModel[nameof(this.ViewModel.SomeNumberViewModel.IntermediateText)]))
    {
        // Update your model and it gets formatted result
        this.ViewModel.SomeNumberViewModel.ModelValue = this.ViewModel.SomeNumberViewModel.IntermediateText;

        // Then, update your IntermediateText to update the UI.
        this.ViewModel.SomeNumberViewModel.IntermediateText = this.ViewModel.SomeNumberViewModel.ModelValue;
    }
}

MainViewModel.cs

public class MainViewModel : ViewModelBase
{
    private SomeNumberViewModel someNumberViewModel;

    public string MyTitle { get => "Stack Overflow Question 65279367"; }

    public SomeNumberViewModel SomeNumberViewModel
    {
        get
        {
            if (this.someNumberViewModel == null)
                this.someNumberViewModel = new SomeNumberViewModel(new MyModel());
            return this.someNumberViewModel;
        }
    }
}

SomeNumberViewModel.cs

public class SomeNumberViewModel : ViewModelBase, IDataErrorInfo
{
    public SomeNumberViewModel(MyModel model)
    {
        this.Model = model;
    }

    private string intermediateText;
    public string IntermediateText { get => this.intermediateText; set { this.intermediateText = value; RaisePropertyChanged(); } }
    public string ModelValue 
    { 
        get => this.Model.SomeNumber.ToString("0.00"); 
        
        set 
        {
            try
            {
                this.Model.SomeNumber = Convert.ToDouble(value);
                RaisePropertyChanged();
            }
            catch
            {
            }
        } 
    }

    public MyModel Model { get; private set; }
    
    public string Error { get => null; }

    public string this[string columnName]
    {
        get
        {
            switch (columnName)
            {
                case "IntermediateText":
                    if (!string.IsNullOrEmpty(this.IntermediateText) && FormatErrors(this.IntermediateText))
                        return "Format errors";
                    break;
            }

            return string.Empty;
        }
    }

    /// <summary>
    /// Only allow numbers to be \d+, or \d+\.\d+
    /// For Example: 1, 1.0, 11.23, etc.
    /// Anything else is a format violation.
    /// </summary>
    /// <param name="numberText"></param>
    /// <returns></returns>
    private bool FormatErrors(string numberText)
    {
        var valid = (Regex.IsMatch(numberText, @"^(\d+|\d+\.\d+)$"));
        return !valid;
    }
}

MyModel.cs

public class MyModel
{
    public double SomeNumber { get; set; }
}