如何禁用按钮，直到在 wpf mvvm 中进行良好验证

Question

如何在文本框验证良好之前禁用按钮？ 我已经做了一些事情，但我不知道如何隐藏按钮，直到使用 MVVM 模式以正确的方式进行良好的验证。 我仍在学习这种 MVVM 模式。 这是我的代码片段：

ValidationRule 类：

public class NotEmptyValidationRule : ValidationRule
{
    public override ValidationResult Validate(object value, CultureInfo cultureInfo)
    {
        return string.IsNullOrWhiteSpace((value ?? "").ToString())
            ? new ValidationResult(false, "Field is required")
            : ValidationResult.ValidResult;
    }
}

xaml:

<TextBox
Style="{StaticResource MaterialDesignFilledTextFieldTextBox}"
AcceptsReturn="True"
TextWrapping="Wrap"
materialDesign:HintAssist.Hint="Content"
IsEnabled="{Binding Path=IsChecked, 
ElementName=MaterialDesignOutlinedTextFieldTextBoxEnabledComboBox}" 
MaxLength="1000" materialDesign:ValidationAssist.UsePopup="True">
        <TextBox.Text>
            <Binding Path="Task.Content" UpdateSourceTrigger="PropertyChanged">
                <Binding.ValidationRules>
                    <domain:NotEmptyValidationRule
                     ValidatesOnTargetUpdated="True" />
                </Binding.ValidationRules>
            </Binding>
        </TextBox.Text>
    </TextBox>

视图模型：

private ICommand _submitCommand;
public ICommand SubmitCommand
{
   get
   {
       if (_submitCommand == null)
       {
          _submitCommand = new RelayCommand(SubmitExecute, anSubmitExecute, false);
       }
       return _submitCommand;
    }
}

private void SubmitExecute(object parameter)
{
     Task.Id++;
     Tasks.Add(Task);
}

private bool CanSubmitExecute(object parameter)
{
     // What can I do to prevent to show this button when validation 
     // failed
     return true;
}

Answer 1

您应该在您的Task 类中实现INotifyDataErrorInfo 接口，或者在您的视图模型类中实现它并包装数据绑定的Task.Content 属性，例如：

private RelayCommand _submitCommand;
public ICommand SubmitCommand
{
    get
    {
        if (_submitCommand == null)
        {
            _submitCommand = new RelayCommand(SubmitExecute, CanSubmitExecute, false);
        }
        return _submitCommand;
    }
}

private void SubmitExecute(object parameter)
{
    Task.Id++;
    Tasks.Add(Task);
}

public string Content
{
    get { return Task.Content; }
    set
    {
        Task.Content = value;
        if (string.IsNullOrEmpty(Task.Content))
            _validationErrors[nameof(Content)] = "Field is required";
        else
            _validationErrors.Remove(nameof(Content));
        ErrorsChanged?.Invoke(this, new DataErrorsChangedEventArgs(nameof(Content)));
        _submitCommand.RaiseCanExecuteChanged();
    }
}

private bool CanSubmitExecute(object parameter)
{
    return HasErrors;
}

#region INotifyDataErrorInfo
private readonly Dictionary<string, string> _validationErrors = new Dictionary<string, string>();

public event EventHandler<DataErrorsChangedEventArgs> ErrorsChanged;

public System.Collections.IEnumerable GetErrors(string propertyName)
{
    if (string.IsNullOrEmpty(propertyName)
        || !_validationErrors.ContainsKey(propertyName))
        return null;

    return new string[1] { _validationErrors[propertyName] };
}

public bool HasErrors
{
    get { return _validationErrors.Count > 0; }
}
#endregion

XAML：

<TextBox Text="{Binding Content, UpdateSourceTrigger=PropertyChanged, ValidatesOnNotifyDataErrors=True}" />

您不应使用验证规则。它们对 MVVM 不友好。请参阅this 博文了解更多信息。

您还需要引发ICommand 的CanExecuteChanged 事件。如何做到这一点取决于实现。大多数实现都有一个名为 RaiseCanExecuteChanged() 或类似名称的方法。

WPF 4.5: Validating Data in Using the INotifyDataErrorInfo Interface