Data Annotation and MVVM Validation Behavior

[borrmann]

I think it would be really great if Data Annotation and validation could be combined. While validation in the ViewModel is backed with ObservableValidator, validation inside the View works also great with the Behaviors.
If I am not missing aynthing, there still needs to be a lot of code written or when using the UI to validate but the backend changes, errors could occur quite easily. It would be wonderful, if an annotation like this:

[ObservableProperty]
[MinLength(2, ErrorMessage = "Min 2 letters required")]
[MaxLength(30, ErrorMessage = "Max 30 letters allowed")]
[RegexPatter("{Mypattern}", ErrorMessage = "Must have format like xxx")]
[Required(ErrorMessage = "First name is required")]
private string firstName;

would be enough and would create some bindable properties like a list of errors (string).

[borrmann]

Actually, I figured out how to already do this with a bunch of availble tools from this toolkit like EventToCommand. The downside is, there has to be an ObservableValidator for each property, but I can live with that for now..

public partial class InputValidator : ObservableValidator
    {
        [ObservableProperty]
        private string error;

        [RelayCommand]
        void Validate()
        {
            ValidateAllProperties();

            if (HasErrors)
                Error = string.Join(Environment.NewLine, GetErrors().Select(e => e.ErrorMessage));
            else
                Error = string.Empty;
        }
        public void ValidateProps()
        {
            ValidateAllProperties();
        }
    }

This can be used in any viewmodel like this:

public partial class FirstNameValidator : InputValidator
        {
            [ObservableProperty]
            [MinLength(2, ErrorMessage = "Name muss mind. 2 Zeichen haben")]
            [MaxLength(30, ErrorMessage = "Name darf max. 30 Zeichen haben")]
            [Required(ErrorMessage = "Vorname ist Pflichtfeld")]
            private string firstName;
        }
        public FirstNameValidator FirstName { get; set; } = new FirstNameValidator();

and then in XAML, like so

                <Entry Margin="10,0"
                                      Text="{Binding Firstname.FirstName}"
                                      Placeholder="Vorname"
                                      ReturnType="Next"
                                      BackgroundColor="Transparent"
                                      mct:SetFocusOnEntryCompletedBehavior.NextElement="{x:Reference E_2}"
                                      HorizontalOptions="Fill"
                                      Style="{DynamicResource MainEntryStyle}">
                    <Entry.Behaviors>
                        <toolkit:EventToCommandBehavior EventName="TextChanged"
                                                        Command="{Binding Firstname.ValidateCommand}" />
                    </Entry.Behaviors>
                    <Entry.Triggers>
                        <DataTrigger TargetType="Entry"
                                     Binding="{Binding Firstname.HasErrors}"
                                     Value="True">
                            <Setter Property="Style"
                                    Value="{StaticResource ErrorEntryStyle}" />
                        </DataTrigger>
                    </Entry.Triggers>
                </Entry>
            </Border>

            <Label Text="{Binding Firstname.Error}"
                   Style="{DynamicResource ErrorTextStyle}"
                   IsVisible="{Binding Firstname.HasErrors}" />

before publishing the model to backend from the viemodel an additional check can be performed like alreay for ObservableValidator with something like this

bool AreFieldsValid()
        {
            Firstname.ValidateProps();

            return !Firstname.HasErrors;
        }

[borrmann]

@jfversluis maybe this is interesting?

[ewerspej]

This seems out of place for the MAUI Community Toolkit. The MVVM Source Generators are part of the .NET Community Toolkit.

[jfversluis]

That is my first thought as well!

[borrmann]

Okay, I will close this and open a discussion in the Community Toolkit

[borrmann]

Here is the new discussion for reference