Improvement for ObservableValidator

[borrmann]

In most scenarios, fields should be validated after the text of each field changes and not an entire form at once after a post attempt. This can already be accomplished with the current ObservableValidator class by inheriting from it for each property. I think the source generators could potentially implement a bit more functionality or the docs could be updated to showcase what is necessary to implement this functionality.

This is how I am currently using the ObservableValidator and I think this would be helpful to others:

First I create a class that implements the typical validation behavior. which can be extended with more functionality.

public partial class InputValidator : ObservableValidator
{
        public abstract string Text { get; set; }

        [ObservableProperty]
        private bool hasValidated;

        [ObservableProperty]
        private string error;

        [RelayCommand]
        void Validate()
        {
            ValidateAllProperties();
            HasValidated = true;

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

here is an example how this can be used with a bindable property and in XAML (for MAUI)

public partial class FirstNameValidator : InputValidator
{
        private string text;

         [ObservableProperty]
         [MinLength(2, ErrorMessage = "Name muss mind. 2 Zeichen haben")]
         [MaxLength(30, ErrorMessage = "Name darf max. 30 Zeichen haben")]
         [Required(ErrorMessage = "Vorname ist Pflichtfeld")]
         public override string Text
         {
            get => text;
            set => SetProperty(ref text, value);
         }
}

public FirstNameValidator FirstName { get; set; } = new FirstNameValidator();

<Entry Margin="10,0"
                                      Text="{Binding Firstname.Text}"
                                      Placeholder="Vorname"
                                      ReturnType="Next"
                                      BackgroundColor="Transparent"
                                      HorizontalOptions="Fill">
                    <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>

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