Error validation for usercontrol properties

[trashhacker]

How to make User Control Show validations Like textbox

[stevemonaco]

Check the TextBox control for implementation details. ie. DataValidationErrors control: https://github.com/AvaloniaUI/Avalonia/blob/master/src/Avalonia.Themes.Fluent/Controls/TextBox.xaml#L110-L124

However, I typically just expose errors as properties on the VM and have a UserControl show it through a TextBlock though. It doesn't make sense to use the same approach as lookless controls to me.

[trashhacker]

thanks

[trashhacker]

i want this what the textbox has for all usercontrols, how to archive that
could u post me a link to the default usercontrol controltemplate?
and tell me how to override the default style controltemplate globaly

[trashhacker]

Oh lord figured it out xD why do u have not implemented it as default why i have to implement it myself ? ...

public static readonly StyledProperty<string> UserTextProperty = 
    AvaloniaProperty.Register<TestUserControl, string>(nameof(UserText), defaultBindingMode:BindingMode.TwoWay, enableDataValidation:true);
public string UserText
{
    get => GetValue(UserTextProperty);
    set => SetValue(UserTextProperty, value);
}
public TestUserControl()
{
    InitializeComponent();
}
protected override void UpdateDataValidation(AvaloniaProperty property, BindingValueType state, Exception? error)
{
    if (property == UserTextProperty)
    {
        DataValidationErrors.SetError(this, error);
    }
}

<ControlTheme x:Key="{x:Type controls:TestUserControl}" TargetType="{x:Type controls:TestUserControl}">
  <Setter Property="BorderBrush" Value="Yellow"/>
  <Setter Property="BorderThickness" Value="2"/>
  <Setter Property="Template">
    <ControlTemplate>
      <DataValidationErrors >
        <Border BorderThickness="{TemplateBinding BorderThickness}" BorderBrush="{TemplateBinding BorderBrush}">
          <ContentPresenter Name="PART_ContentPresenter" Content="{TemplateBinding Content}"/>
        </Border>
      </DataValidationErrors >
    </ControlTemplate>
  </Setter>
</ControlTheme>

[stevemonaco]

Most controls do not participate in error validation, so it's an unnecessary cost to force even more required elements into the visual tree that will need to be measured, rendered, or handle events. As noted by your code, you glued together a few small components that did the work for you which is far less code than a from-scratch implementation. You're free to encapsulate this into your own derived UserControl type for reusability.

[trashhacker]

im sorry (: i was frustrated ...
but it would be very helpfull to write a better documentation on this topic.

[timunie]

but it would be very helpfull to write a better documentation on this topic.

@trashhacker feel free to open a PR on the docs. We need community support to iron out the docs.