Avalonia: Bindings to Text property in TextBox doesnt work

[pelmenstar1]

I don't know why binding to Text doesnt work in TextBox
https://github.com/pelmenstar1/test-projects/blob/master/AvaloniaBindingTest/ViewModels/MainWindowViewModel.cs

public class MainWindowViewModel : ViewModelBase
    {
        private string _str = "#0000";
        public string String
        {
            get => _str;
            set
            {
                if (value!= null && value.Length > 0 && value[0] == '#' && value.Length <= 5)
                {
                    _str = value;
                }

                this.RaisePropertyChanged(nameof(String));
            }
        }
    }

<Window xmlns="https://github.com/avaloniaui"
        xmlns:x="http://schemas.microsoft.com/winfx/2006/xaml"
        xmlns:vm="clr-namespace:AvaloniaBindingTest.ViewModels;assembly=AvaloniaBindingTest"
        xmlns:d="http://schemas.microsoft.com/expression/blend/2008"
        xmlns:mc="http://schemas.openxmlformats.org/markup-compatibility/2006"
        mc:Ignorable="d" d:DesignWidth="800" d:DesignHeight="450"
        x:Class="AvaloniaBindingTest.Views.MainWindow"
        Icon="/Assets/avalonia-logo.ico"
        Title="AvaloniaBindingTest">

    <Design.DataContext>
        <vm:MainWindowViewModel/>
    </Design.DataContext>

    <TextBox Text="{Binding String, Mode=TwoWay}" HorizontalAlignment="Center" VerticalAlignment="Center"/>

</Window>

[FoggyFinder]

Where did you find a bug here?
All works as expected.

[jp2masa]

@pelmenstar1 You probably want to validate value, not _str:

if (value != null && value.Length > 0 && value[0] == '#' && value.Length <= 5)

[pelmenstar1]

TextBox doesnt update text, but I this.RaisePropertyChanged(nameof(String))

[pelmenstar1]

@jp2masa I fixed but it works the same

[FoggyFinder]

What are you trying to achieve? Add validation for String property?

[pelmenstar1]

I'm trying to do so that if String property changed then text in TextBox should be changed as I bind String to Text

[FoggyFinder]

So your Q is: why TextBox doesn't drop value to the previous one?

[pelmenstar1]

So my Q: Why text in TextBox doesn't equal String, but I bound String to Text

[jp2masa]

It seems to be a bug.

[FoggyFinder]

I think this happens due to _ignoreTextChanges field:

https://github.com/AvaloniaUI/Avalonia/blob/18cda6726c0162eb84193f2aaef232e438aed929/src/Avalonia.Controls/TextBox.cs#L999-L1010

Changes that caused by setter in VM are simple ignored.

[pelmenstar1]

Does anybody know how to fix it?

[grokys]

Hmm, that line was added by https://github.com/AvaloniaUI/Avalonia/commit/a222fa9b0c2c14380db40d98dd553f2d744517f2:

Don't modify text due to binding when typing.

If a TextBox is e.g. bound to an int and the user types "02" then the
TextBox text should be "02" - not the value that comes back from the
binding which will be "2".

This was added in #691 back in 2016. However cross-checking with WPF, WPF doesn't seem to have this behavior. If a TextBox is bound to an int and the user types "02" then the textbox will display simply "2".

I think we need to double-check our binding behavior in such cases where the setter doesn't preserve the value written to it.

[pelmenstar1]

Is it fixable now?

[pelmenstar1]

And is it bug or not?

[grokys]

It's a difference in behavior between WPF and Avalonia at least. It needs some investigation.

In the meantime, you have a few options:

• Throw an exception in the setter if the input is in the wrong format. This should cause data validation to kick in and display the TextBox with a red outline and an error message.
• Given that your input is invalid if there's no "#", just display the "#outside theTextBox` so the user doesn't have to enter it, e.g.

<StackPanel Orientation="Horizontal">
  <TextBlock>#</TextBlock>
  <TextBox Text="{Binding String, Mode=TwoWay}"/>
<StackPanel>

• Revert the value after a time delay, e.g.

set 
{
    if (!value.StartsWith("#"))
    {
        throw new Exception("Enter a #");
    }

    DispatcherTimer.RunOnce(() => this.RaiseAndSetIfChanged(ref _stringValue, value), TimeSpan.FromMilliseconds(10))
}

[grokys]

Repoening because this needs investigation.