利用UserControl中的替代来更改属性中断触发器

Question

I have created an instance of a TextBox that implements ICommandSource, I would like to control the IsEnabled property via the DataContext. This portion of my code works, on top of this I would like to control the Text property via this same method or by extension the IsEnabled property.

Basically when the TextBox transitions from IsEnabled="False" to IsEnabled="True" I would like to reset the Text field to an empty string or preferably null.

I have attempted to do this in a handful of ways without success.

Attempt 1

<ctrl:CommandTextBox x:Name="txtSerialNumber"
                     Command="{Binding VMFactory.CreateViewModelCommand, RelativeSource={RelativeSource AncestorType={x:Type UserControl}}}"
                     CommandParameter="{Binding Text, RelativeSource={RelativeSource Self}}" DecoderPrefix="S">
    <ctrl:CommandTextBox.Style>
        <Style TargetType="{x:Type ctrl:CommandTextBox}">
            <Style.Triggers>
                <DataTrigger Binding="{Binding}" Value="{x:Null}">
                    <Setter Property="IsEnabled" Value="True" />
                    <Setter Property="Text" Value="{x:Null}" />
                </DataTrigger>
            </Style.Triggers>
            <Setter Property="IsEnabled" Value="False" />
            <Setter Property="Text" Value="{Binding SerialNumber, Mode=OneWay}" />
        </Style>
    </ctrl:CommandTextBox.Style>
</ctrl:CommandTextBox>

This does work but only when the CommandParameter does not need to be "Decoded". It seems as though when my text property is changed via the override it breaks the trigger until the application is restarted.

CommandTextBox.cs

public class CommandTextBox : DecoderTextBox, ICommandSource
{
    // Additional Fields, Properties, and Methods removed for the sake of brevity.

    protected override void OnKeyDown(KeyEventArgs e)
    {
        base.OnKeyDown(e);

        if (e.Key == Key.Enter && Command != null)
        {
            RoutedCommand command = Command as RoutedCommand;

            if (command != null)
                command.Execute(CommandParameter, CommandTarget);
            else
                Command.Execute(CommandParameter);

            if (CommandResetsText)
                this.Text = String.Empty;

            e.Handled = true;
        }
    }
}

DecoderTextBox.cs

public class DecoderTextBox : TextBox
{
    public static DependencyProperty DecoderPrefixProperty = DependencyProperty.Register("DecoderPrefix", typeof(string), typeof(DecoderTextBox), new PropertyMetadata(String.Empty));

    public string DecoderPrefix
    {
        get { return (string)GetValue(DecoderPrefixProperty); }
        set { SetValue(DecoderPrefixProperty, value); }
    }

    protected override void OnKeyDown(KeyEventArgs e)
    {
        if (e.Key == Key.Enter)
        {
            string text = this.Text;

            // If the if statement returns true the trigger will break.
            if (text.Substring(0, Math.Min(DecoderPrefix.Length, text.Length)) == DecoderPrefix)
                this.Text = text.Remove(0, DecoderPrefix.Length);
        }

        base.OnKeyDown(e);
    }
}

Is there something specific to my implementation of OnKeyDown that is breaking this trigger?

Answer 1

There is an issue related to setting the value of a DependencyProperty locally. It appears as though you have to use SetCurrentValue to maintain the binding.

DecoderTextBox.cs

protected override void OnPreviewKeyDown(KeyEventArgs e)
{
    if (e.Key == Key.Enter)
    {
        if (Text.StartsWith(DecoderPrefix))
            SetCurrentValue(TextProperty, Text.Remove(0, DecoderPrefix.Length));
    }

    base.OnPreviewKeyDown(e);
}