依赖属性绑定和更新到自定义控件 - 它更新，但显然不是两种方式？

Question

ViewModel:

public class MyViewModel : INotifyPropertyChanged
{
    public string MyText { ... }
}

XAML:

<my:MySpecialTextBox Text="{Binding MyText}" />

Custom Control:

public class MySpecialTextBox : TextBox
{
    static MySpecialTextBox()
    {
        TextProperty.OverrideMetadata(typeof(MySpecialTextBox),
            new FrameworkPropertyMetadata
            {
                BindsTwoWayByDefault = true,
                DefaultValue = string.empty,
                PropertyChangedCallback = OnTextPropertyChanged
            });
    }

    private static void OnTextPropertyChanged(DependencyObject d, DependencyPropertyChangedEventArgs e)
    {
        var control = d as MySpecialTextBox;

        if (control != null)
        {
            control.Text = SomeAdjustedValue((string)e.NewValue);
        }
    }
}

---

The problem is that while the DependencyProperty in the custom control does adjust properly, it does not update the ViewModel. I realize that this seems as if it should be a CoerceValueCallback due to the naming of SomeAdjustedValue, but Coercion does not change the ViewModel value either. I can't seem to update the value in my ViewModel if it was the trigger for the OnTextPropertyChanged callback to begin with... I did a debug trace and it does not go through the ViewModel a second time with the new value. Not sure what to do here to fix this.

Answer 1

Did you try this?

private static void OnTextPropertyChanged(DependencyObject d, DependencyPropertyChangedEventArgs e)
{
    var control = d as MySpecialTextBox;

    if (control != null)
    {
        control.SetCurrentValue(TextBox.TextProperty, SomeAdjustedValue((string)e.NewValue));
    }
}

The SetCurrentValue() method ensures that the binding is preserved, where a simple SetValue() which is what is called under the hood if you use the Text property setter will remove any binding.

Answer 2

In the FrameworkPropertyMetadata, there are different constructors you can use. Use one with the FrameworkPropertyMetadataOptions parameter. The FrameworkPropertyMetadataOptions.BindsTwoWayByDefault option will turn on two way binding by default, otherwise it's just one way.

Edit: So you did, I should stop trying to answer questions when I'm sick.

Answer 3

A disclaimer first- this smells like logic that should live inside of the ViewModel, not in the UI binding.

That said, if you are set on doing it this way, I think that you need to first check whether the "Adjusted Value" is different than the one already provided (to avoid looping indefinitely), then use DependencyProperty.SetValue to set the value of the dependency property on the control, rather than just setting it's Text property.

Answer 4

TextBox.Text is a binding, and you are replacing that binding with a string value in OnTextPropertyChanged , so the property is no longer bound to your datasource.

I think you would need to get the binding on TextBox.Text and update the source, however I wouldn't recommend doing that since you'd be mixing your Business Logic layer with your UI layer.

If you only want a display some custom formatting, I would do it in a Converter so it doesn't actually change your data source's value. If you want to change the actual data source value, I would do that with the ViewModel's PropertyChanged event