TextChanged or ContentChanged event in a custom control extending UserControl WPF

Question

I have a custom control in an open source application that I want to change.

The XAML looks like this:

<controls:Scratchpad Grid.Row="1" Grid.Column="2"
                         Text="{Binding DataContext.KeyboardOutputService.Text, RelativeSource={RelativeSource AncestorType=controls:KeyboardHost}, Mode=OneWay}"/>

The codebehind for the Scratchpad control looks like this:

public class Scratchpad : UserControl
{
    public static readonly DependencyProperty TextProperty =
        DependencyProperty.Register("Text", typeof (string), typeof (Scratchpad), new PropertyMetadata(default(string)));

    public string Text
    {
        get { return (string) GetValue(TextProperty); }
        set { SetValue(TextProperty, value); }
    }
}

I want to trigger an event handler each time the text changes in the UserControl. However there is no TextChanged event that I can use in the XAML.

My plan was to do something like this:

<controls:Scratchpad Grid.Row="1" Grid.Column="2"
                         Text="{Binding DataContext.KeyboardOutputService.Text, RelativeSource={RelativeSource AncestorType=controls:KeyboardHost}, Mode=OneWay}"
                         textChanged="EventHandler"/>

However the "textChanged" event does not exist in this custom control.

As you can see, ScratchPad extends UserControl. UserControl also extends ContentControl, and that's why I think it is possible to put text in this control, their might be a "ContentChanged" event that I don't know about.

Best, Peter.

See [How to: Create a Custom Routed Event](https://msdn.microsoft.com/en-us/library/ms752288(v=vs.110).aspx), then raise your custom TextChangedEvent in the PropertyChangedCallback of the Text property (shown in the answer by Eric). — Clemens, Feb 10 '17 at 14:27
Or declare an ordinary CLR event and fire it in the PropertyChangedCallback. — Clemens, Feb 10 '17 at 14:44

score 2 · Accepted Answer · answered Feb 10 '17 at 14:18

Two options:

(MVVM way) If the change is to reflect something in the domain model, perhaps this change is best suited for handling in your viewmodel

(Control way) Have you considered putting a changed handler in your DependencyProperty?

public static readonly DependencyProperty TextProperty = DependencyProperty.Register(nameof(Text), typeof(string), typeof(ScratchPad), new PropertyMetadata(null, OnTextChanged));

private static void OnTextChanged(DependencyObject d, DependencyPropertyChangedEventArgs e)
{
    // Handle change here
}

score 0 · Answer 2 · answered Feb 10 '17 at 15:08

Thanks a lot for your answer Eric.

I ended up putting in an extra line of code in the Setter of the "KeyboardOutputService.Text". However if I were to add an OnTextChanged eventhandler I would try your approach. I might run into the same problem later, so I'll keep this thread up.

Thanks a lot.

score 0 · Answer 3 · answered Jan 13 '23 at 20:08

In your custom control you can add:

public event EventHandler? ValueChanged;

And in parent control add the value to this handler (should even be visible in Designer). Add in xaml file of parent:

ValueChanged="name_ValueChanged"

And in associated cs file:

private void name_ValueChanged(object sender, EventArgs e) {
            //Your logic here
}

And you should be able to see it in Designer:

TextChanged or ContentChanged event in a custom control extending UserControl WPF

3 Answers3