Question

I have am writing a WPF application, and I have a textbox for the user to enter a frames per second value for video playback. The value of this textbox is bound to a dependency property in the code behind (trying to follow MVVM like a good designer). My problem is that the textbox is not updating automatically when the FPS value is changed externally. For example, the user can control the value using a slider. The dependency properties value is changed correctly by the slider, but the textboxes text never updates, unless, of course, i do it manualy using GetBindingExpression(..).UpdateTarget() which is what I have implemented pending a better solution. Does anyone know if this is intended functionality or am I setting something up wrong?

Thanks,
Max

TextBox tag in XAML:

<TextBox Text="{Binding FPS}" Name="tbFPS" FlowDirection="RightToLeft"/>

Code behind for dependency property:

    #region public dependency property int FPS

    public static readonly DependencyProperty FPSProperty =
        DependencyProperty.Register("FPSProperty", typeof(int), typeof(GlobalSettings),
        new PropertyMetadata(MainWindow.appState.gSettings.fps,FPSChanged,FPSCoerce),
        FPSValidate);

    public int FPS
    {
        get { return (int)GetValue(FPSProperty); }
        set { SetValue(FPSProperty, value); }
    }

    private static bool FPSValidate(object value)
    {
        return true;
    }

    private static object FPSCoerce(DependencyObject obj, object o)
    {
        return o;
    }

    private static void FPSChanged(DependencyObject obj, DependencyPropertyChangedEventArgs e)
    {
        //why do i need to update the binding manually? isnt that the point of a binding?
        //
        (obj as GlobalSettings).tbFPS.GetBindingExpression(TextBox.TextProperty).UpdateTarget();
    }

    #endregion

Answer 1

Not sure if this is the issue, but you should pass “FPS” as the property name, not “FPSProperty”, like so:

public static readonly DependencyProperty FPSProperty =
    DependencyProperty.Register("FPS", typeof(int), typeof(GlobalSettings),
    new PropertyMetadata(MainWindow.appState.gSettings.fps,FPSChanged,FPSCoerce),
    FPSValidate);