Propiedad adjunta para actualizar el disparador de estilo en el evento

I'm trying to use an attached property to trigger a style change on a UIElement when an event has fired.

Here is the case scenario:

A user sees a TextBox, and focuses then unfocuses it. Somewhere in an attached property it notices this LostFocus event and sets a property (somewhere?) to say that it HadFocus.

The style on the TextBox then knows that it should style itself differently based on this HadFocus property.

Here's how I imagine the markup to look...

<TextBox Behaviors:UIElementBehaviors.ObserveFocus="True">
    <Style TargetType="TextBox">
            <Trigger Property="Behaviors:UIElementBehaviors.HadFocus" Value="True">
                <Setter Property="Background" Value="Pink"/>

I've tried a few combinations of the attached properties to get this working, my latest attempt throws a XamlParseException indicando "Property can not be null on Trigger."

    public class UIElementBehaviors
    public static readonly DependencyProperty ObserveFocusProperty =
                                            typeof (bool),
                                            typeof (UIElementBehaviors),
                                            new UIPropertyMetadata(false, OnObserveFocusChanged));
    public static bool GetObserveFocus(DependencyObject obj)
        return (bool) obj.GetValue(ObserveFocusProperty);
    public static void SetObserveFocus(DependencyObject obj, bool value)
        obj.SetValue(ObserveFocusProperty, value);

    private static void OnObserveFocusChanged(DependencyObject d, DependencyPropertyChangedEventArgs e)
        var element = d as UIElement;
        if (element == null) return;

        element.LostFocus += OnElementLostFocus;
    static void OnElementLostFocus(object sender, RoutedEventArgs e)
        var element = sender as UIElement;
        if (element == null) return;

        SetHadFocus(sender as DependencyObject, true);

        element.LostFocus -= OnElementLostFocus;

    private static readonly DependencyPropertyKey HadFocusPropertyKey =
                                                    new FrameworkPropertyMetadata(false));

    public static readonly DependencyProperty HadFocusProperty = HadFocusPropertyKey.DependencyProperty;
    public static bool GetHadFocus(DependencyObject obj)
        return (bool)obj.GetValue(HadFocusProperty);

    private static void SetHadFocus(DependencyObject obj, bool value)
        obj.SetValue(HadFocusPropertyKey, value);

Anyone able to direct me?

preguntado el 22 de mayo de 12 a las 14:05

1 Respuestas

Registering a readonly dependency property does not mean to add Key to the property name. Just replace

DependencyProperty.RegisterAttachedReadOnly("HadFocusKey", ...);


DependencyProperty.RegisterAttachedReadOnly("HadFocus", ...);

desde HadFocus is the name of the property.

contestado el 22 de mayo de 12 a las 15:05

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