WPF Attached Property triggering twice

Question

I am trying to learn dependency properties and attached properties, so please forgive me if you find no use in what I am trying to do.

I have a usual MVVM approach with a Window whose datacontext is set to a VM, and View which is a datatemplate containing a usercontrol targetting such VM.

I am trying to make the window container as dumb as possible, as such i'm trying to define some parameters that usually reside in the window XAML (e.g. the height) via the usercontrol using Attached Properties.

For that purpose I created the following class where I define the attached property:

public static class WpfExtensions
{
    public static readonly DependencyProperty ContainerHeightProperty = DependencyProperty.RegisterAttached(
      "ContainerHeight",
      typeof(double),
      typeof(WpfExtensions),
      new FrameworkPropertyMetadata(300.0, FrameworkPropertyMetadataOptions.AffectsParentArrange | FrameworkPropertyMetadataOptions.AffectsParentMeasure | FrameworkPropertyMetadataOptions.AffectsRender, ContainerHeight)
    );
    public static void SetContainerHeight(UIElement element, double value)
    {
        element.SetValue(ContainerHeightProperty, value);
    }
    public static double GetContainerHeight((UIElement element)
    {
        return (double)element.GetValue(ContainerHeightProperty);
    }

    private static void ContainerHeight(DependencyObject d, DependencyPropertyChangedEventArgs e)  
    {  
       if (d is UserControl)
       {
           UserControl l_Control = (UserControl)d;

           Binding l_Binding = new Binding();

           l_Binding.RelativeSource = new RelativeSource(RelativeSourceMode.FindAncestor, typeof(Window), 1);

           l_Binding.Path = new PropertyPath("Height");

           l_Binding.Mode = BindingMode.OneWayToSource;

           BindingOperations.SetBinding(d, e.Property, l_Binding);           
       }
    }  
}

as you can see, to achieve the control of the container window height I am creating a binding in code from the attached property up to the container window.

However the ContainerHeight change get fired twice. The first time I get a change from 300 (the default) to 1024(what is defined in XAML). Then I immediately receive another one from 1024 back to 300 and I am not understanding why.

The window code is very simple:

<Window x:Class="WpfApplication.DialogWindow"
        xmlns="http://schemas.microsoft.com/winfx/2006/xaml/presentation"
        xmlns:x="http://schemas.microsoft.com/winfx/2006/xaml"
        xmlns:lcl="clr-namespace:WpfApplication"
        Title="DialogWindow">
    <Window.Resources>
        <DataTemplate DataType="{x:Type lcl:Dialog_VM}">
            <lcl:Dialog_VM_View />
        </DataTemplate>
    </Window.Resources>

    <ContentPresenter Content="{Binding }" />

</Window>

and finally the simple ViewModel

<UserControl x:Class="WpfApplication.Dialog_VM_View"
    xmlns="http://schemas.microsoft.com/winfx/2006/xaml/presentation"
    xmlns:x="http://schemas.microsoft.com/winfx/2006/xaml"
    xmlns:mc="http://schemas.openxmlformats.org/markup-compatibility/2006" 
    xmlns:d="http://schemas.microsoft.com/expression/blend/2008"
    xmlns:lcl="clr-namespace:WpfApplication"
    mc:Ignorable="d" 
    d:DesignHeight="300" d:DesignWidth="300"
    lcl:WpfExtensions.ContainerHeight="1024">

    <StackPanel>
        <TextBlock Text="This is a test" />
    </StackPanel>

</UserControl>

WpfExtensions, first piece of code I pasted, used withing the UserControl, third piece of code I pasted. — user1464603, Oct 04 '17 at 11:44
Instead of this strange approach, did you consider to simply set the Window's SizeToContent property? — Clemens, Oct 04 '17 at 11:44

score 1 · Accepted Answer · answered Oct 04 '17 at 12:09

1

You should bind the Height property of the parent window to the attached property and not the other way around, shouldn't you?

This sets (binds) the Height of the parent window to 1024 which is the value of the dependency property in the UserControl:

private static void ContainerHeight(DependencyObject d, DependencyPropertyChangedEventArgs e)
{
    if (d is UserControl)
    {
        UserControl l_Control = (UserControl)d;
        if (!l_Control.IsLoaded)
        {
            l_Control.Loaded += L_Control_Loaded;
        }
        else
        {
            Bind(l_Control);
        }
    }
}

private static void L_Control_Loaded(object sender, RoutedEventArgs e)
{
    UserControl l_Control = (UserControl)sender;
    Bind(l_Control);
}

private static void Bind(UserControl l_Control)
{
    Window window = Window.GetWindow(l_Control);

    Binding l_Binding = new Binding();
    l_Binding.Path = new PropertyPath(WpfExtensions.ContainerHeightProperty);
    l_Binding.Source = l_Control;

    BindingOperations.SetBinding(window, Window.HeightProperty, l_Binding);
}

answered Oct 04 '17 at 12:09

mm8

163,881
10
57
88

This achieves exactly what I was expecting. I complicated the things trying to bind the target to the source and using onewaytosource with the target being the control while as you showed, binding with the window being the target and the control being the source is the correct way. I reached a brain-lock blinded by the visual hierarchy. Thank you. – user1464603 Oct 04 '17 at 12:40
@user1464603 You may also find that a Binding is no longer necessary at all. Just set the Window's Height to the current ContentHeight property value in the "Bind" method (as long as ContentHeight doesn't change later). – Clemens Oct 04 '17 at 12:41
Sorry, stackoverflow rookie. – user1464603 Oct 04 '17 at 12:48
@Clemens. I know that works. However if I want to keep the logic self-contained within the UserControl, including further bindings (i.e. from a textbox.Text) to the attached property within the same usercontrol to affect the container height, using the direct value wont help. – user1464603 Oct 04 '17 at 12:51

WPF Attached Property triggering twice

1 Answers1