每次依赖属性设置为 true 时，都不会调用从另一个用户控件绑定到依赖属性的视图模型属性

Question

I have a WPF User Control in which I place another WPF user control. Note that I only post the relevant things here to understand it.

    <UserControl x:Class="myApp.UI.Views.myMainView"
                 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:vm="clr-namespace:myApp.UI.ViewModels"
                 xmlns:v="clr-namespace:myApp.UI"
                 xmlns:p="clr-namespace:myApp.Properties"
                 mc:Ignorable="d"
                 d:DataContext="{d:DesignInstance Type=vm:myMainViewModel}"
                 d:DesignHeight="450" d:DesignWidth="800">
    
            <Button x:Name="myHelpButton"/>

            <v:myAnotherUserControl PlacementTarget="{Binding ElementName=myHelpButton}"
 IsHyperlinkChangedCallback="{Binding Path=IsHyperlinkChangedCallback, Mode=OneWayToSource}" HyperLinkText="{x:Static p:Resources.HyperlinkText}">
            </v:myAnotherUserControl>
    
    </UserControl>

The important thing here you must pay attention is the view model property IsHyperlinkChangedCallback which is bound to a dependency property (which same name) from the myAnotherUserControl user control. This is set to be OneWayToSource because it is only updated within myAnotherUserControl and I want the main user control myMainView to be notified when this happens and if so to execute the ShowHelp() method (see later in this post) in the view model.

Above user control is based on MVVM and its view model associated is the following (only relevant part for this post is shown):

myMainViewModel.cs :

namespace myApp.UI.ViewModels
{
    public partial class myMainViewModel : ViewModelBase
    {

          bool _isHyperlinkChangedCallback = false;

          // This property is never set from this class! it comes always set from
          // the myAnotherUserControl which is the one who can do it. In this class
          // we only process the notification that comes and proceed accordingly.
          public bool IsHyperlinkChangedCallback
          {
             get => this._isHyperlinkChangedCallback;

             set
             {
                this._isHyperlinkChangedCallback = value;

                if (value)
                {
                   this.ShowHelp();
                }
             }
          }

          private void ShowHelp()
          {
               // Do some stuff here
          }
    }
}

In the myAnotherUserControl I only have an WPF Popup. In the WPF Popup below i have some TextBlocks and finally at the bottom an WPF hyperlink (All them within a stackpanel). I only show here the WPF hyperlink (the rest is not necessary for this post):

myAnotherUserControl.xaml :

<UserControl x:Class="myApp.UI.myAnotherUserControl"
             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:local="clr-namespace:myApp.UI"
             mc:Ignorable="d"
             d:DataContext="{d:DesignInstance Type=local:myAnotherUserControl}"
             d:DesignHeight="450" d:DesignWidth="800">
   <Grid>
        <Popup AllowsTransparency="True"                               
               StaysOpen="False"
               Placement="Custom"
               PlacementTarget="{Binding Path=PlacementTarget}">

                <TextBlock Visibility="Visible">
                    <Hyperlink Click="myHyperlink_Click" 
                               TextDecorations="None">
                        <TextBlock Text="{Binding Path=HyperLinkText}"/>                                           
                    </Hyperlink>
                </TextBlock>
        </Popup>
   </Grid>
</UserControl>

In case of myAnotherUserControl, I am not using MVVM, so the code-behind for above myAnotherUserControl.xaml is below. The myAnotherUserControl has some dependency properties defined.

myAnotherUserControl.xaml.cs :

namespace myApp.UI
{
    public partial class myAnotherUserControl : UserControl
    {

        public static readonly DependencyProperty IsHyperlinkChangedCallbackProperty =
            DependencyProperty.Register("IsHyperlinkChangedCallback",
                                        typeof(bool),
                                        typeof(myAnotherUserControl),
                                        new FrameworkPropertyMetadata(false, FrameworkPropertyMetadataOptions.BindsTwoWayByDefault));

        public bool IsHyperlinkChangedCallback
        {
            get => (bool)GetValue(IsHyperlinkChangedCallbackProperty );
            set => SetValue(IsHyperlinkChangedCallbackProperty, value);
        }

        public static readonly DependencyProperty HyperLinkTextProperty =
            DependencyProperty.Register("HyperLinkText",
                                        typeof(string),
                                        typeof(myAnotherUserControl),
                                        new FrameworkPropertyMetadata(defaultValue: "Click me!"));

        public string HyperLinkText
        {
            get => (string)GetValue(HyperLinkTextProperty);
            set => SetValue(HyperLinkTextProperty, value);
        }

        public static readonly DependencyProperty PlacementTargetProperty =
            DependencyProperty.Register("PlacementTarget",
                                        typeof(UIElement),
                                        typeof(myAnotherUserControl),
                                        new FrameworkPropertyMetadata(defaultValue: null,
                                            propertyChangedCallback: OnPlacementTargetChanged));

        public UIElement PlacementTarget
        {
            get => (UIElement)GetValue(PlacementTargetProperty);
            set => SetValue(PlacementTargetProperty, value);
        }

        private void myHyperlink_Click(object sender, RoutedEventArgs e)
        {
             // I want to notify to the myMainView in order it can execute the ShowHelp() method when it is true.
             this.IsHyperlinkChangedCallback = true;
        }
    }
}

What's the problem I have?

When I click on the hyperlink in the Popup, the myHyperlink_Click gets executed, sets the IsHyperlinkChangedCallback to true (it is false previously, I have checked putting a breakpoint there), but the view model property IsHyperlinkChangedCallback is never called (see class myMainViewModel above at the beginning of this post) so method ShowHelp() is not executed. I need ShowHelp() method to be executed each time this.IsHyperlinkChangedCallback is set to true within myHyperlink_Click event handler.

I have tried to replace this line:

IsHyperlinkChangedCallback="{Binding Path=IsHyperlinkChangedCallback, Mode=OneWayToSource}"

By this one:

IsHyperlinkChangedCallback="{Binding Path=IsHyperlinkChangedCallback, Mode=OneWayToSource, NotifyOnTargetUpdated=True}"

But it is not working.

Answer 1

Are you setting the DataContext of myAnotherUserControl.xaml to itself? If so, the following binding probably wouldn't work:

IsHyperlinkChangedCallback="{Binding Path=IsHyperlinkChangedCallback, Mode=OneWayToSource}"

because the binding path points to the myAnotherUserControl.xaml UserControl. I would try something like this:

IsHyperlinkChangedCallback="{Binding Path=DataContext.IsHyperlinkChangedCallback, Mode=OneWayToSource, RelativeSource={RelativeSource FindAncestor, AncestorType=local:myMainView}}"