Question

I have a view model to manage a dialog type of view that allows filtering of a listing (if necessary) and selection of an item. The code works fine whether I set IsSynchronizedWithCurrentItem to true or not. My understanding is that this property is not true by default in a ListView, so I’d like to better understand this property.

Here is the binding setup in the view’s xaml (which works just as well without the synch property setting):

    <ListView  
          ItemsSource="{Binding Projects.View}" 
          IsSynchronizedWithCurrentItem="True"
          SelectedItem="{Binding SelectedProject, Mode=TwoWay}"             
                      >

The view model Projects is actually a CollectionViewSource that is backed by a private ObservableCollection. I think I glommed this idea from a sample project of Josh Smith’s, but I honestly don’t recall right now. Here is the relevant portion of the VM that relates to the xaml binding:

private ObservableCollection<ProjectViewModel> _projectsInternal { get; set; }
public CollectionViewSource Projects { get; set; }

private void _populateProjectListings(IEnumerable<Project> openProjects) {
    var listing = (from p in openProjects 
                   orderby p.Code.ToString()
                   select new ProjectViewModel(p)).ToList();

    foreach (var pvm in listing)
            pvm.PropertyChanged += _onProjectViewModelPropertyChanged;

    _projectsInternal = new ObservableCollection<ProjectViewModel>(listing);

    Projects = new CollectionViewSource {Source = _projectsInternal};
}

/// <summary>Property that is updated via the binding to the view</summary>
public ProjectViewModel SelectedProject { get; set; }

The Filter property of the CollectionViewSource has a handler which returns various predicates on the view model items in the list which is picked up by the bindings correctly. Here is the gist of that code (which is in the same ProjectSelctionViewModel):

    /// <summary>Trigger filtering of the <see cref="Projects"/> listing.</summary>
    public void Filter(bool applyFilter)
    {
        if (applyFilter)
            Projects.Filter += _onFilter;
        else
            Projects.Filter -= _onFilter;

        OnPropertyChanged<ProjectSelectionViewModel>(vm=>vm.Status);
    }

    private void _onFilter(object sender, FilterEventArgs e)
    {
        var project = e.Item as ProjectViewModel;
        if (project == null) return;

        if (!project.IsMatch_Description(DescriptionText)) e.Accepted = false;
        if (!project.IsMatch_SequenceNumber(SequenceNumberText)) e.Accepted = false;
        if (!project.IsMatch_Prefix(PrefixText)) e.Accepted = false;
        if (!project.IsMatch_Midfix(MidfixText)) e.Accepted = false;
        if (!project.IsAvailable) e.Accepted = false;
    }

Setting the Mode=TwoWay is redundant since the ListView’s SelectedItem binding is two way by default, but I don’t mind being explicit about it (I might feel differently about that once I understand WPF better).

What about my code is making the IsSynchronizedWithCurrentItem=True redundant?

My gut is that this is decent code, but I don’t like that pieces of it seem to be working via “magic”, which means I would welcome any constructive feedback!

Cheers,
Berryl

Answer 1

IsSynchronizedWithCurrentItem syncs the CurrentItem of the default CollectionView of the bound collection with the SelectedItem of your control.

Since you never use the CurrentItem of the CollectionView and you do not appear to bind to the same collection twice, setting the property in question has no visible effect at all.

---

Demo of how the property syncs (for XAML viewers like XAMLPad):

<Page xmlns="http://schemas.microsoft.com/winfx/2006/xaml/presentation"
        xmlns:sys="clr-namespace:System;assembly=mscorlib"
        xmlns:x="http://schemas.microsoft.com/winfx/2006/xaml">
    <Page.Resources>
        <x:Array x:Key="Items" Type="{x:Type sys:String}">
            <sys:String>Apple</sys:String>
            <sys:String>Orange</sys:String>
            <sys:String>Pear</sys:String>
            <sys:String>Lime</sys:String>
        </x:Array>
    </Page.Resources>
    <ScrollViewer HorizontalScrollBarVisibility="Auto" VerticalScrollBarVisibility="Auto">
        <StackPanel Background="Transparent">
            <ListBox IsSynchronizedWithCurrentItem="True" ItemsSource="{StaticResource Items}" />
            <ListBox IsSynchronizedWithCurrentItem="True" ItemsSource="{StaticResource Items}" />
            <!-- This TextBlock binds to the CurrentItem of the Items via the "/" -->
            <TextBlock Text="{Binding Source={StaticResource Items}, Path=/}"/>
        </StackPanel>
    </ScrollViewer>
</Page>