SelectedItem binding is not updating the ViewModel

Question

I feel like I'm missing something obvious because this is so simple. Thanks in advance for the help. I'm struggling with binding the SelectedItem of a simple ListView in a Xamarin application. I'm testing on UWP and am using Prism's MVVM BindableBase base class. Here's what I'm experiencing:

The page loads and nothing in the list is selected.
I select an item in the list.
The setter of SelectedGrade is called with a value of null.
After that, selecting items does not cause the SelectedGrade setter to be called.

Here's the relevant XAML:

<ListView BackgroundColor="#7F7F7F"
          CachingStrategy="RecycleElement"
          IsPullToRefreshEnabled="True"
          IsRefreshing="{Binding IsBusy, Mode=OneWay}"
          ItemsSource="{Binding Grades, Mode=OneWay}"
          SelectedItem="{Binding SelectedGrade, Mode=TwoWay}"
          RefreshCommand="{Binding RefreshCommand}"
          RowHeight="50">

    <ListView.ItemTemplate>
        <DataTemplate>
            <ViewCell>
                <StackLayout HorizontalOptions="FillAndExpand"
                             VerticalOptions="CenterAndExpand"
                             Orientation="Horizontal"
                             Padding="10">
                    <Label HorizontalOptions="FillAndExpand"
                           Text="{Binding Title}"
                           TextColor="#272832"/>
                    <Label HorizontalOptions="FillAndExpand"
                           Text="{Binding Score}"
                           TextColor="Aquamarine" />
                </StackLayout>
            </ViewCell>
        </DataTemplate>
    </ListView.ItemTemplate>                
</ListView>

Here's the ViewModel:

Grade _selectedGrade;
public Grade SelectedGrade
{
    get { return _selectedGrade; }
    set { SetProperty(ref _selectedGrade, value); } // this gets set to NULL the first time i select an item then never gets set again
}

Edit: I've also added an ItemSelected event listener in the code-behind and it is not being fired after the initial item is selected. The one time it is fired, the SelectedItemChangedEventArgs reveal that the ListView's SelectedItem is null. I can also see at this time that the ListView's ItemsSource has a collection of three items in it, as I would expect. I'm quite confused as to why the ListView thinks the SelectedItem is null and why it is not broadcasting when the selected item changes.

Have you debugged it putting a breakpoint in the SelectedGrade get ? You should watch if the view "request" it — Juan Carlos Rodriguez
@JuanCarlosRodriguez, the getter only gets called once when the page loads and the SelectedGrade is null. Then the setter is called (with a null value for some reason) when an item is first selected. Then neither the getter or setter are called again, despite other items in the ListView being selected. — NSouth
I've also added an ItemSelected event handler to the code-behind and it is not firing after initial item selection. It seems the ListView is not broadcasting when the selected item changes. — NSouth

NSouth NSouth · Accepted Answer · 2018-07-12T12:37:48

It looks like this was a bug in Xamarin. I upgraded from Xamarin.Forms 3.0.0.550146 to 3.1.0.637273 and now it works.

SelectedItem binding is not updating the ViewModel

1 Answers