CODEWITHSUNDEEP
xamlxamarinxamarin.forms
NSouth
NSouth

Reputation: 5276

SelectedItem binding is not updating the ViewModel

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:

  1. The page loads and nothing in the list is selected.
  2. I select an item in the list.
  3. The setter of SelectedGrade is called with a value of null.
  4. 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.

Upvotes: 2

Views: 533

Answers (2)

Oleg Bondarenko
Oleg Bondarenko

Reputation: 1820

I have faced with the similar issue when List does not bind to view model with SelectedItem and Xamarin forms 5.0.0.2244. It was resolved with setting default value of List property to new List() object instead null value in the view model before setting binding context . After that binding has became working.

Upvotes: 0

NSouth
NSouth

Reputation: 5276

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.

Upvotes: 0

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