更新ObservableCollection之后，Xamarin.Forms ListView发送空事件

Question

As the following approach seems not to work, I might miss a central concept of updating Xamarin.Forms ListView. Maybe you can help, how it should be done correctly.

I have got the following situation in a Xamarin.Forms Page: One ListView like that on a Page:

<ListView x:Name="listView" Margin="0" ItemSelected="OnListItemSelected" ItemsSource="{Binding Items}">
    <ListView.ItemTemplate>
        ...
    </ListView.ItemTemplate>
</ListView>

This ListView's binding is connected to Items in the ViewModel that looks basically like the following:

private ObservableCollection<Item> items = new ObservableCollection<Item>();
public ObservableCollection<Item> Items
{
    get
    {
        items.Clear();
        if (SomeSingeltonSource != null)
        {
            IEnumerable<Item> current = SomeSingeltonSource.GetCurrentItems(viewModelState);
            foreach (Item wf in current)
            {
                items.Add(wf);
            }
        }
        return items;
    }
}

And the method called on ItemSelected starts like the following:

void OnListItemSelected(object sender, SelectedItemChangedEventArgs e)
{
    if (e.SelectedItem is Item selectedItem)
    {
    ...
    }
}

Now on the after the first binding, everything works fine. But if the Items change the List is still correctly updated, however if I select an Item on the list, the OnListItemSelected method is still called, however e.SelectedItem is always null.

Everything worked fine until Xamarin.Forms 3.0.0 Service Release 1 but since Service Release 2 it is not working anymore.

Do I miss an important concept of updating a ListView or is there something else that is implemened the wrong way?

Answer 1

Generally when I use Binding for the ItemsSource , I'll also be using Binding for the SelectedItem . That way you don't need to have any method/event hookup.

So in you example do this in your view:

<ListView x:Name="listView" Margin="0" SelectedItem="{Binding SelectedItem}" ItemsSource="{Binding Items}">
    <ListView.ItemTemplate>
        ...
    </ListView.ItemTemplate>
</ListView>

And add an extra property in your binding context code like:

private Item _selectedItem;
public Item SelectedItem
{
    get { return _selectedItem; }
    set
    {
        _selectedItem = value; // TODO: Add extra code to trigger something because the user selected an item
        OnNotifyPropertyChanged();
    }
}

Answer 2

In the end it turned out that it has been a bug in Xamarin.Forms that has been solved in version 3.1.0.

If you are interested in the details, please have a look at the following issues:

• [UWP] Xamarin Forms List View SelectedItem is not working when bound to ObservableCollection in UWP ( #3017 )
• [UWP] Xamarin.Forms 3.1 ListView ItemTapped event issue for UWP. ( #2996 )