CollectionView SelectionMode SelectionMode="Multiple" Not working

[psp589]

Description

I can't get to work the selection multiple in CollectionView, I can't select more than one item, also I didn't see a SelectedItems property, only SelectedItem

Steps to Reproduce

Having this XAML

<ContentPage>
        <Grid>
            <Grid.ColumnDefinitions>
                <ColumnDefinition />
                <ColumnDefinition Width="auto" />
            </Grid.ColumnDefinitions>
            <CollectionView Grid.Column="0" 
                            SelectionMode="Multiple"
                            SelectedItems="{Binding SelectedItems, Mode=TwoWay}"
                            ItemsSource="{Binding ObservableCollectionTest}">
                <CollectionView.ItemsLayout>
                    <GridItemsLayout Orientation="Vertical" />
                </CollectionView.ItemsLayout>
                <CollectionView.ItemTemplate>
                    <DataTemplate>
                        <Grid>
                            <Label Text="{Binding}"></Label>
                        </Grid>
                    </DataTemplate>
                </CollectionView.ItemTemplate>
                
            </CollectionView>
            <StackLayout Orientation="Vertical" Grid.Column="1">
                  <Label Text="SelectedItems" />
                   <ListView ItemsSource="{Binding SelectedItems}"
                          WidthRequest="400"
                          BackgroundColor="Green"
                          HorizontalOptions="Center">
                    <ListView.ItemTemplate>
                        <DataTemplate>
                            <ViewCell>
                                <Grid>
                                    <Label Text="{Binding}"></Label>
                                </Grid>
                            </ViewCell>
                        </DataTemplate>
                    </ListView.ItemTemplate>    
                </ListView>
            </StackLayout>
        </Grid>
    </ContentPage>

With this data in the Model

public static string SelectedItemsPropertyName() { return nameof(SelectedItems); }
        private ObservableCollection<string>_SelectedItems;
        public ObservableCollection<string>SelectedItems
        {
            get
            {
                return _SelectedItems;
            }
            set
            {
                _SelectedItems = value;
                RaisePropertyChanged();
            }
        }
        public static string ObservableCollectionTestPropertyName() { return nameof(ObservableCollectionTest); }
        private ObservableCollection<string> _ObservableCollectionTest = new ObservableCollection<string>(new List<string>() { "Test1", "test2", "test3" } );
        public ObservableCollection<string> ObservableCollectionTest
        {
            get
            {
                return _ObservableCollectionTest;
            }
            set
            {
                _ObservableCollectionTest = value;
                RaisePropertyChanged();
            }
        }
SelectedItem = ObservableCollectionTest[1];

Expected Behavior

Could select more than 1 item

Actual Behavior

is not possible

Basic Information

• Version with issue: 4.0.0.232914-pre6 ( not working neither in 4.0.0-pre8)
• IDE: Visual Studio Community 2017 version 15.9.9
• Platform Target Frameworks:
  • Android: 8.1
  • UWP: Windows 10 version 1803 Build 17134

[adrianknight89]

#5352

[psp589]

#5352 says Android and IOS, what with UWP ? @adrianknight89

[adrianknight89]

Not sure if @hartez is still updating https://gist.github.com/hartez/7d0edd4182dbc7de65cebc6c67f72e14, but I think the change was implemented only for Android and iOS for now.

[samhouts]

This should be in 4.0.0-pre8. Thanks!

[psp589]

@samhouts I've just tried 4.0.0-pre8 and it is still not working,
in UWP I can't still select more than one item
in Android I can, but the two way binding is not working (it works in mode= "single" but not in "Multiple") so in the UI I can see more than 1 item selected but it doesnt get reflected in the viewmodel
Could this issue be re-opened?

[samhouts]

@psp589 We're currently focusing on implementing all features for Android and iOS before moving on to UWP.

We'll take a look at Android and see what's missing. Thanks!

[psp589]

@samhouts I'm working with both Android and UWP, should I add then different issues when reporting ? one for Android and one for UWP ? Should I add for this issue, a different one for UWP ?

[samhouts]

@psp589 No need to report separate issues. Things that are not implemented in UWP yet will be tracked on the main ticket. Thanks!

[samhouts]

@psp589 To make this easier to reproduce, can you please attach a small project that demonstrates this issue? Thanks!

[psp589]

@samhouts see https://github.com/psp589/XamarinFormsIssuesReproduction, last section marked as "Issue #5625"

[pikausp]

To expand on this issue

• The PropertyChanged gets called
• The binding source is not updated

What helped me was creating a simple bridge behavior that proxied SelectedItems by cloning value of the source SelectedItems property.

I believe the issue here is that the instance of the SelectionList remains the same for the lifetime of the view and so the bindings are not updated. Since there is a manual call to OnPropertyChanged

Xamarin.Forms/Xamarin.Forms.Core/Items/SelectableItemsView.cs

Line 73 in b20afe4

OnPropertyChanged(SelectedItemsProperty.PropertyName);

Xamarin.Forms/Xamarin.Forms.Core/Items/SelectionList.cs

Line 31 in b20afe4

_selectableItemsView.SelectedItemsPropertyChanged(oldItems, Copy());

the PropertyChanged is indeed invoked, but that does not trigger the system to update bindings.

[humhei]

• Set vm selection to List
• Clear VM seletion and items 1 and 2

Nothing will happened

PR #6629 should fixed this issue!

[francipvb]

Hello,

Can someone take a look on this again? I'm stuck trying to get this working on android. I have the screen reader enabled.

Cheers,

[hartez]

@francipvb Since this was closed so long ago and since you're using a screen reader, I would suggest opening a new issue.

[cdavidyoung]

Well, this explains why two way binding is not working in Multiple (at least in Android that I am testing in). I guess I am going to have to implement my own Multiple using Single. Why is this issue closed if it hasn't been fixed?

[cdavidyoung]

So even though two way binding is not working in Multiple I have been able to get multiple selection to work in my MVVM app. The trick is to declare and instantiate the list that is bound to SelectedItems with a type of object and cast it later:

    ObservableCollection<object> selectedWaypoints;
    public ObservableCollection<object> SelectedWaypoints
    {
        get => selectedWaypoints;
        set
        { // note that this is never called
           _userDialogs.Toast($"SelectedWaypoints: {value?.Count}", System.TimeSpan.FromSeconds(1));
           selectedWaypoints = value;
           RaisePropertyChanged(() => SelectedWaypoints);
        }
    }

In constructor:

        SelectedWaypoints = new ObservableCollection<object>();

bound to SelectionChangedCommand - cast to target class messageItem:

    public MvxCommand ItemSelectedCommand => new MvxCommand((() =>
    {
       if (SelectedWaypoints.Count> 0)
          _userDialogs.Toast($"ItemSelectedCommand: {SelectedWaypoints?.Count} {((messageItem)SelectedWaypoints?[SelectedWaypoints.Count-1]).Title}", System.TimeSpan.FromSeconds(1));
       else
          _userDialogs.Toast($"ItemSelectedCommand: {SelectedWaypoints?.Count}", System.TimeSpan.FromSeconds(1));
    }));

[luisyoroslav]

So even though two way binding is not working in Multiple I have been able to get multiple selection to work in my MVVM app. The trick is to declare and instantiate the list that is bound to SelectedItems with a type of object and cast it later:

    ObservableCollection<object> selectedWaypoints;
    public ObservableCollection<object> SelectedWaypoints
    {
        get => selectedWaypoints;
        set
        { // note that this is never called
           _userDialogs.Toast($"SelectedWaypoints: {value?.Count}", System.TimeSpan.FromSeconds(1));
           selectedWaypoints = value;
           RaisePropertyChanged(() => SelectedWaypoints);
        }
    }

In constructor:

        SelectedWaypoints = new ObservableCollection<object>();

bound to SelectionChangedCommand - cast to target class messageItem:

    public MvxCommand ItemSelectedCommand => new MvxCommand((() =>
    {
       if (SelectedWaypoints.Count> 0)
          _userDialogs.Toast($"ItemSelectedCommand: {SelectedWaypoints?.Count} {((messageItem)SelectedWaypoints?[SelectedWaypoints.Count-1]).Title}", System.TimeSpan.FromSeconds(1));
       else
          _userDialogs.Toast($"ItemSelectedCommand: {SelectedWaypoints?.Count}", System.TimeSpan.FromSeconds(1));
    }));

Works like a charm, thanks