Custom selection of Listview using checkbox in GroupHeader

ListView supports selecting each group and items in the group like a checkBox selection by customizing the SfListView.GroupHeaderTemplate and the ItemTemplate respectively. The checkbox state will be update by using converter.

<ContentPage xmlns:syncfusion="clr-namespace:Syncfusion.ListView.XForms;assembly=Syncfusion.SfListView.XForms">
  <syncfusion:SfListView>
    <syncfusion:SfListView.GroupHeaderTemplate>
        <DataTemplate>
            <ViewCell>
              <ViewCell.View>
                <Grid BackgroundColor="#d3d3d3">
                   <Grid.ColumnDefinitions>
                        <ColumnDefinition Width="30" />
                        <ColumnDefinition Width="*" />
                        <ColumnDefinition Width="Auto" />
                   </Grid.ColumnDefinitions>
                   <Label Text="{Binding Key}" Grid.Column="1" VerticalTextAlignment="Center"/>
                   <Image Grid.Column="2" IsVisible="{Binding SelectionMode, Source={x:Reference listView}}" 
                           HorizontalOptions="Center" VerticalOptions="Center"
                           Source="{Binding ., Converter={StaticResource GroupingSelectionConverter}, ConverterParameter={x:Reference listView}}">
                        <Image.GestureRecognizers>
                            <TapGestureRecognizer Tapped="TapGestureRecognizer_Tapped"/>
                        </Image.GestureRecognizers>
                   </Image>
                </Grid>
              </ViewCell.View>
            </ViewCell>
        </DataTemplate>
    </syncfusion:SfListView.GroupHeaderTemplate>
  </syncfusion:SfListView>
</ContentPage>

The checkBox state in the GroupHeaderTemplate will be updated whenever items select and deselect by using converter.

public object Convert(object value, Type targetType, object parameter, CultureInfo culture)
{
    if (value == null)
        return value;

    GroupResult groupResult = value as GroupResult;
    SfListView list = parameter as SfListView;

    var items = new List<MusicInfo>(groupResult.Items.ToList<MusicInfo>());

    if ((items.All(item => item.IsSelected == false)))
    {
        for (int i = 0; i < items.Count(); i++)
        {
            var item = items[i];
            (item as MusicInfo).IsSelected = false;
            list.SelectedItems.Remove(item);
        }
        return ImageSource.FromResource("CustomSelection.Images.NotSelected.png");
    }

    else if ((items.All(item => item.IsSelected == true)))
    {
        for (int i = 0; i < items.Count(); i++)
        {
            var item = items[i];
            (item as MusicInfo).IsSelected = true;
            list.SelectedItems.Add(item);
        }
        return ImageSource.FromResource("CustomSelection.Images.Selected.png");
    }

    else
        return ImageSource.FromResource("CustomSelection.Images.Intermediate.png");
}

To select and deselect all the items of group by tap the checkbox in the group header, follow the code example.

using Syncfusion.ListView.XForms.Control.Helpers;
private void TapGestureRecognizer_Tapped(object sender, EventArgs e)
{
    var image = (sender as Image);
    var groupResult = image.BindingContext as GroupResult;

    if (groupResult == null)
        return;
    var items = groupResult.Items.ToList<MusicInfo>().ToList();

    if ((items.All(listItem => listItem.IsSelected == true)))
    {
        for (int i = 0; i < items.Count(); i++)
           {
                var item = items[i];
                (item as MusicInfo).IsSelected = false;
            }
    }
    else if ((items.All(listItem => listItem.IsSelected == false)))
    {
        for (int i = 0; i < items.Count(); i++)
            {
                var item = items[i];
                (item as MusicInfo).IsSelected = true;
            }
    }
    this.RefreshGroupHeader(groupResult);
    listView.RefreshView();
}

private void RefreshGroupHeader(GroupResult group)
{
    foreach (var item in this.listView.GetVisualContainer().Children)
    {
      if (item.BindingContext == group)
       {
         item.BindingContext = null;
         (item as GroupHeaderItem).Content.BindingContext = null;
        }
    }
}

To know more about grouping, you can refer our documentation here