Why does value of picker disappear while listview is tapped or scrolled?

Question

I have Listview, that use DataTemplateSelector for dynamic adding of custom DataTemplates (containing picker). My issue sets when I scroll or tap listview. In this case are selected items of pickers set to null (there is no value in the row).

Here is part of my code...

xaml:

        <DataTemplate x:Key="PickerTemplate">
            <ViewCell Height="70">
                <ViewCell.View>
                    <StackLayout Orientation="Vertical" Padding="5" >
                        <Label Text="{Binding name}" FontAttributes="Bold"/>
                        <StackLayout Orientation="Horizontal" HorizontalOptions="FillAndExpand">
                            <local:myPicker x:Name="Picker_cell"  ItemsSource="{Binding items}" SelectedIndex="{Binding selectedIndex}" TextColor="{Binding TextColor}" IsEnabled="{Binding enabled}"
                                    HorizontalOptions="FillAndExpand" VerticalOptions="FillAndExpand" HorizontalTextAlignment="End" SelectedIndexChanged="Picker_cell_SelectedIndexChanged"
                                    CommandParameter="{Binding commandParameter}" Focused="Picker_cell_Focused"/>
                        </StackLayout>
                    </StackLayout>
                </ViewCell.View>
            </ViewCell>
        </DataTemplate>
.
.
.
        <local:CustomCellSelector x:Key="CustomCellSelector"
                         PickerTemplate="{StaticResource PickerTemplate}"
                         ButtonTemplate="{StaticResource ButtonTemplate}"
                         EntryTemplate="{StaticResource EntryTemplate}"  
                         SwitchTemplate="{StaticResource SwitchTemplate}"/>
.
.
.
        <ListView x:Name="mainlistView" ItemTemplate="{StaticResource CustomCellSelector}" SelectionMode="None" SeparatorColor="Black" HasUnevenRows="True" CachingStrategy="RecycleElement" 
                  ItemsSource="{Binding CustomCells}" Grid.Row="0" Grid.Column="0"/>

DataTemplateSelector:

public class CustomCellSelector : DataTemplateSelector
{
    public DataTemplate PickerTemplate { get; set; }
    public DataTemplate ButtonTemplate { get; set; }
    public DataTemplate EntryTemplate { get; set; }
    public DataTemplate SwitchTemplate { get; set; }

    public CustomCellSelector()
    {
        this.PickerTemplate = new DataTemplate(typeof(PickerCell));
        this.ButtonTemplate = new DataTemplate(typeof(ButtonCell));
        this.EntryTemplate = new DataTemplate(typeof(EntryCell));
        this.SwitchTemplate = new DataTemplate(typeof(SwitchCell));
    }

    protected override DataTemplate OnSelectTemplate(object item, BindableObject container)
    {
        if (item is PickerCell) return PickerTemplate;
        if (item is ButtonCell) return ButtonTemplate;
        if (item is EntryCell) return EntryTemplate;
        return SwitchTemplate;
    }
}

ViewModel of Picker:

public class PickerCell : CustomCell
{
    .
    .
    .
    private int _selectedIndex;
    public int selectedIndex
    {
        get
        {
            return _selectedIndex;
        }
        set
        {
            if (_selectedIndex != value)
            {
                _selectedIndex = value;
                OnPropertyChanged(nameof(selectedIndex));
            }
        }
    }
    .
    .
    .
}

Does anyone know how to fix this issue?

Thank you for your response!

Answer 1

If value of picker is undesirable value (selectedIndex = -1, selectedIndex = null...), then don't change it.

Edited ViewModel of Picker:

public class PickerCell : CustomCell
{
    .
    .
    .
    private int _selectedIndex;
    public int selectedIndex
    {
        get
        {
            return _selectedIndex;
        }
        set
        {
            if (_selectedIndex != value)
            {
                _selectedIndex = value;
                if(_selectedIndex != -1) OnPropertyChanged(nameof(selectedIndex));
            }
        }
    }
    .
    .
    .
}