Multiselection ListView

Hi developers,

the whole day i am stuck on an (for me) difficult point and i hope there is anybody out there who can help me out

I whould like to have a ContentPage where the user can change some items from a longer list. And it is unknown and left to the user, how many items he can select.
Out of my sight I think it is the best to use the Picker control.

1. So the user can click on a plus (+) button and a new Picker control appears (insight the ListView)
2. The user selects an item from the Picker list
3. If desired he can click again on the plus button and select a second item.

And so on ... maybe until up to 10 Items from 10 different Pickers insight the ListView.

How can I access the user seleceted items in my ViewModel ?

My code:

On the ContentPage I create the ListView:

        private ScoutViewModel ViewModel
        {
            get { return BindingContext as ScoutViewModel; }
        }

        public ScoutPage()
        {
            Title = App.Localization.GetString("Title_FilterPage");

            BindingContext = new ScoutViewModel(Navigation);

            var templateScoutActivityCell = new DataTemplate(typeof(ScoutActivityCell));

            var listViewActivities = new ListView
            {
                AnchorX = 0,
                AnchorY = 0,
                VerticalOptions = LayoutOptions.StartAndExpand,
                HorizontalOptions = LayoutOptions.Start,
                ItemTemplate = templateScoutActivityCell,
                ItemsSource = ViewModel.Activities,
            };

            Content = new StackLayout
            {
                Children = 
                { 
                    listViewActivities 
                }
            };
        }

At the ViewCell (ScoutActivityCell);

        public void CreateView()
        {
            var bindablePickerAllActivities = new BindablePicker 
            { 
                AnchorY = 0,
                AnchorX = 0,
                VerticalOptions = LayoutOptions.Start,
                HorizontalOptions = LayoutOptions.FillAndExpand,
                ItemsSource = from ac in allActivities select ac.Name
            };

            bindablePickerAllActivities.SelectedIndexChanged += new EventHandler(OnSelectedIndexChanged);


            View = new StackLayout 
            { 
                Children = 
                {
                    bindablePickerAllActivities,
                }                
            };
        }