在Listpicker的多选模式下无法更新selectedItems

我试图在multiSelection模式下从lispicker检索selectedItems . 数据显示在FullModelItemTemplate中，但是当我选择项目时，我无法获取值 . 这是我的ListPicker XAML

<toolkit:ListPicker x:Name="location" ItemsSource="{Binding Items,Mode=TwoWay}" ItemTemplate="{StaticResource DataTemplate2}" FullModeItemTemplate="{StaticResource DataTemplate3}"   HeaderTemplate="{StaticResource header}" SelectionMode="Multiple" SelectedItems="{Binding SelectedItem,Mode=TwoWay}" >

MainViewModel

public class MainViewModel : ViewModelBase, INotifyPropertyChanged
   {

       public ObservableCollection<ItemViewModel> Items { get; private set; }
       public MainViewModel()
       {
           Items = new ObservableCollection<ItemViewModel>();
           LoadData();
       }

       public const string SelectedItemPropertyName = "SelectedItem";
       private ObservableCollection<ItemViewModel> selectedItem;
       public ObservableCollection<ItemViewModel> SelectedItem
       {
          get
          {
            return selectedItem;
          }

          set
          {
            if (value != selectedItem)
            {
                selectedItem = value;
                NotifyPropertyChanged(SelectedItemPropertyName);
            }
          }
        }

       public void LoadData()
       {
           // Sample data; replace with real data
           this.Items.Add(new ItemViewModel() { ID = "0", Cuisine = "Thai", CuisineImg = "Assets/Images/thsi.png", Location = "Shantinagar" });
           this.Items.Add(new ItemViewModel() { ID = "1", Cuisine = "Indian", CuisineImg = "Assets/Images/indian.png", Location = "Mirpur" });
           this.Items.Add(new ItemViewModel() { ID = "2", Cuisine = "Chinese", CuisineImg = "Assets/Images/chinese.png", Location = "Dhanmondi" });
        }


      public event PropertyChangedEventHandler PropertyChanged;
      private void NotifyPropertyChanged(String propertyName)
      {
          PropertyChangedEventHandler handler = PropertyChanged;
          if (null != handler)
          {
             handler(this, new PropertyChangedEventArgs(propertyName));
          }
       }
    }

ItemViewModel

public class ItemViewModel : INotifyPropertyChanged
   {
       private string _id;

       public string ID
       {
           get
           {
              return _id;
           }
        set
        {
            if (value != _id)
            {
                _id = value;
                NotifyPropertyChanged("ID");
            }
        }
    }

    private string _location;

    public string Location
    {
        get
        {
            return _location;
        }
        set
        {
            if (value != _location)
            {
                _location = value;
                NotifyPropertyChanged("Location");
            }
        }
    }

    private string _cuisine;
    public string Cuisine
    {
        get
        {
            return _cuisine;
        }
        set
        {
            if (value != _cuisine)
            {
                _cuisine = value;
                NotifyPropertyChanged("Cuisine");
            }
        }
    }

    private string _cuisineImg;
    public string CuisineImg
    {
        get
        {
            return _cuisineImg;
        }
        set
        {
            if (value != _cuisineImg)
            {
                _cuisineImg = value;
                NotifyPropertyChanged("CuisineImg");
            }
        }
    }
    public event PropertyChangedEventHandler PropertyChanged;
    private void NotifyPropertyChanged(String propertyName)
    {
        PropertyChangedEventHandler handler = PropertyChanged;
        if (null != handler)
        {
            handler(this, new PropertyChangedEventArgs(propertyName));
        }
    }

请注意，我正在使用MVVM光，如果selectionMode是单一的，我在获取所选值时没有遇到任何问题，但是多选模式会导致问题