LongListSelector ItemsSource无法正常工作

Question

在我的Windows Phone 8.1商店应用中，我无法在LongListSelector中显示值。这是.xaml和.cs文件。

我错过了什么吗？

<controls:LongListSelector Grid.Row="0" Grid.Column="0"  VerticalAlignment="Stretch" 
                                           DataContext="{Binding ElementName=PageWorld}"
                                           ItemsSource="{Binding Countries}"  RenderTransformOrigin="0.5,0.5" BorderBrush="Blue" BorderThickness="2">
                    <controls:LongListSelector.RenderTransform>
                        <CompositeTransform/>
                    </controls:LongListSelector.RenderTransform>
                    <controls:LongListSelector.ItemTemplate>
                        <DataTemplate>
                            <ListBoxItem Margin="0,6,0,6">
                                <StackPanel>
                                    <TextBlock Text="{Binding Title}" TextWrapping="NoWrap"  Foreground="Black"/>
                                </StackPanel>
                            </ListBoxItem>
                        </DataTemplate>
                    </controls:LongListSelector.ItemTemplate>
                </controls:LongListSelector>

在代码behing中，我的绑定值如下所示。

private ObservableCollection<Country> _countries;

        public ObservableCollection<Country> Countries
        {
            get { return _countries; }
            set
            {
                _countries = value;
                OnPropertyChanged();
            }
        }

        public World()
        {
            InitializeComponent();
            navigationHelper = new NavigationHelper(this);
            navigationHelper.LoadState += this.NavigationHelper_LoadState;
            navigationHelper.SaveState += this.NavigationHelper_SaveState;
            Countries = GetCountries();
        }

 public class Country
        {
            public string Title { get; set; }
        }
        private ObservableCollection<Country> GetCountries()
        {
            ObservableCollection<Country> countries = new ObservableCollection<Country>();
            for (int i = 0; i < 100; i++)
            {
                Country country = new Country();
                country.Title = "Name" + i;
                countries.Add(country);
            }
            return countries;
        }