如何检查itemscontrol项是否在视口中

Question

我正在创建wp8 app并且有一个大型数据集来表示itemscontrol控件。问题是由于元素的大小，我失去了很多记忆。如何检查itemscontrol项是否在视口中？

这是我使用的xaml：

                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                   

                        <ItemsControl x:Name="showList" Margin="0" ItemsSource="{Binding tmp}">
                            <ItemsControl.ItemsPanel>
                                <ItemsPanelTemplate>
                                    <StackPanel Orientation="Vertical"></StackPanel>
                                </ItemsPanelTemplate>
                            </ItemsControl.ItemsPanel>
                            <ItemsControl.ItemTemplate>
                                <DataTemplate>
                                    <StackPanel Orientation="Horizontal" Margin="0,2,0,0" Background="#FF2E2E2E" Width="280">
                                        <Image Source="../Images/edge_shadow_x8.png" HorizontalAlignment="Left" Stretch="Fill" Width="8" Margin="0,0,2,0"/>
                                        <TextBlock x:Name="time" FontSize="16" Text="{Binding StartTime}" Width="50" VerticalAlignment="Center" HorizontalAlignment="Left" Margin="15,0" Foreground="White" TextWrapping="Wrap"/>
                                        <TextBlock x:Name="show" FontSize="16" Text="{Binding title}" Width="160" VerticalAlignment="Center" HorizontalAlignment="Left" Margin="0,5,10,5" TextWrapping="Wrap"/>
                                    </StackPanel>
                                </DataTemplate>
                            </ItemsControl.ItemTemplate>
                        </ItemsControl>
                    </StackPanel>
                </ScrollViewer>
            </DataTemplate>
        </ItemsControl.ItemTemplate>
    </ItemsControl>
</ScrollViewer>