combobox的虚拟滚动选项

Question

我需要在视图中显示可见项，而不是在组合框控件中显示所有行。在滚动时，我们需要加载下一个可见项。

我该怎么做呢？另外，我如何确保它是否被虚拟加载？

示例：

public List<string> items = new List<string>();
public MainWindow()
{
  InitializeComponent();
  DataContext = this;
  for (int i = 0; i < 100000; i++)
  {
    items.Add("item"+ i.ToString());
  }
  combo.ItemsSource = items;
}

前端：

<Grid>
   <StackPanel>
      <ComboBox x:Name="combo" Width="150" HorizontalAlignment="Left" Margin="10,10,0,10" VirtualizingPanel.IsVirtualizing="True" />
    </StackPanel>
 </Grid>

   <ComboBox x:Name="combo" Height="100" Width="150" ScrollViewer.HorizontalScrollBarVisibility="Disabled"
               ScrollViewer.CanContentScroll="True" HorizontalAlignment="Left" Margin="10,10,0,10" 
               VirtualizingStackPanel.IsVirtualizing="True" VirtualizingStackPanel.VirtualizationMode ="Recycling" >
         <ComboBox.ItemsPanel>
              <ItemsPanelTemplate>
                   <VirtualizingStackPanel IsVirtualizing="True"
                                      VirtualizationMode="Recycling" />
              </ItemsPanelTemplate>
         </ComboBox.ItemsPanel>
    </ComboBox>

Answer 1

有几件事你需要做。首先，让我们清理ComboBox并订阅ScrollViewer.ScrollChanged事件。您的许多ComboBox属性在默认情况下已经设置好了，所以我们可以删除它们。我们还可以设置MaxDropDownHeight来指定一次显示多少项。下面是它应该是什么样子的。

 <ComboBox x:Name="combo" ScrollViewer.ScrollChanged="combo_ScrollChanged" MaxDropDownHeight="70" Height="100" Width="150" HorizontalAlignment="Left" Margin="10,10,0,10">
     <ComboBox.ItemsPanel>
           <ItemsPanelTemplate>
                <VirtualizingStackPanel/>
           </ItemsPanelTemplate>
      </ComboBox.ItemsPanel>
 </ComboBox>

接下来，我们需要处理scroll事件，以便跳过3个项目，同时防止在强制scrollviewer更改时出现递归。

    bool scrolling = false;//Used to prevent recursion
    private void combo_ScrollChanged(object sender, ScrollChangedEventArgs e)
    {         
        ComboBox cb = (ComboBox)sender; //Get the sending ComboBox
        ScrollViewer sv = cb.Template.FindName("DropDownScrollViewer", cb) as ScrollViewer; //Find the Comboxes ScrollViewr

        //If scrolling down
        if (e.VerticalChange < 0 && !scrolling)
        {
            scrolling = true; //Set to true to prevent recursion
            sv.ScrollToVerticalOffset(e.VerticalOffset - 2);//Scroll an extra 2 spaces up
            return; //Exit 

        }
        //If scrolling up
        if (e.VerticalChange > 0 && !scrolling)
        {
            scrolling = true; //Set to true to prevent recursion              
            sv.ScrollToVerticalOffset(e.VerticalOffset + 2);//Scroll an extra 2 spaces down
            return; //Exit
        }
        if (scrolling) { scrolling = false; } //Set to false to allow offsets
    }