Question

我一直在尝试以下列方式改进WPF ListBox控件的行为：下面的ListBox会在添加新项目时自动滚动到底部。它使用显示的ScrollToBottom函数执行此操作。使用显示的预览事件，如果用户单击某个项目，即使添加了更多项目，它也会停止滚动。（让它继续滚动是令人讨厌的！）如果用户用鼠标或滚轮手动滚动，那么它会以相同的方式停止滚动。

现在我在下面的代码中有一个按钮，它会再次开始自动滚动。

我的问题是：如果用户将列表框一直向下滚动到底部，我该如何开始自动滚动，或者与鼠标滚轮或键盘。这就是我的旧Borland列表框开箱即用的方式。

using System;
using System.Collections.ObjectModel;
using System.Threading;
using System.Threading.Tasks;
using System.Windows;
using System.Windows.Controls;
using System.Windows.Input;
using System.Windows.Threading;

// Note requires .NET framework 4.5
namespace MMP
{
  public partial class MainWindow : Window
  {
    public ObservableCollection<String> data { get; set; }

    public MainWindow()
    {
      InitializeComponent();
      data = new ObservableCollection<String>();
      DataContext = this;
      BeginAddingItems();
    }

    private async void BeginAddingItems()
    {
      await Task.Factory.StartNew(() =>
      {
        for (int i = 0; i < Int32.MaxValue; ++i)
        {
          if (i > 20) 
            Thread.Sleep(1000);
          AddToList("Added " + i.ToString());
        }
      });
    }

    void AddToList(String item)
    {
        Application.Current.Dispatcher.BeginInvoke(DispatcherPriority.Normal,
            new Action(() => { data.Add(item); ScrollToBottom(); }));
    }

    bool autoScroll = true;
    public void ScrollToBottom()
    {
      if (!autoScroll)
        return;
      if (listbox.Items.Count > 0)
        listbox.ScrollIntoView(listbox.Items[listbox.Items.Count - 1]);
    }

    private void listbox_PreviewMouseDown(object sender, MouseButtonEventArgs e)
    {
      autoScroll = false;
      Console.WriteLine("PreviewMouseDown: setting autoScroll to false");
    }

    private void listbox_PreviewMouseWheel(object sender, MouseWheelEventArgs e)
    {
      Console.WriteLine("PreviewMouseWheel: setting autoScroll to false");
      autoScroll = false;
    }

    private void startButton_Click(object sender, RoutedEventArgs e)
    {
      ScrollToBottom(); // Catch up with the current last item.
      Console.WriteLine("startButton_Click: setting autoScroll to true");
      autoScroll = true;
    }

    private void listbox_ScrollChanged(object sender, ScrollChangedEventArgs e)
    {
      // Can this be useful?
    }
  }
}



<Window x:Class="MMP.MainWindow"
        xmlns="http://schemas.microsoft.com/winfx/2006/xaml/presentation"
        xmlns:x="http://schemas.microsoft.com/winfx/2006/xaml"
        Title="Test Scrolling"
        FontFamily="Verdana"
        Width="400" Height="250"
        WindowStartupLocation="CenterScreen">
  <Grid>
    <Grid.RowDefinitions>
      <RowDefinition Height="*" />
      <RowDefinition Height="Auto" />
    </Grid.RowDefinitions>
    <ListBox x:Name="listbox" Grid.Row="0" 
             PreviewMouseWheel="listbox_PreviewMouseWheel" 
             PreviewMouseDown="listbox_PreviewMouseDown" 
             ItemsSource="{Binding data}" ScrollViewer.ScrollChanged="listbox_ScrollChanged" 
             >
    </ListBox>
    <StackPanel Orientation="Horizontal" Grid.Row="1" HorizontalAlignment="Right">
      <Button x:Name="startButton" Click="startButton_Click" MinWidth="80" >Auto Scroll</Button>
    </StackPanel>
  </Grid>
</Window>

Answer 1

使用以下代码实现了所需的列表框行为，并且感谢Roel提供了初始行为＆lt;＆gt;上面的框架。这是一个包含行为代码的示例项目，以及可用于测试交互性的最小WPF窗口。

测试窗口包含一个ListBox，通过后台任务异步添加项目。行为的重点如下：

列表框会自动滚动显示新项目，因为它们是异步添加的。
用户与列表框的交互会停止自动滚动 - AKA令人讨厌的行为。
完成交互后，要继续自动滚动，用户将滚动条拖动到底部然后放开，或使用鼠标滚轮或键盘执行相同操作。这表示用户希望自动滚动恢复。

AutoScrolBehavior.cs：

using System;
using System.Collections.Specialized;
using System.Windows;
using System.Windows.Controls;
using System.Windows.Interactivity;
using System.Windows.Media;

namespace BehaviorTest.Code
{
  // List box automatically scrolls to show new items as they are added asynchronously.
  // A user interaction with the listbox stops automatic scrolling - AKA obnoxious behavior.
  // Once finished interacting, to continue automatic scrolling, drag the scroll bar to 
  // the bottom and let go, or use the mouse wheel or keyboard to do the same. 
  // This indicates that the user wants automatic scrolling to resume.

  public class AutoScrollBehavior : Behavior<ListBox>
  {
    private ScrollViewer scrollViewer;
    private bool autoScroll = true;
    private bool justWheeled = false;
    private bool userInteracting = false;
    protected override void OnAttached()
    {
      AssociatedObject.Loaded += AssociatedObjectOnLoaded;
      AssociatedObject.Unloaded += AssociatedObjectOnUnloaded;
    }

    private void AssociatedObjectOnUnloaded(object sender, RoutedEventArgs routedEventArgs)
    {
      if (scrollViewer != null)
      {
        scrollViewer.ScrollChanged -= ScrollViewerOnScrollChanged;
      }
      AssociatedObject.SelectionChanged -= AssociatedObjectOnSelectionChanged;
      AssociatedObject.ItemContainerGenerator.ItemsChanged -= ItemContainerGeneratorItemsChanged;
      AssociatedObject.GotMouseCapture -= AssociatedObject_GotMouseCapture;
      AssociatedObject.LostMouseCapture -= AssociatedObject_LostMouseCapture;
      AssociatedObject.PreviewMouseWheel -= AssociatedObject_PreviewMouseWheel;

      scrollViewer = null;
    }

    private void AssociatedObjectOnLoaded(object sender, RoutedEventArgs routedEventArgs)
    {
      scrollViewer = GetScrollViewer(AssociatedObject);
      if (scrollViewer != null)
      {
        scrollViewer.ScrollChanged += ScrollViewerOnScrollChanged;

        AssociatedObject.SelectionChanged += AssociatedObjectOnSelectionChanged;
        AssociatedObject.ItemContainerGenerator.ItemsChanged += ItemContainerGeneratorItemsChanged;
        AssociatedObject.GotMouseCapture += AssociatedObject_GotMouseCapture;
        AssociatedObject.LostMouseCapture += AssociatedObject_LostMouseCapture;
        AssociatedObject.PreviewMouseWheel += AssociatedObject_PreviewMouseWheel;
      }
    }

    private static ScrollViewer GetScrollViewer(DependencyObject root)
    {
      int childCount = VisualTreeHelper.GetChildrenCount(root);
      for (int i = 0; i < childCount; ++i)
      {
        DependencyObject child = VisualTreeHelper.GetChild(root, i);
        ScrollViewer sv = child as ScrollViewer;
        if (sv != null)
          return sv;

        return GetScrollViewer(child);
      }
      return null;
    }

    void AssociatedObject_GotMouseCapture(object sender, System.Windows.Input.MouseEventArgs e)
    {
      // User is actively interacting with listbox. Do not allow automatic scrolling to interfere with user experience.
      userInteracting = true;
      autoScroll = false;
    }

    void AssociatedObject_LostMouseCapture(object sender, System.Windows.Input.MouseEventArgs e)
    {
      // User is done interacting with control.
      userInteracting = false;
    }

    private void ScrollViewerOnScrollChanged(object sender, ScrollChangedEventArgs e)
    {
      // diff is exactly zero if the last item in the list is visible. This can occur because of scroll-bar drag, mouse-wheel, or keyboard event.
      double diff = (scrollViewer.VerticalOffset - (scrollViewer.ExtentHeight - scrollViewer.ViewportHeight));

      // User just wheeled; this event is called immediately afterwards.
      if (justWheeled && diff != 0.0)
      {
        justWheeled = false;
        autoScroll = false;
        return;
      }

      if (diff == 0.0)
      {
        // then assume user has finished with interaction and has indicated through this action that scrolling should continue automatically.
        autoScroll = true;
      }
    }

    private void ItemContainerGeneratorItemsChanged(object sender, System.Windows.Controls.Primitives.ItemsChangedEventArgs e)
    {
      if (e.Action == NotifyCollectionChangedAction.Add || e.Action == NotifyCollectionChangedAction.Reset)
      { 
        // An item was added to the listbox, or listbox was cleared.
        if (autoScroll && !userInteracting)
        {
          // If automatic scrolling is turned on, scroll to the bottom to bring new item into view.
          // Do not do this if the user is actively interacting with the listbox.
          scrollViewer.ScrollToBottom();
        }
      }
    }

    private void AssociatedObjectOnSelectionChanged(object sender, SelectionChangedEventArgs selectionChangedEventArgs)
    {
      // User selected (clicked) an item, or used the keyboard to select a different item. 
      // Turn off automatic scrolling.
      autoScroll = false;
    }

    void AssociatedObject_PreviewMouseWheel(object sender, System.Windows.Input.MouseWheelEventArgs e)
    {
      // User wheeled the mouse. 
      // Cannot detect whether scroll viewer right at the bottom, because the scroll event has not occurred at this point.
      // Same for bubbling event.
      // Just indicated that the user mouse-wheeled, and that the scroll viewer should decide whether or not to stop autoscrolling.
      justWheeled = true;
    }
  }
}

MainWindow.xaml.cs：

using BehaviorTest.Code;
using System;
using System.Collections.Generic;
using System.Collections.ObjectModel;
using System.Threading;
using System.Threading.Tasks;
using System.Windows;
using System.Windows.Documents;
using System.Windows.Input;
using System.Windows.Interactivity;
using System.Windows.Threading;

namespace BehaviorTest
{
  public partial class MainWindow : Window
  {
    public ObservableCollection<String> data { get; set; }
    public MainWindow()
    {
      InitializeComponent();
      data = new ObservableCollection<String>();
      DataContext = this;
      Interaction.GetBehaviors(listbox).Add(new AutoScrollBehavior());
      BeginAddingItems();
    }
    private async void BeginAddingItems()
    {
      List<Task> tasks = new List<Task>();

      await Task.Factory.StartNew(() =>
      {
        for (int i = 0; i < Int32.MaxValue; ++i)
        {
          AddToList("Added Slowly: " + i.ToString());
          Thread.Sleep(2000);
          if (i % 3 == 0)
          {
            for (int j = 0; j < 5; ++j)
            {
              AddToList("Added Quickly: " + j.ToString());
              Thread.Sleep(200);
            }
          }
        }
      });
    }

    void AddToList(String item)
    {
      if (Application.Current == null)
        return; // Application is shutting down.
      Application.Current.Dispatcher.BeginInvoke(DispatcherPriority.Normal,
          new Action(() => { data.Add(item); }));
    }

    private void clearButton_Click(object sender, RoutedEventArgs e)
    {
      data.Clear();
    }

    private void listbox_MouseDoubleClick(object sender, MouseButtonEventArgs e)
    {
      MessageBox.Show("Launch a modal dialog. Items are still added to the list in the background.");
    }
  }
}

MainWindow.xaml.cs：

<Window x:Class="BehaviorTest.MainWindow"
        xmlns="http://schemas.microsoft.com/winfx/2006/xaml/presentation"
        xmlns:x="http://schemas.microsoft.com/winfx/2006/xaml"
        Title="Test Scrolling"
        FontFamily="Verdana"
        Width="400" Height="250"
        WindowStartupLocation="CenterScreen">
  <Grid>
    <Grid.RowDefinitions>
      <RowDefinition Height="*" />
      <RowDefinition Height="Auto" />
    </Grid.RowDefinitions>
    <ListBox x:Name="listbox" Grid.Row="0" 
             ItemsSource="{Binding data}"
             MouseDoubleClick="listbox_MouseDoubleClick" >
    </ListBox>
    <StackPanel Orientation="Horizontal" Grid.Row="1" HorizontalAlignment="Right">
      <Button x:Name="startButton" Click="clearButton_Click" MinWidth="80" >Clear</Button>
    </StackPanel>
  </Grid>
</Window>

Answer 2

您可以尝试创建一个为您执行此操作的Blend Behavior。这是一个小小的开始：

public class AutoScrollBehavior:Behavior<ListBox> 
{
    private ScrollViewer scrollViewer;
    private bool autoScroll = true;
    protected override void OnAttached() 
    {
        AssociatedObject.Loaded += AssociatedObjectOnLoaded;
        AssociatedObject.Unloaded += AssociatedObjectOnUnloaded;      
    }

    private void AssociatedObjectOnUnloaded(object sender, RoutedEventArgs routedEventArgs) 
    {
        AssociatedObject.SelectionChanged -= AssociatedObjectOnSelectionChanged;
        AssociatedObject.ItemContainerGenerator.ItemsChanged -= ItemContainerGeneratorItemsChanged;

        scrollViewer = null;
    }

    private void AssociatedObjectOnLoaded(object sender, RoutedEventArgs routedEventArgs) 
    {
        scrollViewer = GetScrollViewer(AssociatedObject);
        if(scrollViewer != null) 
        {
            scrollViewer.ScrollChanged += ScrollViewerOnScrollChanged;

            AssociatedObject.SelectionChanged += AssociatedObjectOnSelectionChanged;
            AssociatedObject.ItemContainerGenerator.ItemsChanged += ItemContainerGeneratorItemsChanged;
        }
    }

    private void ScrollViewerOnScrollChanged(object sender, ScrollChangedEventArgs e) {
        if (e.VerticalOffset == e.ExtentHeight-e.ViewportHeight) {
            autoScroll = true;
        }
    }

    private static ScrollViewer GetScrollViewer(DependencyObject root) 
    {
        int childCount = VisualTreeHelper.GetChildrenCount(root);
        for (int i = 0; i < childCount; i++) 
        {
            DependencyObject child = VisualTreeHelper.GetChild(root, i);
            ScrollViewer sv = child as ScrollViewer;
            if (sv != null)
                return sv;

            return GetScrollViewer(child);
        }

        return null;
    }

    private void ItemContainerGeneratorItemsChanged(object sender, System.Windows.Controls.Primitives.ItemsChangedEventArgs e) 
    {
        if (e.Action == NotifyCollectionChangedAction.Add || e.Action == NotifyCollectionChangedAction.Reset) {
            if (autoScroll) {
                scrollViewer.ScrollToBottom();

            }
        }
    }

    private void AssociatedObjectOnSelectionChanged(object sender, SelectionChangedEventArgs selectionChangedEventArgs) 
    {
        autoScroll = false;
    }
}

WPF ListBox自动滚动启动和停止行为

2 个答案: