如何以编程方式获取ItemsControl滚动条的位置

Question

我有一个附加到ItemsControl的行为，每当添加新项时，该控件就会向下滚动到底部。由于我正在开发一个聊天类型的程序，如果用户的滚动条不是最底部，我不希望它滚动，否则会很烦人（有些聊天程序会这样做，这很糟糕）。

我该如何做到这一点？我不知道如何访问包装ScrollViewer，也不知道是否需要将其带入视图。

这是我从StackOverflow上的某个人那里得到的行为类。我自己还在学习行为。

public class ScrollOnNewItem : Behavior<ItemsControl>
{
    protected override void OnAttached()
    {
        AssociatedObject.Loaded += OnLoaded;
        AssociatedObject.Unloaded += OnUnLoaded;
    }
    protected override void OnDetaching()
    {
        AssociatedObject.Loaded -= OnLoaded;
        AssociatedObject.Unloaded -= OnUnLoaded;
    }
    private void OnLoaded(object sender, RoutedEventArgs e)
    {
        var incc = AssociatedObject.ItemsSource as INotifyCollectionChanged;
        if (incc == null) return;
        incc.CollectionChanged += OnCollectionChanged;
    }
    private void OnUnLoaded(object sender, RoutedEventArgs e)
    {
        var incc = AssociatedObject.ItemsSource as INotifyCollectionChanged;
        if (incc == null) return;
        incc.CollectionChanged -= OnCollectionChanged;
    }
    private void OnCollectionChanged(object sender, NotifyCollectionChangedEventArgs e)
    {
        if (e.Action == NotifyCollectionChangedAction.Add)
        {
            int count = AssociatedObject.Items.Count;
            if (count == 0)
                return;
            var item = AssociatedObject.Items[count - 1];
            var frameworkElement = AssociatedObject.ItemContainerGenerator.ContainerFromItem(item) as FrameworkElement;
            if (frameworkElement == null) return;
            frameworkElement.BringIntoView();
        }
    }
}

Answer 1

好吧，这是我自己想出的答案。

我发现有一个用于依赖对象的GetSelfAndAncestors方法。使用它，我可以获得我的AssociatedObject（ItemsControl）的ScrollViewer祖先（如果有的话），并用它来操作它。

所以我把这个字段添加到我的行为中

private ScrollViewer scrollViewer;
private bool isScrollDownEnabled;

在OnLoaded事件处理程序中，我为它分配了以下代码

scrollViewer = AssociatedObject.GetSelfAndAncestors().Where(a => a.GetType().Equals(typeof(ScrollViewer))).FirstOrDefault() as ScrollViewer;

在OnCollectionChanged事件处理程序中，我继续将所有逻辑封装在if语句中，如下所示

        if (scrollViewer != null)
        {
            isScrollDownEnabled = scrollViewer.ScrollableHeight > 0 && scrollViewer.VerticalOffset + scrollViewer.ViewportHeight < scrollViewer.ExtentHeight;
            if (e.Action == NotifyCollectionChangedAction.Add && !isScrollDownEnabled)
            {
                 // Do stuff
            }
        }

总之，代码看起来像下面的

public class ScrollOnNewItem : Behavior<ItemsControl>
{
    private ScrollViewer scrollViewer;
    private bool isScrollDownEnabled;
    protected override void OnAttached()
    {
        AssociatedObject.Loaded += OnLoaded;
        AssociatedObject.Unloaded += OnUnLoaded;
    }
    protected override void OnDetaching()
    {
        AssociatedObject.Loaded -= OnLoaded;
        AssociatedObject.Unloaded -= OnUnLoaded;
    }
    private void OnLoaded(object sender, RoutedEventArgs e)
    {
        var incc = AssociatedObject.ItemsSource as INotifyCollectionChanged;
        if (incc == null) return;
        incc.CollectionChanged += OnCollectionChanged;
        scrollViewer = AssociatedObject.GetSelfAndAncestors().Where(a => a.GetType().Equals(typeof(ScrollViewer))).FirstOrDefault() as ScrollViewer;
    }
    private void OnUnLoaded(object sender, RoutedEventArgs e)
    {
        var incc = AssociatedObject.ItemsSource as INotifyCollectionChanged;
        if (incc == null) return;
        incc.CollectionChanged -= OnCollectionChanged;
    }
    private void OnCollectionChanged(object sender, NotifyCollectionChangedEventArgs e)
    {
        if (scrollViewer != null)
        {
            isScrollDownEnabled = scrollViewer.ScrollableHeight > 0 && scrollViewer.VerticalOffset + scrollViewer.ViewportHeight < scrollViewer.ExtentHeight;
            if (e.Action == NotifyCollectionChangedAction.Add && !isScrollDownEnabled)
            {
                int count = AssociatedObject.Items.Count;
                if (count == 0)
                    return;
                var item = AssociatedObject.Items[count - 1];
                var frameworkElement = AssociatedObject.ItemContainerGenerator.ContainerFromItem(item) as FrameworkElement;
                if (frameworkElement == null) return;
                frameworkElement.BringIntoView();
            }
        }
    }
}

正如评论中所要求的那样，要使用行为，我只需要在包含我的行为的代码中的区域的xaml文件中添加一个新的xmlns。

             xmlns:behaviors="clr-namespace:Infrastructure.Behaviors;assembly=Infrastructure"

然后在控制上，我只是添加了行为。

    <ScrollViewer VerticalScrollBarVisibility="Auto">
        <ItemsControl Name="Blah" ItemsSource="{Binding Messages}" ItemTemplate="{StaticResource MessageTemplate}">
            <i:Interaction.Behaviors>
                <behaviors:ScrollOnNewItem />
            </i:Interaction.Behaviors>
        </ItemsControl>
    </ScrollViewer>

i类只是交互性命名空间xmlns:i="clr namespace:System.Windows.Interactivity；assembly=System.Windows.Interactive"

Answer 2

还有另一种方法可以实现这种行为。这种方式比上面更容易。您所要做的就是调用如下方法：

public void AppendText(RichTextBox richTextBox, string data){       
   richTextBox.AppendText(data);
   bool isScrollDownEnabled = richTextBox.VerticalOffset == 0 ||
        richTextBox.VerticalOffset + richTextBox.ViewportHeight == richTextBox.ExtentHeight;
   if (isScrollDownEnabled)
       richTextBox.ScrollToEnd();
}

它也适用于TextBox。