确定ScrollViewer的内部宽度

Question

我在.NET Framework4.7上的WPF应用程序中有一个<ScrollViewer>。在该ScrollViewer内部是一个<Border>。这个边界并不一定会占据它所获得的所有空间。对其应用ScaleTransform以允许放大和缩小。

使用鼠标滚轮进行放大和缩小时，对于新旧缩放，鼠标光标下的内容位置应保持不变。为此，我需要做一些数学运算来确定鼠标光标在新旧缩放中的位置。

我已经有了ScrollViewer的滚动位置，但是我找不到ScrollViewer的虚拟的、可滚动的或内容的宽/高。如何确定ScrollViewer内容区域的总宽度或总高度？这是不显示滚动条时的最小尺寸。我不能只使用内部边框的实际大小，因为它是受约束的，并且可能不会在两个维度上填充可滚动区域。我需要直接从ScrollViewer或其滚动条(我没有访问权限)中获取该信息。

下面是XAML的一部分：

<ScrollViewer HorizontalScrollBarVisibility="Auto" VerticalScrollBarVisibility="Auto"
              PreviewMouseWheel="ScrollViewer_PreviewMouseWheel">
    <Border HorizontalAlignment="Center" VerticalAlignment="Center" Margin="100">
        <Grid>
            <Grid.LayoutTransform>
                <ScaleTransform .../>
            </Grid.LayoutTransform>
            <dt:DrawingCanvas .../>
        </Grid>
    </Border>
</scrollViewer>

DrawingCanvas是决定大小的对象。它在滚动查看器中水平和垂直居中。如果它在一维或两维上超过了可用屏幕空间，则会出现滚动条。

我使用以下代码来获取ScrollViewer_PreviewMouseWheel事件处理程序中的当前位置：

private void ScrollViewer_PreviewMouseWheel(object sender, MouseWheelEventArgs args)
{
    // Determine the mouse cursor position relative to the content
    var sv = (ScrollViewer)sender;
    Point mousePoint = args.GetPosition(sv);
    double x = mousePoint.X + sv.HorizontalOffset;
    double y = mousePoint.Y + sv.VerticalOffset;
    double xFrac = x / sv.ContentWidth;
    double yFrac = y / sv.ContentHeight;

    // Updating scale factor and layout...

    // Move scrollbars to keep the same content under the mouse cursor
    sv.ScrollToHorizontalOffset(...);
    sv.ScrollToVerticalOffset(...);
}

具体地说，我正在从滚动查看器中查找不存在的ContentWidth和ContentHeight属性。如何获取这些值？

Answer 1

扩展应该以Canvas为目标。通过将Canvas和ScaleTransform.CenterY设置为当前鼠标位置，可以在鼠标光标位置缩放ScaleTransform.CenterX。缩放将触发布局过程，该过程将触发ScrollViewer根据更改的内容进行调整。这需要通过滚动到新位置来重新定位内容。

缩放因子适用于相对于缩放元素的完整坐标系，例如Canvas。这意味着当鼠标位置为P1(5;5)并应用比例因子2时，鼠标位置将移动到P2(10;10)。新的鼠标位置P2描述必须添加到当前ScrollViewer偏移量的偏移量：

PcurrentScrollOffset =鼠标* scaleFactor + Pscroll

为了按像素滚动ScrollViewer (以像素为单位返回鼠标位置)，必须将ScrollViewer配置为使用物理滚动单位，而不是逻辑单位。ScrollViewer.CanContentScroll必须设置为false (默认值)。

此外，PreviewMouseWheel事件处理程序还需要能够直接或通过数据绑定访问Canvas的ScaleTransform (或FrameworkElement )。ScrollViewer也是如此。

为了方便起见，我创建了一个附加的行为ZoomBehavior。

此行为将放大和缩小鼠标滚轮上作为FrameworElement输入的每个元素。

要启用缩放，请将ZoomBehavior.IsEnabled设置为true。

可以选择设置缩放因子集ZoomBehavior.ZoomFactor (默认值为0.1)。如果您希望调整滚动位置，可以选择将ScrollViewer绑定或设置为ZoomBehavior.ScrollViewer。请注意，应将ScrollViewer.CanContentScroll设置为false以实现正确的行为。

下面的示例启用对Canvas元素的缩放：

ScrollViewer x:Name="ScrollViewer" 
             CanContentScroll="False" 
             Width="500" Height="500"
             VerticalScrollBarVisibility="Auto"
             HorizontalScrollBarVisibility="Auto">
  <Canvas Width="300" Height="300"
          main:ZoomBehavior.IsEnabled="True"
          main:ZoomBehavior.ZoomFactor="0.1"
          main:ZoomBehavior.ScrollViewer="{Binding ElementName=ScrollViewer}"
          Background="DarkGray">
    <Ellipse Fill="DarkOrange" 
             Height="100" Width="100" 
             Canvas.Top="100" Canvas.Left="100" />
  </Canvas>
</ScrollViewer>

ZoomBehavior.cs

该代码使用C# 8.0的一个特性switch expression。

如果您的环境不支持此语言版本，则需要将表达式转换为经典的switch语句(带有两个标签)。

public class ZoomBehavior : DependencyObject
{
  #region IsEnabled attached property

  // Required
  public static readonly DependencyProperty IsEnabledProperty = DependencyProperty.RegisterAttached(
    "IsEnabled", typeof(bool), typeof(ZoomBehavior), new PropertyMetadata(default(bool), OnIsEnabledChanged));

  public static void SetIsEnabled(DependencyObject attachingElement, bool value) => attachingElement.SetValue(ZoomBehavior.IsEnabledProperty, value);

  public static bool GetIsEnabled(DependencyObject attachingElement) => (bool) attachingElement.GetValue(ZoomBehavior.IsEnabledProperty);

  #endregion

  #region ZoomFactor attached property

  // Optional
  public static readonly DependencyProperty ZoomFactorProperty = DependencyProperty.RegisterAttached(
    "ZoomFactor", typeof(double), typeof(ZoomBehavior), new PropertyMetadata(0.1));

  public static void SetZoomFactor(DependencyObject attachingElement, double value) => attachingElement.SetValue(ZoomBehavior.ZoomFactorProperty, value);

  public static double GetZoomFactor(DependencyObject attachingElement) => (double) attachingElement.GetValue(ZoomBehavior.ZoomFactorProperty);

  #endregion

  #region ScrollViewer attached property

  // Optional
  public static readonly DependencyProperty ScrollViewerProperty = DependencyProperty.RegisterAttached(
    "ScrollViewer", typeof(ScrollViewer), typeof(ZoomBehavior), new PropertyMetadata(default(ScrollViewer)));

  public static void SetScrollViewer(DependencyObject attachingElement, ScrollViewer value) => attachingElement.SetValue(ZoomBehavior.ScrollViewerProperty, value);

  public static ScrollViewer GetScrollViewer(DependencyObject attachingElement) => (ScrollViewer) attachingElement.GetValue(ZoomBehavior.ScrollViewerProperty);

  #endregion
  private static void OnIsEnabledChanged(DependencyObject attachingElement, DependencyPropertyChangedEventArgs e)
  {
    if (!(attachingElement is FrameworkElement frameworkElement))
    {
      throw new ArgumentException("Attaching element must be of type FrameworkElement");
    }

    bool isEnabled = (bool) e.NewValue;
    if (isEnabled)
    {
      frameworkElement.PreviewMouseWheel += ZoomBehavior.Zoom_OnMouseWheel;
      if (ZoomBehavior.TryGetScaleTransform(frameworkElement, out _))
      {
        return;
      }

      if (frameworkElement.LayoutTransform is TransformGroup transformGroup)
      {
        transformGroup.Children.Add(new ScaleTransform());
      }
      else
      {
        frameworkElement.LayoutTransform = new ScaleTransform();
      }
    }
    else
    {
      frameworkElement.PreviewMouseWheel -= ZoomBehavior.Zoom_OnMouseWheel;
    }
  }

  private static void Zoom_OnMouseWheel(object sender, MouseWheelEventArgs e)
  {
    var zoomTargetElement = sender as FrameworkElement;

    Point mouseCanvasPosition = e.GetPosition(zoomTargetElement);
    double scaleFactor = e.Delta > 0
      ? ZoomBehavior.GetZoomFactor(zoomTargetElement)
      : -1 * ZoomBehavior.GetZoomFactor(zoomTargetElement);

    ZoomBehavior.ApplyZoomToAttachedElement(mouseCanvasPosition, scaleFactor, zoomTargetElement);

    ZoomBehavior.AdjustScrollViewer(mouseCanvasPosition, scaleFactor, zoomTargetElement);
  }

  private static void ApplyZoomToAttachedElement(Point mouseCanvasPosition, double scaleFactor, FrameworkElement zoomTargetElement)
  {
    if (!ZoomBehavior.TryGetScaleTransform(zoomTargetElement, out ScaleTransform scaleTransform))
    {
      throw new InvalidOperationException("No ScaleTransform found");
    }

    scaleTransform.CenterX = mouseCanvasPosition.X;
    scaleTransform.CenterY = mouseCanvasPosition.Y;

    scaleTransform.ScaleX = Math.Max(0.1, scaleTransform.ScaleX + scaleFactor);
    scaleTransform.ScaleY = Math.Max(0.1, scaleTransform.ScaleY + scaleFactor);
  }

  private static void AdjustScrollViewer(Point mouseCanvasPosition, double scaleFactor, FrameworkElement zoomTargetElement)
  {
    ScrollViewer scrollViewer = ZoomBehavior.GetScrollViewer(zoomTargetElement);
    if (scrollViewer == null)
    {
      return;
    }

    scrollViewer.ScrollToHorizontalOffset(scrollViewer.HorizontalOffset + mouseCanvasPosition.X * scaleFactor);
    scrollViewer.ScrollToVerticalOffset(scrollViewer.VerticalOffset + mouseCanvasPosition.Y * scaleFactor);
  }

  private static bool TryGetScaleTransform(FrameworkElement frameworkElement, out ScaleTransform scaleTransform)
  {
    // C# 8.0 Switch Expression
    scaleTransform = frameworkElement.LayoutTransform switch
    {
      TransformGroup transformGroup => transformGroup.Children.OfType<ScaleTransform>().FirstOrDefault(),
      ScaleTransform transform => transform,
      _ => null
    };

    return scaleTransform != null;
  }
}