使用 DrawingVisual 对象

项目
2025-05-07

本主题概述如何在 WPF 视觉层中使用 DrawingVisual 对象。

DrawingVisual 对象

DrawingVisual 是用于呈现形状、图像或文本的轻型绘图类。此类被视为轻量级，因为它不提供布局或事件处理，因而提高了性能。因此，绘图非常适合背景和剪贴画。

DrawingVisual 宿主容器

若要使用 DrawingVisual 对象，需要为对象创建主机容器。主机容器对象必须派生自 FrameworkElement 类，该类提供类缺少的布局和事件处理支持 DrawingVisual 。主机容器对象不显示任何可见属性，因为它的主要用途是包含子对象。但是， Visibility 主机容器的属性必须设置为 Visible;否则，其子元素将不可见。

为视觉对象创建主机容器对象时，需要在一个 VisualCollection对象中存储视觉对象引用。 Add使用该方法将视觉对象添加到主机容器。在以下示例中，将创建一个主机容器对象，并将三个可视对象添加到其 VisualCollection中。

// Create a host visual derived from the FrameworkElement class.
// This class provides layout, event handling, and container support for
// the child visual objects.
public class MyVisualHost : FrameworkElement
{
    // Create a collection of child visual objects.
    private VisualCollection _children;

    public MyVisualHost()
    {
        _children = new VisualCollection(this);
        _children.Add(CreateDrawingVisualRectangle());
        _children.Add(CreateDrawingVisualText());
        _children.Add(CreateDrawingVisualEllipses());

        // Add the event handler for MouseLeftButtonUp.
        this.MouseLeftButtonUp += new System.Windows.Input.MouseButtonEventHandler(MyVisualHost_MouseLeftButtonUp);
    }

' Create a host visual derived from the FrameworkElement class.
' This class provides layout, event handling, and container support for
' the child visual objects.
Public Class MyVisualHost
    Inherits FrameworkElement
    ' Create a collection of child visual objects.
    Private _children As VisualCollection

    Public Sub New()
        _children = New VisualCollection(Me)
        _children.Add(CreateDrawingVisualRectangle())
        _children.Add(CreateDrawingVisualText())
        _children.Add(CreateDrawingVisualEllipses())

        ' Add the event handler for MouseLeftButtonUp.
        AddHandler MouseLeftButtonUp, AddressOf MyVisualHost_MouseLeftButtonUp
    End Sub

注释

有关从中提取上述代码示例的完整代码示例，请参阅使用 DrawingVisual 的命中测试示例。

创建 DrawingVisual 对象

创建对象 DrawingVisual 时，它没有绘图内容。可以通过检索对象DrawingContext并在其中进行绘制来添加文本、图形或图像内容。通过调用DrawingVisual对象的RenderOpen方法可以返回DrawingContext。

要将矩形绘制到DrawingContext中，请使用DrawingContext对象的DrawRectangle方法。类似的方法可用于绘制其他类型的内容。完成将内容绘制到其中 DrawingContext后，调用 Close 该方法以关闭 DrawingContext 并保留内容。

在下面的示例中，创建了一个 DrawingVisual 对象，并在 DrawingContext 内绘制了一个矩形。

// Create a DrawingVisual that contains a rectangle.
private DrawingVisual CreateDrawingVisualRectangle()
{
    DrawingVisual drawingVisual = new DrawingVisual();

    // Retrieve the DrawingContext in order to create new drawing content.
    DrawingContext drawingContext = drawingVisual.RenderOpen();

    // Create a rectangle and draw it in the DrawingContext.
    Rect rect = new Rect(new System.Windows.Point(160, 100), new System.Windows.Size(320, 80));
    drawingContext.DrawRectangle(System.Windows.Media.Brushes.LightBlue, (System.Windows.Media.Pen)null, rect);

    // Persist the drawing content.
    drawingContext.Close();

    return drawingVisual;
}

' Create a DrawingVisual that contains a rectangle.
Private Function CreateDrawingVisualRectangle() As DrawingVisual
    Dim drawingVisual As New DrawingVisual()

    ' Retrieve the DrawingContext in order to create new drawing content.
    Dim drawingContext As DrawingContext = drawingVisual.RenderOpen()

    ' Create a rectangle and draw it in the DrawingContext.
    Dim rect As New Rect(New Point(160, 100), New Size(320, 80))
    drawingContext.DrawRectangle(Brushes.LightBlue, CType(Nothing, Pen), rect)

    ' Persist the drawing content.
    drawingContext.Close()

    Return drawingVisual
End Function

为 FrameworkElement 成员创建替代

主机容器对象负责管理其视觉对象的集合。这要求主机容器实现派生 FrameworkElement 类的成员替代。

以下列表描述了必须重写的两个成员：

GetVisualChild：从子元素集合中返回位于指定索引处的子元素。
VisualChildrenCount：获取此元素中的可视子元素数。

在以下示例中，实现了两 FrameworkElement 个成员的替代。


// Provide a required override for the VisualChildrenCount property.
protected override int VisualChildrenCount
{
    get { return _children.Count; }
}

// Provide a required override for the GetVisualChild method.
protected override Visual GetVisualChild(int index)
{
    if (index < 0 || index >= _children.Count)
    {
        throw new ArgumentOutOfRangeException();
    }

    return _children[index];
}


' Provide a required override for the VisualChildrenCount property.
Protected Overrides ReadOnly Property VisualChildrenCount() As Integer
    Get
        Return _children.Count
    End Get
End Property

' Provide a required override for the GetVisualChild method.
Protected Overrides Function GetVisualChild(ByVal index As Integer) As Visual
    If index < 0 OrElse index >= _children.Count Then
        Throw New ArgumentOutOfRangeException()
    End If

    Return _children(index)
End Function

提供命中测试支持

即使主机容器对象不显示任何可见属性，主机容器对象也可以提供事件处理，但是，其 Visibility 属性必须设置为 Visible。这样，便可以为主机容器创建一个事件处理例程，该例程可以捕获鼠标事件，例如释放鼠标左键。然后，事件处理例程可以通过调用 HitTest 该方法来实现命中测试。方法的HitTestResultCallback参数指的是用户定义的过程，可用于确定命中测试的结果动作。

在以下示例中，针对主机容器对象及其子对象实现了命中测试支持。

// Capture the mouse event and hit test the coordinate point value against
// the child visual objects.
void MyVisualHost_MouseLeftButtonUp(object sender, System.Windows.Input.MouseButtonEventArgs e)
{
    // Retrieve the coordinates of the mouse button event.
    System.Windows.Point pt = e.GetPosition((UIElement)sender);

    // Initiate the hit test by setting up a hit test result callback method.
    VisualTreeHelper.HitTest(this, null, new HitTestResultCallback(myCallback), new PointHitTestParameters(pt));
}

// If a child visual object is hit, toggle its opacity to visually indicate a hit.
public HitTestResultBehavior myCallback(HitTestResult result)
{
    if (result.VisualHit.GetType() == typeof(DrawingVisual))
    {
        if (((DrawingVisual)result.VisualHit).Opacity == 1.0)
        {
            ((DrawingVisual)result.VisualHit).Opacity = 0.4;
        }
        else
        {
            ((DrawingVisual)result.VisualHit).Opacity = 1.0;
        }
    }

    // Stop the hit test enumeration of objects in the visual tree.
    return HitTestResultBehavior.Stop;
}

' Capture the mouse event and hit test the coordinate point value against
' the child visual objects.
Private Sub MyVisualHost_MouseLeftButtonUp(ByVal sender As Object, ByVal e As MouseButtonEventArgs)
    ' Retrieve the coordinates of the mouse button event.
    Dim pt As Point = e.GetPosition(CType(sender, UIElement))

    ' Initiate the hit test by setting up a hit test result callback method.
    VisualTreeHelper.HitTest(Me, Nothing, New HitTestResultCallback(AddressOf myCallback), New PointHitTestParameters(pt))
End Sub

' If a child visual object is hit, toggle its opacity to visually indicate a hit.
Public Function myCallback(ByVal result As HitTestResult) As HitTestResultBehavior
    If result.VisualHit.GetType() Is GetType(DrawingVisual) Then
        If (CType(result.VisualHit, DrawingVisual)).Opacity = 1.0 Then
            CType(result.VisualHit, DrawingVisual).Opacity = 0.4
        Else
            CType(result.VisualHit, DrawingVisual).Opacity = 1.0
        End If
    End If

    ' Stop the hit test enumeration of objects in the visual tree.
    Return HitTestResultBehavior.Stop
End Function

通过