使用 DrawingVisual 物件

本主題提供如何在 WPF 視覺層中使用 DrawingVisual 物件的概觀。

DrawingVisual 物件

DrawingVisual 是用來轉譯圖形、影像或文字的輕量型繪圖類別。此類別之所以被視為輕量型，是因為它不提供版面配置或事件處理，這使它有更好的效能。基於此原因，繪圖適合作為背景或剪貼畫。

DrawingVisual 主機容器

為了使用 DrawingVisual 物件，您必須建立這些物件的容器主機。主機容器物件必須衍生自 FrameworkElement 類別，因為該類別提供了 DrawingVisual 類別所缺乏的佈局和事件處理支援。主機容器物件不會顯示任何可見的屬性，因為其主要目的是要包含子物件。然而，必須將主機容器的Visibility屬性設為Visible，否則不會顯示其子元素。

當您為視覺物件創建主機容器物件時，您需要將視覺物件參考儲存於 VisualCollection 中。使用 Add 方法，將視覺物件新增至容器主機。在下列範例中，會建立主機容器物件，並將三個視覺物件新增至其 VisualCollection。

// 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

備註

如需前述程式碼範例的完整程式碼，請參閱「使用 DrawingVisuals 進行點選測試範例」。

建立 DrawingVisual 物件

當您創建一個DrawingVisual物件時，它沒有任何繪圖內容。您可以取得物件的DrawingContext，並在其中繪製以新增文字、圖形或影像內容。呼叫 RenderOpen 方法的 DrawingVisual 物件會傳回 DrawingContext。

在 DrawingContext 中繪製矩形，請使用 DrawingContext 物件的 DrawRectangle 方法。繪製其他類型的內容有類似的方法。當您完成將內容繪入DrawingContext時，請呼叫Close方法來關閉DrawingContext並保存內容。

在以下範例中，會建立 DrawingVisual 物件，並將矩形繪製到其 DrawingContext 中。

// 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

針對 FrameworkElement 成員建立重寫

主機容器物件負責管理其視覺物件的集合。這需要主機容器實現衍生類別的成員覆寫FrameworkElement。

下列清單說明您必須覆寫的兩個成員︰

GetVisualChild：從子元素集合中傳回位於指定索引的子項。
VisualChildrenCount：取得在此元素中視覺子元素的數量。

在下列範例中，兩個 FrameworkElement 成員的覆寫已被實作。


// 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

提供碰撞測試支援

即使主機容器物件未顯示任何可見屬性，也可以提供事件處理，不過，必須將其 Visibility 屬性設為 Visible。這可讓您針對可捕捉滑鼠事件 (例如放開滑鼠左鍵) 的主機容器建立事件處理常式。然後，事件處理例程可以叫用 HitTest 方法來實作碰撞測試。方法的 HitTestResultCallback 參數是指一個使用者定義的程序，您可以用來決定點擊測試結果的動作。

在下列範例中，會針對容器物件及其子物件實作碰撞檢測支援。

// 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

另請參閱

DrawingVisual
HitTest
WPF 圖形呈現概觀
視覺層中的點擊測試

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Last updated on 2026-03-30