使用 DrawingVisual 物件

發行項
09/27/2024

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，繪圖適合背景或美工圖案。

DrawingVisual 主機容器

若要使用 DrawingVisual 物件，您必須建立物件的主機容器。主機容器物件必須衍生自 FrameworkElement 類別，其提供 DrawingVisual 類別所缺少的配置和事件處理支援。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。不過，必須將主機容器的 Visibility 屬性設為 Visible，否則不會顯示其子元素。

建立視覺物件的主機容器物件時，您必須將視覺物件參考儲存在 VisualCollection 中。使用 Add 方法，將視覺物件新增至主機容器。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

注意

如需先前程式碼範例出處的完整程式碼範例，請參閱使用 DrawingVisuals 進行點擊測試範例 (英文)。

建立 DrawingVisual 物件

當您建立 DrawingVisual 物件時，其沒有繪圖內容。您可以藉由擷取物件的 DrawingContext 並在其中繪製，來新增文字、圖形或影像內容。呼叫 DrawingVisual 物件的 RenderOpen 方法會傳回 DrawingContext。

若要在 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。當您完成將內容繪製到 DrawingContext 時，請呼叫 Close 方法來關閉 DrawingContext 並保存內容。

在下列範例中，會建立 DrawingVisual 物件，並將矩形繪製到 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 成員建立覆寫

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

下列清單說明您必須覆寫的兩個成員︰

GetVisualChild：從子元素集合傳回指定索引處的子系。
VisualChildrenCount：取得這個元素內視覺子元素數。

在下列範例中，會實作兩個 FrameworkElement 成員的覆寫。


// 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

提供點擊測試支援

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，來實作點擊測試。方法的 HitTestResultCallback 參數是指使用者定義的程序，您可以用來判斷點擊測試的結果動作。

在下列範例中，會針對主機容器物件和其子系實作點擊測試支援。

// 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

共用方式為