A Drawing Program

Note that the settings chosen from the Brush Properties dialog apply to the next shape that is drawn. Each shape that is drawn is stored in a list of shapes. Figure 11-7 shows a simple drawing created using this program, consisting of a rectangle and two ellipses, with different colors and hatch styles.

Shape Hierarchy

This kind of program can be implemented by using a hierarchy of shapes. For simplicity, our hierarchy only includes rectangles and ellipses. The file Shape.vb provides the code for this hierarchy. The key feature is that the base class CShape is an abstract class with a MustOverride method DrawShape . The DrawShape method of the derived class draws the shape corresponding to the class. Hence DrawShape can be used polymorphically.

' Shape.vb

Imports System.Drawing

Public MustInherit Class CShape
   Protected m_p1 As Point
   Protected m_p2 As Point
   Protected m_pen As Pen
   Protected m_brush As Brush

    Public Sub New(ByVal p1 As Point, ByVal p2 As Point, _
     ByVal pn As Pen, ByVal br As Brush)
       Dim rect As Rectangle
       rect = MakeRectangle(p1, p2)
       m_p1 = rect.Location
       m_p2 = New Point(rect.Right, rect.Bottom)
       m_pen = pn
       m_brush = br
    End Sub

    Public Sub New(ByVal p1 As Point, ByVal p2 As Point)
       m_pen = Pens.Black
       m_brush = Brushes.Transparent
       Dim rect As Rectangle
       rect = MakeRectangle(p1, p2)
       m_p1 = rect.Location
       m_p2 = New Point(rect.Right, rect.Bottom)
    End Sub

Public MustOverride Sub DrawShape(ByVal g As Graphics)

Private Function MakeRectangle(ByVal p As Point, _
     ByVal q As Point) As Rectangle
       Dim top, left, bottom, right As Integer
       top = IIf(p.Y < q.Y, p.Y, q.Y)
       left = IIf(p.X < q.X, p.X, q.X)
       bottom = IIf(p.Y > q.Y, p.Y, q.Y)
       right = IIf(p.X > q.X, p.X, q.X)
       Return New Rectangle(left, top, right - left, _
          bottom - top)
    End Function
End Class

Public Class CRectangle
   Inherits CShape

   Public Sub New(ByVal p1 As Point, ByVal p2 As Point, _
    ByVal pn As Pen, ByVal br As Brush)
      MyBase.New(p1, p2, pn, br)
   End Sub

   Public Sub New(ByVal p1 As Point, ByVal p2 As Point)
      MyBase.New(p1, p2)
   End Sub

Public Overrides Sub DrawShape(ByVal g As Graphics)


Dim rect As New Rectangle(m_p1.X, m_p1.Y, _


m_p2.X


-


m_p1.X, m_p2.Y


-


m_p1.Y)


g.FillRectangle(m_brush, rect)


g.DrawRectangle(m_pen, rect)


End Sub

End Class

An Arraylist of Shapes

The main form can store the shape objects in an array list. The handler for the MouseUp event adds a shape to the list.

...

Private m_list As New ArrayList()

...
Private Function MakeShape(ByVal p As Point, _
 ByVal q As Point, ByVal stype As ShapeType) As CShape
   Select Case stype
      Case ShapeType.Ellipse
         Return New CEllipse(p, q, m_pen, m_brush)
      Case ShapeType.Rectangle
         Return New CRectangle(p, q, m_pen, m_brush)
   End Select
End Function

...
Protected Overrides Sub OnMouseUp(_
 ByVal e As MouseEventArgs)
   m_track = False
   m_shape = MakeShape(m_start, m_end, m_type)

m_list.Add(m_shape)

Invalidate()
End Sub

Displaying the Shapes

The shapes are displayed in the override of OnPaint by looping through the array list. Notice how polymorphism makes this code very concise .

Protected Overrides Sub OnPaint(ByVal e As PaintEventArgs)
   Dim g As Graphics = e.Graphics
   Dim objShape As CShape

For Each objShape In m_list


objShape.DrawShape(g)


Next

MyBase.OnPaint(e)
End Sub