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'Draw the final shape sDraw(g) s = Nothing End If End Class Listing A96: Events for drawing a shape interactively
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If you test the application at this stage, you should nd that you can draw a single line on to the panel, but this will disappear as soon as you start to draw a new one As you draw a line, it should appear interactively, anchored at the point where you rst click on the panel, and stretching out to follow the mouse pointer as you drag it out to de ne the length and orientation
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Activity 4: Building a Shapes collection
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We now need to add the capability to draw more than one line To do this, we will need to add some form of data structure to store all of the drawn objects, and amend the DoPaint() code to enable it to draw the whole collection Whenever a drawing operation ends, the new shape will be added to a collection of shapes and DoPaint() will be called to draw the whole collection First the collection: we will use an ArrayList() to hold all of the shapes, so this will have to be declared as a form-scope variable and created in the form s Load event (Listing A97)
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Public Class frmCAD Inherits SystemWindowsFormsForm 'Existing code ' Private colShapes As ArrayList 'Existing code ' Private Sub frmCAD_Load(ByVal sender As SystemObject, _ ByVal e As SystemEventArgs) _ Handles MyBaseLoad g = pnlOutputCreateGraphics() colShapes = New ArrayList() End Sub End Class Listing A97: Creating the Shapes collection
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Now we can amend the DoPaint() code and arrange for each new shape to be added to the collection (Listing A98)
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Public Class frmCAD Inherits SystemWindowsFormsForm 'Existing code '
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Private Sub DoPaint(ByVal gr As Graphics) Dim s As Shape grClear(MeBackColor) For Each s In colShapes sDraw(gr) Next End Sub Private Sub pnlOutput_MouseUp(ByVal sender As Object, _ ByVal e As SystemWindowsFormsMouseEventArgs) _ Handles pnlOutputMouseUp If Drawing Then 'The interaction is done Drawing = False 'So no need to keep the background backgroundDispose() background = Nothing 'Add the final shape to the collection colShapesAdd(s) 'and draw the lot DoPaint(g) s = Nothing End If End Sub 'Existing code ' End Class Listing A98: Building the Shapes collection
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The application is now capable of dealing with as many shapes as you want to draw on it, but at the moment, any shape as long as it is a line is a good description of its capabilities We should now deal with adding to the Shapes library
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Activity 5: Providing more shapes
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As you saw in Activity 2 and Listing A92, providing a new Shape sub-class is a simple matter For each shape we wish to add, we only need to provide a constructor and a Draw() method We will also need to work out how any particular shape would be drawn given the limitation that the draw operation will need to draw the shape from the sizes of its enclosing rectangle
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Circles and Ellipses
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The GDI+ facilities in the NET framework do not distinguish between circles and ellipses in drawing operations A circle is simply an ellipse which has equal major and minor axes (see Figure A95)
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Major Axis Figure A95 A circle is just a specialized ellipse
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Fortunately for our application, drawing an ellipse ts in well since the
DrawEllipse() method of the Graphics class takes as parameters the position,
width and height of the rectangle that would enclose an ellipse This matches the
Shape class de nition perfectly, and means we will not need to work out any transformations to do the drawing The Ellipse class is shown in Listing A99 Public Class Ellipse Inherits Shape Public Sub New(ByVal xx ByVal wid As ByVal col As MyBaseNew(xx, yy, End Sub
As Integer, ByVal yy As Integer, _ Integer, ByVal hgt As Integer, _ Color) wid, hgt, col)
Public Overrides Sub Draw(ByVal gr As Graphics) MyBaseDraw(gr) grDrawEllipse(New Pen(colour), X, Y, Width, Height) End Sub End Class Listing A99: The Ellipse class
Add Listing A98 to the shapesvb class module All that will then be required is to add a button to the main form that will create a new Ellipse object and assign it to the form s Shape variable s Everything else should be taken care of by the framework we have already built Creating the new Ellipse shape will take just a single line of code, almost exactly the same as for the Line class (Listing A910)
Private Sub btnEllipse_Click(ByVal sender As SystemObject, _ ByVal e As SystemEventArgs) _ Handles btnEllipseClick s = New Ellipse(0, 0, 0, 0, ColorBlack) End Sub Listing A910: Event code for the button to add an ellipse