Copying the Printer canvas

[Rob Cameron]

Hi,
I want to monitor the progress of creating a print job and then decide 
whether to print or not. The drawing is quite lengthy and complex and takes 
a little while to complete, so rather than draw it twice (once to the screen 
and once to the printer canvas) I thought I could do this by drawing to the 
printer canvas, then stretch-blitting the printer canvas to an on-screen 
canvas.

This works OK if the source is a bitmap canvas (see the trivial drawing 
exercise in Button1Click below) but doesn't if the source is the 
printer.canvas (Button2Click). Obviously the printer canvas is handled 
differently (even though it is a TCanvas object).

Have I missed something or is this a bad approach?  Am I stuck with drawing 
twice?

Thanks,
Rob
8< 
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// Put two TButtons,a TImage and a TCheckBox (Name := "I_Want_To_Print") on 
a form
// Add Printers to the uses clause

procedure TForm1.Button1Click(Sender: TObject);
var
 sR, dR: TRect;
 B: TBitMap;
begin
 B := TBitMap.Create;
 try
   B.Width  := 210;
   B.Height := 297;
   sR := Rect(0,0,B.Width,B.Height);
   dR := Rect(0,0,Image1.Width,Image1.Height);
   B.Canvas.Pen.Width := 5;
   B.Canvas.MoveTo(10,10);
   B.Canvas.LineTo(200,200);
   B.Canvas.TextOut(100,100,'Hello world!');
   StretchBlt(Image1.Canvas.Handle,  dR.Left, dR.Top, dR.Right, dR.Bottom,
              B.Canvas.Handle,       sR.Left, sR.Top, sR.Right, sR.Bottom,
              SrcCopy);
   Image1.Refresh;
 finally
   B.Free;
 end;
end;
procedure TForm1.Button2Click(Sender: TObject);
var
 sR, dR: TRect;
begin
 sR := Rect(0,0,Printer.PageWidth,Printer.PageHeight);
 dR := Rect(0,0,Image1.Width,Image1.Height);
 Printer.BeginDoc;
 Printer.Canvas.Pen.Width := 20;
 Printer.Canvas.MoveTo(10,10);
 Printer.Canvas.LineTo(200,200);
 Printer.Canvas.TextOut(200,200,'Hello world!');
 StretchBlt(Image1.Canvas.Handle,  dR.Left, dR.Top, dR.Right, dR.Bottom,
            Printer.Canvas.Handle, sR.Left, sR.Top, sR.Right, sR.Bottom,
            SrcCopy);
 Image1.Refresh;
 if I_Want_To_Print.Checked then
   Printer.EndDoc
 else
   Printer.Abort;
end; 

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