Calling a procedure with objects as arguments - delphi

Question

Trying to save code. I want to display text etc on an image on the form at OnActivate then print the same text on clicking button (Real program is more complicated). To save writing code twice I tried the enclosed code but it won't compile at the "Obj.Canvas" line. If I comment out this line and the enclosed line the program runs but the Obj value is ().
I've tried several other approaches but none work. Can anyone tell me where I'm going wrong.
Badger
unit Unit7;
interface
uses
Winapi.Windows, Winapi.Messages, System.SysUtils, System.Variants, System.Classes, Vcl.Graphics,
Vcl.Controls, Vcl.Forms, Vcl.Dialogs, Vcl.ExtCtrls, Vcl.StdCtrls, printers;
type
TForm7 = class(TForm)
Print: TButton;
Image1: TImage;
PrintDialog1: TPrintDialog;
procedure FormActivate(Sender: TObject);
procedure PrintClick(Sender: TObject);
private
{ Private declarations }
public
DH,DW:Extended;
Procedure DoLayout(Obj:TObject);
{ Public declarations }
end;
var
Form7: TForm7;
implementation
{$R *.dfm}
procedure TForm7.FormActivate(Sender: TObject);
begin
DoLayout(Image1);
end;
procedure TForm7.PrintClick(Sender: TObject);
begin
if PrintDialog1.Execute then
begin
printer.BeginDoc;
DoLayout(Printer);
Printer.EndDoc;
end;
end;
procedure TForm7.DoLayout(Obj:TObject);
begin
if Obj =Printer then //when you run the program Obj is ()
begin
DW:=Printer.PageWidth/Image1.Width;
DH:=Printer.PageHeight/Image1.Height;
end
else
begin
DH:=1;
DW:=1;
end;
With Obj.canvas do //Error here when compiled - tried commenting it out
begin
TextOut(Int(DH*50),Int(DW*30),'This is the text'); //commented this out too
end;
end;
end.

Answer 1

The TPrinter class and the TImage class don't share a common ancestor class except for TObject, as a result that's what you're passing in.
A suggested refactoring is to change the DoLayout code to accept the canvas that you want to use, as well as an parameter to determine if it's a printer or an image that you're passing in e.g.
procedure TForm7.DoLayout(aCanvas : TCanvas; bPrinter : boolean);
begin
if bPrinter then //when you run the program Obj is ()
begin
DW:=Printer.PageWidth/Image1.Width;
DH:=Printer.PageHeight/Image1.Height;
end
else
begin
DH:=1;
DW:=1;
end;
With aCanvas do
begin
TextOut(Int(DH*50),Int(DW*30),'This is the text');
end;
end;
then when you call it, use the printer canvas explicitly, or the image canvas:
DoLayout(Printer.canvas, true);
or
DoLayout(Image1.canvas, false);
this is just a rough estimation based on your code; I don't have a delphi compiler to hand to verify it.