（Delphi 7）在画布（以及画布本身）上绘制的对象没有出现

Question

I've been trying to display a rectangle by creating a canvas on a bitmap. It looks like this:

  TRoom = class
   private
     width, length, X1,X2,Y1,Y2, index: integer;
   public
     plane: TBitmap;
     procedure draw;
     procedure setparam;
     function getparam: integer;
  end;


procedure TRoom.draw;
begin
  plane:= TBitmap.create;
  plane.canvas.Pen.Color:= 1791767;
  plane.Canvas.pen.Width:= 3;
  plane.canvas.Rectangle(10,10,20,20);
end;

As stated in the title, neither the canvas, nor the rectangle appear. I have never worked with the canvas in Delphi before so I expect it to be something rather trivial.

Answer 1

A TBitmap is a non visual class that represents a raster image, a 2D array of pixels. On its own it is never visible. You would need to paint it on the screen in order to see it.

What you should do is create a visual control to which you can paint. For instance a TPaintBox . Put one of those on your form and add a handler for its OnPaint event.

procedure TForm1.PaintBox1Paint(Sender: TCanvas);
begin
  PaintBox1.Canvas.Pen.Color :=. 1791767;
  PaintBox1.Canvas.Pen.Width := 3;
  PaintBox1.Canvas.Rectangle(10, 10, 20, 20);
end;