function CheckDC  error on closing a Form
lazarus / 39-Graphics

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Jeff_t · 2025-09-25 12:33

I have a simple form with a button and a TPaintBox
In the PaintBox's Paint event, I draw on the canvas, having used from
TColors on the way

If I close the Form using the close gadget, I get an exception in
function CheckDC - almost as if it's still trying to draw after the
TPaintBox has been destroyed.
Have I missed something important?

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Thaddy · 2025-09-25 13:05

Did you not use beginpaint/endpaint?

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Jeff_t · 2025-09-25 13:12

QuoteDid you not use beginpaint/endpaint?
I did not.
Haven't seen that mentioned in any online examples , such as this one
from Lazarus Wiki


Code: Pascal  [Select][+][-]window.onload = function(){var x1 =
document.getElementById("main_content_section"); if (x1) { var x =
document.getElementsByClassName("geshi");for (var i = 0; i procedure
TMyDrawingControl.Paint;var  x, y: Integer;  Bitmap: TBitmap;begin 
Bitmap := TBitmap.Create;  try    // Initializes the Bitmap Size   
Bitmap.Height := Height;    Bitmap.Width := Width;     // Draws the
background    Bitmap.Canvas.Pen.Color := clWhite;   
Bitmap.Canvas.Rectangle(0, 0, Width, Height);     // Draws squares   
Bitmap.Canvas.Pen.Color := clBlack;    for x := 1 to 8 do      for y
:= 1 to 8 do        Bitmap.Canvas.Rectangle(Round((x - 1) * Width /
8), Round((y - 1) * Height / 8),          Round(x * Width / 8),
Round(y * Height / 8));           Canvas.Draw(0, 0, Bitmap);  finally 
  Bitmap.Free;  end;   inherited Paint;end;
I'm on a Mac.

A search for BeginPaint gave me this:

QuoteIn the Lazarus IDE and Free Pascal's LCL (Lazarus Component
Library), BeginPaint is a low-level Windows API function, accessed via
the LCLIntf unit, used in conjunction with EndPaint to manage the
process of drawing or updating a control's visual appearance. It
prepares a window for painting, fills a PAINTSTRUCT structure with
painting information, and is often called from the OnPaint event
handler or overridden Paint method to ensure that custom drawing
persists correctly.
So is it needed on all OS's?

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Thaddy · 2025-09-25 13:23

try Code: Pascal  [Select][+][-]window.onload = function(){var x1 =
document.getElementById("main_content_section"); if (x1) { var x =
document.getElementsByClassName("geshi");for (var i = 0; i procedure
TForm1.FormPaint(Sender: TObject);var  P: Array[0..4] of TPoint;  i:
Integer;  phi: Double; begin  form1.canvas.LockCanvas;  for i := 0 to
4 do  begin    phi := 2.0 * pi / 5 * i + pi * 0.5;;    P[i].X :=
round(100 * cos(phi) + 110);    P[i].Y := round(100 * sin(phi) +
110);  end;  Canvas.Brush.Color := clRed;  Canvas.Polygon(P); 
form1.canvas.UnlockCanvas;end;Or
Code: Pascal  [Select][+][-]window.onload = function(){var x1 =
document.getElementById("main_content_section"); if (x1) { var x =
document.getElementsByClassName("geshi");for (var i = 0; i procedure
TForm1.FormPaint(Sender: TObject);var  P: Array[0..4] of TPoint;  i:
Integer;  phi: Double; begin  form1.BeginFormUpdate;  for i := 0 to 4
do  begin    phi := 2.0 * pi / 5 * i + pi * 0.5;;    P[i].X :=
round(100 * cos(phi) + 110);    P[i].Y := round(100 * sin(phi) +
110);  end;  Canvas.Brush.Color := clRed;  Canvas.Polygon(P); 
form1.EndFormUpdate;end; Both are blocking calls, I guess the second
would also work on mac.

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Jeff_t · 2025-09-25 13:38

Well, Lock and Unlock didn't affect things.
BeginPaint and EndPaint  report not available on my canvas

Invalid Access exception.

The problem line appears to be this:

if TObject(dc) is TCocoaContext

dc does have a longint value at this point, and that line is quite
happy when the form opens.


If I remove all code from the Paint, it works.
The presence of these lines (which do produce the required rectangle)
brings the error back

Code: Pascal  [Select][+][-]window.onload = function(){var x1 =
document.getElementById("main_content_section"); if (x1) { var x =
document.getElementsByClassName("geshi");for (var i = 0; i           
    // Set the fill color                Canvas.Brush.Color
:=palette[cul1].palRGB;  //palRGB is a prefilled TColor              
  // Draw the rectangle                Canvas.Rectangle(R);   //R is a
prefilled TRect

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Jeff_t · 2025-09-25 20:14

The exact same code works perfectly if pasted into the Form's Onpaint
event.
Confused.

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korba812 · 2025-09-25 21:07

It seems that you are drawing on form canvas. Try drawing on the
PaintBox canvas.

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Jeff_t · 2025-09-26 06:12

QuoteIt seems that you are drawing on form canvas.
If I was drawing on the form's canvas, the drawing would be relative
to the top left corner of the form, yes?
It is not - when I draw in the PaintBox methods, drawing begins top
left corner of the Paintbox.
But if I prefix the  calls to   Canvas.<something>   with the name of
the Paintbox, all seems well.
eg if the Paintbox is named Brian, then using Brian.Canvas.Rectangle()
is OK

This is hugely unintuitive,  as I would have expected any reference to
an unqualified 'Canvas'
within a method of the TPaintBox , to refer to the TPaintbox, not the
form.

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cdbc · 2025-09-26 06:35

Hi
In the code-editor, if you hover the mouse over the method-name,
'Code-Tools'*) will show you to whom the method belongs...

*) If you [Ctrl] + Click on the method-name, code-tools will take you
to its
    declaration.
Regards Benny

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wp · 2025-09-26 09:50

Quote from: Jeff_t on September 25, 2025, 02:33:38 pmI have a simple
form with a button and a TPaintBox
Quote from: Jeff_t on September 25, 2025, 03:12:10 pmCode: Pascal
 [Select][+][-]window.onload = function(){var x1 =
document.getElementById("main_content_section"); if (x1) { var x =
document.getElementsByClassName("geshi");for (var i = 0; i procedure
TMyDrawingControl.Paint;[...] Your introductory statement about issues
in drawing on a Paintbox and the code shown later are contradictory
because that code is for a custom control, TMyDrawingControl. Are you
sure that your issues are not rooted somewhere in that custom control?
I've never seen issues that you are reporting for a plain TPaintbox.
Just tested again on a mac-VM, and the Paintbox is working fine.