MacOSX does not accept Canvas Output outside OnPaint Message
lazarus / 39-Graphics

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Martin V · 2010-06-23 12:22

Hello,

in Win32 and gtk2 (Linux), you can simply draw something e.g. to the
Form.Canvas in an arbitrary event like OnClick:

Code: [Select]procedure tForm1.Button1OnClick;
  begin
     Canvas.MoveTo (1,1);
     Canvas.LineTo (100,100);
  end;

In MacOSX, there's no output shown and the only way to get Canvas
output is to do it in the OnPaint message. This is a really big
difference to the other OS. Is it a Lazarus bug or a system-specific
limitation? How can I modify my application (an editor) which often
writes to the Canvas forced by the user and not in a PAINT message
procedure?
I made a dirty workaround by executing Form.Invalidate after every
Canvas Output. Invalidate generates a PAINT message. But in this case,
the whole window gets repainted.

Martin

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theo · 2010-06-23 12:56

Yes, that's the correct way imho for all widgetsets.
Invalidate -> Paint.

You can also invalidate only parts using InvalidateRect (LCLIntf) and
Canvas.ClipRect in overridden procedure Paint.

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zeljko · 2012-07-04 05:24

You are drawing outside of paint event and that's why it does not
work. Same thing is with qt widgetsets on any platform. To get it work
you must do paint stuff in Form.OnPaint() , and in ButtonClick just
call Update;
MacOSX (carbon and cocoa) and Qt does not support painting outside of
paint event.

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felipemdc · 2012-07-04 05:41

Quote from: Martin V on June 23, 2010, 02:22:57 pmIn MacOSX, there's
no output shown and the only way to get Canvas output is to do it in
the OnPaint message. This is a really big difference to the other OS.
Is it a Lazarus bug or a system-specific limitation? How can I modify
my application (an editor) which often writes to the Canvas forced by
the user and not in a PAINT message procedure?
I made a dirty workaround by executing Form.Invalidate after every
Canvas Output. Invalidate generates a PAINT message. But in this case,
the whole window gets repainted.
In general most platforms do not support painting outside the paint
event, examples: Carbon, Cocoa, Qt, LCL-CustomDrawn (in all platforms,
including Android). If you want to make a continuous draw, then store
it in a TBitmap and call .Invalidate or InvalidateRect (I would avoid
messing with the clip rects, InvalidateRect should be all you need).
Invalidate is not a workaround, it is the proper cross-platform
solution.

Your OnPaint event would then draw this stored bitmap to the canvas.