TStringGrid.OnPrepareCanvas - when does it fire
lazarus / 31-LCL

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Graeme · 2015-07-03 05:45

As the title say, when does OnPrepareCanvas fire in the grand scheme
of things. Unfortunately the API Class documentation[1] doesn't
elaborate on that. We have a StringGrid with custom drawing using
OnDrawCell, but where does OnPrepareCanvas fit in, and why would I use
the OnPrepareCanvas, when I could do all the Canvas stuff I need in
the OnDrawCell?


Links:
API Class Documentation
http://lazarus-
ccr.sourceforge.net/docs/lcl/grids/tcustomgrid.onpreparecanvas.html

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balazsszekely · 2015-07-03 09:31

OnPrepareCanvas is fired just before OnDrawCell(grids.pas-->line
9838).
If you use custom drawing, you don't need OnPrepareCanvas(basically
it's useless), as you said you can do all the canvas stuff in the
OnDrawCell. On default draw instead, just before OnPrepareCanvas is
called, a  lot of things are going on, like painting the selected raw,
setting canvas font, etc...(for more details see line 3562).

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Graeme · 2015-07-03 10:53

Many thanks for the information. So in our case OnPrepareCanvas is
definitely not needed.

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wp · 2015-07-03 11:08

The grid's wiki page is extremely useful for someone trying to learn
more about the grid. I definitely prefer it over the "official"
documentation.

Grid drawing is explained in
http://wiki.lazarus.freepascal.org/Grids_Reference_Page#Description_of
_grid.27s_drawing_processand http://wiki.lazarus.freepascal.org/Grids_
Reference_Page#What_happens_in_the_TCustomGrid.Paint_method.3F