[Lazarus] Calling DrawFocusRect() outside the Paint() event

[Michael Van Canneyt]

On Mon, 10 Oct 2016, Mattias Gaertner via Lazarus wrote:

> On Mon, 10 Oct 2016 10:21:48 +0100
> Graeme Geldenhuys via Lazarus <lazarus at lists.lazarus-ide.org> wrote:
>
>> Hi,
>> 
>> Working on a visual report designer for the upcoming fpReport. We use
>> Canvas.DrawFocusRect() to draw a selection rectangle around a reporting
>> element (eg: a memo). We can drag that rectangle around in the report
>> designer (which itself is a custom widget). We call the
>> Canvas.DrawFocusRect() from within the MouseMove event. It seems to work
>> fine under LCL-GTK2, but could this be problematic on other platforms,
>> as painting outside the Paint event can normally have undefined results.
>
> Some widgetsets like Carbon can only paint during the Paint event.
>
> 
>> Or has LCL somehow resolved the issues of painting outside the Paint event?
>
> No.
> If you want to optimize for mini changes like the focusrect you might
> want to use a bitmap.

Please explain ?

>
>> On a side note:
>>   The Paint() event for the report designer can potentially have very
>> lengthy code, so we definitely don't want to repaint everything simply
>> because we are dragging a focus rectangle around on the canvas. So if we
>> must, we can obviously work around this by using various states in the
>> Paint method.
>
> You need to optimize the Paint event anyway.
> You don't need to draw everything when using InvalidateRect instead
> of Control.Invalidate.
> And in Paint use Canvas.GetClipRect or lclintf's GetClipRGN to avoid
> unnecessary calculations.

So, if I understand you correctly, when I want to draw a focus rect, I must 
(roughly):
- calculate the focusrect, 
- Call canvas.invalidaterect() with the focusrect
- in the paint event, check ClipRect (which should equal the focusrect)
- and redraw anything that intersects with the cliprect ?

Correct ?

Michael.