Re: flicker with viewport and hbox

[Bert <bert ioniq co za>]

On 02 Jan 2009, at 4:17 AM, Yu Feng wrote:

> Hi,
>
> I am not sure if I understand your scenario correctly.
>
> Why don't you put the icons out of the viewport, aka:
>
> VBOX
> + up indicator icon
> + Viewport
> + down indicator icon
>
> in your key-press handler, you scroll the viewport, and show/hide the
> icon.

I unfortunately don't have vertical space left, this is a 320x240  
embedded application. However, your suggestion has triggered me into  
thinking of doing the following:
HBOX
+ viewport
+ VBOX
++ up indicator
++ down indicator

and then show/hide the indicators as necessary, still keeping them out  
of the viewport as per your suggestion.

> Because the visibility of the icon rarely changes, the allocation of  
> the
> Viewport is not changed very often. Neither will the allocation of the
> widgets inside the viewport change.
>
> The problem of your implementation, (perhaps) is that you add/remove
> those indicator icons in every keypress event. The add/remove causes a
> resize to be queued, thereafter every widget is redrawn, and flicker
> occurs.

I believe you've hit the nail on the head with this last insight --  
this (the queueing of the resize) is probably the crucial thing that I  
had not realized, and explains why things were drawn twice.