Fl_Scroll: Add possibility to get notified when scroll position changes?

[Jakub Holý]

Hello, awesome FLTK devs!

I find myself needing to get notified and run custom code when scrolling is happening / stops. Currently there seems no way to achieve that. Perhaps it would be generally useful if Fl_Scroll had callback(s) triggered when its x/yposition changes.

Background: I am making a file explorer for pictures, displaying thumbnails. For performance reasons I only show thumbnails of pictures in the visible viewport. When the user scrolls to a new position (or is scrolling slowly), I want to load and show also these newly visible pictures' thumbnails. Thus I want to trigger the image loading function when scrolling ends.

Obviously FLTK lived fine without this for many years and I was able to implement this by querying the scroll's yposition every 100ms, so it might not be worth the effort. But it would be nice.

Best regards, Jakub Holy

[imacarthur]

On Friday, 9 September 2022 at 08:28:23 UTC+1 jakub wrote:

I find myself needing to get notified and run custom code when scrolling is happening / stops. Currently there seems no way to achieve that. Perhaps it would be generally useful if Fl_Scroll had callback(s) triggered when its x/yposition changes.

This post is off-topic for this list (this list is about developing the library itself, not about developing using the library) and so this would be more appropriate in the  fltk.general list - though I note you have cross-posted there, so I'll pick this up over there...

[Jakub Holý]

I meant it not as a question but as a proposal to extend the existing api of Fl_Scroll, which only core devs can do.

I got now an advice to subclass it and leverage its scrollbar's callback, which sounds as a nice small solution. So perhaps there is no need to change anything.

[Bill Spitzak]

You could probably check this when draw() is called.

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[Bill Spitzak]

Yea actually draw is right as it will also get resizes.

Just check at draw all the images that are now visible and load them if necessary.