widget re-use, style sheets, and memory usage

[Ray Froehlich]

Hi,

We've written an application that uses style sheets to "skin" buttons, etc., using PNG files. There are multiple instances where we re-use widgets (i.e., have multiple instances of the same kind of widget) where each instance must have the style sheet "re-declared" due to the widget hierarchy. Is there any way to make widgets whose style sheets point to the same image "share" one single pixmap within memory?

I see Qt has "implicit data sharing," but I struggle to figure out any easy way to leverage it for style pixmaps while using style sheets.

Any help would be much appreciated! Thanks!

-Ray Froehlich

[wysota]

I think Qt for stylesheets uses a pixmap cache in the background so all buttons using the same images should in fact use the same pixmap.

[Ray Froehlich]

First of all, thank you for the quick response. I really do appreciate it!

Are you sure about the pixmap cache? That was my assumption as well, but all the data I've collected so far indicates this is not the case. Or, is there something I need to do to leverage this cache that I may not be doing now?

Looking at the Qt source code 4.6.1, QRenderRule constructor, it looks like a regular QPixmap is constructed when the bg member of type QStyleSheetBackgroundData is instantiated.

Thanks again for the help!

-Ray

[wysota]

Are you sure about the pixmap cache?

No, but it would be silly of the Trolls not to do it this way. I don't want to dig into Qt code right now so I can't be certain.

That was my assumption as well, but all the data I've collected so far indicates this is not the case.

If you mean that the more widgets you have the more memory is used this proves nothing. Surely each button has its own instance of the final pixmap associated with it but it doesn't mean the intermediate pixmaps are not shared between instances.

Looking at the Qt source code 4.6.1, QRenderRule constructor, it looks like a regular QPixmap is constructed when the bg member of type QStyleSheetBackgroundData is instantiated.

You should look into QPixmap code as well.

[Ray Froehlich]

I agree, it would be silly. Also, I can't blame you for not wanting to dig into Qt source code. Finally, thanks for the QPixmap suggestion. I'll take a look.

If anyone else has anything to add, please feel free.