@palinek, do you know of a reason for having an internal icon engine? Have I missed something obvious?

If I'm not mistaken, it is here exactly for being a bridge for using glib's icon resolving. I didn't look into the code thoroughly now...how do icons work with the engine removed?

If I'm not mistaken, it is here exactly for being a bridge for using glib's icon resolving.

The bridging is done by Fm::IconInfo (especially, by IconInfo::internalQicon). Fm::IconEngine just used it.

how do icons work with the engine removed?

They're OK. Moreover, the problem described in #788 is resolved because, now, LXQt's icon engine does its job without any interference.

I'll use the patch for a while. If you find a reason for having an internal icon engine, please tell me!

The bridging is done by Fm::IconInfo (especially, by IconInfo::internalQicon). Fm::IconEngine just used it.

Yes, but the themed GIcon supports multiple (prioritized) names, whereas the QIcon does not. So the Fm::IconEngine here is used to make the bridge for it.
Without the IconEngine if you put most precise QIcon to any qt-world component (by using IconInfo::qicon()), that qt component lost track of the "parent" IconInfo object -> now if there would an icon theme change occur, the qt component (with its QIcon object) may show wrong icon (low priority).

E.g.:

• you need icon themed "AAA"
• current theme T1 doesn't have such icon, so the first one found of fallback ("unknown", "application-octet-stream", "application-x-generic", "text-x-generic") will be used and returned as QIcon q
• icon theme is changed
• current theme T2 does have icon "AAA"
• the qt component with QIcon will still show the wrong fallback icon image (if any in the T2 theme) or even some QIconEngine fallback (if the T2 doesn't have such icon)

Where if there is the QIcon with Fm::IconEngine used, upon the theme change the most suitable icon is shown for that theme.

Very good argument! Yes, it's all about icon set change. Thank you very much!

Now, we have a reason to tolerate #788.

Yes, but the themed GIcon supports multiple (prioritized) names, whereas the QIcon does not.

Using IconEngine doesn't make a difference in that respect. IconEngine::virtual_hook calls IconInfo::internalQicon(), as is the case without IconEngine: in both cases, the first icon is picked up.

However, that doesn't change the value of your main argument about changing the icon set.

Using IconEngine doesn't make a difference in that respect. IconEngine::virtual_hook calls IconInfo::internalQicon(), as is the case without IconEngine: in both cases, the first icon is picked up.

No. The first no-null/valid icon is taken in internalQicon() and the validity of each one could have been changed after icon theme update.

No. The first no-null/valid icon is taken in internalQicon() and the validity of each one could have been changed after icon theme update.

Yes, I meant the first existing icon, and it's picked up with or without an internal icon engine. The whole reason for an internal icon engine is the case of icon theme change, whether a theme is updated or it's switched.

I might add a comment to the code later.