I came to GNOME after using KDE Plasma for many years, but then I was trying to figure out the most streamlined way to produce cross-platform apps, and then someone suggested that GNOME is closest to Xcode on other platforms.
But I think that GNOME is still geared towards desktop apps and web apps to some extent. However, what about Android?
Then I found Waydroid which gives an Android “runtime” on a desktop Linux.
Is there or can we have a GNOME “app engine” that can build for all platforms from GNOME Builder etc.? I think it would be like a hybrid of Java JDK and Xcode-like design.
It could make an excellent use case for Vala because one could try to make all of it have a Vala API so that it resembles what Apple users get with Objective-C and what Microsoft users get with C#.
I’m not sure where you get the name “app engine” from. Its not a term I’ve heard to describe technology for cross-platfrom development with.
Anyway, about cross development:
The UI toolkit GNOME uses, GTK, has initial support for Android now. Its still experimental and only available from the development version.
Other than that, GTK can be used for Windows and macOS as well (and is used, like by Inkscape or Gimp).
That aside:
Apple and XCode itself only provide macOS and iOS target, and Microsoft and C# to my knowledge only desktop. So not sure why you mention them.
The only cross-platfrom toolkits which do provide mobile and desktop on all major platforms which I know of are Flutter and web-based applications (think React-Native and Electron).
Well, the GNOME “app engine” would be an improvement in this regard and has the advantage that Android and, for example, Debian are already closely linked.
Commonly, professionals would use Qt for this use case, but it’s much less integrated with the core Linux ethos like GObjects are.