[toast0]

If Apple (or Google) wanted to make this work, it wouldn't be that hard. An app could list the dependencies, with a name perhaps, but definitely a hash. The developer would have to upload the dependencies, too. If the hash matched, the server would also do a byte by byte comparison, and if that matched, mark the dependencies as sharable. When downloading an app with a sharable dependency, the phone would check if it already had installed it, otherwise download it and install it at the same time as the app. Refcount the sharable dependencies at install/remove/upgrade time and rove unused dependencies after a reasonable time.

You would only get a benefit if two or more apps you installed used the exact same version of the dependency, but that seems like it might happen enough; especially once it's expected to be in the ecosystem.

[stickfigure]

Sounds like you just invented https://nixos.org/nix/

[on_and_off]

Google is actually working on this for the support libraries. All the Android apps use them, so it is a good target (and of course it might be possible to extend it to other libs). I guess it makes the most sense in the markets where data is very costly. By itself the support lib is not really big but if each KB counts, let's not download it several times.