App interoperability the Sailfish way. What is the way to share between apps (UI and API wise)?

Certainly that would work, but it requires you to know each and every service's dbus interface you need to connect to in advance. A more standardized (at an OS level) way to do it would be much better IMHO: if an app provides some sort of "share" functionality it register itself as a provider; on the other hand, if an app wants to share something, it can request a list of known providers and simply let the user choose which one to use.

D-Bus is missing a "find all objects implementing this interface" function. Never understood why it was left from the initial design.

I think first and foremost we should push for a dbus improvement in this regard.

Then, while we wait for it to be implemented, we build a workaround that registers every apps dbus interface on install, by introspection.

Then this register will itself publish an dbus interface, which can be used to query about available dbus services, etc.

Jolla could publish an standard expected interface, if required.

I just think, that we shouldn't reinvent the wheel, specially now that dbus is going zero-copy efficiency with kernel integration!