but it is almost unusable on ios because it can not run in background
Unfortunately, the only way around that is to integrate with Apple's Push Service, which means you need to run a server-side component which notifies Apple (and which in turn notifies your app). This is not specified for XMPP, so most "XMPP clients" for iOS instead store your credential on the app developer's infrastructure. However, this problem is being worked on:
I am working on a protocol-agnostic, federated push messaging layer for ChatSecure-iOS but it will be interesting to see if anything else pops up in the meantime.
