Since the Realtime pub/sub channels are a great fit for chat apps (simply map them to chat rooms), I must say you have already given the first step in the right direction by browsing this help center 😉
The next step is deciding in which programming language you'll be developing your iOS app.
If you are familiar with ObjectiveC or Swift maybe you should go the native way, pickup Xcode and start coding a native app.
In Realtime we believe in learning by example so luckily for you we have a group chat example for the native and hybrid approach. And both include Push Notifications through APNS so you can engage with your users when they are not using your chat app.
Download your preferred example, follow the instructions in the repository README file and you'll be up and running in no time.
As soon as you understand the main concepts and see the code examples working in your own device, it'll be much easier to move forward and start coding your own chat app.
Here are the links to the examples:
- Native ObjectiveC: https://github.com/realtime-framework/MessagingObjectivecChat
- Native Swift 3: https://github.com/realtime-framework/MessagingSwift3Chat