What is the best approach for writing unit tests for iPhone / iPad?

To fix the issue you can do When tasked with developing against a third party API, the first thing I do is code a prototype (integration test, if you will) to obtain the expected / desired result. I then create some unit tests that enforce that expectation and refactor the prototype into the actual code I will use moving forward.
So yes, IMO, it's pretty typical. It may not be TDD in the purest sense, but I'm OK with that.

Hope that helps Unit testing is not about catching possible bugs, its about 'feeling a little bit safe'. Good example is a project with code base larger than hello world with multiple developers. It is only a matter of time, when somebody breaks something, what was working before. The question is - when will you know it? After the application is deployed or during development?

this one helps. You probably have simple ruby classes inside your /app/services folder. Then just write very simple unit test for each single service. Place them under /test/services/ or spec/services/ folder. You should have one unit test file for each class you define for your application.

help you fix your problem There is more than one reason, so here is the ones I can think of:
Modularity

I wish did fix the issue. I think the easiest route to go would be to use two different storyboards, 1 for iPad and 1 for iPhone. You set the default start up screen in the AppDelegate viewDidLoad withOptions method. and in there check to see what device you are running on by