Okay, now for the bite-the-hand-that-feeds-me post. This gun.io post on How to hire an iOS Developer is somewhat wrong on the major points to the point where its somewhat the clueless leading the clueless.
First point, there are two types of mobile applications. Those that do not require a-lot of interactivity and can be developed using a mobile web framework. Those that do require high interactivity require native code such as ObjectiveC, Java, etc and are based on the platform target.
Second point, you are always attempting to compute the return on investment early in the process of picking a developer. Why? Because if you have no idea of the ROI than you probably have no clue about what a proper budget might be for said app development.
In fact most professional firms in the fortune 500 category compute the ROI first before fully fleshing out a plan to show the developer.
Sorry, to rant but the mistakes were not tiny.