Basic CS Knowledge
I don’t believe that being a computer science PHD is a requirement for getting into mobile application development, In my experience, heavy CS concepts don’t often come up in day-to-day work, but a lot of employers still like to run their interviewees through CS puzzles. So, for the novice, I would recommend watching the Harvard CS50 courses on YouTube as a great place to get started, they are easy to follow and actually pretty interesting to watch.
If you are planning to interview with one of the big companies like Google or Apple, you might benefit by picking up a couple algorithm books to study.
Cracking the Coding Interview by Gayle Leekman
The Algorithm Design Manual by Steven S. Skiena
Swift or Objective-C?
You don’t have to be an expert, but you should be able to write in at least one of these languages without having to look up syntax very often. Just get yourself to a point where you can write classes, structs, loops, functions, assign variables and evaluate expressions without help.
Right now we are in a strange time where it’s still acceptable to be hired only knowing Objective-C. Swift is pretty new and unless you are applying for a company that has a lot of project turnover, you will mostly be writing in Objective-C anyways. That said, there are plenty of companies taking the plunge to Swift (including the one I work for) and I don’t think it will be too difficult to find a job if it’s the only language you know. The important thing is to be fairly proficient in whichever you choose, and get familiar enough with the other that you can at least read the code.
Frameworks and API
Cocoa Touch is updated yearly and with it things come and go. It is not unusual for me to work with the Apple Documentation open most of the time; there is just too damn much to try to keep in your head.
I don’t think its necessary that you memorize all of the iOS API’s, but you should have a good idea of what is available to you, and be fluent in a few of the big ones.
UIKit (IUViewController, UITableView, UIButton, UINavigationController, GestureRecognizers)
Interface Builder (Storyboards, Segues, and the odd .xib)
Foundation Types (NSArray, NSDictionary, NSString) and their Swift counterparts (Array, Dictionary and String)
HTTP API (NSURLSession, Basic REST API concepts, JSON Parsing with NSJSONSerialization)
Grand Central Dispatch (GCD, NSOperationQueue)
Persistence (NSCoding, NSUserDefaults, CoreData)
Memory Management (what Retain Cycles are and ARC fundamentals)
Third-Party frameworks can also fun, but try to not depend on them too much; if it’s something I can do myself, I usually do.
Patterns are important, they make development easier and they make your code cleaner. Make sure you understand these basic patterns, they are used A LOT in the iOS Frameworks and it is not likely that you will be able to do much without knowing them. There are many more, but you can learn those as you go.
Delegation (This is sort of the workhorse of most iOS API’s, you should DEFINITELY understand this)
Model View Controller (MVC, I don’t think Apple did the best job of encouraging best MVC separation, but it’s an important pattern that can help improve your code if you take the time to implement it properly. Also, it’s pretty much guaranteed to be on any iOS interviewer’s question list.)
Subclassing or Object-Oriented Programming (The most part you will be writing apps in this style)
Singleton (This one can definitely be abused… use sparingly.)
Familiarity with the Environment
This might seem obvious, but if you don’t have a Mac, get one! If you don’t have an iOS device, get one! While it’s not impossible to get started just writing apps for the simulator, you will eventually want a real device for development. As for computer, it’s gonna be pretty hard to learn Xcode without a mac to run it on. I started out with a 2009 MacBook Pro 13″ and a first-gen iPod Touch. It is quite possible to get by with the lower end devices to start, it was nearly a $1500 investment, but totally worth it in the long run.
All mobile developers should at least know some of the basic concepts of design. You should know the difference between mockups and wireframes and how to use both in your development process. Knowing what Apple considers usable UI will help as well; for this you will definately want to read the Apple Human Interface Guidelines.
You should be familiar with a few of the common development tools.
Xcode (of course)
Source Control (Git, Subversion or Mercurial… probably just git though)
Issue Tracking Software (JIRA is the big one here, but there are others. Just play with some of them if you can.)
Having an opinion about iOS, Swift, or even a specific API is a great way for interviewers to understand how deep your knowledge about a topic goes. It also allows us to see your passion. If you are having an interview and you are asked, “So what do you think of Swift?” saying “It’s alright, I guess” is not the correct answer, you should tell them what you think of optionals, how you like using a specific feature. There aren’t many wrong answers here, the important thing is to have something to say.
Actions can speak louder than words. If you really want to nail that interview, put together a couple of simple apps (or even better, launch them on the AppStore); they really like to see that you have the ability to complete a project. GitHub is nice too, but make sure your code is easy for the interviewers to compile if they wanna test it out.
If you are wanting more resources to get started, please checkout my iOS Developer Resources page where I link to blogs and pages I found helpful when I started out.
I guess the last thing I would add is JUST DO IT! Right now iOS developers are in high demand and if you can get yourself to a decent skill level, you won’t be without work for a while. It’s a rewarding job that allows you to practice both engineering skills and creativity.