iOS is a mobile operating system that is developed and sold by Apple Inc. It was introduced in 2007. The iOS operating system is currently the most widely used mobile OS worldwide, with an estimated 85% market share on smartphones as of September 2018.
iOS development is by far the dominant market in several regional markets, including the US, Canada, Norway, and France. Remote developers might not be a bad idea if you are hiring an iOS developer as many tech talent communities can provide help to find your ideal candidate remotely.
You may also already realize several reasons to choose iOS development. Android is less standardized than iOS and has more types of devices, which means you need more time and money to do the designing, but apps designed for it have better UI/UX.
Best iOS Developer Interview Questions And Answers
We have compiled a list of suggested interview questions to ask iOS developers for you. These questions are designed to weed out the most competent candidates from the least qualified ones, and as such should be helpful for your needs.
We have broken these interview questions into three levels of difficulty: Basic, Intermediate, and Advanced. We suggest picking whichever question level you’re most comfortable with to start off.
Basic Interview Questions
1 What is Swift programming?
Swift is an advanced and easy-to-use programming language for iOS, iPadOS, macOS, tvOS, and watchOS. Swift is a user-friendly and exciting programming language that is concise yet powerful. Swift provides modern capabilities that developers enjoy while being inherently safe by design.
What are some key features of Swift programming? (Bonus)
Features available in Swift to make your code more expressive:
- Generics are powerful and user-friendly.
- Protocol extensions that make generating generic code quicker and more painless
- Functions with first-class status and a lightweight closure syntax
- Iterations over a range or collection that is fast and succinct
- Tuples and multiple return values are examples of nested types.
- Protocols, extensions, and methods can all be supported by this design.
2 Which JSON framework is supported by iOS (iPhone OS)?
SBJson is a JSON parser and generator for Objective-C. It offers OS X, iOS compatibility (Objective-C is the programming language we use when writing software developed for Apple’s OS X and iOS. It’s a superset of the C Programming Language). SBJson provides a powerful API for handling JSON in a robust, simple-to-use fashion.
3 Name the tool that is used to construct the interface for iOS.
UIKit is the framework that Apple provides for constructing iOS user interfaces.
The UIKit framework provides the Touch Events, such as gesture recognizers and in-app delegates for handling user interactions.
The UIKit framework provides a Drawing Model to render the Views with custom drawing techniques (i.e., deferred rendering) specific to devise screen resolution and allows the developer to create its Controls that are delivered like any other object within an app’s bundle.
4 Which application thread should UIKit classes be used?
Unless specified, use UIKit classes only from your application’s main thread or main dispatch queue. This restriction applies in particular to classes derived from UIResponder or that require modifying the interface of your app at some point.
5 What are UI Elements in iOS?
The interface elements in our applications are known as UI elements. Some of these components, such as buttons and text fields, will respond to user interactions, while others that include pictures and labels provide information.
6 What is the iOS SDK?
The iOS Software Development Kit (SDK) includes tools, compilers, and frameworks for creating applications that run on Apple mobile devices such as iPhone. The current version of the kit supports all four versions of the OS: iOS 12, 11.x-12.0, which support iPad Pro models running both in landscape orientation and portrait orientation; iPad mini with Retina display model; iPod Touch sixth generation running iOS 12 or later; and earlier iPod Touch generations running iOS 11 or later.
7 What is the difference between Synchronous & Asynchronous tasks?
Working synchronously means that everything after the operation waits until it is completed before proceeding.
Asynchronous means “out of order” and can be defined as running code that initiates an action without waiting for it to complete.
8 How can you respond to state transitions on your app?
To respond appropriately to state changes, the app’s delegate object must be informed when the app enters certain states.
9 What are some common execution states in iOS?
The states that are most commonly used in iOS apps, for example, might be active or inactive:
- The Active state means an app is running and has been visually presented on screen by the user (the application’s process is alive).
- An Inactive state indicates an app is executing but not currently visible on the device’s screen.
10 You’ve been alerted that your application is prone to crashing. How should you respond?
It’s essential to know whether you want an iOS developer who is perfect at solving problems, or just someone who can follow a standard approach.
- To determine the version and make or model of the device where you are encountering a problem, gather enough information to reproduce it.
- Acquire any logs available on the device (or if possible, on your development machine).
- Once you have a sense of how to debug the problem, download any necessary tools before beginning.
Intermediate Interview Questions
11 What is the difference between Objective-C and Swift?
Objective-C, which was developed in 1983 by Brad Cox and Tom Love for a new project at Stepstone Computer Systems (later acquired by NeXT) to make it easier for C programmers to learn than other languages such as C++. This language has since been superseded in Apple’s development tools.
Swift is an object-oriented programming language that requires fewer lines of code than typical applications written with either Objective-C or Java; this makes writing iOS apps much quicker. Apps also run on a more secure system because no runtime errors are possible when developers write error-free codes while coding using the Swift compiler toolkit.
12 What are the advantages of using Objective-C?
Objective-c has several distinctive features, including type safety and strict memory management.
Some other benefits which make Objective C one of the most popular programming languages: compile-time checks for errors in code (thereby providing quick feedback to developers), automatic reference counting, powerful object-oriented capabilities, and fast execution speed among others.
13 Name four important data types found in Objective-C.
Here are 4 data types that you want your developer to take into account.
14 What are the four data types found in Objective-C?
The 4 data types in Objective-C are:
- NSString: Represents a string.
- CGfloat: Represents a floating-point value.
- NSInteger: Represents an integer.
- BOOL: Represents a boolean.
15 What are the advantages of using Swift?
First, Swift is a modern language. It was introduced in 2014 and has been updated for iOS 11 use. This means that it’s designed to work with Apple’s latest operating system updates without any modifications required from the user or developer.
Second, its speed of execution is comparable to Objective-C: Swift provides safe automatic memory management through Automatic Reference Counting (ARC), which eliminates crashes due to dangling pointers; this offers developers an easy way out when coding applications that are not prone to crashing – one less thing they worry before publishing their app into the App Store!
Third, according to surveys conducted by Stack Overflow developers rank Swift as being more concise than Java while offering much better performance than both C++ and Java.
Fourth, Swift is a safe programming language and utilizes dynamic dispatch which allows it to improve its performance by avoiding runtime checks for the most common cases.
Fifth – because of this unique combination of safety-focused but also powerful features in swift – iOS developers can quickly create apps that will not only be stable and beautiful looking on the outside as well as having an elegant structure from within!
16 Explain the difference between raw and associated values in Swift.
The raw value of a variable is set when the program begins, and it never changes unless explicitly changed by the programmer. Associated values are assigned to data types to provide different representations of the same type at different times during execution. In Swift, associated values must be initialized before they can be used, but their initializers need not take any parameters themselves.
17 What are UI elements and some common ways you can add them to your app?
The UI elements that make up an iOS app are buttons, text fields, images, labels, and any other items visible within the application. These can range from interactive to not-so-interactive. Interactive elements could be a simple button that clicks when pressed or another element like a slider bar for volume control.
When they’re coded from scratch using things such as NSLayoutConstraints and Auto Layout they’re called screens which comprise the user interface of your iPhone application. Alternatively, you can use the UIView “initWithFrame” method (id)to position these at exact coordinates on screen without having to specify them by Xcode’s interface builder or anything else.
18 What are different ways that you can specify the layout of elements in a UIView?
The ways that you can specify the layout of elements in a UIView are Auto Layout and NSLayoutConstraints.
You use constraints to visually position your views, while auto-layout let’s iOS automatically calculate those positions for you based on other criteria such as being horizontally or vertically adjacent to another element.
Auto-Layout is one of the most powerful approaches because it’s dynamic and fluid enough to adapt when new models come out with various screen sizes – so if X comes out with an iPhone 11SX+, then iOS would be able to dynamically adjust all child layouts accordingly without any developer intervention.
NSLayoutConstraint maintains fixed distances between two points (e.g., top edge of a view to the leading edge of another) and can constrain one element within a rectangular area.
19 What is an efficient way to cache data in memory?
There are several methods to build caches, and the most basic is a simple dictionary, but whatever you pick, be prepared to discuss why you like it. Make sure to consider how you’re going to remove data from the cache: either explicitly or by running out of memory.
NSCache is far preferable to a simple dictionary since it gets erased automatically by the system when memory runs low.
20 How does Swift handle memory management?
ARC is the Automatic Reference Counting that’s built into iOS and OS X. It manages how objects or classes hold references to other objects, ensuring they don’t sit in memory uselessly after they’re done being used by your app.
The idea behind ARC is simple: you focus on writing code for what you want it to do rather than having to worry about managing its lifecycle yourself. As long as you follow a few guidelines (like not creating reference cycles), then ARC will take care of everything else at compile time so you never have to think about it again!
Advanced Interview Questions
Here are some interview questions to ask when hiring an experienced iOS developer with five or more years of expertise.
21 What is CoreData?
Core Data is a graph management framework that works with objects. It manages a potentially large graph of object instances, allowing an app to work with graphs that would not fit into memory because entities are faulted in and out of memory as needed. Core Data also maintains consistency among references (e.g., keeping forward and backward links consistent when objects are added/removed to/from a relationship), making it an ideal foundation for the “model” component of an MVC architecture.
The database used by Core Data to manage its graph is SQLite, as you might guess. It may have been done using a different relational database or even a non-relational database like CouchDB. Core Data isn’t a data store engine so much as it is an API that hides the underlying data store behind abstracted functionality. You can tell Core Data to save in an SQLite database, plist file, binary file, or even a custom data storage type.
22 What is a Serial Queue?
The Serial Queue enables us to complete only one task at a time, regardless of execution method, i.e., Synchronous or Asynchronous. The code executed by the Serial DispatchQueue is not parallel and must wait for the first task to finish. This style of execution is also known as First In, First Out (FIFO). The queues must all wait for the conclusion of the previous queue. By default, DispatchQueue is a serial queue.
23 Explain code signing for iOS apps
Signing an application allows the system to determine who signed it and to assure that it hasn’t been tampered with since then. Signing is necessary for submitting apps to the App Store (both on iOS and Mac). OS X and iOS check whether applications downloaded from the App Store are valid by verifying their signatures. This ensures that users know that the application was signed by an Apple source, so they can have confidence in its integrity.
Your digital identity is used by Xcode to sign your application during the build process. A public-private key pair and a certificate are included in this digital identity. The private key is employed by encryption algorithms to produce the signature. Apple issues the certificate, which includes the public key and indicates you as the owner of the key pair.
The signature on an application’s executable code is protected because the signature becomes invalid if any of the programs compiled components changes. The signatures of images and nib files are not signed; their modifications do not affect the validity of the seal.
24 What is the iOS App States
Every iOS app goes through the five states at some point. The operating system manages the application state, but the software is in charge of critical operations to maintain a fluid flow between states. The following are the five possible states of an iOS app, as defined in the iOS App Programming Guide:
- Non-running – The app isn’t active.
- Inactive – The app is active but not receiving events. When a call or SMS message is received, an iOS application may be put into an inactive state, for example.
- Active – The app is in the foreground, listening for events.
- Background – The app is in the background, and it’s executing code.
- Suspended – The app is running in the background, but no code is being run.
25 Explain the difference between functions vs methods in Swift
- Functions are chunks of code that do one thing and only one thing. A name is assigned to a function, and this name is used to “call” the function when it’s needed to execute its operation. There is no self in functions.
- Methods are functions that refer to a specific type. Methods, which encapsulate certain operations and features for working with instances of a given type, may be defined in classes, structures, and enumerations. The method may call self.
26 Explain Responder Chain
The UIView associated with the event is sent the event by the view. The event propagates up the chain of UIResponder objects connected to the UIView until it reaches a responder that can handle it. If none of those handle it, then the UIWindow is asked whether it can manage it, and if not, the UIApplicationDelegate is contacted.
27 What is Autolayout?
Autolayout is a layout system that allows you to create user interfaces such as iPhone apps and iPad apps, which can adapt dynamically depending on the size at runtime.
How does Autolayout work?
When an iOS app runs it has one main window associated with it. This application’s UIWindow object handles all of your views for you by calling methods like addSubview(), insertSubview(), and removeFromSuperView().
Don’t expect someone with no technical background to master this technology, but they should be able to explain how and when they would use it, as well as the benefits it offers.
28 How do you handle async tasks?
Asynchronous programming is an important component of every iOS app. To avoid blocking the user interface and causing users to wait or system-kill your application due to a lack of memory, networked functions, local storage, and other major computations should be carried out in the background.
There’s no one-size-fits-all solution, and what may work well for you and your team may not work so well if applied to a larger group. The most common answers include NSOperations, GCD, Promises, and RAC. A competent developer knows many ways to execute async operations and when they are required (i.e., with networking, local persistence, etc.). We hope for more specialized tools from a senior developer such as ReactiveCocoa and PromiseKit.
29 How do you debug and profile things on iOS?
Nobody writes flawless code, and debugging and profiling are two of the methods we use to find the appropriate technical fix. On iOS, we have all of the traditional “manual” debugging tools available such as NSLog/print functions to print information in the console. However, Apple provides us with a more sophisticated set of tools and instruments to assist us in detecting where issues exist.
The ability to explain how one would utilize breakpoints and logs to obtain runtime data should be a fundamental knowledge required of all iOS developers. You should expect to hear things about Instruments and Crash Logs from a senior developer.
30 Do you test your code? What steps do you take to make your software testable?
This is mortifying, to say the least, but we recognize that we don’t do testing as often as we should. We all know how important it is to do it more frequently. In this case, I’m talking about Unit Testing and Integration Testing in particular.
There are only two alternatives: they do it or they wish they could. Because iOS users are less inclined to test, the community isn’t as large as that of Ruby. The majority of it is due to a lack of testing culture passed down through generations, but there have been improvements in recent years.
We hope that these interview questions will assist you in finding a great iOS developer. Remember, no one person has all of the answers, but a good programmer should be able to explain all of these questions in their own words.
These iOS developer interview questions cover the majority of the topics and tools that a seasoned iOS developer would encounter on a typical project.
Always ask for code samples; it’s really useful to look at what someone has done and even request them to explain it to you.
Keep the conversation informal or more conversational; people are more inclined to respond when they’re comfortable.