iOS Developer Interview Questions

iOS Interview Questions for Freshers (Beginners)

These basic iOS interview questions and answers are an excellent resource for beginners with little experience working with iOS or basic knowledge of iOS concepts. 

1. What is iOS?

iOS is a software platform that runs on mobile devices like cell phones and functions like a computer system. An iOS is intended to be more power-efficient, quicker, and smaller. A mobile operating system (OS) maintained and developed by Apple Inc. and published only for Apple hardware is called iOS (formerly iPhone OS).

The following are some major features of iOS:

• Integrated search capability allows simultaneous file, media, application, and email searches.
• Device shaking to undo most recent activity, all thanks to gesture detection.
• Navigation tools (Google Maps)
• Closed-source makes things more secure.
• iCloud service enables users to store all their data in the cloud. 
• Notification Center provides app notifications. 

2. What are the abstraction levels of iOS?

There are four different abstraction layers of iOS:

• Core OS Layer: Provides low-level capabilities and security frameworks for interacting with external hardware.
• Core Service Layer: Provides services top layers need from the core services layer.
• Media Layer: Offers tools required for graphics, audio, and video.
• Cocoa Touch Layer: Location for frameworks, frequently used while designing applications.

3. Differentiate between asynchronous and synchronous calls. 

Synchronous:

Synchronous API calls entails pausing the code execution and waiting for the API call to complete. This means your application won’t continue to run until the API returns a response, which the user may experience as latency or performance lag. However, synchronous API calls can be advantageous if your app functions only once the API response has been received.

Asynchronous:

Asynchronous calls don’t pause (or wait) for the server to respond to the API call. Your program continues to run, and a “callback” function is then called when the server responds to the call.

4. What are properties in iOS?

Values are connected to certain classes, structures, or enumerations called properties. Unlike computed properties, which calculate (rather than save) a value, stored properties save constant and variable values in an instance. Enumerations, classes, and structures all offer computed properties. However, only classes and structures offer stored properties.

5. What are enums?

A variable with an enum symbol can only take one of a set of predetermined values. For instance, the day of the week can only be one of the following: Monday, Tuesday, Wednesday, Thursday, Friday, Saturday, or Sunday. Consequently, a list of all potential values must be provided when declaring an enum. These are the only possible values that can be assigned to the variable anywhere in a program, according to the compiler.

6. Differentiate between strong and weak references in Objective-C. 

Strong Reference: 

Strong references (most common) imply you want to “possess” the object that a property or variable is addressing. As long as you reference any object you assign to this property with a strong reference, the compiler will take precautions to prevent its destruction. The object won’t be destroyed until you set the attribute to nil (unless one or more other objects also hold a strong reference to it).

Weak Reference: 

With a weak reference, however, you indicate that you do not wish to be in charge of the object’s lifetime. Why? Because at least one other object has a strong reference to the item you are weakly referencing. The object is destroyed when that’s no longer the case, and your weak property is then set to zero by default.

7. What is code coverage?

Code coverage calculates how much your production code ran while your automated test suite was being run. It’s usually stated as a percentage of lines that were actually executed out of all the lines that could be performed. For example, if your tests covered 75% of 100 lines of code, only 25% of the code was executed. In only 25 lines, bugs can be simple to conceal. It provides a general indication of “how well tested” your code is and, thus, “how certain we may be that it will operate.”

8. What are different application states in iOS?

iOS has five application states: 

Not running

The system shut down the app, whether it had already started or not.

Inactive

Despite being active in the foreground, the app is not currently getting events. (It might, however, be running some other code). A typical app only spends a short time in this stage before moving on to another.

Active

The application is getting events while it is active in the foreground. The default setting for foreground apps is this.

Background

The application runs code in the background. Most apps briefly experience this status before being suspended. However, a program requiring more execution time can stay in this condition for a while.

Suspended

No code is being executed even if the program is running in the background. Before the system converts them to this mode automatically, apps are not informed. A paused program still has memory, but it is not actively running any code.

9. What is the difference between Android and iOS?

iOS was built on the Unix foundation by Apple Inc.. The fixed UI and predetermined way the iOS system operates limit the user’s ability to alter the device’s functionality. 

Android: Google owns Android OS, a more adaptable and open-sourced platform. The operating system is open-sourced because several devices with various hardware configurations use it. This makes it challenging to stay bug-free and maintain the OS for all types of devices.

10. What is Regex?

A regex is the nickname for “regular expression.” It outlines guidelines for formatting a pattern that will apply to text searching. 

For example: “a?z” would match “acz”, “abz”, “adz”, or any other three-letter combination with ‘a’ at the beginning and a ‘z’ at the end. 

We can apply more rules for our use cases, particularly those dealing with character groups, string positions, etc. In practice, in all computer languages, the REGEX syntax is the same.

11. What is Deep Linking in iOS?

Deep linking in mobile apps is the future frontier for exponentially increasing product and service sales. It is comparable to clickable links on web pages, except, in this case, it directs consumers to certain pages within mobile apps. Instead of just opening a mobile app and navigating to a specific page inside it, this technology instantaneously integrates mobile applications, giving users direct access to services. 

The current state of mobile app technology prevents smooth navigation and interaction between several apps. Deep linking provides a solution by making it easier for users to find certain landing pages within the app.

12. What is Grand Central Dispatch (GCD)?

Grand Central Dispatch (GCD) optimizes software for multi-core computers. In essence, it aids in concurrent code execution by delegating thread pool management from the developer to the OS. It works with OS X 10.6 and iOS 4 and later.

GCD provided several advantages as iOS matured, including

• Making an app more responsive by carrying out complicated activities in the background.
• Offering a better concurrency model than the typical locks and threads to prevent concurrency errors.

13.  What is Automatic Reference Counting?

Automatic reference counting is a compile-time memory management technique. It guarantees objects are kept in memory only as long as they are required in object-oriented software development (removed otherwise). 

Typically, an object is allocated to carry out a task or preserve some data or state. Ideally, the object is deallocated once the task is complete, if the data is no longer needed. Programmers or system designers can create large graphs of interconnected objects. The issue of when to deallocate a specific object arises as object relationships become more complex.. 

Failure to release an object when it is no longer useful is wasteful and can cause an application to “consume” more memory than it actually needs. This can result in significant software malfunction (and, eventually, deplete available memory). Reference counting is giving each object a “reference count”— a running track of how many other objects depend on it to stay in memory. A given item gets deallocated when its total is decreased until it reaches zero.

14. What is the difference between C and Objective C?

Here is how C and Objective C differ from each other: 

• In contrast to C, Objective C incorporates classes.
• While members of C structures are always public, instance variables (data members) in Objective C can be specified as public, private, or protected.
• Object-oriented programming is made possible by Objective-C’s addition of syntax and semantics.
• C is procedural programming, whereas Objective C is object-oriented programming.
• Separating issues into smaller, independently generated, and testable subproblems and submodules is possible with Objective C.
• Objective C has the capabilities of remote invocation and delegating methods to other objects, and they are straightforward to implement using categories and message forwarding.
• Similar to Java, Objective C does not allow for multiple inheritances.
• A user of Objective C can define a protocol by declaring an Objective C class.

15. What is Cocoa Touch?

To create iOS applications with Objective-C (or Swift), Apple Inc. created the Cocoa Touch framework (Cocoa for OSX- Cocoa Touch for IOS). Classes in Cocoa Touch that begin with “NS” or “UI” include NSObject (Basic Class), NSArray, UIView, and UITableViewController, among others. Cocoa is a prerequisite for running applications on the iPhone. Think of cocoa or any framework as a bridging mechanism between a device and a programming language (Objective-C).

16. What is the difference between a thread and a process?

Process

A process is a program in execution or a dynamic instance of a program waiting in the queue or using the CPU. Many components make up a process, as follows:

• Process ID
• Memory sections(stack, heap, text, and code section)
• Process state
• Program counter
• List of open files

Thread

The fundamental component of CPU usage is a thread. The process itself is a component of a thread. There might be one thread or several threads in a process.

Here are some crucial parts that make up a thread:

• Thread ID
• Program Counter
• Register set
• Stack

iOS Intermediate Interview Questions and Answers

Now, let’s move on to the next-level iOS interview questions and answers for more experienced individuals.

17. What are Design Patterns?

In essence, design patterns are repeatable code solutions that may be applied repeatedly to address typical software issues. Using design patterns in your projects will result in more modular, more scalable, and optimized software. You’ll better comprehend other people’s code since you’ll be able to recognize the design pattern right away. 

Design patterns do not serve as guidelines for creating better software, as design patterns and best practices are totally unlike one another. Moreover, they aren’t intended to provide instructions on how to approach challenges. Rather, they merely serve as documentation of observed usual responses to typical engineering and architectural concepts. Common design patterns include Facade, Decorator, Factory Method, Singleton, etc.

18. How well do you understand assign and retain keywords? 

Assign: This keyword enables us to construct a reference from one object to another without increasing the source’s retain count. The retain count keeps track of how many objects are clinging to one another. The value is immediately assigned to the instance variable. 

Retain: It is the opposite of the keyword assign. It establishes a reference between two objects, but it also raises the source’s retain count. 

19. What different programming languages will you choose for iOS development? 

The following are different programming languages we can use for iOS development: 

• C
• C++
• Objective-C
• Swift
• .NET
• HTML5
• JavaScript

20. What is Objective-C in iOS? 

Apple Inc. created Objective-C — a high-level programming language that adheres to the object-oriented programming concept. It is the most basic programming language iOS developers leverage to build software applications for iOS and OS X. It is an amalgamation of C and Smalltalk languages. 

The following are the salient features of Objective-C: 

• Supports both static and dynamic typing
• Object-oriented
• Offers excellent memory management
• Supports binary frameworks
• Automatically creates and manages meta classes
• Interoperates with both C++ and Objective C++

21. What are the data types in Objective-C? 

Here are some commonly employed data types in Objective-C

• BOOL: It returns either true or false for a specific condition. BOOL and _Bool are both valid keywords. 
• NSInteger: It represents an integer. 
• NSUInteger: It indicates an unsigned integer. 
• NSString: It indicates a string. 

22. How is Bundle ID different from App ID?

Bundle IDs serve as the applications’ exclusive IDs within the Apple ecosystem. The result? No two applications can share the same identification. You can use the bundle ID (for both OS X and iOS programs), to identify app upgrades.

The App ID string identifies any apps from the same development team. The string comprises two segments: the Team ID and the Bundle ID. These segments are separated by a period (.). While developers provide Bundle IDs to identify a single program or a group of apps, Apple provides Team IDs to identify a certain development team.

23.  What is the Singleton Design Pattern?

Singleton is an object construction design pattern. It comes under the “creative” design patterns. It allows us to have one and only one instance of a class. Singleton objects are typically used to give your application configuration options or a global environment. 

For instance, when your application launches, it creates a global environment and fills it with client-supplied or pre-configured configuration parameters. When necessary, several application components may use these configuration options.

24. What are the features of Swift Programming Language?

Apple created Swift, a compiled programming language used to make apps for iOS, macOS, watchOS, and tvOS. Here are some of the features of Swift Programming Language:

• Simple to Understand: Swift reads very similarly to English, making it simple for newcomers to learn.
• Scalable: Swift is one of the most scalable programming languages and can extend features to both iOS and OSX platforms. 
• Easy to maintain: xCode checks your code for problems before building your app, making it quick and easy to manage. 
• Quick: Swift is statically typed, making it faster than other programming languages. 
• Improved Memory Use: Swift’s memory management is automated.

25. What is a Facade Design Pattern?

Facade Pattern makes a system easier to use by disguising its complexity. Under a structural design pattern, it belongs. In this architecture, client programs can access the system, but it hides how it functions by giving clients a more user-friendly interface. 

In this approach, a single class (FACADE) is constructed and contains user-defined functions as well as delegates that call other classes belonging to the system. As a result, the client code just communicates with Facade and not the underlying system.

26. Which framework will you leverage to develop the application interface for iOS? 

UIKit is a framework specifically designed for iOS development. This framework lets developers design an application’s graphical infrastructure and user interface. It comprises the following: 

• User interface
• Event handling
• App structure
• Graphics, printing, and drawing

Advanced iOS Interview Questions and Answers for Experienced Developers

Next stop, iOS experienced interview questions for the developer with a few projects and resume entries under their belt! 

27. What is MVC?

The main MVC principle is the division of a program into three parts: 

Model

Data shape and business logic are referred to as models, which serve the database blueprint. The model maintains application data and accesses and stores the object model state in a database.

It updates the data and reacts to user requests for data reading. In this case, the Model is communicates with the database to receive or publish data. Moreover, the it takes requests linked to the database, processes them, and then returns the data to the user via the browser.

Controller

The controller controls the View-Model interactions. What happens if a user wants to ask for data and receive a response? They would require the right business logic — an algorithm that accepts a user request, verifies it, or forwards it to another component. Then, the algorithm delivers the client response. 

Controllers are responsible for this task.

View 

The view is the user interface and presentation that gives end-users access to the application’s resources and lets them send requests to the application’s server or backend. Normally, the user can use a web browser or a mobile app to send the query to get certain resources, such as a specific web page. You can write the view entirely in HTML or with tools and frameworks like React, Vue, etc. 

28. Can you tell me about the SpriteKit and SceneKit frameworks? 

SpriteKit: This framework enables game developers to quickly and easily create 2D animated objects. It lets them create text, videos, images, shapes, and particles in two dimensions. 

SceneKit: It is an iOS framework for creating 3D graphics. iOS game developers can use this framework to produce appealing 3D animated effects and scenes for iOS game applications.

29. What is Swift in iOS? 

Developed by Apple Inc., Swift is an intuitive programming language that lets iOS developers build applications for Apple TVs, iPhones, iPads, iPods, Apple Watches, and Macbooks. It is an open-source language Apple created to provide iOS developers with more flexibility and freedom. 

30. What are the different pattern matching techniques in Swift? 

The pattern matching techniques in Swift are as follows: 

• Tuple patterns
• Type-casting patterns
• Wildcard patterns
• Optional patterns
• Enumeration case patterns
• Expression patterns

31. What are generics in Swift?

Code that uses generics does not specify underlying data types. We can predict the type it will contain thanks to generics. Additionally, generics optimize code.

32. What is a lazy stored property in Swift?

A property is considered lazy stored if its initial value isn’t determined until the first time it’s utilized. By adding the lazy modifier before a stored property’s declaration, you may indicate a property as a lazy stored property.

33. What is concurrency?

Concurrency entails two tasks running simultaneously. For example, when the kernel on a computer assigns two processes to distinct cores, both cores execute the process instructions at the same time. 

Likewise, concurrency occurs when new connections arrive before older connections have finished and must be handled right away. In a broader sense, it occurs when we must manage several tasks at roughly the same time. Parallel execution occurs when many tasks begin running simultaneously.

34. What is Guard Statement in Swift?

The conditional statement “guard” offers a few more promises than a typical conditional. Execution will unquestionably depart the current scope if the condition is false (or, at the very least, skip this iteration of a loop, break out of a loop, return from the current block/function, or terminate the entire program). 

Additionally, it has the advantageous effect of maintaining whatever bindings you create in the condition for the remainder of the scope, preventing you from needing ever more nested if statements simply to say unwrap four optional values.

35. What is @synthesize in Objective C?

@synthesize generates getter and setter methods for properties.

36. What is UUID?

UUID stands for Universal Unique Identifier, a 128-bit number used to uniquely identify an online object. Any number of variants can be used to specify the meaning of each bit. UUIDs are designed to help applications uniquely identify data without a lot of central coordination.

37. What is the Observer Design Pattern?

The “Subject” in the Observer design pattern keeps track of all the Observers waiting for Subject updates. When a subject is updated, all observers will receive an alert.

38. What is JSON?

Simple data structures and objects are represented using JSON, a text-based, human-readable data transfer standard in web browser-based programs, as well as sometimes client-side and server-side programming environments. 

JSON was initially developed as a page scripting language for the Netscape Navigator Web browser and was based on JavaScript. You can serialize objects, arrays, numbers, characters, booleans, and null with JSON syntax.

39. What do you understand about KVC and KVO in Swift? 

KVC: Key-Value Coding, which lets you leverage strings to retrieve properties of objects at runtime. 

KVO: Key-Value Observation, an approach to observing program state changes. 

40. Can you explain completion handlers? 

Completion handlers are functions used as parameters to pass to another function. They are primarily tasked with handling asynchronous event responses. In addition, completion handlers provide applications with information about an operation’s completion. 

Bonus Tips

Here are some bonus tips for you to ace your next iOS developer interview: 

• Leverage courses and online resources: online resources, such as iOS courses or tutorials to gain a basic understanding of the concepts. 
• Practice with a friend: Try out mock interviews to master even those iOS tricky interview questions before the big day.
• Update your resume: Your skills won’t have a chance to shine if your interviewers can’t assess your experience when they review your full application. 
• Be confident: You’ve got this!