50 Top Swift Interview Questions and Answers For 2024

Swift is a user-friendly programming language for creating applications for iOS, iPadOS, macOS, tvOS, and watchOS. Developers also use Swift for Linux development. It is an alternative language to Objective-C, both designed to create applications for Apple devices.

Swift syntax is simultaneously concise and expressive. It entails cutting-edge features, and lets developers conduct front- and back-end application development. Did you know Xcode, one of the most popular IDEs for macOS, has integrated Swift since version 6.

Swift and iOS developers are in high demand in the job market. If you’re planning to pursue an iOS developer career, it is essential to learn and master Swift.

Preparing for a Swift interview? Don’t worry; we’ve got you covered with these top iOS Swift interview questions to score your dream job.

Top iOS Swift Interview Questions and Answers

Swift newbie? Seasoned veteran? We categorized our iOS interview questions on swift into basic, intermediate, and advanced to fit all experience types.

Let’s start with the basics:

Swift Basic Interview Questions and Answers

Still a beginner? These are some questions you’ll likely hear in an interview for an entry-level developer role.

1. What is iOS Swift?

Apple Inc. created Swift in June 2014 to help developers create apps for both mobile and desktop platforms. The language is functional for operating systems like macOS, iOS, watchOS, and tvOS. Apple developed Swift to support the Cocoa framework (Cocoa Touch and Cocoa) and the Objective-C library.

Swift also supports numerous operating systems like FreeBSD, Linux, and Darwin.

2. What are the advantages of Swift?

Here are some advantages of using Swift:

• Readability

Perhaps Swift’s biggest benefit is its simple syntax, which makes it easy to read and write. For example, Swift requires much fewer lines of code to implement an option than Objective-C does. Moreover, Swift does away with numerous legacy traditions, like semicolons to end lines and parentheses to enclose conditional expressions inside if/else statements.

Another significant change is that method calls no longer nest within one another, preventing a bracket jumble. Instead, Swift method and function calls employ a list of parameters that use commas for seperation. You’ll find these nestled within parantheses. As a result, the grammar is simpler, and the code is clearer.

• Maintenance

Objective-C cannot advance unless C advances. Swift, in contrast, lacks these dependencies, which makes it much simpler to use. To speed up build-time and increase code efficiency, C mandates that programmers maintain two code files, which likewise applies to Objective-C.

• Speed

Swift offers speedy development, which reduces expenses. For instance, a sophisticated object sort will execute 3.9 times faster than a Python implementation of the same algorithm. Additionally, it is faster than Objective-C, which is 2.8 times faster than Python.

• Open-Source

Swift's open-source status was declared in 2015, making it possible for backend infrastructure and a wide range of platforms. By making Swift open source, Apple can access community feedback to continuously improve the language as independent developers contribute to its success.

3. Can we use an iOS device to test Apple iPhone apps?

If we don't own an iOS device, you can test our applications on Apple's Mac system simulators.

4. What are Swift's key characteristics?

The key characteristics of Swift are:

• Additional structs and enums

• Optional Types with a protocol focus

• Type Safety and Type Inference

• No requirement for semicolons

• Mandatory initializers

• Tuples and Closures

• Speed

5. Describe the typical iOS app execution states for a Swift app (iOS Application Lifecycle).

The five most typical execution states are as follows:

Not Running: The application has ended, not been started, or turned off entirely.

Inactive: This is a temporary, inactive state where the application operates in the background and can’t receive events.

Active: This is the main execution state, where the software is running in the background and able to receive events.

Background: The application operates in the foreground while still being able to run the background code.

Suspended: The program is running in the background and has been suspended by the operating system.

6. Is Swift a language for object-oriented programming?

Swift is an object-oriented programming language.

7. What kinds of objects fall under Swift's core data types?

Swift uses a standard set of fundamental data types, including texts, integers, and Boolean values:

• int: The integer value is stored in an int.

• double and float:  When working with decimal numbers in Swift, double and float are accounted for. 

• boolean: The binary value is stored using the boolean type. The 1 and 0 conditions are used in Swift for this data type.

• String: The text encased in double quotations in Swift is defined by the user in string literals. Here we store a list of characters.

• Arrays: An array is a grouping of list items.

• Dictionaries: A dictionary is a collection of unarranged elements of a specific sort bound together by a specific key-value pair.

8. How can you add an element to an array?

One of the application’s most popular data types is an array (app). Arrays organize application data. 

Using an array literal in Swift makes generating code arrays easy. Simple commas denote array elements, and square brackets demarcate lists of values.

9. Which JSON framework does iOS support?

iOS supports the SBJson framework, which offers more control and a versatile API to make handling JSON simpler. It is a strong and very adaptable framework that facilitates APIs' adaptable operation.

10. Why does iOS use PLIST?

PLIST stands for property list. You can save a file with the extension.plist in the system as PLIST, which is essentially a dictionary of values and keys. This is a portable and lightweight method for storing a smaller quantity of data, usually with XML.

Related Course

iOS & Swift - The Complete iOS App Development Bootcamp

11. Describe a dictionary.

Dictionary associations are collections of unordered, key-value pairs. Each value has a distinct key, which is a hashable type like an integer or text, attached to it. You can use the dictionary idea to get values based on a key value at any time.

12. What is a protocol in Swift?

Swift protocol is akin to a Java interface. It’s a common component in Swift and outlines the qualities, procedures, and other specifications necessary for a specific task. A Swift protocol is like an interfaced describing operations and characteristics. However, the protocol is only described in terms of its attributes or basic procedures. 

You can define enumerations, functions, and classes to implement properties and methods. Moreover, you must declare protocols (either single or multiple) after the enumeration, structure, or class type names. Finally, you can use commas to distinguish different protocols.

13. What is a delegate in Swift?

Delegate is a design pattern that conveys data or facilitates communication between classes or structs. It’s used to handle table view and collection view events, and enables message delivery from one object to another when a specified event occurs. Delegates interact and communicate directly with one another.

14. What does optional chaining do?

Optional changing invokes the methods, properties, and subscripts on the optional. We might attempt to extract a value from a sequence of optional values during this operation. Calling an optional’s subscript, method, or property will return a value if the optional has a value. Likewise, calling the subscript, method, or property will return nil if optional is nil.

15. How does optional binding work?

The optional binding concept makes an optional’s value available as a variable or temporary constant. You can also use the process to determine whether an optional includes a value. You can use ‘if’ and ‘while’ statements together with optional binding to test for a value inside an optional.

16. Describe the Swift module.

A module is a discrete piece of distributed code. A framework or application is created and distributed as a single piece, and another module can import it using the swift ‘import’ keyword. Swift treats each build target as a separate module in the Xcode tool.

17. What are the advantages of inheritance?

The advantages of inheritance are as follows: 

• Offers code reusability.
• Significantly reduces code redundancy. 
• Reduces the size of source code which improves code readability. 
• Easy to split the code into parent and child classes. 
• Support code extensibility by overriding the base class functionality within the child class. 

Intermediate Swift Interview Questions and Answers for Experienced Developers

Next up, we’ll cover medium-difficulty Swift interview questions for experienced developers. You’ll be well prepared to answer these if you have a working knowledge of the language. 

18. How do you make a property optional?

You can make a property optional in Swift code by declaring a question mark ("?"). When a property lacks a value, the question mark "?" helps to prevent the runtime error.

19. During the app launch cycle, who calls the main app function?

Our application’s primary thread calls the primary function. 

20. How do UI elements work?

The term "UI element" refers to any component of an application visible to the user, including images, buttons, labels, text fields, and others.

21. Which superclass does each view controller object belong to?

The UIViewController is the superclass to which all view controller objects belong. In response to device rotations, functionality is provided for showing them, loading views, and rotating them. The UIViewController class provides all the typical system activity.

22. What brand-new features does Swift 4.0 offer?

Here are some of Swift 4.0’s latest features: 

• Speedy use of Unicode-compliant strings
• Multiple return values and tuples
• Native throw/try/catch error handling
• Serialization to a struct by extending

23. How do I use Swift to write a multi-line comment?

You can type a multi-line comment between the (/*) at the beginning and the (*/) at the finish.

24. What source objects does Xcode use?

Xcode employs four source objects: Framework, Supplier Group, Data File, and Original Folder.

25. What is "defer"?

The execution of a defer statement is postponed until the current scope finishes. 

Deferred actions are carried out in the opposite order from which they are listed in your source code. In other words, the first defer statement’s code runs last, followed by the code in the second defer statement, and so on.

26. What is a Tuple in Swift?

A tuple is a collection of many values combined into a single compound value. It contains elements in an organized list. You can access a tuple's object data in two ways: by name or position. 

A Swift tuple can accommodate two values, one of the string and one of the integer types.

27. What different collection types does Swift support?

Swift has three main collection forms to store a collection of values: 

• Arrays: An array is an ordered collection of values stored in an ordered list of the same kind of data.

• Sets: Sets are undefined collections of different values of the same type stored in distinct values of the same type, but not arranged in any particular order.

• Dictionaries: Dictionaries are unordered collections of associations between key and value pairs.

28. What is the difference between synchronous and asynchronous tasks?

Synchronous:

Synchronous API calls cause code to pause while it waits for the call to finish. This means that until the API response, your application won't keep running, which you might construe as latency or performance lag. If your app features functionality that can only be used after receiving the API answer, synchronous API calls may be advantageous.

Asynchronous:

Asynchronous calls don't halt (or wait) for the server to answer an API call. Instead, when the server responds to the call, a "callback" function is then invoked while your application is still running.

29. What are enums?

An enum symbol indicates that a variable can only accept a predetermined range of values, i.e., the days of the week could only be one of seven from Monday to Sunday. 

Therefore, you must provide a list of all possible values when declaring an enum. According to the compiler, these are the only possible values to be applied to the variable.

30. What are Generics in Swift?

Generic code does not identify underlying data types. Thanks to generics, we can anticipate the type it will contain. Generics also help us optimize code.

31. What is ‘Lazy’ in Swift?

A lazy stored property's initial value is only calculated when first called. 

32. What is the ‘Guard’ statement in Swift?

The conditional statement "guard" makes a few more commitments than a standard conditional. 

If the condition is false, execution will surely leave the current scope (or, at the very least, skip this loop iteration, exit the loop, return from the current block/function, or end the program). It also has the benefit of keeping whatever bindings you establish in the condition throughout the remainder of the scope, saving you from having to use ever more nested “if” statements to accomplish something as simple as "unwrap four optional values."

33. What are classes in Swift?

Classes (and structures) are the general-purpose, essential building blocks of any programming language that supports the object-oriented paradigm. They allow you to specify a "template" for creating objects (also known as instantiating a class) that has the same set of properties and actions (sometimes referred to as functions or methods) common for a particular type of object. 

You use the same syntax to define properties and functions in your classes and structures as you do for regular, unconnected constants, variables, and functions. 

It's important to note that in Swift, the term instance is favored over the term object, primarily because it is appropriate for both instances of classes and structures. 

Advanced Swift Interview Questions for Senior Developers

Maybe you’re already a Swift pro — you’ll likely be scouring more senior iOS developer job ads. If you’re armed with extensive Swift knowledge and have a few years of experience, these are the Swift interview questions for you! 

34. Differentiate between ‘let’ and ‘var’ in Swift

You can declare real variables—parts of your code whose contents you can alter over time, with the keyword “var.” It may start off as an integer containing the number 10, and you may later modify it to 5 or any other value you like.

You can declare constant variables with the keyword “le.” Constants are parts of your code whose contents, once launched, may not alter over time. Initiated as an integer storing the value 10, it is unchangeable and retains this value throughout program execution.

35. What are the basic categories of types in Swift?

Swift requires each object to have a type, which must be known during compilation. Swift types fall into two groups:

• Value types, which are typically described as a struct, enum, or tuple and allow each instance to maintain a separate copy of its data.

• Reference types, where a single copy of the data is shared by all instances, and where the type is often expressed as a class.

The type identifies the kind of objects that can be stored inside an object.

36. What are optionals in Swift?

In Swift, every object must either have a value or be expressly defined as optional (possibly no value). By giving it the unique value “nil.” you can make an optional variable valueless. 

In Swift, nil is used when a (proper) value of a particular type is not present. You may argue that nil indicates no correct value. You can set optionals of any type to nil. Nil, on the other hand, is incompatible with non-optionals. You must declare a variable in your code as the right type of an optional value if it has no required value.

37. What is a nil-coalescing operator in Swift?

When dealing with variables that might have nil values, the nil-coalescing operator, represented by the symbol "??," is helpful. 

For instance, it unwraps an optional “a” if it includes a value when used in the expression (a?? b), or returns a default value “b” when “a” is nil. The type stored inside of ‘a’ must match the expression ‘b’.

38. Is it possible to return multiple values from a function in Swift? If yes, how?

Like the majority of programming languages, Swift only allows each function to return a single value. If this element is a primitive type, you will only return one value. 

Additionally, a thing could be a complex type, such as a class, struct, tuple, or array. You can pack several values into a complex type in this situation. After that, you formally return a single item with numerous values kept inside this data structure. 

Here’s how we can return multiple values kept inside a tuple:

func functionWithMultipleReturnValues(
val1: Int,
val2: Int
) -> (sum: Int, product: Int) {
let sum = val1 + val2
let prod = val1 * val2
return (sum, prod)
}
let result = functionWithMultipleReturnValues(val1: 10, val2: 20)
let s = result.sum
let p = result.product

39. What is the difference between parameter and argument in function?

These two words are often used interchangeably in programming languages but are slightly different. While argument (also known as a real parameter) refers to the actual input supplied, parameter (also known as a formal parameter) refers to the variable found in the function declaration. 

For instance, the variable "x" is a parameter in the function definition "f(x) = x+1" while the function argument in the function call "f(10)" is the value "10". A parameter can be thought of as a type and an argument as an instance.

40. What are closures in Swift?

Closures are independent units of code. Blocks in Objective-C and lambdas in other programming languages are comparable to closures in Swift. Closures are essentially Swift's anonymous functions, a method for putting lexically scoped name binding into practice. 

The value or storage location to which the name was bound when the closure was constructed is associated with each variable used locally but specified in an enclosing scope. Unlike a simple function, a closure enables the function to access those captured variables via the closure's reference to them, even when the function is called outside their scope.

41. What are mutating methods?

Implementing mutating methods for structures (and enumerations) allows you to change value type characteristics) from within their instance methods. By design, an instance method for a value type cannot change any of its properties. You must use the “mutating” keyword must to implement such a technique.

42. What is concurrency?

The term "concurrently running" refers to two active tasks at once. When a computer's kernel allocates two processes to different cores, both of those cores carry out the instructions for those processes simultaneously. Similar situations include when new connections begin before existing ones have ended and demand immediate attention. It happens when we have to do numerous things at roughly the same time in a broader sense. When several tasks start operating concurrently, parallel execution—a specific type of concurrent execution occurs.

43. What is polymorphism?

Polymorphism is an essential building block of any object-oriented programming language. It uses a single symbol to represent many different kinds of entities or the availability of a single interface to entities of diverse types. With polymorphism, your code can operate on either the parent class or one of its offspring, depending on the supported hierarchy (family of objects).

44. What is access control?

Access control limits who can access certain sections of your code. You can use access control to hide implementation specifics and enable code access through a selected interface. 

Swift offers various levels of access:

• Open access: You can use entities from any source file from their defining and other module modules using open access (“open” and “public” access). 

• Internal access: You can use entities within any source file from their defining module but not within source files outside of that module. In Swift, this is the default access specifier.

• File-private access: You can only use an entity within its defining source file.

• Private access: Limits your resource use to the declaration that encloses it.

45. What is the Singleton Design Pattern?

Singleton is a design pattern for object production in the “creative design patterns” category. It permits us to have a single instance of a class. Singleton objects are frequently used to provide application configuration options.

For instance, when your application starts up, it creates or initializes a global environment and fills it with pre-configured parameters. Several parts of your application may use these configuration settings when appropriate.

46. What is JSON?

JSON is a text-based, human-readable data transmission standard that expresses simple data structures and objects. JSON is also occasionally used in client-side and server-side programming contexts.

Based on the JavaScript programming language, JSON was initially created as a page scripting language for the Netscape Navigator Web browser. The JSON syntax allows for the serialization of objects, arrays, numbers, characters, booleans, and null.

47. What is code coverage?

Code coverage measures how much of your production code was executed concurrently with your automated test suite. usually expressed as a ratio. For example, only 25% of the code was executed if your tests covered 75% of the 100 lines of code. It's possible to hide bugs in just 25 lines easily. Code coverage indicates your code’s ability to run and how well tested it is.

48. Write a code to remove an element from an index in Swift.

// Initialize Array with 5 elements
var array = [1, 2, 3, 4, 5]
// Remove element at index: 2
array.remove(at: 2)
// Print array element
print(array)

49. What is a Facade Design Pattern?

A facade pattern hides a system's complexity, making it simpler to use. It fits within a structural design pattern. Client programs can access the system with this design, but it hides how it works by providing clients with a more user-friendly interface. This method creates a single class (FACADE) with delegates that call other system-related classes and user-defined functions. Because of this, the client code only talks to Facade and not the actual system.

50. What is a type alias?

You can rename an existing data type in your program with a type alias. Once you declare a type alias, you can use it throughout the program in place of the original type. You cannot produce a new type with type aliases.

Bonus Tips

Here are some bonus tips for you that might be the chef’s touch to perfecting an iOS Swift interview:

• Opt for a Certification: A Swift certification showcases your skills and impresses employers.
• Build Projects: If you’re a beginner, try out simple and easy Swift projects and add them to your portfolio.
• Keep Your Resume Updated: Make sure to adequately describe your skills in your resume. Try your best to describe specific outcomes your skills helped you achieve in previous roles.
• Mock Interviews: Try mock interviews to emulate an actual interview and test how you respond to difficult questions.

Conclusion

If you review and practice this list of Swift interview questions, we’re confident you’ll achieve your dream iOS developer career!

Keep studying Swift topics related to core Swift programming logic, object-oriented design, design patterns, and implementation. And if you’re open to other developer opportunities, you might also consider reviewing other interview questions.

Frequently Asked Questions (FAQs)

1. Is Swift a Real Programming Language?

Swift is a general-purpose programming language created with a contemporary perspective on software design principles, performance, and safety.

2. What Type of Code Does Swift Use?

Swift primarily uses C++ code.

3. Is Swift a High-Level Language?

Yes, Swift is a high-level programming language that enables efficient application development.

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