Swift iOS Interview Questions and Answers – Part 1 – Language Features

Welcome to the first part of our series on Swift iOS interview questions and answers! In this blog post, we will focus on language features, and cover some common questions you may be asked about Swift during an iOS interview. As a junior iOS developer, mastering the language features of Swift is crucial to your success in this field. This is why “Swift IOS Interview Questions and Answers – Language Features” is a vital resource for anyone looking to develop their skills and grow their career in iOS development. Interview questions on Swift language features are often used by hiring managers to assess a candidate’s level of expertise and understanding of the language. Junior iOS developers must be well-versed in these questions and answers in order to perform well during job interviews. By having a good understanding of the language, you can demonstrate your ability to write efficient and reliable code, which is essential in the iOS development field.

However, knowing the language features of Swift helps junior iOS developers to mid-level iOS developers write better code. Understanding the syntax and structure of the language will allow you to write cleaner and more readable code, making it easier for others to understand and work with your code in the future. Swift is known for its simplicity, which makes it easier for developers to write code quickly, but it is also powerful and flexible, allowing you to create complex applications that meet the needs of your users.

Part 1 consists Swift iOS interview questions and answers of language features. It will cover Swift language features such as control flow, access modifier, type safety, struct, generics, tuples etc. Follow along and learn those basic to advanced Swift interview questions and answers that every iOS Developer shall know before next iOS interview.

What is the difference between weak and strong references in Swift? Explain with example.

In Swift, references to objects can either be strong or weak. A strong reference creates a strong relationship between two objects, meaning that the reference will keep the object alive as long as it exists. A weak reference, on the other hand, does not prevent the object from being deallocated, and is used to avoid retain cycles.

For example, consider the following code:

class Person {
    let name: String
    var friend: Person?
    init(name: String) {
        self.name = name
    }
}

let john = Person(name: "John")
let mike = Person(name: "Mike")
john.friend = mike
mike.friend = john

In this code, we have two Person objects, john and mike, each with a strong reference to the other. This creates a retain cycle, as each object is keeping the other alive, even though they are no longer needed. To avoid this, we can use a weak reference instead:

class Person {
    let name: String
    weak var friend: Person?
    init(name: String) {
        self.name = name
    }
}

let john = Person(name: "John")
let mike = Person(name: "Mike")
john.friend = mike
mike.friend = john

Now, the friend property is a weak reference, so it does not prevent john and mike from being deallocated when they are no longer needed.

What is mutating function in Swift? 

The understanding of the mutating function depends on the understanding of reference and value types. In Swift, The properties of value types cannot be changed within its instance methods by default. You need to use a mutating keyword in the instance method to change the properties of a value type. The mutating function has the power to mutate the values of the properties.

Also described here Grasp mutating function in Swift under three minutes!

Why does a struct need mutation?

You need to use the mutating function if you will change any state contained within the struct. Calling a mutating function returns a new struct in place as Struct is immutable. It works the same as passing an inout parameter to a function. The mutating keyword lets callers know that the method is going to make the value change.

What are tuples and in which scenarios are they useful?

Tuples are a way to group multiple values together into a single compound value. They are useful for returning multiple values from a function or for creating complex data structures.

How do dictionaries differ from arrays in terms of functionality and usage?

A dictionary is a collection data type in which values are stored in key-value pairs, whereas an array is a collection of values stored in a specific order. Dictionaries use keys to access values, whereas arrays use indexes.

What purpose does the Codable protocol serve?

The Codable protocol is used to encode and decode custom data types in Swift. It allows you to easily convert data between different formats, such as JSON and property lists.

What are the key distinctions between classes and structs in the Swift programming language?

Classes and structs are both used to create custom data types in Swift, but they have some key differences. Classes are reference types and are passed by reference, whereas structs are value types and are passed by value. Classes can have inheritance, whereas structs cannot. Classes can have deinitializers, whereas structs cannot.

How do arrays and sets differ in terms of functionality and usage?

An array is an ordered collection of values, whereas a set is an unordered collection of unique values. Sets are useful for checking for membership or removing duplicates.

What is the significance of key decoding strategies when using the Codable protocol?

Key decoding strategies are used to handle cases where the keys used in the encoded data do not match the keys used in the decoding model. There are several key decoding strategies available in Swift, such as using the CodingKeys enum, custom key decoding functions, and more.

Can you explain the differences between the Float, Double, and CGFloat data types?

Float, Double and CGFloat are all data types for storing numerical values in Swift. The main difference between them is the level of precision they provide. Float has less precision than Double, and CGFloat is a type-alias for either Float or Double, depending on the platform.

How does the map() function differ from compactMap() when working with arrays?

The map() function is used to apply a transformation to each element in an array and returns a new array with the transformed values. The compactMap() function is similar, but it also filters out any nil values.

Why is immutability considered important in programming?

Immutability is important because it helps to prevent unintended changes to data, which can lead to bugs and other issues. Using immutable data structures can make code more predictable and easier to reason about.

Can you explain what one-sided ranges are and when they would be used?

One-sided ranges are a way to create ranges that include either the first value or the last value, but not both. They are useful for creating ranges that are open or closed on one end.

Explain OOP in the context of an iOS developer

Object-oriented programming (OOP) is a programming paradigm that uses objects and their interactions to design applications and computer programs. In the context of iOS development, OOP is used to build the structure and functionality of iOS applications using the Objective-C or Swift programming languages. iOS developers use OOP principles such as inheritance, encapsulation, and polymorphism to create classes and objects that interact with each other to provide the desired functionality of the application. OOP also allows for code reuse and makes it easier to maintain and update the application.

Read more: Refine your OOP skills for iOS Development

How do strings differ from other collections in the Swift programming language?

When we say “strings are collections in Swift”, it means that strings can be treated like collections of characters. This means that you can use for-in loops, subscripts, and other collection methods to work with strings in Swift.

What is a UUID and in which situations would it be useful?

A UUID (Universally Unique Identifier) is a 128-bit value used to identify resources in a unique and consistent manner. They are useful for generating unique keys for data, such as primary keys in databases.

In which scenarios would you use the Result type in Swift?

Result type is a Swift enumeration that encapsulates either a success value or a failure value. It is useful for representing the outcome of an operation that can fail.

Can you explain the concept of type erasure and when it would be used?

Type erasure is a technique used to hide the underlying type of a value while still preserving its behavior. This is useful when working with generic types and protocols where the specific type is not important.

What is the benefit of using map?

The power of map in Swift

Can you explain what phantom types are and in what situations they would be useful?

Phantom types are types that don’t carry any value but are used to add extra type-safety to the code. They are often used in functional programming and allows the type system to enforce certain constraints or behavior. For example, a phantom type could be used to distinguish between two different kinds of objects, such as “read-only” and “read-write” versions of the same object, or an object that has been initialized and one that has not.

What is the benefit of using reduce?

The Power of reduce in Swift

What’s the difference between Self vs self?

Self with a capital S refers to whatever type is conforming to the protocol. self with a lowercase S refers to whatever value the type holds. More on this in stackoverflow

What is optional chaining in Swift?

Optional Chaining is a process of querying and calling properties. The power of optional chaining is that developer can chain multiple queries together, but if any link in the chain is nil then, the entire chain generates nil. Optional Chaining provides value if the entire chain succeeds.

What are the control transfer statements in Swift?

break, continue, fallthrough, return, throw

What is lazy in Swift? / What is lazy stored procedure in Swift and when is it used?

An initial value of the lazy stored properties is calculated only when the property is called for the first time.

What is defer ? 

defer is block of code that will be executed in the case when execution is leaving the current scope.

What is benefit of using guard statement?

There are two main benefits of using guard statement. The first one is avoiding the pyramid of doom – lots of annoying if let statements nested inside each other moving further and further to the right. The other benefit is provide an early exit out of the function using return.

What is variadic parameter?

A variadic parameter accepts zero or more values of a specified type. When creating API it seems more developer-friendly as we don’t needs to generate an array at the caller site in order to pass multiple arguments.

Also described here – How to use Variadic Parameters in Swift 5.4?

What is final keyword into the class?

By adding the keyword final in front of the method/class name, we prevent the method/class from being overridden.

What is the benefit of .xib over Storyboad?

If several people are working on same the Storyboard, there will be conflicts which would be hard to solve.

What is the difference between static vs final?

A static variable/function is shared through all instances of a class. static is used for a property/method that can be accessed without creating a class instance and final is a modifier which can not be inherited by others.

What is the difference between open & public access?

open allows other modules to use the class and inherit the class. Member of the open class can be overridden by other modules. public only allows other modules to use the public classes and but it can not be subclassed or overridden by other modules.

Describe some common ways to convert an array of objects to other objects in Swift.

How to convert one array of objects to another in Swift?

What is the difference between the == operator and the === operator in Swift?

The main difference between the double = operator and the triple = operator is that the equal to “==” operator compares value types to check if the values are the same, and the triple = “===” operator compares reference types to check if the references point to the same instance.

Why can’t we declare struct as open?

Because it is not possible inheritance between structs. “Open” means that you can override a parent method/property of a given class outside of module.

What are the common ways to merge two Swift arrays?

There are several ways to merge two arrays. One way is to use the + operator to concatenate the two arrays. Read more: How to Merge Two Arrays in Swift?

What is the difference between fileprivate, private and public private(set) access level ?

fileprivate is accessible within the current file. private is accessible within the current class. public private(set) means getter is public, but the setter is private.

What is internal?

Internal access enables entities to be used within any source file from their defining module, but not in any source file outside of the module. Internal access is the default level of access.

How to remove nil elements from an array effectively?

A common way to remove nil elements from an array is by using the compactMap function. Read more: How to remove nil elements from an array in Swift?

Explain the difference between struct and class in Swift

structs are value types whereas classes are reference types. When you copy a struct, you end up with two unique copies of the data. When you copy a class, you end up with two references to one instance of the data.

What is value/reference type in Swift?

A value type contains the data within its own memory. A reference type holds a pointer to another memory location that holds the actual data.

Also described here – Value and reference types in Swift -A deep dive

What is an optional in Swift?

An optional is a type that can hold either a value or nil, indicating the absence of a value. Optionals are used to handle the possibility of null or missing values in Swift.

What is the difference between lazy and eager loading in Swift?

Lazy loading is when the initialization of a property is delayed until it is accessed for the first time. Eager loading is when the property is initialized as soon as the object is created.

What is the difference between a computed property and a stored property in Swift?

A stored property is a variable or constant that is stored as part of an instance of a class or structure, whereas a computed property is a property that is computed each time it is accessed, rather than being stored.

What is the difference between a protocol and an interface in Swift?

In Swift, protocols are similar to interfaces in other languages. They define a set of methods or properties that a class or struct must implement, but they do not provide any implementation themselves.

What is the difference between a singleton and a global variable in Swift?

A singleton is a design pattern that ensures that a class has only one instance, whereas a global variable is a variable that is accessible from anywhere in your code. Singletons are useful for creating shared resources, such as a shared network manager, whereas global variables should generally be avoided in favor of dependency injection.

What is the difference between a mutable and an immutable collection in Swift?

An immutable collection is a collection whose size and contents cannot be modified after it is created, whereas a mutable collection is a collection whose size and contents can be modified.

What is the difference between a synchronous and an asynchronous function in Swift?

A synchronous function is a function that blocks the current thread of execution until it completes, whereas an asynchronous function is a function that returns immediately and performs its work on a different thread. Asynchronous functions are useful for performing long-running tasks or tasks that require a lot of CPU or I/O, to avoid blocking the main thread.

What is the difference between a static and a non-static method in Swift?

A static method is a method that is associated with a class, rather than an instance of the class. This means that it can be called without creating an instance of the class, and it does not have access to the instance variables or methods of the class. A non-static method, also known as an instance method, is a method that is associated with an instance of a class and has access to the instance variables and methods of the class.

What is the difference between assign and retain?

Assigned elements get a dedicated place in memory while retain is dependent on the lifecycle of something further up the hierarchy.

What is the simple way to implement abstraction in your project?

By using protocol and separating modules.

What is first class function?

A programming language such as Swift is said to have First-class functions when functions in that language are treated like any other variable.

Also described here – First-class functions in Swift

Map vs Filter vs Reduce

Map, FlatMap, Filter, Reduce – High order functions in Swift

What is the difference between Delegates and Callbacks ?

Delegation simply means that you pass yourself to someone else, when you want that someone to notify you of changes, so you can react to them. For instance, if a ViewController talks to a network service, and wants to get notified when that service is done with some request, it would make itself the network service’s delegate. The network service would then call the delegate methods when it’s done.

Callbacks are similar in function to the delegate pattern. They do the same thing: letting other objects know when something happened, and passing data around.
What differentiates them from the delegate pattern, is that instead of passing a reference to yourself, you are passing a function.

What is the difference between guard and if let?

if let and guard let both unwrap optionals if they contain a value. guard let is normally used for an early exit from the current scope, so any values you unwrap using it will stay around after the check. While using if let, the value needs to use inside the if scope.

What is copy or write?

Copy-on-write is a strategy used in Swift for optimising memory usage. The main idea of COW is that when multiple callers want to access the resources which are same, you can put them pointing to the same resource. The state of the resource will be maintained until a caller tries to modify its “copy” of the resource.

Also described here – What is Copy On Write(COW) in Swift?

What is trailing closure syntax?

In swift functions allow multiple closures as parameter. Trailing closure syntax is a little piece of syntactic sugar that makes the code pleasant to read and write. Trailing closure syntax is used as the final parameter to a function.

// Without trailing closure syntax
func greeting(title: String, greetingCallBack: () -> ()) {
    print("Hello world, \(title)")
    greetingCallBack()
}

// With trailing closure syntax
func greeting(title: String) {
    print("Hello world, \(title)")
}

What is associated type?

The associated type in Swift gives a placeholder name to a type that will be used as part of the protocol. When the protocol is conformed, the actual type to use for that it is specified. The associated type makes the protocol generic by providing a type placeholder.

Also described here – What does the associated type mean in Swift 5.4?

When to use a set rather than an array in Swift?

We should use set when the items of the set are hasbable and unique and we don’t care about the order of the items.  Set does allow duplicate item.

What is inout parameter?

The inout parameters can be updated inside the function. The updated value replaces the original value outside the function.

Also described here – How to use inout parameters?

What is the difference between upcast and downcast in Swift?

In swift, upcast will be checked on compile time and it goes from a derived class to a base class.Upcasting is casting to a supertype, while downcasting is casting to a subtype. Upcasting is always allowed, but downcasting involves a type check. We can downcast using optional with as? or forced failable with as!.

What is difference between as?, as! and as in swift?

• “as” is used for upcasting.

• as? produces an optional value. It will produce nil when the casting can not be done successfully.
• as! doesn’t produce an optional value, it will create a value of the specified type. The program will crash if the casting can not be done successfully.

In the end, I must say that having a good understanding of Swift’s language features will allow you to take advantage of its many advanced features, such as error handling, optionals, and automatic reference counting. These features provide powerful tools for handling errors, optimizing performance, and streamlining the development process, making it easier for junior iOS developers to build high-quality applications that run smoothly and efficiently.

“Swift IOS Interview Questions and Answers – Language Features” is an essential resource for junior to expert iOS developers. It helps them prepare for job interviews, write better code, and take advantage of the language’s advanced features. By mastering the language features of Swift, iOS developers can build their skills and grow their careers in the field of iOS development.

Fellow iOS Developers, Please Keep In Mind

• It’s important to keep in mind a few key points as you prepare for your interview. Firstly, it’s worth noting that there are over 1000 interview questions available in the interview section for you to review and prepare for. While reading the question, take the time to carefully consider your answer and think about the information that you want to convey. The answer provided here in this blog can be explained in a different way. You should also prepare your examples.
• It’s also important to remember that these interview questions are not meant to be difficult. The interviewer is not looking to challenge you, but rather to start a conversation that will allow your abilities and interests to come to the forefront. They want to get to know you and your experience better.
• Finally, it’s crucial to avoid simply answering questions with a “yes” or “no.” Interviewers are looking for more in-depth responses that include basic understanding, reasoning, explanation, and examples. So, make an effort to elaborate on your answers and provide specific, relevant information to support your response. This will demonstrate your thoughtfulness and show the interviewer that you are well-prepared for the interview.

These links are for a series of interview questions and answers related to SwiftUI and iOS development. Part 1 covers UI basics, Part 2 covers advanced UI topics, and Part 3 covers data-related concepts. The Swift iOS interview questions and answers series is also included, with 5 parts covering various topics related to iOS development. Additionally, there is a bonus section for iOS developer interview questions.

SwiftUI Interview Questions and Answers – Part 1 – UI Basics

SwiftUI Interview Questions And Answers – Part 2 – UI Advance

SwiftUI Interview Questions And Answers – Part 3 – Data

If you are interested in 5 parts Swift Series –

Swift iOS interview questions and answers

Swift iOS interview questions and answers – Part 1

Swift iOS interview questions and answers – Part 2

Swift iOS interview questions and answers – Part 3

Swift iOS interview questions and answers – Part 4

Swift iOS interview questions and answers – Part 5

and a bonus

iOS Developer – Bonus Interview Questions

Planning to apply for iOS job? Check out this article to uplift your resume! Happy job hunting!

✍️ Written by Ishtiak Ahmed

👉 Follow me on X ● LinkedIn

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Get Ready to Shine: Mastering the iOS Interview

• Ace iOS Coding Interview
• Uplift iOS Interview - A Comprehensive Guide to iOS Interview

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