Swift Interview Questions and Answers in 2024

If you want to work as a successful Swift developer for a top Silicon Valley firm or build a team of talented Swift developers, you've come to the right spot. We've carefully compiled a list of Swift developer interview questions for your Swift interview to give you an idea of the kind of Swift interview questions you can ask or be asked.

Last updated on Jun 14, 2024

Swift, an intuitive and high-level compiled programming language, was designed to enhance the development experience by complementing Apple's Objective-C library and Cocoa framework.
It has revolutionized the landscape of mobile and desktop app development, seamlessly powering apps across Apple platforms including watchOS, macOS, iOS, and tvOS.

Although relatively new, Swift has become hugely popular and has created a demand for talented developers. Whether you're a job seeker preparing for a Swift developer job or are a recruiter seeking talent, our curated list of Swift interview questions can serve as a valuable resource.

Basic Swift interview questions and answers


What is Swift primarily used for in programming?

Swift is a powerful and intuitive programming language primarily used for developing applications within Apple's ecosystem, encompassing iOS, macOS, watchOS, and tvOS. It was created by Apple with a focus on safety, performance, and software design patterns, making it an excellent choice for crafting everything from mobile apps and desktop software to cloud services.

Swift's syntax encourages developers to write clean and maintainable code, which has led to its popularity not only for consumer-facing apps but also for enterprise and education software development.


Explain the difference between 'let' and 'var' in Swift.

In Swift, 'let' and 'var' are keywords used to declare constants and variables, respectively. When you declare a constant using 'let', you're telling the Swift compiler that this piece of data won't change throughout its lifetime in your code. In contrast, a 'var' declaration indicates that the data can be altered or reassigned at a later point.

let pi = 3.14159 // This is a constant, and its value cannot be changed.

var counter = 0 // This is a variable, and its value can be updated.

counter = counter + 1 // This is valid as counter is declared with 'var'.

Attempting to change the value of 'pi' after its initial declaration would result in a compile-time error, ensuring that constants remain immutable as the developer intended. Making appropriate use of 'let' and 'var' can help make your code safer and clearer, indicating the intended mutability of values.


How do you write single-line and multi-line comments in Swift?

In Swift, comments are used to include non-executable notes in your code. These are intended for developers to document the purpose, logic, or any other information useful for understanding the code.

For a single-line comment, you use two forward slashes //. The Swift compiler ignores any text following // on the same line. Here's an example of a single-line comment:

// This is a single-line comment.

For multi-line comments, you enclose the text between /* and */. The compiler ignores everything between these markers, regardless of the number of lines it spans. Here’s how you write a multi-line comment:

/* This is a multi-line comment.

It can span multiple lines.

These lines of text are not executed by the compiler. */

Comments are an essential part of programming, improving the readability and maintainability of the code by providing context or explaining complex logic to other developers who may read the code later.


Define type inference in Swift.

Type inference is a powerful feature in Swift wherein the compiler deduces the type of a variable or constant based on the initial value assigned to it. This capability allows developers to write less code while maintaining strong typing in the language, a characteristic that makes Swift code both concise and less prone to type-related errors.

For example:

Image 08-12-23 at 2.00 PM.webp

In these cases, the Swift compiler infers studentName to be of type String and score to be of type Int without the need for explicit type annotations. Type inference also comes into play when determining the type of expressions:

Image 08-12-23 at 2.00 PM (1).webp

Here, although 3 is an Int and 0.14 is a Double, Swift uses type inference to deduce the type of result as Double to accommodate the more precise representation of the expression.

Type inference optimizes the development process, enabling cleaner code and reducing potential verbosity. It also helps in situations where the type might be cumbersome to write out, like with complex generic types or closure expressions.


Compare 'print' and 'debugPrint' in Swift.

In Swift, both 'print' and 'debugPrint' functions are used for logging values to the console, but they serve slightly different purposes and provide different output formats, especially useful while debugging.

The print function outputs the given variable or message to the console in a human-readable form. This is the function you would typically use for quick debugging messages or for simple logging of information during development:

Print op 1.webp

On the other hand, debugPrint is designed to provide more detailed debugging information about the data it prints. This function outputs the textual representation of an instance that is suitable for debugging. For types that conform to the CustomDebugStringConvertible protocol, debugPrint will use the debugDescription property to get a more detailed representation of the instance, including additional debugging information like explicit quotes around strings and visual representation of escape characters:

Print op 2.webp

To sum up, you would typically use print for most day-to-day logging, but when you need precise format output for complex data types or want to inspect details that are not normally visible, debugPrint is the preferable choice.


Differentiate 'if let' from 'guard' statements.

In Swift, both 'if let' and 'guard' statements are used to safely unwrap optional values, but they are used in different contexts and have different implications for control flow within a function.

An 'if let' statement is used to try to unwrap an optional, and if it contains a non-nil value, the code within the braces {} gets executed. It’s a way to handle optional values safely without forcibly unwrapping them. The unwrapped optional is only available within the scope of the 'if' block:

if let vs guard 1.webp

On the other hand, a 'guard' statement is commonly used to exit a function early if certain conditions are not met. While it can also unwrap an optional, its main purpose is to provide a sort of 'early return' pattern. If the optional can’t be unwrapped, the code within the 'else' block must exit the enclosing scope, typically with a 'return', 'break', or 'continue'. Unlike 'if let', the unwrapped variable from a 'guard' is available for the rest of the function or block after the 'guard' statement:

block after guard.webp

To summarize, use 'if let' when you need to unwrap an optional within a local scope and don’t necessarily want to exit the entire function if the optional is nil. Use 'guard' for cases where you want to affirm that certain conditions are met before proceeding in a function and handle the failure case early on.


Perform basic arithmetic operations using Swift.

In Swift, you can perform basic arithmetic operations such as addition, subtraction, multiplication, and division using standard arithmetic operators. These operations work with numerical values such as integers and floating-point numbers. Below are the basic arithmetic operations demonstrated with examples:

basic arithmatic operations.webp

Note that the division operation in Swift when used with integers will perform an integer division and will truncate any decimal part of the result. If you need a floating-point division result, make sure at least one of the numbers is a floating-point number:

let floatDivision = 12.0 / 5 // Divides 12.0 by 5 to get 2.4.

It's also important to handle division by zero, which can cause a runtime error if not managed appropriately.


What is a tuple in Swift? Provide an example.

A tuple is a lightweight data structure that groups multiple values into a single compound value. Tuples can contain elements of any type and are useful when you want to return multiple values from a function or when you need a quick way to group related values together without creating a more complex structure like a class or a struct.

Here’s an extended example:

tuples in swift.webp

Tuples are particularly helpful for temporary groupings of related values. However, for more formal data structures where you need to include functionality (methods) or enforce specific types and names, a class or a struct might be more appropriate.


What are the fundamental data types available in Swift?

Swift provides a rich set of fundamental data types that cover a wide range of use cases. These include:

  • Int: Represents whole numbers, both positive and negative. Example: var age: Int = 25
  • UInt: Represents unsigned whole numbers (positive numbers including zero). Example: var itemsCount: UInt = 5
  • Float: Represents a 32-bit floating-point number. It's less precise than Double and used when floating-point precision isn't as critical. Example: var height: Float = 1.75
  • Double: Represents a 64-bit floating-point number and is used when higher precision is needed. Example: var distance: Double = 384_400.0 // distance to the Moon in km
  • Bool: Represents a Boolean value that is either true or false. Example: var isCompleted: Bool = false
  • String: Represents a sequence of characters for text manipulation. Example: var message: String = "Hello, Swift!"
  • Character: Represents a single-character string literal. Example: var grade: Character = 'A'


How does Swift handle memory management?

Swift handles memory management through Automatic Reference Counting (ARC), a mechanism that automatically keeps track of an object's reference count. When an object is instantiated, it has a reference count of one. As other parts of the code create references (or strong links) to the object, the reference count increments. Conversely, when the references are removed or go out of scope, the reference count decrements.

Once the reference count reaches zero, meaning there are no more active references to the object, ARC deallocates the memory used by that object, effectively cleaning it up from the memory. This helps prevent memory leaks—the condition where unused objects are not released from memory—ensuring efficiencies and performance for your Swift applications.

Here's a simple illustration of ARC:

illustration of ARC.webp

ARC is efficient and handles most of the work of memory management, but developers still need to be mindful of strong reference cycles—situations where two object instances reference each other, which can prevent ARC from deallocating them. Swift provides features like weak and unowned references to help break these cycles.

In addition to ARC, Swift uses compile-time checks to prevent many common memory issues that arise in programming, further solidifying its capabilities for high-performance and safe coding.


Compare '==' and '===' operators in Swift.

comparing operators.webp


comparing operators 1.webp

On the other hand, the '===' operator checks for identity equality, which means it's used to compare whether two references (such as class instances) are exactly the same instance in memory. It's known as the 'identity' operator and is used only with reference types.


comparing operators 2.webp


Explain the 'switch' statement in Swift.

The 'switch' statement in Swift provides a powerful and flexible way to respond to different values of a variable or constant. It evaluates an input against multiple 'case' conditions and executes the associated block of code for the matching case.

One distinct feature of Swift's 'switch' statement is that it must be exhaustive, which means that every possible value of the input needs to be accounted for. If not all values are covered by 'case' statements, a default 'case' must be provided to handle any potential unhandled cases.

Additionally, 'switch' in Swift supports a variety of pattern matching techniques, allowing you to match ranges, tuples, and even cast types. You can also use 'where' clauses to check for additional conditions.


Create a dictionary in Swift.

In Swift, a dictionary is a collection type that stores associations between keys of the same type and values of the same type in an unordered collection with no defined sequence. Each value is associated with a unique key which acts as an identifier for that value within the dictionary. Here's how you can create and manipulate a dictionary:

dictionary swift.webp

In this example, we start by creating a dictionary with two key-value pairs. We then add a third pair, update the value for "apple", and remove the pair with the key "banana". Note that dictionaries are mutable, meaning you can change them by adding, removing, or modifying their items as shown.

When defining a dictionary, you can also use dictionary literals with a colon : separating each key from its value, and each key-value pair separated by a comma. The entire dictionary is wrapped in square brackets [].


Explain 'for-in' loop and its usage in Swift.

The 'for-in' loop iterates over a sequence (like arrays, ranges, or collections) and performs a set of statements for each element.


let numbers = [1, 2, 3, 4, 5] for number in numbers { print(number) }


How do you check if two strings are equal in Swift?

You can use the '==' operator to compare strings for equality:

let string1 = "Hello" let string2 = "World" if string1 == string2 { // Strings are equal }


What's the difference between 'nil' and 'null' in Swift?

In Swift, 'nil' is a special value that represents the absence of a value for any type that is optional. Optionals are a powerful feature of Swift that allows you to store a value that might be absent; they can either contain a value or be 'nil' to express that no value exists. 'nil' is not a pointer, as used in some other programming languages, but a specific value that an optional can take on to signify 'no value'.


var optionalNumber: Int? = nil // optionalNumber does not hold a value

optionalNumber = 10 // optionalNumber now holds a value of 10

On the other hand, 'null' is a concept that exists in other programming languages like Java and JavaScript, often representing the absence of a value or a 'null' pointer. It is important to recognize that Swift does not use 'null'; instead, 'nil' is the keyword provided by the language to deal with the absence of a value. If you try to use 'null' in your Swift code, you will get a compile-time error since it is not recognized by the language.

By effectively using 'nil' and optionals, Swift programmers can write safer code that explicitly handles cases when data may be absent, which helps prevent common runtime errors such as null pointer exceptions found in many other programming languages.


Describe the 'fallthrough' keyword in a 'switch' statement.

In Swift, the switch statement is designed to be more safe and predictable than in many other languages, executing only the code within the matched case block and then exiting the switch statement. However, there are times when you might want the program to continue execution into the subsequent case after the current one finishes executing. To handle this scenario, Swift provides the fallthrough keyword.

The fallthrough keyword allows the code flow to continue to the next case after the matched case, effectively creating a 'fallthrough' effect. It's important to note that fallthrough does not check the next case's condition; it simply passes control to the code in the next case block directly.


How is an optional binding different from force unwrapping?

In Swift, optionals allow you to represent the absence of a value, but when you come to use the value contained within an optional, it needs to be safely accessed. There are two common ways to handle this: optional binding and force unwrapping.

Optional binding is a safe method to unwrap optionals. It can be done using 'if let' or 'guard let' constructs. With optional binding, the optional is checked for a value, and if one exists, it is 'unwrapped' and made available within the scope of the 'if let' or 'guard let' statement.

For example:


With optional binding, if myOptional contains nil, the print statement inside the 'if let' won't execute, and in the case of 'guard let', the function will return early.

Force unwrapping is the straightforward extraction of an optional's value using the '!' operator. It’s a more dangerous action and should be done only when you are certain the optional contains a non-nil value. Failing to do so (i.e., if the optional is nil when you force unwrap it) will cause a runtime crash with a fatal error.

Example of force unwrapping:

var anotherOptional: Int? = 10

let forcedValue = anotherOptional! // This is safe because we know it's not nil

In general, the safe and recommended approach is to use optional binding to handle optional values. Force unwrapping should be used sparingly when you can guarantee that the optional isn’t nil. Avoiding force unwrapping can help prevent runtime crashes and results in more robust and predictable code.


Define 'break' and 'continue' statements in loops in Swift.

In Swift, 'break' and 'continue' are control transfer statements used to change the flow of execution within loops.

The 'break' statement immediately ends the execution of an entire control flow statement (i.e., for, while, or repeat-while loop) and transfers control to the first line of code after the loop's closing brace. It's useful when you need to terminate a loop based on a condition other than its built-in termination case.

Example using 'break':

break in swift.webp

The 'continue' statement, on the other hand, stops the current iteration and proceeds with the next one. It only ends the current loop iteration, not the entire loop itself. This is handy when you want to skip over an iteration based on a specific condition and move directly to the next iteration of the loop.

Example using 'continue':

continue in swift.webp

Both 'break' and 'continue' contribute to flexible loop control, allowing more nuanced execution paths based on runtime conditions and logic.


Explain the concept of a half-open range in Swift.

A half-open range is created using the ..< operator and defines a range that includes the first value but excludes the last value. This kind of range is useful when you want to iterate over a sequence without including the final element, which is common when dealing with zero-based sequences such as arrays.

Here's the syntax of a half-open range:

let start = 0

let end = 5

let range = start..<end // This will create the range [0, 1, 2, 3, 4]

In this example, the range includes 0 (the start) and goes up to, but does not include, 5 (the end). It is said to be "half-open" because one end of the range (the start) is included while the other end (the end) is not.


What is the 'is' operator used for in Swift?

In Swift, the 'is' operator is used for type checking. It allows you to check whether an instance is of a certain class or conforms to a protocol. When used, it returns a Boolean value (true or false) indicating whether the instance is of the specified type or a subclass thereof. This is particularly useful when working with class inheritance or when an instance's type might be unknown.


Describe the 'stride' function in Swift.

In Swift, the stride function is used to create a sequence of evenly spaced values within a specified range. This function can be particularly useful when you need to iterate over a range of numbers with a specific interval between them, which is especially common in for-loops.

There are two variations of the stride function to be aware of:

  • stride(from:to:by:): Creates a sequence from a start value up to, but not including, an end value, stepping by a specific increment. It generates a so-called half-open range, which is suitable when the end value is not meant to be included in the sequence.
  • stride(from:through:by:): Similar to the prior function, but this time the sequence includes the end value. It creates a closed range.


What's the purpose of the 'where' clause in Swift?

In Swift, the 'where' clause serves as a powerful conditional extension to various control flow and type declaration statements. It allows developers to specify one or more conditions that act as a filter or a constraint, refining the scope of the operation to which it's applied. The 'where' clause is used in various constructs like 'for-in' loops, 'switch' cases, and function declarations to add additional conditions that must be satisfied for the loop, case, or function to execute. It refines the conditions for execution.

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Intermediate Swift interview questions and answers


Explain Automatic Reference Counting (ARC) in Swift.

Automatic Reference Counting (ARC) is a memory management mechanism in Swift that automatically tracks and manages the memory usage of class instances. ARC keeps track of how many references to an instance exist and releases the memory when the instance is no longer needed. When the reference count of an object drops to zero, ARC automatically deallocates the memory associated with that object.


Define closures in Swift with an example.

Closures are self-contained blocks of code that can be passed around and used in Swift. They can capture and store references to variables and constants from the surrounding context in which they are defined.

Here's an example of a closure that adds two numbers:

let addClosure: (Int, Int) -> Int = { (a, b) in return a + b } let result = addClosure(5, 3) // Result will be 8


Distinguish between value types and reference types in Swift.

In Swift, the difference between value types and reference types is central to understanding how data is managed and passed around in a program.

Value types include structures (struct), enumerations (enum), and standard library data types like strings (String), arrays (Array), and dictionaries (Dictionary). When a value type is assigned to a variable, passed to a function, or when you create a copy of it in any way, a new instance is created and copied bit for bit.

Exemplifying value types:

value types swift.webp

After the modification of copy, the original remains unchanged, demonstrating that each has its own unique copy of the data.

Reference types, on the other hand, include classes (class). They allow multiple references to the same instance, meaning if you assign a class instance to a variable or pass it to a function, you're passing a reference to the same existing instance.

Exemplifying reference types:

reference types swift.webp

In this case, modifying referredPerson also affects originalPerson since both variables refer to the same instance in memory.

The distinction between value and reference types influences how you design your applications, especially with regard to understanding object mutability, thread-safety, and performance considerations. Knowing when to use value or reference types can significantly impact the predictability and efficiency of your code.


Define a protocol in Swift and its purpose.

Protocols are similar to interfaces found in other programming languages, and they serve as a blueprint or contract for methods, properties, and other functionality. A protocol does not provide implementation for any of these requirements, it only specifies the required names and types. It’s then up to conforming types, such as classes, structures, or enumerations, to provide concrete implementations of these requirements.

Protocols enable polymorphism where different types can be treated as being of a protocol type if they conform to it, fostering code reuse and abstraction. Additionally, protocols support the composition of behaviors, which can lead to more flexible and reusable code designs.

Here’s an example of defining a protocol named ‘Printable’:

protocol Printable { func printDetails() }


How does Swift handle asynchronous tasks?

Swift handles asynchronous tasks using closures, async/await, and other concurrency mechanisms. With Swift's structured concurrency model, you can write asynchronous code that's easier to read and maintain, and make use of the async and await keywords to manage asynchronous operations.


Describe the 'defer' statement in Swift.

The defer statement in Swift is used to define a block of code that will be executed just before the current scope is exited, regardless of whether it has exited normally or due to an error. It's often used to ensure cleanup code runs even in error scenarios.


Explain associated values in Swift enums (enumerations).

Associated values in Swift enums (enumerations) allow you to attach additional data to enum cases. This makes enums more versatile and enables them to represent a wide range of scenarios. For example, an enum representing different types of errors could have associated values providing specific error details.


Implement delegation in Swift.

Delegation in Swift involves defining a protocol that the delegate conforms to. The delegating object then calls methods on the delegate to notify or ask for certain actions. This is commonly used for communication between view controllers and other objects.


What are generics in Swift? Provide an example.

Generics in Swift allow you to write flexible and reusable functions, types, and classes that work with different types without sacrificing type safety. Here's an example of a generic function that swaps two values:

swift generics.webp


Compare 'map', 'filter', and 'reduce' in Swift.

  • map: Transforms each element in a collection based on a given closure.
  • filter: Creates a new collection containing only the elements that satisfy a given condition.
  • reduce: Combines elements in a collection into a single value by applying a combining closure sequentially.


What is function currying in Swift?

Function currying in Swift is a technique of transforming a function that takes multiple parameters into a sequence of functions that each take a single parameter. By applying these functions partially, you create intermediate functions that "remember" the applied arguments. This can be particularly useful for creating specialized functions from more general ones and can help in creating more modular and reusable code.

Currying is often used in functional programming to break down complex functions into simpler, more manageable pieces. In Swift, this process involves returning a function from another function.


How does Swift manage memory in closures?

In Swift, closures can capture and store references to any constants or variables from the context in which they are defined. Closures do this to maintain access to those variables when the closure is executed at a later time, even if the execution occurs in a different scope. By default, when a closure captures variables or constants, it creates strong references to them to ensure they are not deallocated while the closure needs to execute.

While this behavior is often desirable, it can lead to strong reference cycles—especially when the captured variables include a reference to the closure itself, such as a class instance property referring to a closure, and the closure captures the instance. A strong reference cycle occurs when two or more objects reference each other strongly, preventing Swift’s Automatic Reference Counting (ARC) system from deallocating them, potentially leading to memory leaks.

To mitigate this issue, Swift provides capture lists, which allow you to define the rules under which captures occur. You can use capture lists to establish weak or unowned references to captured instances within the closure’s capture list, thus preventing strong reference cycles.


Explain the concept of protocol-oriented programming (POP) in Swift.

Protocol-oriented programming (POP) is a design paradigm that Swift emphasizes, leveraging the power of protocols to achieve polymorphism and encapsulation. Unlike traditional object-oriented programming, which relies heavily on inheritance hierarchies, POP encourages developers to define behaviors using protocols and then use protocol extensions to provide default implementations. This approach can lead to more flexible and reusable code.

In POP, protocols are the core around which functionality is built. They define a contract with the required methods, properties, and other requirements. Classes, structures, and enumerations can then adopt these protocols and gain behaviors without being tied to a specific inheritance chain.


Describe the concept of lazy initialization in Swift.

Lazy initialization is a design pattern in Swift that defers the instantiation of an object until the first time it's needed. This approach can be particularly beneficial when initializing an object is computationally expensive or resource-intensive, and you want to delay this process to improve the startup performance of your application or to reduce memory footprint.

In Swift, you declare a property as lazy with the lazy keyword. A lazy property's initial value is not calculated until the first time it's accessed. It's important to note that lazy properties must have an initializer or be provided with a closure that defines how they are to be initialized.


How does Swift manage memory with reference cycles and how to break them?

Swift uses ARC to manage memory, but reference cycles can lead to memory leaks when objects reference each other strongly. To break these cycles, you can use weak or unowned references.

Weak references allow the referenced object to be deallocated. Unowned references assume the referenced object won't be nil during the lifetime of the reference.


How can you achieve dependency injection using property wrappers in Swift?

Property wrappers in Swift are a powerful feature that can be leveraged to implement dependency injection. They allow you to add behavior or encapsulate logic around the access to a property, which can be used to inject dependencies into a class or structure.

To achieve dependency injection with property wrappers, you can create a property wrapper that takes the dependency as an initializer argument. When you apply this property wrapper to a property of a class or struct, you effectively inject the dependency into the instance. This means when the instance is created, it's supplied with the necessary dependency without needing to know how it was created or where it came from, thus separating the concern of object creation from its use—this is the essence of dependency injection.


Implement a custom view transition animation.

Here's a simplified example of a custom view transition animation using UIViewPropertyAnimator:

custom view transition animation.webp


Explain the role of the 'as' keyword for typecasting in Swift.

In Swift, the 'as' keyword serves multiple purposes in the context of typecasting. Typecasting is the process of checking the type of an instance, and/or treating that instance as a different type from somewhere else in its own class hierarchy or a protocol it conforms to.

  • Safe Downcasting (Optional): as? The 'as?' operator attempts to downcast the specified value to a subclass or to a type it conforms to. Because downcasting can fail (the instance might not be of the target subclass or type), 'as?' returns an optional. If the cast fails, the result of 'as?' is nil.
  • Forced Downcasting (Unsafe): as! The 'as!' operator performs a forced downcast and should only be used when you know the downcast will succeed, such as when you're certain of the underlying type of the instance. Using 'as!' with an instance that does not match the expected subclass or type will trigger a runtime error.
  • Upcasting and Protocol Conformance: as The plain 'as' keyword is used for upcasting: casting an instance to one of its superclass types or to a protocol type it is known to conform to. Since upcasting can never fail (an instance can always be considered as its superclass or a conforming protocol type), 'as' does not return an optional.


Describe the role of 'NotificationCenter' in Swift.

NotificationCenter is a design pattern enabling a publish-subscribe form of communication between different parts of an application. It allows one component to broadcast notifications to any other part of the application that's registered to listen for those notifications. This means publishers can send out notifications without needing to know about the subscribers, and vice versa.

NotificationCenter is used in a wide variety of scenarios, such as to respond to external events (like changes to device orientation, or keyboard visibility), user actions, or to relay information between different components (such as controllers, models, and views) in a loosely coupled way.


How do you create and use enumerations in Swift?

Enumerations (enums) in Swift are a way to group related values together in a type-safe way. Swift enums are highly versatile and can be enhanced with additional functionality, such as computed properties, instance methods, and initializers. Enums in Swift can have raw values, which are the same for all cases, or associated values, which can be different for each case.

Enums with raw values:

enums with raw values.webp

In this example, each case of the CompassDirection enum has a raw value of type String. The raw values can be accessed using the .rawValue property.

Enums with associated values allow you to store additional custom information about each case, which can vary each time you use that case in your code:

enums barcode.webp

Here, Barcode can represent a product code in one of two different formats: a UPC format with an associated tuple of four integers, or a QR code format with an associated string.

In addition to defining cases, enums in Swift can be iterated over by conforming to the CaseIterable protocol. When an enum conforms to CaseIterable, Swift automatically creates an array of all the cases in the order they were defined.

Example of enum iteration:

enums iteration.webp

This example declares an enumeration with three cases (coffee, tea, and juice) and prints the count of all possible values of the enum as well as their names.


Explain the concept of capturing values in closures.

Closures in Swift have the ability to capture and hold on to values from their surrounding context, a feature often referred to as 'closure capturing'. When a closure captures variables or constants, it essentially 'closes over' those values, hence the name 'closure'. This behavior enables closures to retain access to the values they capture and manipulate them even after the scope in which the variables were originally declared has ended.

Here’s how closure capturing works in Swift:

  • When you create a closure in a certain context, such as within a function, and the closure refers to variables or constants from that context, the closure captures those values.
  • Swift manages the memory of captured values by keeping the variables or constants around for as long as the closure itself is retained. This can lead to strong reference cycles if not handled carefully, especially with class-instance properties or when closures capture self within class methods.
  • To mitigate potential strong reference cycles with closures, Swift offers capture lists, allowing developers to define how values should be captured—strongly, weakly, or unowned.


How do you sort an array in Swift?

In Swift, you have multiple options to sort an array depending on whether you need a new sorted array or want to sort the existing array in place.

Using sorted(): This method returns a new array that is sorted according to the criteria defined by a closure you provide. The original array remains unchanged. If no closure is provided, the array is sorted in ascending order by default.

Example with sorted():

let unsortedArray = [3, 1, 4, 1, 5, 9]

let sortedArray = unsortedArray.sorted() // [1, 1, 3, 4, 5, 9]

Or with a custom sorting closure to sort in descending order:

let reversedArray = unsortedArray.sorted { $0 > $1 } // [9, 5, 4, 3, 1, 1]

Using sort(): This method sorts the array in place, modifying the original array. Similar to sorted(), you can provide a closure to determine the sorting criteria.

Example using sort():

var numbers = [3, 1, 4, 1, 5, 9]

numbers.sort() // numbers is now [1, 1, 3, 4, 5, 9]

Or with a custom sorting closure to sort in descending order:

numbers.sort { $0 > $1 } // numbers is now [9, 5, 4, 3, 1, 1]

Both sorted() and sort() can accept a closure that takes two arguments of the same type as the array's elements and returns true if the first argument should be ordered before the second. This closure allows for a flexible sorting mechanism that can work with custom rules and complex types.


Describe the purpose of 'optional chaining' in Swift.

Optional chaining in Swift is a technique that allows you to call properties, methods, and subscripts on an optional that might currently be nil. It is a way to attempt an operation while safely handling the possibility of the optional being nil at any point in the chain. If at any stage, the optional is found to be nil, the entire chain fails gracefully, and the result of the entire expression is nil.

This is preferable to forcibly unwrapping each optional in the chain, which could lead to a runtime crash if any one of them happens to be nil. Optional chaining provides a concise, readable, and safe way to interact with nested optionals.

Example of optional chaining:

optional chaining.webp

In this example, because john.residence is nil, the optional residence chain ?.numberOfRooms will fail, and the if let check will result in the else block being executed, outputting a message that the number of rooms cannot be determined.

Optional chaining thus provides a graceful way to navigate hierarchies of optionals without risking unwrapping a nil value and in a way that avoids cumbersome and deeply nested conditionals.

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Advanced Swift interview questions and answers


Describe copy-on-write in Swift.

Copy-on-write is a memory optimization technique employed by Swift to manage value types like arrays and dictionaries efficiently. When a value type is assigned or passed to a new variable, the actual data isn't immediately duplicated.

Instead, the original and the new variables share the same data. When one of them is modified, a separate copy is created to ensure that modifications don't affect the other variable. This approach reduces unnecessary copying and memory usage, which improves performance when dealing with large data structures.


What are property wrappers? Provide a use case.

Property wrappers are a powerful feature in Swift that allow you to add custom behavior to property access and modification. They encapsulate common property behavior in a reusable and declarative manner.

For instance, you can create a @Clamped property wrapper that automatically ensures a numeric property stays within a specified range. This enhances readability and reduces error-prone manual range checks throughout your code.


How does Swift handle errors?

Swift uses a comprehensive error-handling model to manage errors. Instead of exceptions, Swift employs the throws keyword to mark functions that can potentially throw errors. Errors are represented as values conforming to the Error protocol.

When calling a throwing function, you use a try keyword. Errors can be caught using do-catch blocks, allowing for precise error handling and recovery strategies.


Describe opaque return types in Swift.

Opaque return types, denoted by some Type, allow functions to hide their implementation details while still providing a consistent, well-defined return type to the caller. They're particularly useful when you want to return a value that conforms to a protocol without exposing the exact type. This enhances encapsulation and reduces coupling between components.


Implement a custom operator in Swift.

Swift allows you to define custom operators, which can be infix, prefix, or postfix. An infix operator is used between two operands and performs an operation on them. Here's a step-by-step explanation of how to create a custom infix operator that computes exponentiation:

  • Declare the operator and specify its precedence group. In this case, we use MultiplicationPrecedence because exponentiation is generally at the same precedence level as multiplication in mathematics.

infix operator **: MultiplicationPrecedence

  • Next, implement the operator function by defining the logic you want the operator to perform. The following function takes two Double values as arguments (the base and the exponent) and uses the built-in pow function to calculate the result:

built in pow fucntion.webp

  • Finally, you can use the custom operator in the same way you would use any other infix operator:

let result = 2.0 ** 3.0 // Result will be 8.0.


How is Swift's result type used in error handling?

The result is an enum in Swift that's used to handle success and failure outcomes in a more structured way. It has two cases: .success(value) and .failure(error). It's commonly used in asynchronous operations to provide a clear and consistent way of handling results and errors, avoiding the confusion that can arise from using options alone.


Explain the 'Any' and 'AnyObject' types in Swift.

Any and AnyObject are Swift's ways of dealing with values of unknown types. Any can represent an instance of any type including value types and reference types. AnyObject represents an instance of any class type.

Note: While they can be convenient, it's essential to use them judiciously as they bypass Swift's type safety features.


Explain how Swift's opaque return types differ from generics.

Opaque return types and generics both allow you to abstract over types. However, generics are used when you need to maintain type information across different parts of your code. Opaque return types are more about hiding implementation details while exposing a consistent interface.
They are often used for returning protocol conformances without revealing the exact types.


What is the purpose of Swift's 'keyPath' type, and how can it be used?

keyPath is a type that represents the path to a property or subscript of a type without accessing the value. It's used to refer to properties dynamically and perform various operations like sorting and filtering collections based on properties. It's especially useful in SwiftUI for data binding and dynamic UI updates.


Describe trailing closures in Swift.

Trailing closures are a syntax feature in Swift that allow you to move a closure expression outside the parentheses of a function call if the closure is the last argument. This enhances code readability and can make the code feel more fluent. For instance, when working with functions like map, filter, or forEach on arrays, trailing closures make the code more concise.


Implement a custom collection type in Swift.

To implement a custom collection in Swift, you must conform to the Collection protocol, implementing required properties and methods such as startIndex, endIndex, and the subscript method to access elements, along with methods like index(after:).

custom collection type in swift.webp


How does Swift handle error propagation and what are the different ways you can handle errors in Swift?

In Swift, errors are represented by values of types that conform to the Error protocol. Error handling is done using a combination of the try, catch, and throw keywords. There are several ways to handle errors in Swift:

  • Propagating errors using a throwing function, which allows the error to be passed to the code that calls it, where it can be handled using a do-catch block.
  • Using optional try? to convert an error to an optional value—if there's an error, the result is nil.
  • Unchecked error handling with try!, which can be used when you are sure that the function will not throw an error; if an error is thrown, your program will crash.
  • Handling with a guard statement or an if let to gracefully handle nil values from try?.


Role of '@escaping' attribute in closure parameters.

The @escaping attribute is used when you pass a closure as a parameter to a function and that closure is retained beyond the lifetime of the function call. It tells the compiler that the closure "escapes" the scope of the function. This is necessary when the closure is stored, passed as an argument, or otherwise used outside of the function immediately.


Describe the 'Set' type in Swift.

A Set in Swift is an unordered collection of distinct values. It's used when you need to store a collection of items without any duplicates. Swift's Set conforms to the Hashable protocol, allowing it to efficiently manage uniqueness and provide operations like insertion, deletion, and membership checking in constant time on average.


Create a custom dynamic framework in Swift.

Creating a dynamic framework involves several steps:

  • Open Xcode and create a new project.
  • Choose the framework template.
  • Add your Swift files.
  • Build the framework for a simulator and a device.
  • Locate the built .framework file in the derived data folder.
  • Distribute the framework, along with its header files.


How to use availability attributes in Swift?

Availability attributes in Swift are annotations you can add to your code to indicate the lifecycle of certain functionalities—specifying the version of iOS, macOS, watchOS, or tvOS that a piece of code can be used with. These attributes prevent the use of APIs on unsupported platforms or versions, and they are critical for writing code that is compatible with a range of operating system versions.

Availability attributes like @available are used to specify the minimum deployment target and version where a particular code block or API can be used.

For instance:

if #available(iOS 15, *) { // Code for iOS 15 and above } else { // Fallback code }


Describe the use of closures as first-class citizens in Swift.

Closures in Swift can be assigned to variables, passed as parameters, and returned from functions just like any other data type. This makes them first-class citizens, allowing for more flexible and expressive coding patterns. Closures are often used for tasks like encapsulating functionality, asynchronous programming, and functional programming techniques.


Explain the concept of lazy loading in Swift.

Lazy loading, often achieved using the lazy keyword, is a mechanism where a property's value is only computed or loaded when it's accessed for the first time. This is particularly useful for performance optimization as it prevents unnecessary work until the value is needed, especially in cases of expensive computations and resource-intensive operations.


How would you implement a custom subscript in Swift?

You can create custom subscripts in Swift for your custom types. Here's an example with a custom Matrix type:

custom subscript in swift.webp


Describe the concept of pattern matching in Swift.

Pattern matching in Swift is an expressive and powerful feature that enables you to check for matches against particular patterns. It goes beyond simple equality checks and allows for more complex and flexible ways of handling data. You can use pattern matching in various contexts, including switch statements, if-case statements, for-in loops, and more.


How does Swift implement computed properties?

Computed properties in Swift are properties that do not store a value directly. Instead, they calculate (compute) a value when accessed. Computed properties must be declared with the var keyword because their value is not fixed. They provide getters and optional setters to retrieve or set other properties and values indirectly.

Here’s the structure of a computed property with both getter and setter components:

computed properties in swift.webp

In this example:

  • The area computed property calculates the area of a rectangle by multiplying width by height.
  • The get block is used to read the value of the computed property. This block must be present for a computed property.
  • The optional set block defines how the new value is interpreted and stored. It takes a default parameter newValue if no parameter name is specified.
  • When rect.area is set to a new value, under the assumption of maintaining a square shape, both the width and height are recalculated.


Explain Swift's 'switch' statement patterns.

Swift's switch statement supports a wide range of patterns for matching values. These include:

  • Value matching: case 1, case "apple"
  • Range matching: case 1...10
  • Tuple matching: case (0, 0)
  • Enum case matching with associated values: case .success(let value)
  • Type casting patterns: The case is Int
  • Where clauses: case let x where x % 2 == 0


What's the purpose of 'mutating' functions in Swift?

In Swift, when a method of a value type (struct or enum) needs to modify the instance itself, you must mark that method as mutating. This informs the compiler that the method might change the instance's internal state, ensuring that value semantics are maintained and appropriate copying occurs.


How does Swift handle memory management for value and reference types?

Swift uses Automatic Reference Counting (ARC) to manage memory for reference types (classes) and ensures that objects are deallocated when there are no strong references to them.

For value types (structs, enums), they're automatically copied when assigned to another variable or passed as an argument, ensuring that each instance is independent and has its own memory space.


Describe the role of the 'unowned' keyword in Swift.

The unowned keyword is used when you have a reference that will never become nil during its lifetime. It creates a non-optional reference that doesn't keep a strong hold on the instance it refers to. It's crucial to use unowned carefully as accessing a released instance using an unowned reference leads to a runtime crash.


Explain the concept of function builders in Swift.

Function builders are used in Swift to enable a DSL-like (domain-specific language) syntax for creating complex structures such as SwiftUI views or Combine publishers. They allow you to use a more declarative and readable syntax to build hierarchies of objects. The @ViewBuilder attribute is an example of a function builder used in SwiftUI.


What is Key-Value Observation (KVO) and how is it used in Swift?

KVO is a pattern that allows an object to be notified when a property of another object changes. In Swift, KVO is typically supported by NSObject and classes that inherit from it, for properties marked with @objc and dynamic to make them observable.


key value observation.webp


How does Swift handle optional chaining in complex expressions?

Optional chaining allows you to safely access properties, methods, and subscripts on optional values. In complex expressions involving multiple optional properties, the chaining will automatically short-circuit and return nil if any of the involved optional is nil. This prevents runtime crashes due to force-unwrapping of nil values.


Describe the concept of 'phantom types' in Swift and their application.

Phantom types are types that don't hold any data but are used to enforce specific type constraints during compilation. In Swift, you can use phantom types to impose additional checks or constraints on your code's behavior, which enhances type safety. For example, you could create phantom types to ensure units of measurement are used correctly.


What is the purpose of Swift's 'dynamic' keyword?

In Swift, the dynamic keyword is used when you want to indicate that a method or property should be dynamically dispatched using Objective-C runtime mechanisms. This is primarily used in cases where you need to interact with Objective-C code or when you need to enable features like method swizzling.


Describe Swift's 'switch' statement patterns and their uses.

Swift's switch statement supports various patterns for matching values. These patterns allow you to match against different types of values and conditions. For instance, value patterns match specific values, tuple patterns match combinations of values, and type-casting patterns allow checking types.


Explain the concept of value semantics in Swift.

Value semantics refer to the behavior where instances of a type are copied when assigned to a new variable or passed as arguments. This ensures that each instance is independent and modifications to one instance don't affect others. Value types, like structs and enums, exhibit value semantics, promoting safer and more predictable code.


Describe the role of lazy properties in Swift.

Lazy properties are properties that are only computed or initialized when they are accessed for the first time. They're particularly useful for delaying expensive computations or resource-intensive operations until they are needed, which can lead to performance improvements in certain scenarios.


How does Swift implement subscripting for collections?

Swift allows you to define custom subscripts for your types using the subscript keyword. This enables you to provide a convenient syntax for accessing elements within your type. Subscripts can have multiple parameters and can be used with varying types, making them a powerful tool for customizing element access.


Explain the concept of key paths in Swift.

Key paths are a way to reference properties or subscripts of types in a type-safe and efficient manner. They allow you to access and manipulate properties without invoking the property's getter or setter directly. Key paths are particularly useful for tasks like sorting, filtering, and key-value observing.


Describe the role of defer statements in Swift's error handling.

In Swift, the defer statement is used to execute a set of statements just before the current scope exits. Its primary role is to allow you to write cleanup code that is guaranteed to run regardless of whether an error is thrown. It ensures that necessary cleanup actions, such as releasing resources, are performed even when the execution flow is interrupted by an error.

Here's how the defer statement works in the context of error handling:

  • Resource Management: defer is often utilized to manage resources. When you acquire a resource that needs to be actively released, such as opening a file or a network connection, you can use defer to ensure that the resource is properly cleaned up.
  • Maintaining Consistency: Maintaining consistency within a block of code is another use case for defer. You can ensure that certain states are reverted back to their original form before the block exists.


How can you achieve thread safety in Swift?

You can achieve thread safety in Swift by using mechanisms like serial queues or DispatchSemaphore to ensure that only one thread accesses a particular resource at a time. Additionally, you can use concurrent collections like DispatchQueue to safely manage shared resources among multiple threads.


Explain the usage of access control keywords in Swift.

Swift's access control keywords (private, file private, internal, and public) define the visibility and availability of types, properties, methods, and other elements in your codebase. They help you encapsulate and control the visibility of your code to ensure appropriate data hiding and maintainability.


Describe Swift's approach to method overloading and overriding.

Method overloading in Swift allows you to define multiple methods with the same name but different parameters. The correct method is chosen based on the parameters' types and labels.

Method overriding is used in subclasses to provide a new implementation for an inherited method. The override keyword is used to indicate that you're intentionally replacing a superclass's method.


Swift has many powerful features such as guards and where clauses. Can you provide an example of when and how you would use a guard ... else statement along with a where clause in a loop?

A guard statement is used to exit the current scope quickly when certain conditions are not met, promoting early returns. A where clause in a loop helps filter elements based on specific conditions.

An example using guard and where clauses:

guard and where clauses.webp


Describe what is a memory leak and provide an example of how a memory leak can occur in Swift.

A memory leak occurs when memory that is no longer needed by the program is not released, typically due to objects being kept alive by strong reference cycles. This can occur in Swift when two objects hold strong references to each other and are not deallocated when they should be.

An example of a memory leak caused by a strong reference cycle:

memory leak.webp


Describe the role of initializer delegation in Swift.

Initializer delegation in Swift is a powerful mechanism that allows one initializer to call another initializer to perform part of an instance's initialization. This is especially useful in class hierarchies and when working with multiple initializers that share common initialization logic.

Within the same class, initializer delegation takes two forms:

  • Convenience Initializers: These are secondary, supportive initializers for a class. A convenience initializer must call another initializer (convenience or designated) from the same class. The convenience initializer calls the designated initializer to ensure the properties are correctly set up. It simplifies instance creation when default values suffice.
  • Designated Initializers: Every class must have at least one designated initializer. These initializers are the primary initializers for a class and must ensure that all properties introduced by that class are initialized.


Explain the use of custom operators in Swift.

Swift allows you to define your custom operators using the operator keyword. While this feature should be used judiciously, it can be beneficial in cases where custom domain-specific syntax or operations need to be encapsulated in a more readable manner.


Describe the concept of protocol composition in Swift.

Protocol composition in Swift is a powerful feature that allows developers to combine multiple protocols into a single type requirement. By using protocol composition, you can require that an object or type conform to all of the specified protocols at once. This is particularly useful when you want to work with types that share functionality across different protocols or when you create API interfaces that require their implementers to conform to several protocols.


How does Swift handle associated types in protocols?

Associated types in Swift protocols allow you to define placeholders for types that will be specified by the conforming type. This makes protocols more flexible and adaptable as they can work with different concrete types while maintaining type safety. Conforming types provide the actual types for associated types.


Explain the benefits of using value types in Swift.

Value types in Swift, such as structs and enums, offer several benefits:

  • Value semantics: Instances are copied, which prevents unintended side effects.
  • Thread safety: Multiple threads can work with separate copies.
  • Performance: They can be more memory-efficient due to stack allocation.
  • Predictable behavior: No shared mutable state, which reduces bugs.


Explain the difference between the throws and rethrows keywords in Swift function declarations.

The throws keyword in Swift indicates that a function can throw an error, requiring the caller to handle potential errors using do-try-catch. On the other hand, rethrows is used for functions that take a throwing function as a parameter and will only throw an error if that parameter throws.

An example illustrating throws versus rethrows:

throws vs rethrows.webp


Explain the concept of type erasure in Swift.

Type erasure is a technique used to hide the specific underlying type of a value while still conforming to a protocol. It's achieved by wrapping the value in a container type that conforms to the protocol. This is particularly useful when you want to abstract away implementation details or create a uniform interface for different types.


How does Swift handle multiple optional bindings in a single if-let statement?

Swift allows you to use multiple optional bindings in a single if let or guard let statement, separated by commas. This provides a concise way to unwrap multiple options and execute code only if all of them have non-nil values.


Describe the difference between Swift's struct and class.

Structs and classes in Swift share similarities, but they have some key differences:

  • Inheritance: Classes support inheritance, while structs don't.
  • Reference vs value: Classes are reference types, while structs are value types.
  • Mutability: Properties of a struct are not mutable by default when assigned to a constant, but class properties are.
  • Copying: Structs are copied when assigned or passed, while classes are not.
  • Deinitialization: Classes have deinitializers, while structs don't.


Explain the usage of the willSet and didSet observers in Swift.

In Swift, willSet, and didSet are property observers that allow you to execute code before or after a property's value changes. This is helpful for tasks like updating UI elements when a value changes, or performing additional actions when a property is modified.


Describe the role of the Self type in Swift protocols.

The Self type in Swift protocols is a powerful feature that represents the conforming type within the protocol's own definition. When you use Self within a protocol, you are referring to the eventual type that will conform to that protocol. It's particularly useful for writing generic code within a protocol, where the operations or methods need to return or manipulate instances of the implementing type itself, rather than instances of a fixed type.


Explain Swift's assert function and its purpose.

The assert function in Swift is a runtime check that you can use to enforce certain conditions or assumptions in your code. It's a debugging tool designed to catch errors and bugs during the development phase by ensuring that essential conditions hold true. When an assertion fails, it means that one of your assumptions is violated, and the application will halt immediately, providing a clear signal that there's a bug to be fixed.


Describe the concept of escape analysis in Swift.

Escape analysis is a compiler optimization technique used to determine if a local variable's reference escapes the current function scope. If it's guaranteed that the reference won't be used outside the scope, the compiler can optimize the memory management of that reference, often avoiding unnecessary heap allocations. This contributes to improved performance and memory efficiency.

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Wrapping up

This guide on Swift interview questions and answers aims to give significant insights into the world of Swift development to both job applicants and hiring managers. They can serve as the foundation for evaluating your Swift development technical competency and problem-solving abilities.

We've highlighted crucial topics to consider when evaluating potential team members, ensuring that you can find top talent who can successfully contribute to your Swift projects. For hiring managers looking to access the top 1% of Swift developers, Turing is your gateway to exceptional talent. Partner with us today to streamline your recruitment process and find the right Swift developer to elevate your projects.

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