Top Swift Interview Questions and Answers (2024)

Swift is an open-source programming language used to build applications in the Apple ecosystem which includes iOS, Mac, Apple TV, and Apple Watch. Swift is also used for server-side development, systems programming, and cross-platform projects.

Being relatively new, the demand for skilled Swift developers is very high. Here are some basic and advanced Swift interview questions and answers.

1. What are the key features of Swift?

The following are some of the key features of Swift:

1. Type safety and inference  - Swift enforces type safety to reduce errors. It also allows the compiler to infer the type based on the value assigned.

2. Optional for handling nil values - This forces developers to explicitly handle nil cases reducing the crash related to null values.

3. Protocols and protocol-oriented programming - A protocol is a blueprint that defines methods, properties, and other requirements.

4. Closures are first-class citizens, meaning they can be passed as arguments, returned from other functions, and assigned to variables.  

5. Automatic Memory Management with ARC (Automatic Reference Counter) - The ARC automatically frees up memory used by class instances when it is no longer needed.

6. Generics - They allow the creation of flexible and reusable functions and types that can work with any type, subject to your defined requirements.

7. Enumerations with associated values - This allows storing additional custom information along with enumeration cases. 

8. Structs and Classes - Swift provides value type (structs) and reference type (classes) objects.

9. Extensions - allows adding new functionality to existing types

10. Error handling - Error handling in Swift can be done in four ways: by propagating, with a do-catch statement, handling the error as an optional value, or asserting that the error will not occur. 

2. What are the different types of data structures available in Swift?

Swift provides the following built-in data structures - arrays, dictionaries, sets, tuples, strings, ranges, and optionals. The following are the user-defined structures you can define in Swift - classes, enums, and structs. 

3. Explain the memory management in Swift.

Memory management is primarily handled through ARC which stands for Automatic Reference Counting. It tracks and manages the app’s memory usage by counting the number of references or “strong references” to objects.

By default, all references in Swift are strong references. Each time you create a strong reference to an instance, ARC increases its reference count. The reference count is decreased when the reference is removed.

In contrast, weak and unowned references do not increase the reference count. Weak and unowned references are used to prevent reference/retain cycles. A reference cycle occurs when two or more instances hold strong references for each other preventing ARC from deallocating them when they are no longer needed.

Structs, enums, and tuples are value types in Swift. They are copied when assigned or passed as arguments, simplifying memory management. 

4. What is upcast and downcast in Swift?

Upcast and downcast in Swift are related to type casting of objects.

Upcast is used to convert an instance of a subclass to a superclass or a protocol that the class conforms to.

Downcast is used to convert an instance of a superclass to that of a subclass. Since downcasting can potentially fail, Swift provides two ways to handle downcasting - safe (as?) and forced (as!).

5. What does the keyword mutating mean in Swift?

The keyword mutating is used to modify the properties of a value type. With the keyword ‘mutating’, a method can mutate a property’s value.

6. What is the difference between == and ===?

== and === are two of the comparison operators in Swift. == is used for comparison by value and === is used for comparison by reference. 

== compares the values of two instances and === compares the memory addresses of two instances.

8. How do you declare a property in Swift?

There are different types of properties in Swift and they can be declared as: 

1. Stored properties

Constant property:

let propertyName: Type = value

Variable property:

var propertyName: Type = value

2. Computed properties

var propertyName: Type {     get {         // Return the computed value     }     set(newValue) {         // Set the value     } }

3. Lazy properties

lazy var propertyName: Type = initialValue

4. Property observers

var propertyName: Type = value {     willSet(newValue) {         // Called before the value is set     }     didSet {         // Called after the value is set     } }

5. Type properties 

Static type property for structs and enums:

static var propertyName: Type = value

Class type property:

class var propertyName: Type {     // Return the computed value }

8. What are the different types of collections available in Swift?

The different types of collections in Swift are:

Arrays: they store values of the same type in an ordered list

For example:

var numbers: [Int] = [1, 2, 3, 4, 5]

Dictionaries: They store value in key-value pairs where unique keys hold a specific value.

For example:

var countryCodes: [String: String] = ["US": "United States", "FR": "France"]

Sets: They store values of the same type in an unordered collection. The values in sets are unique, unlike arrays where values can be duplicates.

For example:

var uniqueNumbers: Set = [1, 2, 3, 4, 5]

9. What is defer in Swift?

The defer statement is used to execute a set of code before the execution of the current scope (such as a function or loop) ends.

10. How do you append two arrays together in Swift?

Here are some of the ways you can append two arrays together:

1. Using the + operator

Example: 

let array1 = [1, 2, 3] let array2 = [4, 5, 6] let array3= array1 + array2

The resultant array will be array3 with value [1,2,3,4,5,6]

2. Using the method append(contentOf)

Example: 

var array1 = [1, 2, 3] let array2 = [4, 5, 6] array1.append(contentsOf: array2)

The value of array1 will now be [1,2,3,4,5,6]

3. Using the += operator

Example:

var array1 = [1, 2, 3] let array2 = [4, 5, 6] array1 += array2

The value of array1 will now be [1,2,3,4,5,6]

Conclusion

Swift interview preparations become easier with each interview question and answer you go through. Impressing your next hiring manager to get your dream job in iOS Swift development will now be a piece of cake.