Swift Interview Questions

Intermediate Swift Interview Questions and Answers

26. What is plist in swift? 

Plist is a property list dictionary made from key-value teams and stores the data in the project file system, i.e., info. plist. 

27. What do you mean by type inference in swift? 

The swift-type inference is the ability to deduce the expression automatically at the time of compilation. 

28. Explain protocols in swift. 

Protocols in swift are like blueprints of the methods used by a class for a particular task 

Syntax: 

protocol home {
       func room()
       func hall()
}
class kitchen: home{
      var making food = false
      func room(){
        sleeping = true
  }
}  

29. Why is mutability used in swift?

In the swift programming language, the variables are mutable. On the other hand, the constants are immutable. Thus mutability can be considered a factor that can differ a variable from a constant in swift. 

30. Define dynamic dispatch used in swift. 

The term dynamic dispatch is essential from the object-oriented perspective and is the core primary mechanism that can perform runtime polymorphism. 

31. What access control is in swift? 

Access control is used or restricts the parts of the code from the code in other source files and modules. This hides the code implementation from the source code.

32. Define open access control and public access control.

• open-access control: in the open-access control, the class and the class members can be overridden both within and outside of the module.
• public access control: the public access controls class and class members can only be overridden within the defined module. 

33. What is the use of extensions in swift?

An extension in swift adds new functionality to the predefined class, structure, or protocol type. It also holds the ability to extend the kind for which there is no requirement for source code. The extensions in swift are very similar to that in objective- c  only difference is that they do not have names.

34. What is a type alias in swift? 

The type alias introduces a named alias of an existing class into the program. The type alias is declared using the “typealias” keyword. 

typealias my_list = dictionary<string, int > 

35. What is forced unwrapping in swift?

When a variable is defined as optional, the unwrap is used in swift to get the value from the optional variable. 

36. What is an enum in Swift?

Enums define a common type for a group of related values and enable you to work with those values in a type-safe way. Swift enums can also have associated values.

enum Direction {
    case north
    case south
}

37. What is the purpose of the defer keyword in Swift?

The defer keyword is used to define a block of code that will be executed when the current scope exits. It's often used for cleanup tasks.

38. What is deinit in Swift?

The deinit keyword is used to define a deinitializer for a class, which is called just before an instance of the class is deallocated from memory. It is useful for cleanup tasks.

class Example {
    deinit {
        print("Instance is being deallocated")
    }
}

39. What is the difference between Any and AnyObject in Swift?

Any can represent an instance of any type, including function types and optional types. AnyObject can represent an instance of any class type.

40. What is a lazy property in Swift?

A lazy property is one that is not initialized until it is accessed for the first time. It is useful when the initial value requires complex setup or resources.

lazy var expensiveCalculation = performExpensiveCalculation()

41. What is a convenience initializer in Swift?

A convenience initializer is a secondary initializer that must call another initializer from the same class. It's marked with the convenience keyword and is used to provide shortcuts for initializing an instance in common ways.

42. What is subscripting in Swift?

Subscripting is used to access elements of collections, lists, or sequences using index notation. You can also define subscripts for custom classes and structs.

struct Matrix {
    subscript(row: Int, col: Int) -> Int {
        return row * col
    }
}

let matrix = Matrix()
print(matrix[2, 3]) // Outputs 6

43. How does memory management work in Swift?

Swift uses Automatic Reference Counting (ARC) to track and manage the app's memory usage. ARC automatically frees up the memory used by class instances when they are no longer needed.

44. What are property observers in Swift?

Property observers monitor changes to a property’s value and respond accordingly. They are willSet (called before the value is changed) and didSet (called after the value is changed).

var score: Int = 0 {
    didSet {
        print("Score updated to \(score)")
    }
}

45. What is the difference between final, static, and class keywords when used with methods?

• final: Prevents a method from being overridden in subclasses.
• static: Defines a method that belongs to the type itself, not instances. It can't be overridden in subclasses.
• class: Similar to static, but allows the method to be overridden in subclasses.

46. What is the difference between lazy var and computed property?

A lazy variable is only initialized when accessed for the first time, and its value is stored after initialization, while a computed property recalculates its value every time it is accessed.

47. What are Property Wrappers in Swift?

Property Wrappers add a layer of separation between code that manages how a property is stored and the code that defines the property. They are defined using the @propertyWrapper attribute and can be used to reuse property implementation across multiple properties.

48. Explain optional chaining in Swift.

Optional chaining is used to call properties, methods, and subscripts on optionals that might currently be nil. If the optional contains nil, the call returns nil and does not trigger any further actions.

let name = person?.address?.city

49. What is the difference between map, flatMap, and compactMap in Swift?

• map: Transforms each element of a collection using a provided closure.
• flatMap: Transforms each element of a collection and flattens the result one level.
• compactMap: Transforms each element of a collection and removes any nil results.

50. What are protocols with associated types?

Protocols with associated types define one or more placeholder types, which are specified when conforming to the protocol. This allows for flexibility in defining generic code.

protocol Container {
    associatedtype Item
    func add(item: Item)
}

51. Explain the concept of Protocol-Oriented Programming in Swift.

Protocol-oriented programming is a programming paradigm that focuses on designing your code around protocols and protocol extensions. It emphasizes the use of protocols to define shared behavior and functionality, promoting code reuse and flexibility.

52. What is a failable initializer in Swift?

A failable initializer is used when there is a possibility that initialization might fail. It returns nil if the initialization fails.

class Person {
    var age: Int
    init?(age: Int) {
        if age < 0 { return nil }
        self.age = age
    }
}

53. What are Phantom Types in Swift?

Phantom Types are types that don't have any storage associated with them but are used to enforce type constraints at compile-time. They're typically implemented using generic types with unused type parameters.

54. What is a capture list in Swift?

A capture list in closures is used to prevent strong reference cycles by specifying how captured variables should be treated (weak, unowned). It is placed before the parameter list.

let closure = { [weak self] in
    print(self?.name ?? "No name")
}

55. How does method dispatch work in Swift?

Swift uses three types of method dispatch:

1. Direct dispatch: Fastest, used for final methods and methods in structs/enums.
2. Table dispatch: Used for class methods that can be overridden.
3. Message dispatch: Used for Objective-C interoperability.

56. What are static properties and methods in Swift?

Static properties and methods belong to the type itself rather than an instance. They are declared using the static keyword.

struct Math {
    static let pi = 3.14
}

57. What is the @dynamicMemberLookup attribute in Swift?

@dynamicMemberLookup allows instances of a type to access members dynamically at runtime using subscript syntax. It's often used for creating more flexible APIs or for interoperability with dynamic languages.

58. What is protocol extension in Swift?

Protocol extensions allow you to provide default implementations for methods or computed properties, so any type conforming to the protocol can adopt the default behavior.

protocol Describable {
    func describe() -> String
}

extension Describable {
    func describe() -> String {
        return "Default description"
    }
}

59. Explain the concept of Opaque Return Types in Swift.

Opaque Return Types, introduced with the some keyword, allow a function to return a value of some specific type without specifying the exact type name. This preserves type identity while hiding implementation details.

60. What is a strong reference cycle in Swift?

A strong reference cycle occurs when two or more instances hold strong references to each other, preventing ARC from deallocating them and causing a memory leak.

Advanced Swift Interview Questions and Answers

61. What is optional binding? 

The optional binding is used to find whether the optional variable contains the value. Or any discount is available, either temporary or permanent.

if let home = someoptional {
   home decode
}

62. What are identical operators?

In swift, there are three or more identical operators mentioned below 

1. === ( or ==!) 

• It is used for checking the value for both points to the same memory address 
• It is used for comparison of the reference type. 
• it is used similar to == used in objective -c. 

2. ==(or !=) 

• It is used to check whether the values are the same and to compare the value type.
• It is similar to is equal: in objective-c

63. From the following code, answer the following question.

var arr1 = [ a,b,c,d,e,f]
var arr2 = arr1 
arr2.append(g)
var len = arr1.count

output:

arr1 = [a,b,c,d,e,f]
arr2 = [a,b,c,d,e,f,g]

64. What do you understand about the higher-order functions?

The higher-order function takes one or more functions as arguments or returns a function. Higher-order functions are also used to enhance 

readability and reduce the length of the code. 

65. Differences between array and nsarray?

66. What is the function of init()?

The inti() in swift is used to initialize a structure or class; it's a property inside the initial value. This method allows leaving stored properties to be avoided in the intermediate state. This function also checks the line or symbol causing an error in the execution process of the code.  

67. Mention the key difference between structure and classes.

68. Why swift uses immutability? 

Immutability is one of the important and core for swift coding. Immutability is important because it ensures that the data cannot be changed and creates consistent variable usage and optimized data storage. All the unnecessary errors and wasted time is also avoided using immutability.

69. What is a generic in Swift?

Generics allow you to write flexible and reusable code by specifying placeholder types. They work with any type and reduce code duplication.

func swapValues<T>(_ a: inout T, _ b: inout T) {
    let temp = a
    a = b
    b = temp
}

70. What are Result Builders in Swift?

Result Builders (also known as Function Builders) are a feature that allows you to create domain-specific languages (DSLs) within Swift. They're commonly used in SwiftUI for declarative UI construction.

71. What is as!, as?, and as used for in Swift?

• as! is used for forced casting and will crash if the cast fails.
• as? is used for safe casting, returning nil if the cast fails.
• as is used for upcasting or when casting to a superclass type.

72. How does Memory Safety work in Swift?

Swift ensures memory safety through various mechanisms:

• Automatic Reference Counting (ARC)
• Strict initialization rules
• Array bounds checking
• Exclusive access to memory

73. What is the difference between escaping and non-escaping closures?

An escaping closure is a closure that is called after the function returns, meaning it escapes the function’s scope. Non-escaping closures must be called within the function.

func performTask(completion: @escaping () -> Void) {
    DispatchQueue.main.async {
        completion()
    }
}

74. What are Associated Types in protocols?

Associated Types are placeholder names for types used in a protocol. They allow protocols to be more flexible by not specifying concrete types, but rather relationships between types.

75. What is Key-Value Observing (KVO) in Swift?

KVO allows an object to observe changes to specific properties of another object. In Swift, you can observe properties with the @objc dynamic keyword and use addObserver to monitor changes.

@objc dynamic var name: String

76. How do you implement Dependency Injection in Swift?

Dependency Injection can be implemented in Swift through:

1. Initializer injection
2. Property injection
3. Method injection
4. Using protocols and extensions

77. What is the difference between inout parameters and return values in Swift?

An inout parameter allows a function to modify the value passed to it directly, whereas a function with a return value simply returns a new value without modifying the original input.

func increment(_ value: inout Int) {
    value += 1
}

78. What are Variadic Parameters in Swift?

Variadic Parameters allow a function to accept zero or more values of a specified type. They're indicated by three dots (...) after the parameter's type annotation.

79. How do you implement method overloading in Swift?

Method overloading in Swift is done by defining multiple methods with the same name but different parameter types or number of parameters.

func display(value: Int) {
    print("Int: \(value)")
}
func display(value: String) {
    print("String: \(value)")
}

80. What is the @autoclosure attribute in Swift?

The @autoclosure attribute is used to automatically create a closure from an expression passed as an argument to a function. It's often used to delay evaluation of an expression.

81. How do you handle thread safety in Swift?

Thread safety can be ensured using GCD (Grand Central Dispatch) or other synchronization mechanisms like locks. The DispatchQueue API is commonly used to ensure thread-safe access to resources.

let queue = DispatchQueue(label: "com.example.serialQueue")
queue.sync {
    // Thread-safe code
}

82. What is the difference between Equatable and Hashable protocols?

Equatable requires types to implement the == operator for equality comparison. Hashable extends Equatable and also requires a hash(into:) method for use in hash-based collections like Set and Dictionary.

83. What is a retain cycle, and how do you prevent it in Swift?

A retain cycle occurs when two objects hold strong references to each other, preventing ARC from deallocating them. This can be avoided using weak or unowned references.

class A {
    var b: B?
}
class B {
    weak var a: A?  // Prevents retain cycle
}

84. How does Swift handle method swizzling?

Swift doesn't natively support method swizzling like Objective-C does. However, it can be achieved using the Objective-C runtime and the @objc attribute on Swift methods.

85. What is KeyPath in Swift, and how is it used?

KeyPath is a feature introduced in Swift 4 that allows you to reference properties of types dynamically, without explicitly accessing them.

struct Person {
    var name: String
}
let keyPath = \Person.name
let name = person[keyPath: keyPath]

86. Explain the concept of Conditional Conformance in Swift.

Conditional Conformance allows a generic type to conform to a protocol only when its type parameters meet certain requirements. This enables more flexible and reusable code.

Example:

extension Array: Equatable where Element: Equatable {
   static func == (lhs: [Element], rhs: [Element]) -> Bool {
       return lhs.elementsEqual(rhs)
   }
}

87. What is a mutating function in Swift?

In Swift, a mutating function is one that modifies properties of a value type (such as struct). It’s needed because value types are immutable by default.

struct Point {
    var x: Int
    var y: Int
    mutating func move(dx: Int, dy: Int) {
        x += dx
        y += dy
    }
}

88. What are the benefits and drawbacks of using Protocol-Oriented Programming?

Benefits:

1. Improved code reuse
2. Better abstraction
3. Easier testing and mocking
4. More flexible than class inheritance

Drawbacks:

1. Can lead to complex type relationships
2. Potential performance overhead compared to direct method calls
3. Steeper learning curve for developers new to the paradigm

89. How do you implement a singleton in Swift?

A singleton in Swift can be implemented using a static property to create a shared instance.

class MySingleton {
    static let shared = MySingleton()
    private init() {}
}

90. How does Swift's memory layout work, and how can it affect performance?

Swift's memory layout includes:

1. Inline storage for small value types
2. Heap allocation for reference types and large value types
3. Tail-allocated arrays for collections

Understanding memory layout can help optimize performance by:

1. Choosing appropriate types (struct vs class)
2. Minimizing heap allocations
3. Leveraging value semantics for better performance

91. What are dynamic member lookups in Swift?

Dynamic member lookup allows access to properties or methods that aren’t explicitly declared, using key-value coding, useful when interacting with dynamically typed data.

@dynamicMemberLookup
struct Dynamic {
    subscript(dynamicMember member: String) -> String {
        return "Hello \(member)"
    }
}
let dynamic = Dynamic()
print(dynamic.name) // Outputs "Hello name"

92. Explain the concept of Type Erasure and when it's useful.

Type Erasure is a technique used to hide the concrete type of an object while still preserving its behavior defined by a protocol. It's useful when you want to use a protocol with associated types as a type, which is not directly possible in Swift.

Example:

protocol Presenter {
   associatedtype View
   func present(on view: View)
}
struct AnyPresenter<View>: Presenter {
   private let _present: (View) -> Void
   
   init<P: Presenter>(_ presenter: P) where P.View == View {
       _present = presenter.present
   }
   
   func present(on view: View) {
       _present(view)
   }
}

93. How do you handle JSON parsing in Swift?

JSON parsing is handled using Codable, which simplifies encoding and decoding of data. JSONDecoder is used to decode JSON data into Swift types.

struct User: Codable {
    var name: String
    var age: Int
}
let data = jsonString.data(using: .utf8)!
let user = try? JSONDecoder().decode(User.self, from: data)

94. How does Swift handle binary compatibility, and what are its implications for library authors?

Swift uses module stability and library evolution to maintain binary compatibility. This allows libraries to evolve without breaking existing clients. Key aspects include:

1. Resilient structs
2. @frozen attribute for performance-critical types
3. @inlinable and @usableFromInline for cross-module optimization

Library authors need to be aware of these features to balance API evolution and performance.

95. How does ARC handle memory management for closures?

Closures in Swift capture variables from their surrounding context by default. If the closure retains a strong reference to self, it can create a retain cycle, which can be resolved by using weak or unowned references.

{ [weak self] in
    self?.doSomething()
}

96. Explain the concept of Function Currying in Swift.

Function Currying is the technique of converting a function that takes multiple arguments into a sequence of functions, each taking a single argument. It allows for partial application of functions.

97. What is Equatable in Swift, and why is it important?

Equatable is a protocol that allows comparing two instances of a type for equality using the == operator. It’s essential for types where equality comparisons are necessary.

struct Point: Equatable {
    var x: Int
    var y: Int
}

98. Explain the concept of Ownership in Swift and how it relates to memory management.

Ownership in Swift refers to the set of rules that govern how memory is managed and accessed. It's closely tied to Swift's memory safety features and includes concepts like:

1. Exclusive access to memory
2. Move-only types (proposed feature)
3. Borrowing (proposed feature)

These concepts aim to prevent issues like data races and use-after-free errors while maintaining performance.

99. How do you conform to Codable for custom decoding?

You can conform to Codable and implement custom decoding logic when the default behavior doesn’t meet your needs, using init(from:).

struct User: Codable {
    var name: String
    var age: Int

    init(from decoder: Decoder) throws {
        let container = try decoder.container(keyedBy: CodingKeys.self)
        name = try container.decode(String.self, forKey: .name)
        age = try container.decode(Int.self, forKey: .age)
    }
}

100. What are the differences between indirect enums and class for recursive data structures?

1. Memory allocation: indirect enums use heap allocation only for recursive cases, while classes always use heap allocation.
2. Reference semantics: Classes have reference semantics, while enums (even with indirect) maintain value semantics.
3. Inheritance: Classes support inheritance, indirect enums don't.
4. Performance: indirect enums can be more performant for small recursive structures.

Swift MCQs

1. Which keyword is used to define a constant in Swift?

A) let

B) const

C) define

D) var

Answer: A) let

2. What does the guard statement do in Swift?

A) Terminates the program

B) Executes code unconditionally

C) Transfers control out of the scope if a condition is false

D) Creates a loop

Answer: C) Transfers control out of the scope if a condition is false

3. What is the default access level in Swift?

A) public

B) internal

C) private

D) fileprivate

Answer: B) internal

4. Which of the following data types is not present in Swift?

A) Int

B) Float

C) char

D) String

Answer: C) char

5. What is the primary use of the defer keyword in Swift?

A) To delay code execution until a later point

B) To handle errors

C) To ensure code is executed right before exiting the current scope

D) To pause a thread

Answer: C) To ensure code is executed right before exiting the current scope

6. In Swift, how do you define a function that can accept variable number of arguments?

A) Use ... after the parameter type

B) Use & before the parameter

C) Use * after the parameter

D) Use @varargs

Answer: A) Use ... after the parameter type

7. What is the significance of @escaping in Swift closures?

A) It allows closures to run only once

B) It allows the closure to capture the value of external variables

C) It prevents the closure from returning a value

D) It allows the closure to outlive the function it is passed to

Answer: D) It allows the closure to outlive the function it is passed to

8. Which of the following is true about optional in Swift?

A) Optional values cannot be nil

B) Optionals must always have a value

C) Optionals allow variables to have either a value or be nil

D) Optionals do not need to be unwrapped

Answer: C) Optionals allow variables to have either a value or be nil

9. What operator is used for optional binding in Swift?

A) if let

B) while

C) do-try

D) guard var

Answer: A) if let

10. Which of the following is used to mark a class as inheritable in Swift?

A) open

B) public

C) internal

D) final

Answer: A) open

Conclusion

This article covered the Swift Interview Questions. Swift, as a powerful and evolving programming language, has become a critical skill for iOS developers. In this blog, we covered various Swift interview questions ranging from basic concepts to advanced-level topics, providing a solid foundation for your interview preparation.

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