Top 50 Swift Interview Questions (Swift iOS Interview)

Top Swift (iOS) Interview Questions and Answers with Examples:

Swift is a powerful and interactive programming language created for iOS, macOS, tvOS, watchOS, and Linux development by Apple Inc.

Swift is a fantastic way to develop software, it is an interactive programming language that is fast, safe, and friendly to new programs. Swift is the most popular language for developing apps for Apple’s devices. It is an alternative to the Objective-C language with some added object-oriented features.

Quiz on Swift Interview Questions

Try this expert quiz with a curated list of the top Swift interview questions. This exclusive quiz is structured to guide you right from fundamental topics to more complex ones, thereby helping you with comprehensive preparation for your upcoming Swift interview.

Swift defines major classes of usual programming errors by adopting recent programming patterns, as shown below:

• Open Source.
• Swift is easy to learn.
• It is approachable.
• Built-in error handling.
• Memory management happens automatically.
• Developers always initialize variables before use.
• They verify integers to prevent overflow.
• Provides modern programming features.
• The switch function offers an alternative to the “if” statement.
• High speed & safer performance, code complexity.
• Support of dynamic libraries.

Basic Swift (iOS) Interview Questions

Here is a list of the top iOS Swift interview tips and questions, along with the answers.

Let’s Explore!!

Q #1) What is iOS Swift?

Answer: Swift is a compiled new programming language developed by Apple Inc. in June 2014 to develop apps for mobile and desktop. This language works for watchOS, macOS, iOS, and tvOS.

Apple created the Swift language to work with both Cocoa Touch and Cocoa. Swift supports multiple operating systems, such as FreeBSD, Linux, Darwin, etc. This language worked along with the Objective-C library and Cocoa framework in the Apple products.

Q #2) What are the advantages of using Swift?

Answer: Swift programming language has speedily become one of the fastest-growing languages in memoir. Swift makes us develop software that is incredibly fast, robust, and secure.

This language is the most useful programming language that is used to develop an application for macOS and iOS (iPad and iPhone).

There are some important advantages offered by developing apps for macOS and iOS using Swift.

• Open-source language: The Swift programming language has been created as an open-source language and is open to everyone. This makes it simple for the program to upgrade all the source codes, email lists, and bug trackers at regular intervals.
• Easy to learn and maintain: Swift programming language is simpler and more accurate when compared to C/C++. Apple evolved its programming language to be easy to use, and it took syntax from programming languages such as C#, Python, and Ruby. The simple syntax of this programming language makes it more meaningful. In Swift, all the content of the implementation (.m) and header (.h) files is combined in a single file that is (.swift).
• Supported by multiple devices: The Swift programming language is not just limited to supporting Apple devices; it will also support multiple devices in the technology world, like Linux and Windows devices.
• Supports dynamic libraries: Dynamic libraries are the executable chunks of code that can be connected to an app. This feature allows the latest Swift programming language. In Swift, the system directly uploads dynamic libraries to memory, thus reducing the app’s initial size and improving performance.
• Optional types: An optional in Swift is a type that can be held either as a value or not. To declare an option, we can use a question mark.
• Closures: Closures are self-contained blocks of functionality that can be passed around and used in our code.

Q #3) Explain Swift vs Objective-C.

Answer: Listed below are the differences between Swift and Objective-C.

1. Swift is an object-oriented and functional programming language, whereas Objective-C is a class-based object-oriented programming language.
2. Swift supports dynamic libraries, whereas Objective-C does not support dynamic libraries.
3. Swift supports Tuples, whereas Objective-C does not support them.
4. Semicolons are not required in Swift, whereas semicolons are required in Objective-C.
5. Swift is an open-source programming language, whereas Objective-C is limited to Apple, and it’s not an open-source language.
6. We have to use the “let” keyword to declare a constant and the “var” keyword to declare a variable, whereas in objective C, we have to declare the variable as “NSString” and the constant as “int”.
7. Swift enables us to define methods in structures, classes, or enumerations, whereas Objective C does not allow this.
8. In Swift, we can define classes in a single file (.swift), whereas in Objective C we create separate interface (.h) files for classes and implementation (.m) files for classes.

Q #4) Where can we test the Apple iPhone apps if we don’t have an iOS device?

Answer: If we don’t have an iOS device, we can test our application on simulators provided by Apple on the Mac system.

Q #5) What are the tools that are required to develop iOS applications?

Answer: These are some of the essential tools that we should have:

• Mac/MacMini: We must get a Mac with an Intel-based processor running on Mac OS. Not to worry, if we have our PC, we can still develop iOS apps through a Mac Mini.
• Xcode: Xcode is the Apple IDE (Integrated Development Environment) that is used for both iOS apps and MAC OS. It provides us with a visual layout editor and a code editor that can deal with the logic, user interface, and response behind the scenes.
• Swift Programming Language: In the code editor, the logic will be written in a programming language that is invented by Apple, called Swift.
• Apple Developer Program: This program allows the developer to push our app live on the App Store so that customers and downloaders all over the world can download our app and use it.

Q #6) What are the most important features of Swift?

Answer: Some important features of Swift are given below:

• More impressive structs and enums
• Protocol oriented
• Optional Types
• Type Safety and Type Inference Language
• Not required to use semicolons
• Enforced initializers
• Safe by default
• Less code, fewer files
• Forced Unwrapping
• Tuples
• Closures
• Much faster when compared to other languages.

Q #7) Explain the common execution states for a Swift iOS App (iOS Application Lifecycle).

Answer: The 5 common execution states are:

• Not Running: This is a simple state in which our app is not launched, or no code is being executed and terminated by the system, and the application is completely switched off.
• Inactive: This state is just a transitional state. An inactive state means our application is running in the background but cannot receive events.
• Active: The active state is the main execution state, where our app is running in the background and can receive events.
• Background: This is the state where our App is running in the background and can still execute code in the background.
• Suspended: This state means that our app is running in the background state, and the system suspends this app, and the application cannot execute any code.

Q #8) Is Swift an object-oriented programming language?

Answer: Yes, Swift is an object-oriented programming language.

Q #9) What type of objects are basic data types in Swift?

Answer: Swift uses a standard set of basic data types for different purposes, such as Boolean values, numbers, and strings.

• Int: int is used to store an integer value.
• Double and Float: Double and Float in Swift are considered when working with decimal numbers.
• Bool: The bool type is used to store the Boolean values. Swift uses true and false conditions.
• String: In String literals, the user defines the text that is enclosed by double quotes in Swift.
• Arrays: Arrays are a collection of list items.
• Dictionaries: A dictionary is an unordered collection of items of a particular type that is connected with a unique key.

Q #10) What is init() in Swift?

Answer: Initialization is preparing an instance of an enumeration, structure, or class for use.

Initializers are also called to create a new instance of a particular type. An initializer is an instance method with no parameters. Using the initializer, we can write the init keyword.

nit()
{
// perform some New Instance initialization here
}

Q #11) What are the control transfer statements that are used in iOS Swift?

Answer: The control transfer statements that are used in iOS Swift include:

1. Return
2. Break
3. Continue
4. Fallthrough

Q #12) What is the difference between Let and Var in Swift?

Answer: In swift language, we can declare a constant and a variable using the Let and Var keyword.

(i) Let: Let keyword is immutable, it’s used to declare a constant variable, and the constant variable cannot be changed once they are initialized.

For Example, let myAge = 25

We cannot change the value of age; you can declare the constant value of it only once using the let keyword.

(ii) Var: Var keyword is mutable, and is used to declare a variant variable. These variant variables can change the run time.

For Example:

var myName = “Dell”

we can change the value of the name = “Apple”.

Q #13) How to add an element to an Array?

Answer: Arrays are one of the most used data types in an application (app). We use arrays to organize our application (app) data.

Swift makes it easy to create an array in our code using an array literal. Square brackets separate the list of values, and you can simply surround array elements with a comma.

For Example,

// Add ‘Int’ elements in an Array
let natural number = [1, 2, 3, 4, 5, 6, 7]

// Add ‘String’ elements in an array
let countryName = [“India”, “Japan”, “Malaysia”, “Peru”, “Russia”]

Q #14) Which JSON framework is supported by iOS?

Answer: SBJson framework is supported by iOS. SBJson framework provides additional control and a flexible API, which makes JSON handling easier. It is a well and highly flexible framework that supports the flexible functioning of APIs.

Q #15) What is PLIST in iOS?

Answer: PLIST stands for Property List. PLIST is a dictionary of values and keys that can be stored in our file system with a .plist file extension. The property list is used as a portable and lightweight means to store a smaller amount of data. They are normally written in XML.

Different types of property lists are mentioned below:

• Binary Property List
• XML Property List
• ASCII Legacy Property List

Scenario-Based Swift Interview Questions

Q #16) What is a dictionary?

Answer: Dictionaries are an association of an unordered collection of key-value pairs. Each value is associated with a unique key, which is a hashable type, such as a number or string. We can use the dictionary concept in the Swift programming language whenever we want to obtain the values based on a key value.

Syntax of Swift Dictionaries:

Following is the syntax for defining a dictionary in the Swift programming language.

Dictionary<Key, Value> ()

Or

[Key: Value] ()

Creating Dictionaries in Swift:

The following are the different ways of creating a dictionary with key-value pairs in the declaration.

// using Dictionary
Var names = Dictonary<String,Int> ()
Names = [“Ajay”: 1, “Mohit”: 2]
print(names)
// it prints [Ajay: 1, Mohit: 2]

Q #17) What is a Protocol in Swift?

Answer: The protocol is a very common feature of the Swift programming language, and the protocol is a concept that is similar to an interface from Java. A protocol defines a blueprint of properties, methods, and other requirements that are suitable for a particular task.

In its simplest form, the protocol is an interface that describes some methods and properties. The protocol is just described as the properties or methods skeleton instead of implementation. Properties and methods implementation can be done by defining enumerations, functions, and classes.

Protocols are declared after the structure, enumeration, or class type names. A single and multiple protocol declaration can be possible. Multiple protocols are separated by commas.

We can define a protocol in a way that is very similar to structures, enumerations, and classes:

Protocol Someprotocol
{
// protocol definition goes here
}

We can define multiple protocols, which are separated by commas:

Class SomeClass: SomeSuperclass, Firstprotocol, Secondprotocol
{
// Structure definition goes here
}

Q #18) What is a delegate in Swift?

Answer: Delegate is a design pattern that is used to pass data or communication between structs or classes. Delegate allows sending a message from one object to another object when a specific event happens, and is used for handling table view and collection view events.

Delegates have one-to-one relationships and one-to-one communication.

Q #19) What is the use of the double question mark “??” in Swift?

Answer: The double question mark “??” is a nil-coalescing operator. It is mainly a shorthand for the ternary conditional operator that we used to test for nil. A double question mark is also used to provide a default value for a variable.

stringVar ?? “default string”

This exactly does the common thing, if stringVar is not nil then it is returned, otherwise the “default string” is returned.

Q #20) What is a GUARD statement? What is the benefit of using the GUARD statement in Swift?

Answer: A GUARD statement is used to transfer the program control out of the scope when one or more conditions are not met. Using this statement helps in avoiding the pyramid of doom.

A GUARD statement will be in the following form:

guard condition else
{
Statements
}

Q #21) What are the collection types that are available in Swift?

Answer: Three primary collection types are available in Swift for storing a collection of values. They are dictionaries, sets, and arrays.

1. Arrays: Arrays are an ordered collection of values, which are stored in the same type of values in an ordered list.
2. Sets: Sets are an unordered collection of unique values, which are stored in a distinct value of the same type in a collection without any defined ordering.
3. Dictionaries: Dictionaries are an unordered collection of key and value pair associations in an unordered manner.

Q #22) What is “defer”?

Answer: The “defer” is a keyword that provides a block of code that can be executed while the execution is leaving the current scope.

Q #23) What is Tuple? How to create a Tuple in Swift?

Answer: A tuple is a group of different values in a single compound value. It is an ordered list of elements. There are two ways of accessing the object data in a tuple, i.e., by name or by position.

In Swift, a tuple can consist of multiple different types. It can support two values, i.e. one of integer type, and the other of a string type. It is a legal command.

For Example, let ImplementationError = (501, “Not implemented”).

We can create a basic tuple like this:

let person = (name: “Ajay” , age: 34)

Q #24) What is the difference between Array and NSArray?

Answer: The differences between Array and NSArray are given below:

• An array can hold only one type of data, whereas NSArray can hold different types of data.
• An array is a value type, whereas NSArray is an immutable reference type.

Q #25) What is the difference between class and structure?

Answer: The difference between class and structure is given below:

1. Classes are reference types, whereas structs are value types.
2. Classes can be built on other classes, whereas a struct cannot inherit from another struct.
3. Classes have an inheritance, whereas structs cannot have an inheritance.
4. In class, we can create an instance with “let” keywords and attempt to mutate its property, whereas there is no Mutability in Structs.
5. Classes have Type Casting, whereas struct don’t have Type Casting.

Q #26) What are the best ways of achieving concurrency in iOS?

Answer: Here are the three best ways to achieve concurrency in iOS:

• Dispatch queues
• Threads
• Operation queues

Q #27) How to create a constant in Swift programming?

Answer: We have to use the “let” keyword to declare a constant in Swift programming.

Q #28) How to pass the data between view controllers?

Answer: There are three ways to pass data between view controllers, as shown below.

1. Using Segue, in prepareForSegue method (Forward).
2. Setting the variable directly (Backward).
3. Using Delegate (Backward).

Q #29) How can we define a base class in Swift?

Answer: In the Swift programming language, classes are not inherited from the base class. The classes are defined by the developer without specifying the superclass and it will become the base class automatically.

Q #30) How can we make a property optional in Swift?

Answer: Declaring a Question mark “?” in the Swift code can make a property optional. This question mark “?” helps to avoid the runtime error when a property doesn’t hold a value.

Q #31) Who calls the main function of our app during the app launch cycle?

Answer: The main thread is called the main function of our app. During the app launching cycle, the system will create a main thread for the app and call the app’s main function on that main thread.

Q #32) What are UI elements?

Answer: Images, buttons, labels, text fields, and any other elements that are visible to the user within the application are called UI elements.

Q #33) Which is the superclass of all the view controller objects?

Answer: UIViewController class is the superclass of all the view controller objects. The functionality for presenting them, loading views, and rotating them is a response to the device rotations. All the standard system behavior is provided by the UIViewController class.

Q #34) What are the new features in Swift 4.0?

Answer: The new features in Swift 4.0 are given below:

• Faster and easier to use strings that keep Unicode correctness.
• Tuples and multiple return values.
• Native error handling using throw/try/catch.
• Extends to support serialization to a struct.

Q #35) How to write a multiple-line comment in Swift?

Answer: A multiple-line comment is written between the (/*) at the starting point and (*/) at the endpoint.

Q #36) What are the source items used by Xcode?

Answer: Xcode uses four different types of source items, as mentioned below:

• Framework
• Source Group
• Source File
• Source Folder

Experienced Level Questions for Swift Interview

Q #37) What are the different ways to pass data in Swift?

Answer: There are several ways to pass data in Swift, such as KVO, Delegate, NSNotification & Callbacks, Target-Action, etc.

Q #38) Explain the usage of Class and the benefits of Inheritance.

Answer: They are:

• Reuse implementation
• Subclass provides dynamic dispatch.
• Subclass provides the reuse interface.
• Modularity
• Overriding provides the mechanism for customization.

Q #39) Explain some common features of Protocols & Superclasses.

Answer: Some common features of Protocol & Superclass are given below:

• Interface reuse.
• Implementation reuse.
• Supporting modular design.
• Provides points for customization.

Q #40) Explain some biggest changes in UserNotifications.

Answer: Some bigger changes in UserNotifications are given below:

• Allows adding images, audio, and videos.
• Allows creating custom interfaces for notifications.
• Allows managing the notifications with an interface in the notification center.

Q #41) What is optional chaining?

Answer: Optional chaining is a useful process that we can use in combination with the optional to call the methods, properties, and subscripts on the options, and these values may or may not be nil. In this process, we may try to retrieve a value from a chain of optional values.

• If the optional contains a value, then calling the subscript, method, or property of an optional will return a value.
• If optional is nil, then calling the subscript, method, and property will return nil.

Q #42) What is Optional binding?

Answer: Optional Binding concept is used to find out whether an optional contains a value, and it makes that value available as a variable or temporary constant. We use an optional binding concept to check if the optional contains a value or not.

Optional binding can be used with the condition (if and while) statements to check for a value inside an optional.

Q #43) What are the Higher-Order functions in Swift?

Answer: The higher-order functions are given below:

• Map: Transform the array contents.
• Reduce: Reduce the values in the collection to a single value.
• Sort: Sorting the arrays.
• Filter: Transform the array contents.

Q #44) Explain some design patterns that we normally use during app development.

Answer: The design patterns that are used during app development are given below:

• Behavioral: Memento and Observer.
• Creational: Builder, Factory, and Singleton.
• Structural: Façade, Adapter, and Decorator.

Q #45) What are the various ways to unwrap an optional in Swift?

Answer: There are seven ways to unwrap an option in Swift. They are:

• Guard statement: safe.
• Forced unwrapping: using “!” operator is unsafe.
• Optional binding: safe.
• Optional pattern: safe.
• Nil coalescing operator: safe.
• Implicitly unwrapped variable declaration: unsafe in many cases.
• Optional chaining: safe.

Q #46) What mechanism does iOS support for multi-threading?

Answer: They are:

• NSThread: It can create a low-level thread that can be started by using the “start” method.
• NSOperationQueue: It allows a pool of threads to be created and is used to execute “NSOperations” in parallel.

Q #47) What is the Swift module?

Answer:

• A module is a single unit of code distribution.
• A framework or application is built and shipped as a single unit and that can be imported by another module using the swift import keyword.
• Each build target in the Xcode tool is treated as a separate module in Swift.

Q #48) Explain Core Data.

Answer: Core data is one of the most powerful frameworks provided by Apple for macOS and iOS apps. Core data is used for handling the model layer object in our applications. We can treat Core Data as a framework to filter, modify, save, and track the data within the iOS apps. Core Data is not a relational database.

Using core data, we can easily map the objects in our app to the table records in the database without knowing any SQL. Core data is the M in the MVC structure.

Some features of Core data are given below for your reference:

• Effective integration with the iOS and macOS toolchains.
• Organizing, filtering, and grouping data in memory and in the UI (User Interface).
• Automatic support for storing objects.
• Automatic validation of property values.
• First framework for managing an object graph.
• Core Data framework for managing the life cycle of the object in the object graph.

Q #49) Explain the Grand Central Dispatch (GDC).

Answer: GCD (Grand Central Dispatch) is a low-level API for managing concurrent operations. This concept is used to help improve application performance. This process is used to manage multiple tasks at the same time. Grand Central Dispatch (GCD) is the most useful API for multitasking with Async and Sync programming in iOS.

• Dispatch Queue: The Dispatch Queue manages the tasks in FIFO(First In First Out) order. Dispatch queues are thread-safe, as we can access them from multiple threads simultaneously.
• Concurrent: This process has started multiple task,s but is not sure of their termination at the same time. It can be finished in any order. They execute one or more tasks at the same time, concurrently. Complexity determines task completion, not queue order.
• Serial: Only one task will execute at a time. It can be used to synchronize access to a specific resource.
• Sync: A synchronous function returns control to the caller after the task is complete.
• Async: An asynchronous function returns immediately by ordering the task to start, but will not wait for it to complete.

Q #50) Explain the MVC structure.

Answer: MVC stands for the model view controller. MVC is a powerful software architecture pattern for developing apps.

MVC builds on top of the Object-Oriented Programming concepts. It structures the flow of data and interaction in our app. Model-View-Controller is an important fundamental concept to be understood in iOS development. Many iOS frameworks, like UIKit, and Cocoa Touch, use the MVC pattern for messaging and structured data flow.

Model-View-Controller is used to pass the data from one part of our app to another.

It’s a design pattern used to assign objects in an application to any one of the three roles:

• Model: A model is a wrapper of data. The model is a representation of our app data and it holds the information to a specific user like birthdate, username, etc. The model manages the application state. This also includes writing and reading data. The Model encapsulates an individual set of data and applies some logic to manipulate that data.
• View: View is a representation of a UI (User Interface). A View is an object that is visible to the user and interacts with a user interface (UI).
• Controller: The controller is an intermediary between the View and the Model. The controller controls all the logic that goes between the Model and the View. It inter-communicates messages between the Model and the View, and vice versa.

Conclusion

We have explored all possible and important Swift interview questions along with their answers and examples in this article.

We hope the answers provided here will indeed be very helpful in understanding the iOS Swift basics and advanced topics. Clear knowledge of these Swift and iOS developer interview questions would assist any beginner or experienced professional in successfully clearing the interview on the first attempt.

All the best for your interview!!