iOS Interview Questions and Answers
iPhone Operating System is becoming popular and there are many billion active Apple devices around the world.
The future of the iPhone is very bright as the learning of iOS app development grows exponentially with the integration of innovative technologies for devices like Apple TV, Apple Smartphones, and Apple Watch.
Here we have created popular and frequently asked iOS Interview Questions and Answers that help you ace the technical rounds effectively.
- Define iOS
- What are the specialized features of iOS?
iOS has become popular as it has the following unique features for its users.
- Multi-tasking capabilities: iPhone users can easily switch between applications using the multi-tasking feature or multi-finger gesture.
- Easy integration with social networks: iOS offers an easy integration facility with social network interactions with apps that displays an activity stream and share content.
- iCloud Service of iOS: It enables users to store data on the internet as it offers a high level of encryption and a backup option that ensure the user about their data.
- In-app purchases: Apple products will be available on all platforms and they offer additional services and materials that include digital items like iOS, macOS, iPad, premium content, and subscriptions.
- Editable notification settings: The user can view all the alerts in the notification center in iOS and it can also be modified as per the user’s needs.
- Closed System: The source code of iOS apps will not be available for the developers and the owner of the iPhone or iPad can’t modify the code for their devices. It makes iOS harder to hack.
- What is the architecture of iOS?
iOS will be operated in a Layered structure and it is composed of four layers that each of which offers a programming framework for developing applications on top of the hardware.
It will enhance the communication between the Application Layer and the Hardware Layer. A lower-level layer offers the services for all the required applications and an upper-level layer provides interface-related and graphics services.
Application (Core OS) Layer: It sits directly on top of the device hardware and it is the bottom layer of the iPhone OS.
It has additional services like memory management, file handling systems and threads, low-level networking, and external resource accessing along with a basic operating system.
Service Layer: The main purpose of this layer is to design the services for the upper layers according to the user’s demand.
It has some significant services like object blocking, grand central dispatching, iCloud storage, and in-app purchases.
It has been powered by the addition of ARC (Automatic Reference Counting).
Media Layer: This layer is used to manage media contents like audio, video, graphics, etc and it allows users to use all graphics, video, and audio technologies for the system.
Cocoa Touch Layer: This is the layer where frameworks are created when applications are developed.
Additionally, it functions as the interface for iOS users to perform with the operating system. It includes touch and motion capacities
- Explain Cocoa and Cocoa Touch.
Cocoa is the application development environment for OS X. It includes the foundation and AppKit frameworks.
It is used to refer to any class or object that is based on the Objective-C runtime and inherits from the root class.
It is used to refer to the application development using any programming interface.
- What is a design pattern and explain its importance?
Design Patterns are reusable solutions to general problems in software design and they’re templates designed for helping users to write codes that are easy to understand and reuse.
The most commonly used design patterns are creational with Singleton, Structural with Decorator, Façade, Adapter, and Behavioral with Observer and Memento.
- Explain Singleton pattern
The singleton pattern is to ensure that only one instance is there for a given class and that there’s a global access point for that instance.
It uses lazy loading for creating a single instance when it is required for the first time.
- Describe the Façade Design pattern
- Explain the Decorator Design pattern
The Decorator Design pattern adds behaviors and responsibilities to an object without editing its code and it is an alternative to subclassing where the developer modifies a class’s behavior by wrapping it with another object of a class.
- Define Adapter pattern
This Adapter pattern enables classes with incompatible interfaces to work together and it wraps around an object and exposes a standard interface to communicate with that particular object.
- Explain Observer Pattern and Memento Pattern for behavioral design
The Observer pattern notifies an object of any state changes and cocoa applies the observer pattern in the popular two ways such as notification and key-value observing (KVO).
The Memento Pattern stores the stuff somewhere and this externalized state will be restored without violating encapsulation.
The private data will have remained private and one of Apple’s specialized implementations of the memento pattern will be archived as part of State Restoration.
- What JSON framework is supported by iOS?
iOS supports the SBJson framework as it has a JSON parser and generator for Objective-C.
- Differentiate app ID and Bundle ID
An app ID is a two-part string that is used to define one or more apps from a single development team and the string contains a Team ID and a Bundle ID search string with a period (.) to separate the two parts.
Apple supplies the Team ID that is unique to a particular developer team and the developer supplies a bundle ID search string to match either the bundle ID of a single app or a set of bundle IDs for a group of apps.
The Bundle ID is used to denote each app with its specified XCode and a single XCode project will have multiple targets for multiple apps.
The common use case is an app that contains both lite or free and pro or full version or is branded multiple ways.
- What are the ways to achieve concurrency in iOS?
There are three popular ways to achieve concurrency in iOS that are Threads, Dispatch Queue, and Operation Queue.
Threads: Threads are the independent sequence of instructions that can be executed separately from other codes within a program.
One can execute multiple code paths parallelly in a single application through threads and having a thread is useful for performing a lengthy task without affecting the execution of the rest of the program.
Dispatch Queues: Dispatch Queues are used to manage tasks in first-in-first-out (FIFO) order and execute tasks sequentially or concurrently.
It is an easy way to manage asynchronous and concurrent tasks in the applications.
Operation Queues: They are objects that invoke with priority and readiness.
The operation queues are high-level abstractions of queuing models that are built-in top of GCD (Grand Central Dispatch) and it is possible to execute tasks concurrently like GCD but in an object-oriented manner.
- Differentiate Atomic and Non-Atomic properties and mention the default one for the synthesized property.
Atomic Property is the default property that guarantees the valid value to be returned from the getter or set by the setter.
It ensures that only one thread will access the setter/getter of a given property at a time and they should wait until the first thread releases the setter/getter.
It is not fast but thread-safe for ensuring the process will be finished completely.
The non-atomic property allows multiple threads to access the getter/setter method for a given property at the same time and it is the potential for inconsistency between the existing values.
It comes with enhanced access without any guarantee of the returned value.
- Explain the various types of iOS Application States
An iOS application goes through a set of states that mention as an application’s lifecycle state and they are as follows.
- Not Running State: Either an application is not yet launched or it has been closed or shut down by the system
- Inactive State: Inactivity occurs while the app is leaving or entering into its active state and it is not yet ready to receive user inputs or events despite running in the foreground. It means that the application remains inactive at this state.
- Active: It indicates that the app is running in the foreground and accepting events from the users and this is the normal mode for foreground users and the accessible user interface.
- Background: The user interface of the application will be hidden in this state but it continues to run in the background of the iOS system. It passes through this state before the application is suspended.
- Suspended: The application will be in the background but it will not run the code during this state. It stays in “my memory”. The system can remove apps in this suspended state without any alerts under low memory conditions.
- What framework will be used to develop the application’s user interface for the iOS platform?
iOS is using the UIKit framework for developing the application’s user interface and it offers event handling, windows, drawing model, views, and controls that are designed especially for a touch screen interface.
- Explain about deep linking in iOS
Deep links are used to send links directly to an app instead of a website or store using URL (Uniform Resource Locator) or universal links.
The URL scheme is the popular and well-known method of sending deep links but Universal Links is the new approach of Apple iOS for connecting the web page to the app under the same link.
Deep Links are involved not only to create a clickable link that opens up the app but also a smart one for navigating to the resource that the user wants.
Users are directed straight to in-app locations using these links which saves them the time and effort of finding those pages for improving the User Experience effectively.
- Define GCD
GCD is the acronym for Grand Central Dispatch which means a low-level API to allows users to run concurrent tasks by managing threads in the background.
It is the solution of Apple to develop concurrency and parallelism into iOS apps for working on multiple background tasks in the background without affecting the main thread.
It was launched in iOS 4 to avoid the complicated process of a serial execution of tasks.
- Explain ARC
ARC stands for Automatic Reference Counting which is used to manage apps memory usage in the Swift Programming Language.
It initializes and deinitializes system resources for releasing memory allocated by a class instance when it no longer requires it.
ARC manages the properties, constants, and variables that are currently referred to in each class instance.
ARC will not deallocate instances when there is at least one active reference to an instance.
The use of ARC concepts is significant for iOS app development.
- What are the important functions of ARC?
The functions of ARC are as follows
- ARC creates new class instances using init() and allocates a chunk of memory to save the information.
- Memory saves information about the instance type and its values.
- ARC automatically frees up memory by calling deinit() as soon as the class instance is no longer needed.
- ARC ensures that deinit() is only implemented to those instances that are unused by tracking properties, variables, and constants of the current referring classes.
- What are the programming languages used in iOS app development?
- Explain the roles of SpriteKit and SceneKit frameworks for game development
SpriteKit framework is designed to make it easier and faster for game developers for creating animated 2D assets or objects in casual games.
The developer can draw shapes, images, texts, and videos in 2D with the SpriteKit framework.
SceneKit is an iOS framework inherited from OS X that helps in creating 3D graphics.
The developer can build 3D animated scenes and effects for iOS games and apps with the SceneKit framework.
- Differentiate Assign and Retain keywords
Assign: A reference is created from one object to another without increasing the source’s retain count with the Assign keyword.
It is the number that keeps track of how many objects are “holding onto” another object. It will not copy or retain the value but assigns it directly to the instance variable.
if (object != object)
object = nil;
object = object;
In this example, the assigned keyword will generate a setter that assigns the value to the instance variable directly instead of copying or retaining it.
Retain: The developer can create a reference from one object to another and increase the retain count of the source object using this “retain” method.
if (object != object)
object = nil;
object = [object retain];
In this example, the object is prevented from being deallocated until the user uses it with a retain message.
- Explain iBeacons
iBeacon.com identifies iBeacon as Apple’s technology standard that enables mobile apps to listen for signals from beacons in the physical environment to react accordingly.
iBeacon technology enables mobile apps of Apple to understand its position on a micro-local scale to deliver hyper-contextual content to users according to the location.
The communication technology underlying here is Bluetooth Low Energy.
- Differentiate strong, weak, read-only, and copy attributes
There are some attributes in iOS to define how memory for that property can be managed and they are strong, weak, read-only, and copy.
Strong denotes that the reference count will be increased and the reference to it will be managed through the life of the object.
A weak attribute denotes that the user is pointing to an object but not increasing its reference count.
It is often used when creating a parent-child relationship. The parent has a strong reference to the child while the child has a weak reference to the parent.
It will be in three cases like every time used on var, every time used on an optional type, and automatically changes to nil.
A Read-only attribute is used to set the property initially but then it can’t be changed.
Copy attribute is used to copy the value of the object once it is created and it also prevents the value from changing.
- Explain Completion Handler
It can be seen in Apple’s APIs like dataTaskWithRequest as pretty handy in writing codes.
The completion handlers are taking a chunk of code with three major arguments namely, NSData?, NSURLResponse?, and NSError that returns nothing: Void and it is a closure.
for i in 0….50
if i == 25
if let url = URL(string: “https:/softlogicsys.in”)
(data, response, error) in
print(“I =”, i)
let newInstance = CompletionHandler()
Here, all the numbers are printed in the console as the code runs but the received response will be printed only after all the numbers have been printed.
- Differentiate the frame and the bounds
Prioritize usability for breaking down the designing process in the following four steps
- Think like a user and design the UX (User Experience)
- Remember that users are not demographics but people
- Consider all the situations that could be useful when promoting the app.
- Continue to work on the utilities of the app even after deployment.
- What is Swift and what are the important features of Swift?
Swift is the widely used programming language created by Apple. Swift has a leading position in iOS development over other programming languages like Objective-C.
It is specifically created for developing apps for Apple Operating System (iOS). It implements all the features of modern programming languages like type inference, generics, and other higher-order functionalities.
Swift is compatible with macOS, iOS, tvOS, and watchOS and the features of Swift are as follows:
- Safety: Swift is an efficient way to write programs and checking code before it is used in production is very essential. It removes any unsafe code before it is used in production.
- Simple Syntax: The syntax of swift is simple and easy to use just as developers can expect it. The features of Swift allow developers to write more impressive code.
- Readability: Swift is easier to read and write as it has a simple syntax. It is easy for developers to write Swift code as it is more similar to basic English and enables them to spend less time looking for complex and problematic code
- Multiplatform Support: Swift is compatible with iOS, watchOS, tvOS, and macOS and the developers can be able to develop apps that work on all major operating systems.
- Open-source: Swift is developed in swift.org as an open-source framework. The technology must be open to all as swift supports all Apple platforms to make programming easier.
- Compatible with Objective-C: Swift is enabling developers to import frameworks from Objective-C using the Swift Syntax. The developers can utilize libraries and classes to Swift codes from Objective-C.
- Explain generics in Swift along with its usage
The main feature of Swift is generics and many of the libraries in Swift is written in Generic Code.
The arrays and dictionaries of Swift constitute generic collections and the generic code enables the developers to create reusable, flexible functions, and types for working with any data type.
The developer can create code that doesn’t get too particular about underlying data types which leads to cleaner and more efficient codes.
func Swapping(x: inout Int, y: inout Int)
let temp = x
x = y
y = temp
var num1 = 20
var num2 = 40
print(“Before Swapping: \(num1) and \(num2)”)
Swapping(x: &num1, y: &num2)
print(“After Swapping: \(num1) and \(num2)
Before Swapping: 20 and 40
After Swapping: 40 and 20
- Explain the @dynamic and @synthesize commands in Objective-C
@Synthesize: It is the command that generates setter and getter methods within the property and performs in conjunction. It creates a variable with the same name as the target of set/get by default as follows
@property (nonatomic, retain) NSButton *someButton;
It is easy to specify a member variable as it get / set the target as follows
@property (nonatomic, retain) NSButton *someButton;
@synthesize someButton = _homeButton;
In this example, the set/get method points to the member variable _homeButton.
@dynamic: It tells the compiler that the getter and setter methods are not implemented within the class itself but elsewhere like the superclass or that they will be available at runtime.
@property (nonatomic, retain) IBOutlet NSButton *someButton;
The iPhone users are rising rapidly and it makes a promising career for the learners around the world.
Check out the iOS Interview Questions and Answers that have been prepared meticulously by gathering them from top companies.
Hope it will be useful for you to prepare for your technical rounds confidently.