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Top 50 iOS Interview Questions and Answers

29/Sep/2020 | 10 minutes to read

QFLES is listing some Essential iOS Interview Questions and Answers for Freshers and mid level of Experienced Professionals prepared by Industry Experts. All answers for these iOS interview questions are given based on standard documents and tried to explain in simple and easiest way.


Best Answers to iOS Interview Questions


1. What is @dynamic in Objective-C? 

@dynamic tells the compiler that getter and setter methods are not implemented by itself , these are implemented by superclass or will be provided at run time.

2. What is the Difference Between ViewDidLoad and ViewDidAppear?

ViewDidLoad is called after the controller's view is loaded into the memory. It does not depend on the view hierarchy that it is loaded from the nib or loaded programmatically in the loadView Method. Important thing is that it is loaded only once when the view controller is loaded in the memory.
ViewDidAppear is used to notify the controller that the view is loaded in the Hierarchy. It can be called multiple times during the life cycle of viewcontroller.

3. Explain the Difference between Array and Ns Array.

An array can hold one type of data while nsArray can hold multiple type of data. An array is a value type while NSArray is an immutable reference type.

4. Explain Difference between class and structure.

There are some differences between classes and structures.

  • Class is a reference type while the structure is a value type.
  • Class can inherit from another class while structure can not inherit from another structure.
  • Class has typecasting property while structure does not have.

5. How can we achieve concurrency in iOS in better Ways?

We have 3 ways to achieve Concurrency in iOS as below.

  • Dispatch queue - it is an abstraction layer on the top of the gcd Queue which allows you to perform various tasks synchronously or asynchronously in our iOS application. Tasks are always executed in the order they are added to the queue.
  • Operation queue - Simply put, Operation is a wrapper around some work that we would like to execute. Speaking more formally it is an abstract class that when subclassed will perform a given task for us. It’s built atop of the Grand Central Dispatch, which allows us to focus less on details of how concurrent execution will be done and more on the implementation of our business logic. Operations can help us with tasks that require more time to complete such as network calls or data processing.
  • Threads - An application is not functional unless the code we wrote is executed. A thread is nothing more than a context in which commands are executed. It's a line or thread of execution through the application we created. The stack trace of a thread illustrates this. Each frame of the stack trace is a command that is being executed by the thread.  Threads have many more properties such as unique identifiers such a stack and a collection of register. 
    Every application has a minimum one thread that is the main thread, the application starts its life from the main thread. in the list of threads in the debug navigator the topmost thread is the main thread.

    Debug navigator shows us how the application uses multiple threads to carry out its task. We can say that the application is multi threaded when it uses more than one thread.  multithreading is used to increase the performance of the application.when work is scheduled on multiple threads more work can be performed parallel.

6. Describe the best way to pass data between view controller. 

    You can pass the data between view controller as below.
  • forward ( using segue)
  • backward (using a protocol and delegate)

7. Name the superclass for all the view controllers.

UiViewController class is the superclass of all the view controller objects.

8. What are the higher Order functions in swift?

These below are the higher-order functions in swift.

  • Map
  • Reduce
  • Sort
  • Filter

9. Name the mechanism iOS support for multithreading.

  • NSOperationqueue - it allows the pool of thread to be created,which is used to create NsOperations in parallel
  • NSThread - it creates low-level threads that can be started using the start method.

10. Difference between Explicit and implicit.

The type of the variable is automatically assigned to the variable is known as an implicit variable.
For example, Var a = 10 When we initialize the variable with a defined data type it is known as an explicitly declared variable.
For example, Var a:Int = 10

11. Closures in swift?

Closures are self-contained blocks of functionality that can be passed around and used in your code.
In swift mostly we used functions using the keyword Func but there is another type of function named a closure that can be used without using Func keyword and function name. The closure is having some similarities with functions like Accept the parameter and return the values and contains set of the statement which executed after you call it and can be assigned to the variable or constant as per our requirement.
Why we use closure?
Completion Block - It helps to notify you when some task has finished his execution.

12. What is the difference between Cocoa and cocoa-touch?

Cocoa is commonly referred to as the combination of the Foundation and AppKit frameworks, while Cocoa Touch is the combination of the Foundation and UIKit frameworks.
Cocoa is for Mac development and Cocoa Touch is for iOS development.
If something is only in Cocoa, you can’t use it on iOS, and if something is only in Cocoa Touch, you can’t use it on Mac OS X.

12. What is the difference between frame and Bound.

14. What are the Benefits of Realm?

Realm has following benefits.

  • It is an open source Database Framework.
  • You have to implement it from scratch.
  • Zero copy object store.
  • It is fast too.
  • No cost, free of charges.
  • There is no limit to data storing.
  • Availability in both language Objective-C and swift.

15. Why do we use Defer? explain it.

defer keyword provides a block of code which is executed at the end of the current scope, method, or loop exiting. for example,


                func deferExample(){
                defer{
                Print (“leaving”)
                }
                Print(“are u feeling well”)
                }
                Output :
                Are u feeling well 
                Leaving 

16. What is the difference between synchronous and asynchronous tasks? 

In short, we can explain it as synchronous waits until the task has completed and asynchronous completes its task in the background and notify you when the task completed.

17. What are the advantages in swift?

Swift comes with many advantages as below.

  • Readability - there is no doubt to say that because of its clean syntax it is easy to read and write.
  • Maintenance - Objective-C having a dependency on c while swift is not having any dependency.
  • Speed - swift provide various speed advantages during the development. for example, a complex object sort will run 3.9x faster than implementing the same algorithm in python .
  • Support dynamic libraries
  • Open-source.
  • Optional type - which make it crash resistance. 
  • Type safe language.
  • Support pattern matching.

18. What is a lazy property in swift?

Lazy property is a property whose initial value is not calculated until the first time it is used.We can indicate a lazy property with the help of a lazy modifier. Lazy properties are useful when the initial value for a property is dependent on outside factors.

19. What is UiKit in iOS?

Uikit is the framework, You will use it at the time of developing an iOS Application. Core Components of an iOS application are defined in it like label, button, views, navigation controller.

20. What is the foundation?

Foundation is a framework work as a bread and butter in the toolbox of an iOS Developer.which provide a large number of building blocks and Nsobject root class. It provides the NSObject root class and a large number of fundamental building blocks for iOS development. It accesses essential data types, operating system service, and collections to the base layer of the functionality of the application.

21. What is a Singleton Pattern?

It is the most commonly used design pattern in iOS and it is easiest to use. Singleton pattern we only used to create a single instance of a class, in case of a singleton pattern a class can have only one instance.

22. What is spritekit?

It is the general-purpose framework for drawing the objects in 2D, objects are text, images, video. It helps to create games and other graphics-intensive apps.

23. What is scenekit?

It is also a framework that helps to create 3D games. Add 3D content to the application. easily add Physics simulation, Animation, particle effect. realistic physically based rendering.

24. Name the Json framework supported in iOS Application Development?

SbJson framework is supported in iOS Application Development.  It is the Strict JSON Parser and generator for Objective-C.   It helps to add categories to the existing objective objects for a super simple Interface. More flexible Apis is also provided for added control.

25. Which object is created by the UIApplicationMain function at app launch time?

An app delegate object is created by the UIApplicationMain function at app launch time. App delegate object's job is to handle state transitions within the app.

26. What is the Use of UIWindow in iOS Application.

UiWindow Object coordinates the presentation of one or more Views on the screen. 

27. What is the Difference between Bundle Id And App Id? 

An app id is a two-part string that is used to identify that one or more apps are from a single development team. The string consists of team id and bundle id. a dot (.) is used to separate both parts. one part is team id and another part is bundle id. Team id is supplied by apple and it is unique to a specific development team while the bundle id search string is supplied by you. The bundle id is used to uniquely identify your app. you can easily register, delete, and modify it. we need the bundle id before we assign the capabilities with the Bundle id capabilities resources or when we create a provisioning profile for the application with the profile resource.

28. Explain About the Atomic And Non-Atomic Properties.

Atomic. It is the default behavior. here only one thread can access the variable that’s why it is threaded safe.but it is slow in performance. Non-Atomic. non-atomic means multiple threads can access the variable that’s why it is not threaded safe but its performance is fast.

29. Where can we test the iOS application if we don’t have an Apple Device?

If we don’t have an Apple Device we can test the application on a simulator.

30. Explain the Execution States for iOS Application.

iOS application has the following execution states.

  • Not Running - In this condition the application is not launched or Terminated.
  • Suspended - App is still in memory.
  • BackGround - App is running in the background. 
  • InActive - transition state, App is running but did not receive any user event because of call and message. 
  • Active - App is on-screen and in running state.

31. What is the difference between let and var?

We use let Keyword in swift to declare a constant while var keyword is used to declare a variable. The Basic difference between both of them is that value of a variable can be changed after it set but the value of constant does not change.


                Ex. let maximumNumber = 10 
                Var maximumNumber = 0  

                Here we can change the value of var many times but the value for let is fixed and it can not
                be changed.

32. What is plist in iOS? 

An information property list, initially it was developed by apple to use in iPhone devices and later spread to other applications. it is an XML file. We can simply use a text editor to translate it and it is very tricky to handle because if a single character misplaces makes it non-usable.

33. What is Delegate in swift? 

It is a Designing Pattern, It allows one object to send a message to another object when any particular event takes place. 
Imagine an object A calls object B to perform any action and as the actions completed by B then A should know that the action has been completed by B now A can take necessary action. This can be achieved by Delegate. 

34. What is the use of (optional)? mark and (exclamation)! mark?

An optional is a type in swift which can have a value or no value. While (exclamation) ! is used to force unwrap the optional value, sometimes we don’t have a value in the optional variable so it is not safe until we are sure that the variable is having a value or not.

35. What are the Ui elements in iOS? 

UI elements are the elements that we can see in our Application. Some of the elements respond to user Actions such as labels, buttons, text fields, and other information such as images.

36. What is optional Binding? 

Optional Binding is used to find out whether an optional contains a value or not if the value is available then hold it in a constant or in a variable with the help of if let. constant and variable created in the if statement available only in the if block.

37. What is optional chaining? 

It is the process for querying And calling properties, subscripts, and methods on an optional which may currently be nil. If the optional the property method, value or subscript call succeeds. In case the optional is All property, methods or subscript call returns nil. If multiple queries chain together and one of the links in the chain is nil then the entire chain fails gracefully. 

38. Explain the Design pattern used in iOS application development? 

Following design patterns are used commonly in iOS development.

  • Creational design pattern - Singleton
  • Structural design pattern Decorator - facade, adapter
  • Behavioral design pattern - observer and memento

39. What are the various ways to unwrap an optional value in swift.

  • Guard statement (safe way)
  • Optional Binding 
  • Optional Pattern 
  • Forced unwrapping
  • Optional chaining

40. Name the control transfer statement used in swift? 

Continue, Break, Return, fallthrough

41. What is core data in iOS? 

Core data is a framework provided by Apple to save, fetch, delete, and modify the data. it is used to manage the model layer object in the application. Core data is not a database. With the help of core data’s data model editor, we can easily define our data type and relationship, and we can also generate the respective class definitions. 

42. What is the difference between static Binding And Dynamic Binding? 

Static Binding: Event Occurs at compile time. Execution is fast in static binding. There is early Binding
Dynamic Binding: Event occurs At run time  Execution is slow in dynamic binding There is late Binding. 
Static Binding: It is resolved at “Compile-time” Method overloading is an example of static binding.
Dynamic Binding: It is virtual binding resolved at a “Run Time”. Method overriding is an example of Dynamic Binding.

43. What is Entity Inheritance? 

Entity inheritance works similarly to class inheritance. Sometimes we have a large number of entities and some of them are similar. In this case, we can factor the common properties into the supernity and we can say it is the parent entity. It helps to reduce the work when subentities inherit the parent entity.
Example: As we defined an entity as a person name with attributes first Name and Last Name and sub-entity employee and customer can inherit those attributes.

44. Multithreading can be possible in core Data?

Yes, multithreading can be possible in core Data.

45. What is Deinit? 

DeInitializer is called immediately before an instance of a class is deallocated. The keyword used by DeInitializer is deinit similar to the initializer used with Init keyword. Important thing is that DeInitializer is only available for class type not for structure type.

46 Why we use reuse identifier in the UITableViewCell constructor?

If we do not set a reusableidentifier then Tableview will force to allocate new uitableviewcell.
The use of ReuseIdentifier is that it is used to group similar rows content in an uitableView but they can differ in data content.

47. What are generics and which problem do they solve?

Generic code Helps you to write reusable, flexible functions And type that can work with any type. Generics are the most powerful feature of the Swift language. 

48. What is @synthesize in Objective-C? 

@synthesize tells the compiler to generate the setter and getter property for the variable.

49. What is ARC? 

ARC Automatic Reference Count is used in iOS Application to track and manage your application memory. And Arc automatically manages the memory it automatically releases the memory held by the class instance when it is no longer needed. 

50. What is Live Rendering?  

The Attributes IBDesignable and IBInspectable used by the Interface builder, which helps to directly render the element in the interface builder, for specific attributes like colors/widths/shadows and border to be configured live in the storyboard.

Some General Interview Questions for iOS

1. How much will you rate your self in iOS?

When you attend an interview, Interviewer may ask you to rate your self in specific Technology like iOS, So It's depend on your knowledge and work experience in iOS.

2. What challenges did you face while working on iOS?

This question may be specific to your technology and completely depends on your past work experience. So you need to just explain the challenges you faced related to iOS in your Project.

3. What was your role in last Project related to iOS?

It's based on your role and responsibilities assigned to you and what functionality you implemented using iOS in your project. This question is generally asked in every interview.

4. How much experience do you have in iOS?

Here you can tell about your overall work experience on iOS.

5. Have you done any iOS Certification or Training?

It's depend on candidate like you have done any iOS training or certification. Certifications or trainings are not essential but good to have.

Conclusion

We have covered some frequently asked iOS Interview Questions and Answers to help you for your Interview. All these Essential iOS Interview Questions are targeted for mid level of experienced Professionals and freshers.
While attending any iOS Interview if you face any difficulty to answer any question please write to us at info@qfles.com. Our IT Expert team will find the best answer and will update on portal. In case if we find any new iOS questions, we will update the same here.

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