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What is singleton class? Pros and Cons.

Ans : The singleton pattern guarantees that only one instance of a class is instantiated. We are aware of following singleton classes,

// Shared URL Session
let sharedURLSession = URLSession.shared

// Default File Manager
let defaultFileManager = FileManager.default

// Standard User Defaults
let standardUserDefaults = UserDefaults.standard

Above classes are instantiate only once, if we try to create another instance, it returns also first instance. So through out application, only one instance is generated for singleton class.

Custom Singleton Class example: 

Different ways : 
1. When we need default configuration  (Using closure) : 

class NetworkManager {

    // MARK: - Properties

    private static var sharedNetworkManager: NetworkManager = {
        let networkManager = NetworkManager(baseURL: API.baseURL)
       // ....
       // Configuration
       // ....         return networkManager

    let baseURL: URL(string : "")

    private init(baseURL: URL) {
        self.baseURL = baseURL

    class func shared() -> NetworkManager {
        return sharedNetworkManager

Use  : NetworkManager.shared().baseURL 

2. Using only single statement : 

class mpiosapp {
    static var shared = mpiosapp()
    let baseURL: URL(string : "")
    private init(){}


Use : mpiosapp.shared.baseURL

Q : When static variable will be deinitialized?
A : Static variable will be deinit when program or app stops working.

Disadvantage of Singleton :

1. One of the main disadvantages of singletons is that they make unit testing very hard. They introduce global state to the application. The problem is that you cannot completely isolate classes dependent on singletons. When you are trying to test such a class, you inevitably test the Singleton as well. When unit testing, you want the class to be as loosely coupled with other classes as possible and all the dependencies of the class should be ideally provided externally (either by constructor or setters), so they can be easily mocked. Unfortunately, that is not possible with singletons as they introduce tight coupling and the class retrieves the instance on its own. But it gets even worse. The global state of stateful singletons is preserved between test cases.

2. It is static so memory will not be freeze until app will killed. It has it's own creation time and its own lifecycle.

3. Singletons create hidden dependencies. As the Singleton is readily available throughout the code base, it can be overused. Moreover, since its reference is not completely transparent while passing to different methods, it becomes difficult to track. (To solve that we should pass singleton object variable as parameter and this way is called dependency injection).

Write program of fast enumeration in swift

Ans : 

  1. enum Beverage: CaseIterable {
  2. case coffee, tea, juice
  3. }
  4. let numberOfChoices = Beverage.allCases.count
  5. print("\(numberOfChoices) beverages available")

  1. for beverage in Beverage.allCases {
  2. print(beverage)
  3. }

Write a program to convert character array to string in Swift


Convert character array to string :

let characterArray: [Character] = ["i", "o", "s", "i", "q", "a", ".", "c", "o", "m"]
let string = String(characterArray)


// prints ""

Convert string array to String : 

let stringArray = ["iosiqa", ".", "com"]
let characterArray = stringArray.flatMap { String.CharacterView($0) }
//let characterArray = stringArray.flatMap { $0.characters } // also works
let string = String(characterArray)


// prints ""

How many ways to pass data from one view controller to another view controller?

Ans :

Through following methods, we can pass data from one viewcontroller to another viewcontroller.

1. Using segue (PrepareForSegue) method,
2. Delegate
3. Setting variable

Let's assume we have 2 viewcontroller named VC1 and VC2. We want to pass data from VC1 to VC2 and some times VC2 to VC1. For both cases, we can use segue to pass data. But in general practise, to pass data from VC1 to VC2 (Forward data passing), we use segue and from VC2 to VC1(Backward data passing), we use protocol - delegate way.

1. Using segue (PrepareForSegue) steps :  VC1 --->  VC2

  •   Draw segue from VC1 to VC2 and give identifier string 'InputVCToDisplayVC'  to segue.
  •   Take variable in VC2.swift as :

           var fullName : String?

  •    In VC1, write following code in PrepareForSegue method,

       override func prepare(for segue: UIStoryboardSegue, sender: Any?) {
        if(segue.identifier == "InputVCToDisplayVC"){
let displayVC = segue.destination as! DisplayViewController
displayVC.fullName = nameTextField.text

  •       In VC2, viewDidLoad, we can get  value of fullName and access it.
2. Using delegate steps :    VC2  --->  VC1

Import UIKit

class VC1: UIViewController, VC2Delegate {

       override func viewDidLoad() {

       func doSomething(data : String) {

      override func prepare(for segue: UIStoryboardSegue, sender: Any?) {
      if(segue.identifier == "InputVCToDisplayVC"){
      let displayVC = segue.destination as! DisplayViewController
      displayVC.delegate = self


Import UIKit
protocol VC2Delegate {
      func doSomething(data : String) 

class VC2 : UIViewController {
      var fullName : String?
      weak var delegate : VC2Delegate?
      override func viewDidLoad() {

     @IBAction func btnPassToVC1_TouchUpInside(_ sender : UIButton) {
              if let delegate = delegate {
                     delegate.doSomething(data: btn.titleLabel.text)       

In above, method we make custom protocol and create its object as delegate.We set delegate reference to VC1. So on VC2 button pressed event, VC1 doSomething method will be called.

3. Setting Variable steps :     VC1   --->  VC2

  • Take variable in VC2.swift as :  
           var fullName : String?

  • On buttonPressed action of VC1, write code as :
           let vc2 : VC2 =  UIStoryboard(name: "Main", bundle:                                                                                                        nil).instantiateViewController(withIdentifier: "VC2_ID") as! VC2
           vc2.fullName = "iOS iQA"
           self.present(vc2, animated: true, completion: nil)

Note : There are also persistent storage, by which we can store data and access that data any where in our project. But this is not correct answer or solution for above question or problem. We can store data in NSUserDefault, PList, Coredata, SQLite, KeyChain Access, File as persistent storage.


Write a program to remove duplicate elements from array

Ans : 

    Method - 1 : 

    extension Array where Element: Equatable {
        mutating func removeDuplicates() {
            var result = [Element]()
            for value in self {
                if !result.contains(value) {
            self = result

   Use : 

var faa = [3, 0, 1, 0, 3, 1, 2, 0, 1, 2]

output = [3,0,1,2]

Method - 2

 let unique = Array(Set(originals))

Set is collection of unique and unordered elements. So we have converted array to set here to remove duplicate. Output array does not have same order as original.

Write a program to find first 10 prime numbers

Ans : 

     let N = 10
    let maxP = 1000
    var isPrime: [Bool] = []
    var primes: [Int] = []
     for i in 0...maxP {
    isPrime[0] = false
    isPrime[1] = false
     for i in 2...maxP {
       if isPrime[i] == true {
          var j = i*i
          while j <= maxP 
               isPrime[j] = false
               j += i
    for i in 0..<N {

Delete record from Core-data


Deleting a record from a persistent store involves three steps:

  1. Fetch the record that needs to be deleted
  2. Mark the record for deletion
  3. Save the changes
Code : 

let fetchRequest: NSFetchRequest<Profile> = Profile.fetchRequest()
fetchRequest.predicate = Predicate.init(format: "profileID==\(withID)")
let objects = try! context.fetch(fetchRequest) // Step 1
for obj in objects {
    context.delete(obj)  // Step 2

do {
    try //  <- remember to save data // Step 3
} catch {
    // Do something... 

Difference between Core Data and Sqlite

Ans : Core-data is not database that we understand. Sqlite is Database. 

Lets have a look over difference between Core-data and Sqlite (or Database) :

Core Data:

  1. Primary function is graph management (although reading and writing to disk is an important supporting feature).
  2. Operates on objects stored in memory (although they can be lazily loaded from disk).
  3. Works with fully-fledged objects that self-manage a lot of their behavior and can be sub classed and customized for further behaviors.
  4. Non-transactional, single threaded, single user (unless you create an entire abstraction around Core Data which provides these things).
  5. Only operates in memory.
  6. Requires a save process.
  7. Can create millions of new objects in-memory very quickly (although saving these objects will be slow).
  8. Leaves data constraints to the business logic side of the program.

Database or SQLite:

  1. Primary function is storing and fetching data.
  2. Operates on data stored on disk (or minimally and incrementally loaded).
  3. Stores "dumb" data.
  4. Can be transactional, thread-safe, multi-user.
  5. Can drop tables and edit data without loading into memory.
  6. Perpetually saved to disk (and often crash resilient).
  7. Can be slow to create millions of new rows.
  8. Offers data constraints like "unique" keys.
Credit :

difference between coredata and sqlite

Advantage of Core-data : 

  • Much better memory management. With a plist you must load the entire thing into memory; with Core Data only the objects you're currently using need to be loaded. Also, once objects are loaded, they're normally placeholder "fault" objects whose property data doesn't load until you need it.
  • Related to the above, when you have changes, you can save only the changed objects, not the entire data set.
  • You can read/write your model objects directly instead of converting them to/from something like an Dictionary.
  • Built-in sorting of objects when you fetch them from the data store.
  • Rich system of predicates for searching your data set for objects of interest.
  • Relationships between entities are handled directly, as properties on the related objects. With a plist you would need to do something like store an object ID for a relationship, and then look up the related object.
  • Optional automatic validation of property values.
More about Core-data : 

  • CoreData isn't a Database. It's an object persistence layer. There is no concept of primary keys or foreign keys in CoreData.
  • If you want to establish a relationship between two entities. You'll define a relationship, CoreData takes care of how that relationship is stored.
  • Select an entity, use plus button at the bottom of the entities attributes list, select add relationship, select the destination entity from the dropdown.
  • Select the destination entity and define an inverse relationship in the same way.

What is lazy property?

Ans : 

When to use lazy initialization is when the initial value for a property is not known until after the object is initialized.

For example, if you have a Person class and a personalizedGreeting property. The personalizedGreeting property can be lazily instantiated after the object is created so it can contain the name of the person.

class Person {      
  var name: String
  lazy var personalizedGreeting: String = {
    return "Hello, \(!"
  init(name: String) { = name

When you initialize a person, their personal greeting hasn’t been created yet:

let person = Person(name: "John Doe") // person.personalizedGreeting is nil But when you attempt to print out the personalized greeting, it’s calculated on-the-fly:


Bonus Tip : You do need to declare your lazy property using the var keyword, not the let keyword, because constants must always have a value before initialization completes.

Benefit of lazy property increase performance in terms of speed.

Explain defer keyword or defer statement in Swift

Ans : Defer block is executed just before return statement or exit of block. Let's have a look to following chunk of code for better understanding

      func defer()  { 
          var value: String? 
         defer { 
              if let v = value { 
                   print("Ending execution of \(v)")
      value = "defer function" 

So printing sequence is like following :

Ending execution of defer function

So in short defer block will be called lastly in the current block. It will be executed in any case like before return, before the break, before throw exception. Yes, if exception comes, then also defer block will be called before program crash or going to exception.

If there is multiple defer block in same method, Then defer block will execute from down to up side.

Write a program to find missing number of array of 1 to n.

Ans : 

      import Foundation
        let arr = [1,2,4,5,8,6,9,7]
        let n = arr.count
        let total = ((n + 1)*(n + 2))/2
        var missingNum = total
        for item in arr {
            missingNum = missingNum - item