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Showing posts with label Swift. Show all posts
Showing posts with label Swift. Show all posts

What is capture and capture list in closure?

Ans :

According to apple document :

Closures are self-contained blocks of functionality that can be passed around and used in your code. Closures in Swift are similar to blocks in C and Objective-C and to lambdas in other programming languages. 

Closures can capture and store references to any constants and variables from the context in which they are defined. This is known as closing over those constants and variables. Swift handles all of the memory management of capturing for you.

Closure is reference type

Capturelist is used to stop memory leakage.

Code for Example of memory leakage :

class Increment {
    var number = 0

     deinit {
    lazy var incrementNumber: (Int) -> () = { value in
        self.number += value

 do {
  let increment = Increment()


This will cause memory leak, the closure refers back to the object itself, it refers to self in order to increment the number, and that will create a reference cycle:

We have an object and the object has a stored property that refers to a closure.
That closure refers back to self (means Increment instance)

In above example, deinit should be called. But it never due to retain cycle created.

To Stop memory leakage we use capture list :

1. [unowned self]
2. [weak self]
3. [strong self] - Default


lazy var incrementNumber: (Int) -> () = { [unowned self] value in
        self.number += value


If I use [unowned self] there here less chance to crash. But if we use
let increment = Increment().incrementNumber(3)
then there will be more chance to be crashed. We can not immediately call incrementNumber method after object instantiated. Because when the stored property has returned, the object (increment instance) can be deallocated, nothing else is referring to it.


 let’s change [unowned self] to [weak self], that means that everywhere that self is accessed, we treat it as a weak property. While using weak, we should use optional self? to access property.

When the stored property has returned, if the object be deallocated, mean self is nil, then the number will not be incremented. This code will make it easy to handle if self is nil

So when no clue to what to use, we should use [weak self] as capture list.

What is Generics in Swift? Write a program to make and use generics.

Ans :

Apple doc says: Generic code enables you to write flexible, reusable functions and types that can work with any type. You can write code that avoids duplication by using generics in most cases.

Code for Example :

 let intArray = [1,2,3,4,5] // type int
 let stringArray = [“abhi”, “iOS”] // type string

If we want to print all values, we have to make 2 functions accepting integer array or string array.
But using Generic, we can make 1 method accepting generic type.

 func printAnyArray<T>(arr:[T]) { { print($0) }


 <T> after the function name represents a generic function. A generic function can work with whatever type. We can put whatever instead of T. T is placeholder type. We can use for dictionary like Dictionary<key,value> and for array like Array<element> .

Generic function with parameter type : 

func anotherMethod<T, U> (first: T, second: U) { print("first one called")}
func anotherMethod<T> (first: T, second: T) { print("second one called")}
anotherMethod(first: 123, second: "456"// T - Int, U - String
anotherMethod(first: 123, second: 456// T - Int

In above methods, we use different placeholder type in method1 and same placeholder type in method2. If out passing parameter types are different then 2nd method will be called, and if same then 1st method will be called.

Generic function with type constraints :

func middleVal<T>(array: [T]) -> T? {
  guard !array.isEmpty else { return nil }
  return array.sorted()[(array.count - 1) / 2]


Sometimes, we require specific types only i.e type which confirm comparable protocol

func middleVal<T: Comparable>(array: [T]) -> T? {
  guard !array.isEmpty else { return nil }
  return array.sorted()[(array.count - 1) / 2]


Above method will only accepts. int, float, double.

If you have any comment, question, or recommendation, feel free to post them in the comment section below!  

Json serialization and deserialization

Ans :  

JSON is a format that encodes objects in a string. Serialization means to convert an object into that string, and deserialization is its inverse operation (convert string -> object).
When transmitting data or storing them in a file, the data are required to be byte strings, but complex objects are seldom in this format. Serialization can convert these complex objects into byte strings for such use. After the byte strings are transmitted, the receiver will have to recover the original object from the byte string. This is known as deserialization.

import UIKit

var str = "Hello, playground"

// Starting decode -> json to class or object
// ------------- De-Serialization ----------------

let singleDict = """
    "foodName" : "Banana"
    "calories" : 100
""".data(using: .utf8)

class Food : Codable {
    let foodname : String
    let calories : Int
    init(foodname: String, calories: Int) {
        self.foodname = foodname
        self.calories = calories

let jsonDecoder = JSONDecoder()
do {
    let foodResult = try jsonDecoder.decode(Food.self, from: singleDict!)
} catch {
    print("failed to decode \(error.localizedDescription)")

// Starting encode -> class or object to json
// ------------- Serialization ----------------

let apple = Food(foodname: "apple", calories: 80)
let jsonEncoder = JSONEncoder()
jsonEncoder.outputFormatting = .prettyPrinted
do {
    let jsonData = try jsonEncoder.encode(apple)
    if let jsonString = String(data: jsonData, encoding: .utf8) {
} catch {


If you have any comment, question, or recommendation, feel free to post them in the comment section below!

What is subscript?

Ans : 

Subscripts are used to access information from a collection, sequence and a list in Classes, Structures and Enumerations without using a method.

These subscripts are used to store and retrieve the values with the help of index without the use of separate method.

To access elements via subscripts write one or more values between square brackets after the instance name.

For example: Array elements are accessed with the help of someArray[index] and its subsequent member elements in a Dictionary instance can be accessed as someDicitonary[key].

Code :

In coding, subscript is same as computed property.

 class monthsInYear {
 private var months = [“Jan”, “Feb”, “Mar”, “Apr”, “May”, “Jun”, “Jul”, "Aug",  "Sep" , "Oct", "Nov" , "Dec"
 subscript(index: Int) -> String {

 get {
       return months[index] // getter is mandatory
 set(newValue) {
       self.months[index] = newValue // setter is optional
 var p = monthsInYear()
 print(p[0]) // prints Jan
 p[0] = “Jan”
 print(p[0]) // prints Jan

If you have any comment, question, or recommendation, feel free to post them in the comment section below!

Meaning of ??, !!, !?, ?!

Ans : 

1. ?? is  Nil-Coalescing Operator. It is used to provide default value if value is nil.

let val = values["value1"] as? String ?? "default"

2. !!

struct StockFullData: Codable
           var ok : Bool?  
           var sn : StockName?

struct StockName : Codable
           let display_name : String?

Now Check :


Output :


-> See third print, where we have used !! for extract value. I am not sure it is correct answer because it is like !)! .

3. !? : am finding answer or We can make custom operator using this .
4. ?! : am finding answer or We can make custom operator using this .

If you have any comment, question, or recommendation, feel free to post them in the comment section below!

What is Operator overloading?

Ans : 
Operator overloading is the practice of adding new operators and modifying existing ones to do different things. Operators are those little symbols like +*, and /, and Swift uses them in a variety of ways depending on context – a string plus another string equals a combined string, for example, whereas an integer plus another integer equals a summed integer.
To create a new operator, try adding this to a playground:
infix operator **
That’s the exponentiation operator, designed to raise one number to the power of another. Normally we’d use the pow() function for that job, but with operator overloading we can make ** work instead.
Now you need to tell Swift what to do when it sees that operator. For example, when we write something like 2 ** 4 what does that mean?
Syntax of making operator : 
func **(lhs: Double, rhs: Double) -> Double {
    return pow(lhs, rhs)
Use : 
let result = 2 ** 4
We can specify associativity and a precedence group also but it is very deep level.
If you have any comment, question, or recommendation, feel free to post them in the comment section below!

What is meaning of _ in Swift func ?

Ans : 

The _ is used to define that the parameter is not named
If you have multiple _ it states that you do not need to name the parameters in your function call
func myFunc(name:String, _ age:String){       }

myFunc(Milo", "I'm a really old wizard")
If you do not use the underscore you would use 
myFunc(Milo, age: "I'm a really old wizard")
The _ is not necessary in function calls. It is just used to indicate that something does not to have a name.

Difference between normal function and function ending throw?

Ans : 

Function ending with throw can throw error but normal function can not throw error it can just return value.

Lets take example :

1. Custom error enum

enum ErrorsToThrow: Error {
    case fileNotFound
    case fileNotReadable
    case fileSizeIsTooHigh

2. Make function which can throw error

func readFiles(path:Stringthrows  ->String {
        if path == "" {
            throw ErrorsToThrow.fileNotFound
        return "Data from file"


3. Calling readFiles function
   do {
            let dataFromString = tryreadFiles(path: "")
        catch ErrorsToThrow.fileNotFound {
            print("error generated1")
        catch ErrorsToThrow.fileNotReadable {
            print("error generated2")
        catch ErrorsToThrow.fileSizeIsTooHigh {
            print("error generated3")
        catch {

Now, let's analysis.

1. If in #2, function is not ending with throws can not throw error. But our function can throw error.
2. In #3, we have used try?, so if readFiles function return nil instead of throwing error. So in our case print(dataFromString) statement executed and it will nil means printing nil.
3. If try! is written and if function throw error, then fatal error will be occured and program will be crashed as we ensure that error will not occur by putting ! .
4. So if we want to execute catch statement then we have to use only try . try always used with do-catch.
5. try? and try! does not need do-catch block.

Some truths about protocol

Q1 : Can structure confirm protocol?
A1 : Yes

Q2 : Can enumeration confirm protocol?
A2 : Yes

Q3 : Can we declare variable in protocol?
A3 : Yes -> It must be var and it must be read-Only or readAndWrite . Property declaration is like following : 

protocol someprotocol {
     var gettable : Int { get }
      var setAndGettable : Int { get set }

Q4 : Can we add function in enumeration?
A4 : Yes

Q5 : Can protocol has own init method?
A5 : Yes

Q6 : Can protocol inherit another protocol?
A6 : Yes
protocol someprotocol : anotherprotocol {


Q7 : Can we make class specific protocol?
A7 : You can limit protocol adoption to class types (and not structures or enumerations) by adding the AnyObject or class protocol to a protocol’s inheritance list.

protocol someprotocol : AnyObject, Someanotherprotocol {

Now someprotocol can only be confirmed by class-type. No strcture or enum type can confirm this protocol.

Q8. Can we declare optional protocol method in swift?
A8. Yes. In that protocol name and optional methods should be followed by @objc due to it consider as objective c code.

@objc protocol someprotocol {
        @objc optional func somemethod()
        @objc optional var someName : String { get set }

Q.9 Can protocol be fileprivate?
A9. Yes. Protocol can be fileprivate, private, public. Private and fileprivate protocol can be confirmed only within current file.

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).

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.