iOS interview questions

Wrapping and UnWrapping in Swift

Ans :

An Optional is basically means that the variable can be nil

i.e:

var itCanBeNil: String? = “You can make me nil”
itCanBeNil = nil
Question Mark(?) indicates the fact that itCanBeNil Variable can be nil

Let’s try to make nil a variable which declared without Optional ?

i.e

var itCantBeNil: String = “You can’t make me nil”
itCantBeNil = nil //Compiler Throws Exception as “Nil cannot be assigned to type ‘String’ ‘”

If we need to get the value from the Variable if it is Optional, we need to unwrap it,
unwrapping is nothing but just putting an exclamation mark at the end

i.e

var itCanBeNil: String? = “Unwrap me”
print(itCanBeNil) //Output: Optional(Unwrap me)
print(itCanBeNil!) //Output: Unwrap me

We can also declare optionals to automatically unwrap by using exclamation mark instead of a question mark

i.e

var itCanBeNil: String! = “Automatic unwrap”

print(itCanBeNil) //No Wrapping needed

What method is called after tap back button in ios?

Ans :

When you go back the following methods will be called:

Notifies the view controller that its view is about to be added to a view hierarchy :

override func viewWillAppear(_ animated: Bool) {
super.viewWillAppear(animated)
}

Notifies the view controller that its view was added to a view hierarchy :

override func viewDidAppear(_ animated: Bool) {
super.viewDidAppear(animated)

}

What is stored property and computed property?

Ans :

1. Stored Property :

A stored property is a constant or variable that is stored as part of an instance of a particular class or structure. Stored properties can be either variable stored properties (introduced by the var keyword) or constant stored properties (introduced by the let keyword).

struct FixedLengthRange {
var firstValue: Int
let length: Int
}

So here, firstValue is mutable (var) storedProperty that can have value and change value,
length is immutable (let) computedProperty that can have value but can not be changed.

We can add property observers to any stored properties you define, except for lazy stored properties.

You have the option to define either or both of these observers on a property:
willSet is called just before the value is stored. (newTotalSteps)
didSet is called immediately after the new value is stored. (oldValue)

2. Computed Property :

Classes, structures, and enumerations can define computed properties, which do not actually store a value. Instead, they provide a getter and an optional setter to retrieve and set other properties and values indirectly.

struct Point {
var x = 0.0, y = 0.0
}
struct Size {
var width = 0.0, height = 0.0
}
struct Rect {
var origin = Point()
var size = Size()
var center: Point {
get {
let centerX = origin.x + (size.width / 2)
let centerY = origin.y + (size.height / 2)
return Point(x: centerX, y: centerY)
}
set(newCenter) {
origin.x = newCenter.x - (size.width / 2)
origin.y = newCenter.y - (size.height / 2)
}
}
}

What is priority in constraints?

Ans :

Constant priority is a number to determine how important is that constraint. The number can range from 1 to 1000, the higher the number goes, the more important a constraint is. Lower priority in screen seen like dashed blue line.

This is useful when two constraint make conflict.

Example :

Let's take one UIView and place them in main view.

Set Leading and Trailing constraint is 40,40.
Set width and height is 240,240.

Now if iPhone SE is selected, then all goes fine. No red line will show about conflicting constraints due to iPhone SE width is 320. So if it takes 40,40 as leading and constraints, it will calculate view's width as 240. And we also set width constraint as 240. So there is no conflicting.

But if iPhone 8 is selected then view's width constraint is conflicting with leading, trailing constraints.
iPhone 8 width = 375 - 40 - 40 = 295 But we have set width constraint as 240. So here conflicting occurs.

Solution :

If we set width constraint priority 900 then its priority is lower than leading and trailing priority(1000) So width constraint will be ignored here. So when we do some auto layout and we feel conflicting constraint, we can lower priority of constraint according to requirement.

Device token will be changed or not if we uninstall and reinstall the app ?

Ans :

The Device token is change in the following conditions.

If the user restores backup data to a new device.
Reinstalls the App

So update to server from app delegate method didRegisterForRemoteNotificationsWithDeviceToken every time. (Just suggestion)

String Interpolation in Swift

Ans :

String interpolation means creating string from mix of constants, variables, literals, expressions.

let length:Float = 3.14
var breadth = 10
var myString = "Area of a rectangle is length*breadth"
print(myString)
myString = "\(myString) i.e. = \(length)*\(breadth)"
print(myString)

Output :

Area of a rectangle is length*breadth
Area of a rectangle is length*breadth i.e. = 3.14*10

Another way of string interpolcation in Swift :

Use the Swift String initializer:
String(format: <#String#>, arguments: <#[CVarArgType]#>)

For example: let stringFromNumber = String(format: "%.2f", number)

Swift 5.0 has super powerful string interpolation :

1. Normal custom String interpolation

Code for Example :

Using the new string interpolation system in Swift 5.0 we can extend String.StringInterpolation to add our own custom interpolations

extension String.StringInterpolation {
    mutating func appendInterpolation(_ number: Int) {
        let formatter = NumberFormatter()
        formatter.numberStyle = .spellOut

        if let result = formatter.string(from: number as NSNumber) {
            appendLiteral(result)
        }
    }
}

Use : print("My age is \(age).")
Output : My age is twenty nine.
Above custom interpolation tells to convert integer to word.

Amazing....hua.....

We can also write above function as following :

mutating func appendInterpolation(format number: Int) {

So, we can use like : print("Hi, I'm \(format: age)."). So there will be no confusion between default interpolation and custom interpolation.

2. String interpolation with parameters. :

mutating func appendInterpolation(linkedin: String) {
    appendLiteral("<a href=\"https://linkedin.com/in/\(linkedin)\">@\(linkedin)</a>")}

output :
"You should follow me on linkedin : <a href="https://linkedin.com/in/twostraws">@twostraws</a>.\n"

We can also pass multiple parameters.

3. We can pass auto closure as parameter :

extension String.StringInterpolation {

    mutating func appendInterpolation(_ values: [String], empty defaultValue: String) {

        if values.count == 0 {

            appendLiteral(defaultValue())

        } else {

            appendLiteral(values.joined(separator: ", "))

}

}

}

let names  = [String]()

print("Crew: \(names, empty: "None").")

Using @autoclosure means that we can use simple values or call complex functions for the default value. Here defaultValue() function called for set default value.

extension String.StringInterpolation {
    mutating func appendInterpolation(if condition: @autoclosure () -> Bool, _ literal: StringLiteralType) {
        guard condition() else { return }
        appendLiteral(literal)
    }
}

let amIiOSDeveloper = true
print("Swift rocks: \(if: amIiOSDeveloper, "(*)")")
print("Swift rocks \(amIiOSDeveloper ? "(*)" : "")")

Normally we use 2nd print option at above. But due to custom string interpolation, we can directly use 1st print option.

4. String interpolation with custom type (class or struct)

struct Employee {
    var name: String
    var designation: String
}

extension String.StringInterpolation {
    mutating func appendInterpolation(_ emp: Employee) {
        appendLiteral("I am \(amp.name) and my designation is \(amp.designation).")
    }
}

let manan = Employee(name: "Manan", designation: "Senior Software Engineer")
print("\(manan)")

Print("\(manan)") gives output as :

I am Manan and my designation is Senior Software Engineer.

if we use print(manan) then it gives class debug description because here no string interpolation used.

Awesome...
Thanks Swift 5.0...

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

Difference between Cocoa and Cocoa Touch

And :

Application Framework For
i) Cocoa is the application framework for Mac OS X.
ii) Cocoa Touch is the application framework for iPhone and iPod Touch.

Frameworks
i) Cocoa: Foundation and AppKit.
ii) Cocoa Touch: Foundation and UIKit

Absence of certain classes
Cocoa has NSHost and Cocoa Touch doesn't

API
i) Cocoa: All the classes used in Cocoa have the NS prefix Ex: NSTextField
ii) Cocoa Touch: classes used in Cocoa have the UI prefix Ex: UITextField

MVC patterns
i) Cocoa: Cocoa has multiple alternative design patterns – in addition to MVC
ii) Cocoa Touch: The iPhone SDK has a reinforced MVC system, which performs better than the default MVC in Cocoa

Other Differences
There Are also Differences In App Lifecycle, Sandboxing ,Memory Footprint

Difference between POP and OOP

Ans :

POP - Protocol oriented programming
OOP - Object oriented programming

Swift has both power. POP has more advanced features having object oriented feature itself.

Base class of different class or views

Ans :

Base class of

UITableView : UIScrollView : UIView : UIResponder : NSObject
UIButton : UIControl : UIView : UIResponder : NSObject
NSObject : It is top most super class in swift. NSObject class confirm NSObject protocol.

Q : Is it compulsory to mention NSObject in swift inheritance?
A : No. Any class which is not inheriting any other class have NSObject as super class by default.

How to handle error in Swift?

Ans :

Error Protocol is just a type for representing error values that can be thrown.

Lets declare our custom error enum by confirming Error protocol.

enum UserDetailError: Error {
case noValidName
case noValidAge
}

Now we make one function that throw our error type.

func userTest(age: Int, name: String) throws {

guard age > 0 else {
throw UserDetailError.noValidAge
}

guard name.count > 0 else {
throw UserDetailError.noValidName
}
}

In function signature, throws keyword is used and in function throw keyword is used to throw error.

Now we cal our function userTest that can throw error.

do{
try userTest(age: -1, name: "")
}
catch UserDetailError.noValidName
{
print("The name isn't valid")
}
catch UserDetailError.noValidAge
{
print("The age isn't valid")
}
catch let error {
print("Unspecified Error: \(error)")
}

Swift try, try? and try!

Swift try is the most basic way of dealing with functions that can throw errors. try is only used within a do-catch block. However, try? and try! can be used without it.
try? is used to handle errors by converting the error into an optional value. This way if an error occurs, the function would return a nil and we known Optionals can be nil in Swift. Hence for try? you can get rid of do-catch block.
try! is used to assert that the error won’t occur. Should be only used when you’re absolutely sure that the function won’t throw an error. Like try?, try! works without a do-catch block.

var t1 = try? Student(name: nil)

var t2 = try! Student(name: "Anupam")