What is protocol? Why it is used?

Ans : Protocols are way to specify a set of methods that class has to implement if class want to work with this protocol. Protocol has 2 types of methods like Required type and Optional type methods.

1. If @required are tagged above methods, then these methods must be implemented on class which confirm this protocol.

2. If @optional are tagged above methods, then these methods is not necessary to implement on it.

Why it is used? 

Protocol provide blueprint methods for classes. It has certain methods and parameters that classes have to implement in its body. Its like that classes should have certain characteristic if it confirm protocol. Like if you want to become human(class) you must be sleep, eat, breathe(characteristics).

For more detail,
I have created Delegate and Protocol. See example.

What is category?

Ans : It is used to extend the functionality of the class. Implementation of category is in different file with name like classname + categoryname.h and .m file.

Read : Differnce between Category and Extension

Difference between Static Constructor and Private Constructor

Ans : I have asked this question for C# language.

Private Constructor :  It is used to prevent class to be instantiated and to be inherited. Class can have multiple private constructor and can be called by any other constructor.

Static Constructor :   It is used to initialise static members of a class. We can't say when it is called and It is called by CLR and not called by manually. It is called just before first instance of class is created. Class can have a only one static constructor. This constructor is called only once in lifetime of application.

What are higher order functinos in swift?

Ans : Higher order functions are functions that operate on other functions by taking function as argument or returning function.

 Sorted : Sorting array. It may takes closure as argument and returns array .

let numbers : [Int] =  [1,5,2,4,3]

let arrNumbers = numbers.sorted()

By default it returns in ascending order.

numbers.sorted(by: (Int,Int) -> Bool)  - > How we want to sort array then sorted(by:)

numbers.sorted((a,b) -> Bool in return a > b } // descending order

numbers.sorted(by : >)

      Map : It iterates through the array that is calling it and changes each element of the array based on the closure passes to the method.

Use map to loop over a collection and apply the same

operation to each element in the collection. It returns after

applying transform.

let arrInt = [1,8,4,6]

I want add 1 in every number in array.

Without loop, using map,

arrInt.map( $0 + 1)  // <- This is shortest code for doing map

output : [2,9,5,7]

How map works :

The map function has a single argument which is a closure (a

function) that it calls as it loops over the collection. This

closure takes the element from the collection as an argument and

returns a result. The map function returns these results in an

array.

Another example :

let chocolateAmt = [“Dairy Milk”:20.0, “Munch”:10]

I want to increase price 5%,

so using map,

chocolateAmt.map{ (key, value) in

   value + value*0.05

      }

output : [“Dairy Milk”:21.0, “Munch”:10.5]

Another Example :

If we want index in map,

let arrInt = [1, 2, 4, 5]

let indexElement = arrInt.enumerated().map { (index,element) in

return "\(index):\(element)"

}

print(indexElement) // [“0:1”, “1:2”, “2:4”, “3:5”]

CompactMap: Use this method to receive an array of nonoptional values when your transformation produces an optional value.

let scores = ["1", "2", "three", "four", "5"]

let mapped: [Int?] = scores.map { str in Int(str) }

// [1, 2, nil, nil, 5] - Two nil values as "three" and "four" are strings.

let compactMapped: [Int] = scores.compactMap { str in Int(str) }

// [1, 2, 5] - The nil values for "three" and "four" are filtered out. 

FlatMap : Flatmap is used to flatten a collection of collections.

Flatmap is joined word of Flat + Map. So as per name, it applies map function over collection and do flatten collection.

Test 1 :

let codes = [["abc","def","ghi"],["jkl","mno","pqr"]]

let newCodes = codes.flatMap { $0.map {$0.uppercased()} }

print(newCodes)

Output : ["ABC","DEF","GHI","JKL","MNO","PQR"] 

Test 2 :

let codes = ["abc","def","ghi"]

let newCodes = codes.flatMap { $0.uppercased() }

print(newCodes)

Output : ["A","B","C","D","E","F","G","H","I"]

-> In Test 2, first it applies map so half output is like 

["ABC","DEF","GHI"] and after that it applies flat so now full 

output is : ["A","B","C","D","E","F","G","H","I"]

Tip : 

1. String is collection from Swift 4.

2.If you do flatmap a collection containing optional values, 

flatmap will only consider the non-nil values.

let codes = [1,2,nil,3,nil,5]

let newCodesFlatMap = codes.flatMap { return $0 }

output : [1,2,3,5]

let newCodesFlatMap = codes.map { return $0 }

output : [optional(1),optional(2),nil,optional(3),nil,optional(5)]

The output of map became a collection of optional int ([Int?]) 

only because the array had nil — value in it. Otherwise it would 

have been an Int array. It is benefit of using flatMap.

Filter : Return array with elements which fulfill filter condition.           

let numbersLessThanFive = numbers.filter { (a) -> bool in return a  <  5 }

let numbersLessThanFive = numbers.filter { $0 < 5}

          Reduce : It is used to combine all element in array to make one single value.

let sumOfNumbers = numbers.reduce(0, { $0 + $1 }) 

Difference between map and flatmap : 

Big difference is Map considers nil but flatmap  remove nil value from collection.

Difference between Delegate and NSNotification

Ans :  A delegate uses protocol and creates a has-a relationship between two classes. Benefit of delegate is that we can return something back to the owning class.

Notification is like point to multi-point communication. Notification is one way  of message transmitting way.

Delegates create relationship between two classes. Notifications are used to send events to one or many classes.

We have to use delegate to specified known object. Notification for all object.

Delegate is like talking over telephone. Notification is like radio station.  

Coding of NSNotificationCenter :

[[NSNotificationCenter defaultCenter] addObjserver:self selector:@selector(useNotificationWithString:)  name:@”TimeOut” object:nil];

For BroadCast,

[[NSNotificationCenter defaultCenter] postNotificationName:@”TimeOut” object:nil userInfo:dict];

-(void) useNotificationWithString:(NSNotification *)notification

{

            dict = [notification userInfo];

}

To Remove observer,

[[NSNotificationCenter defaultCenter] removeObserver];

Coding of Delegate

What is delegate?

Ans : Delegate is means of communication between objects of iOS Applications. Delegate allows one object to send message to another object when an event occurs.

i.e

UIAlertView *alert = [[UIAlertView alloc] initWithTitle:@”title” message:@”message” delegate:self cancelButtonTitle:@”Ok” otherButtonTitle:nil];

Here delegate is self. So now self is responsible for handling  all event fired by this instance of UIAlertView class.

Which button of UIAlertView is clicked, for that, event is clickedButtonAtIndex is called in or by Self or currentViewController.

Create PreDefined delegate :

1. There are two ViewController NameVC and SurNameVC.
2.  In NameVC, there are 2 textfield named as Name and FullName and 1 button as Submit.
3. If I write in name and click on submit, it went to SurNameVC to take Surname parameter.
4. On SurNameVC, after write Surname, on clicking of Submit, It call delegate method and went back to NameVC and Print Full Name in FullName textfield.

Implement above delegate and protocol in Objective-C : 

I have made protocol on SurNameVC like

    @protocol SurNameVCDelegate

      -(void)setSurName:(NSString *) strSurName;

    @end

    

    @property (nonatomic, retain) id delegate;

Now on NameVC submit button click, choose delegate of  SurNameVC object as self.

   objSurNameVC.delegate = self

and create method -(void)setSurName:(NSString *) strSurName;

on NameVC and it is called from surNameVC submit button. So setSurName is delegate method. We can print fullname by concatenating Name and Surname in FullName textfield.

So we delegate just pass message from one view controller to another view controller by delegate method.

Implement delegate and protocol in Swift : 

I have made custom UISlider. I want to send some value from custom UISlider value changed to view controller in which it is used. So for that, I have used delegate - protocol method.

customSlider.swift  Custom Slider file

import UIKit

protocol SliderDelegate: class {

    func sliderValueChanged(_ sender : UISlider)

}

class mpgpsSlider: UIView {

     

      weak var delegate : SliderDelegate?

      required init?(coder aDecoder: NSCoder) {

        super.init(coder: aDecoder)

        

        let bundle = Bundle.init(for: type(of: self))

        let nib = UINib(nibName: "Slider", bundle: bundle)

        let view = nib.instantiate(withOwner: self, options: nil)[0] as! UIView

        view.frame = bounds

        view.autoresizingMask = [.flexibleWidth,.flexibleHeight]

        addSubview(view)

        

        slider.addTarget(self, action: #selector(sliderValueChanged(_:)), for:                  .valueChanged)

    }

     @objc func sliderValueChanged(_ sender : UISlider)  {

        delegate?.sliderValueChanged(sender)

      }

}

ViewController.swift ViewController in which custom slider is used.

import UIKit

class VehicleProfileVC: BaseViewController,SliderDelegate{

    

    override func viewDidLoad() {

        super.viewDidLoad()

        slider.delegate = self

    }

    

    func sliderValueChanged(_ sender: UISlider) {

        label.text = String(sender.value)

    }

}

Difference between Any and AnyObject

ANS : Swift has 2 types for working with  nonspecific type.
1. AnyObject
2. Any

1. AnyObject is for reference type(class) and Any is for both reference and value type.

2. AnyObject represent instance of only any class type, Any represent for any type including function type.

Note : It is always good practice to use specific type instead of Any, AnyObject.

After Swift 3.0, Objective C I'd type can be compatible with Swift Any type. Before that it is equivalent to AnyObject.

I.e

I have created dictionary in which I don't know what will be Value type.

[String : ?? ]

then ?? may be int, float, Array, Dictionary type.

So here we should use Any in replace of ??, because int, float are value type and array, dictionary are class type.

[ String : Any]

What is the nil coalescing operator in swift?

Ans : 

Swift has very powerful in terms of safety because it has feature of optional.

Optional may be has value or may be not. So we try to assign some optional to any another string or any type, then it comes some problem.

Nil Coalescing operator is solution of above problem.

let myname : String = nil
let unwrappedname = myname ?? "Default"

It checks if myname is nill then Default will be assigned to unwrappedname.

So unwrappedname is type of String instead of String?.

Such way nil Coalescing operator ?? is very useful in Swift.

UIViewcontroller Lifecycle

Ans : A view controller manages set of views and making user interface. It will coordinate with data and other controller. Views are automatically loaded when view property is accessed in the app.

Following methods are used to mange view controller's view.

1. LoadView : It is automatically called when it's view property is accessed. It loads or create a view and assigned to property.

2. ViewDidLoad : It is automatically called when view controller completely loaded into memory. Override this method to perform additional initialization on views that were loaded from xib.
I.e instance variable initialization, database access, network request

Event Management to Views :

1. ViewWillAppear : It is called when View is about to added on view hierachy. If we want to change some, then we have to override this method.
Like change orientation, change screen data

2. ViewDidAppear : It is called when view was added on view's hierachy.
When we need to display loader, start UI animation ,then override this method.

3. ViewWillDisAppear : It is called when view is about to removed from hierachy. We can hide keyboard,  commit changes ,revert changes in this method by overriding.

4. ViewDidDisappear : It is called when view is removed from hierachy. We can remove cache data in this method.

Memmory Management method :

1. didReceiveMemoryWarning :
It is called automatically when system determine that the system has low amount of available memory.
Override this method remove not essential data from memory.

Ordering of excecuting methods :

1. Init(coder:)
2. (void)loadView
3. (void)viewDidLoad
4. (void)viewWillAppear
5. (void)viewDidAppear
6. (void)didReceiveMemoryWarning
7. (void)viewWillDisappear
8. (void)viewDidDisappear

What is app thining?

Ans : App thinning automatically detects the user’s device type (i.e. model name) and only downloads relevant content for the specific device. In other words, if you’re using an iPad Mini 1 (which does not have a retina display but rather a 1x resolution), then only your 1x files (more on this in a moment) will be downloaded.  The assets for more powerful and sharper iPads (such as the iPad Mini 3 or 4) will not be available for download).  Because the user needs only download the content that targets his/her specific device, this speeds up the download process and saves space on the device.

There are three aspects of app thining.

1. App Slicing : Slicing is the process of creating and delivering variants of the app bundle for different target devices. App Slicing delivers only relevant assets to each device (depending on screen resolution, architecture, etc.)  In fact, app slicing handles the majority of the app thinning process.
When you’re ready to submit the app, you upload the .IPA or .App file to iTunes Connect, as you typically would (but must use Xcode 7 as it contains the iOS 9 SDK with support for app thinning). The App Store then slices the app, creating specific variants that are distributed to each device depending on its capabilities.

2. On Demand Resources : On demand resources are files that can be downloaded after the app’s first installation.  For example, specific levels of a game (and these levels’ associated content) could be downloaded only when the player has unlocked them.  Further, earlier levels that the player has not engaged with after a certain set time can be removed to save storage on the device.
Enabling on demand resources involves changing the “Enable On Demand Resources” boolean to “Yes” in Xcode settings (under Build Settings).

3.  Bitcode : Bitcode makes apps as fast and efficient as possible on whatever device they’re running.  Bitcode automatically compiles the app for the most recent compiler and optimizes it for specific architectures. Can be turned on by the project settings under Build Settings and selecting bitcode to YES.

App thining process can be test by TestFlight software and install in various device. Size may be vary.

What is size class? Explain with Example.

Ans : 
Size Classes are groups of screen sizes that are applied to the width and height of the device screen. The two Size Classes that exist currently are Compact and Regular.

The Compact Size Class refers to a constrained space. It is denoted in Xcode as wC (Compact width) and hC (Compact height).

The Regular Size Class refers to a non-constrained space. It is denoted in Xcode as wR (Regular width) and hR (Regular height).

iPhone : Most iPhone devices will use the Compact width (wC) size class in each orientation with some exceptions.
The iPhones 6/7 Plus can inherit the Regular width (wR) size class resulting in the ability to use functionality normally reserved for the iPad, such as the split-view pane. Split-VIew pane is speciall made for iPad.

iPad : All iPad devices use the Regular width (wR) size class regardless of landscape or portrait orientation. In both horizontal and vertical landscape, the iPad will use a split-view pane to take advantage of the available screen space because it falls within this unconstrained size class.

Controlling the UI with Size Classes

In most cases, specifying the width class is sufficient when laying out your app; however, you can use the height class to customize your app as well. For example, if you use a split-view pane in an iPad app, users will see the same pane on iPhone Plus in landscape orientation because they both inherit the Regular width (wR) size class.

If that is not your intention, you can exclude a split-view pane from appearing on the 7 Plus by specifying both the width and height Size Classes. This works because the height of an iPhone Plus in landscape orientation is compact (hC) while all iPad devices have a regular height (hR).

You can also use Size Classes to target specific UI elements like font colors, font sizes, drop shadows, view colors and more to adapt to users’ device screen. These variations give you added control as to how your UI can adapt to different devices and orientations that your user may be using.

Difference between frames and bounds.

Ans : 

Frame :  View's location and size using the parent view's coordinate system
Needed while placing the view in the parent

bounds = View's location and size using its own coordinate system
Needed while placing the view's content or subviews within itself

The bounds of an UIView is the rectangle, expressed as a location (x,y) and size (width,height) relative to its own coordinate system (0,0).

The frame of an UIView is the rectangle, expressed as a location (x,y) and size (width,height) relative to the superview it is contained within.

So, imagine a view that has a size of  50x50 (width x height) positioned at 15,15 (x,y) of its superview. The following code prints out this view's bounds and frame:

NSLog(@"bounds.origin.x: %f", label.bounds.origin.x);
NSLog(@"bounds.origin.y: %f", label.bounds.origin.y);
NSLog(@"bounds.size.width: %f", label.bounds.size.width);
NSLog(@"bounds.size.height: %f", label.bounds.size.height);

NSLog(@"frame.origin.x: %f", label.frame.origin.x);
NSLog(@"frame.origin.y: %f", label.frame.origin.y);
NSLog(@"frame.size.width: %f", label.frame.size.width);
NSLog(@"frame.size.height: %f", label.frame.size.height);

Output : 

bounds.origin.x: 0
bounds.origin.y: 0
bounds.size.width: 50
bounds.size.height: 50

frame.origin.x: 15
frame.origin.y: 15
frame.size.width: 50
frame.size.height: 50

So, we can see that in both cases, the width and the height of the view is the same regardless of whether we are looking at the bounds or frame. What is different is the x,y positioning of the view. In the case of the bounds, the x and y coordinates are at 0,0 as these coordinates are relative to the view itself. However, the frame x and y coordinates are relative to the position of the view within the parent view (which earlier we said was at 25,25).

For more understanding : Visit this 

Difference between Objective-C Category and Extension

Ans : 

1.Category is a way to add methods to a class whether or not source code is available implies you can add category to foundation classes like NSString and also to your own custom classes.

2.We can add extra instance variable and property in class extension but not in Category.

3.Any variable and method inside the extension is not even accessible to inherited class.

4.Category and Extension both are basically made to handle large code base but category is a way to extend class API in multiple source file while extension is a way to add required methods out side the main interface file.

5.Use category when you have to break your same class code into different source file according to different functionality and Extension when you just need to add some required methods to existing class outside the main interface file. also when you need to modify a publicly declared instance variable in a class. for ex: readonly to readwrite you can re declare it in extension.

Read : What is Category?

What is extension and How to use it?

Ans :
Swift Extension :

Add a new swift file with File > New > File... > iOS > Source > Swift File, but you can call them what you want.
The general naming convention is to call it TypeName+NewFunctionality.swift

Make extension of Double

Double+Conversions.swift

import Swift // or Foundation

extension Double {

    func celToFahren() -> Double {
        return self * 9 / 5 + 32
    }

    func fahrenToCel() -> Double {
        return (self - 32) * 5 / 9
    }
}

How to make extension:

let boilingPointCel = 100.0
let boilingPointFaren = boilingPointCel.celToFahren()
print(boilingPointFaren) // 212.0

Make extension of UIColor

UIColor+CustomColor.swift

import UIKit

extension UIColor {

    class var customGreen: UIColor {
        let darkGreen = 0x008110
        return UIColor.rgb(fromHex: darkGreen)
    }

    class func rgb(fromHex: Int) -> UIColor {

        let red =   CGFloat((fromHex & 0xFF0000) >> 16) / 0xFF
        let green = CGFloat((fromHex & 0x00FF00) >> 8) / 0xFF
        let blue =  CGFloat(fromHex & 0x0000FF) / 0xFF
        let alpha = CGFloat(1.0)

        return UIColor(red: red, green: green, blue: blue, alpha: alpha)
    }

}
See here also.

Using extension :

view.backgroundColor = UIColor.customGreen

Summary : Once you define an extension it can be used anywhere in your app just like the built in class functions. In Objective-C extensions are known as categories.

Objective C Extension : 

In objective c, when you want to make behavior of some property private you use class extension.
-> it comes with .m file only.
-> mainly for properties.
The implementation of the extension must be in the main @implementation block of the file.
Extension can only be added to the classes whose source code is available because compiler compile the source code and extension at same time.

Difference between Delegate and Datasource

Ans :

A delegate type object responds to actions that another object takes.

i.e  the UITableViewDelegate protocol has methods such as didSelectRowAtIndexPath for performing actions upon a user selecting a particular row in a table and willDisplayCell which called before delegate use cell to draw row.

DataSource type object gives data to another object.

i.e UITableViewDataSource protocol has methods such as cellForRowAtIndexPath and numberOfSectionInTaboeView dictating what should be displayed in the table.

Understanding Delegate in More Detail : 

If Object X call Object Y to perform an action. Object X should know when Object Y complete task and take action after that. 

Here we can tell that X is delegate object of Y. Y will have a reference of X. So X will implement delegate methods of Y. So Y can notify to X via delegate method.

One more point we can say that Delegate about controlling of UI and DataSource about controlling data.

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Difference between Sqlite and CoreData

Ans  : Sqlite is database and Coredata is Object Relational Model which is layer between UI and Database. Coredata is memory efficient and Querying to Sqlite is not so efficient.

So we can't compare Sqlite and Coredata. Both are different thing.

What are persistent storage in iOS and Which one is most secure?

Ans : There are SIX types of persistent storage.

1. Userdefaut
2. Property List
3. Sqlite
4. Keychain
5. Files
6. Coredata

If brief answer they asked then follow :

1. Userdefaut : NSUserDefault class allow us to store small amount of data. It can store NSData, NSString, NSArray, NSDictionary,  NSNumber,

The maximum data can we saved depends on iOS. Currently it can store 4GB of data.

But if file is too large, then it takes too much time for retrieve and write data in file. So we can save small amount of data only. Otherwise it waste time.

We can also store our custom objects in userDefaults. We achieve this by conforming our class to NSCoding protocol. We can then convert our custom object into NSData with the help of NSKeyArchiver class. The NSData is then stored into userDefaults like other objects. Similarly we can get NSData from userDefaults and then using NSKeyUnarchiver convert the NSData back to our custom objects.

2. Property List : As userdefaut save data in plist file, so like userdefaut, Property List is not also made for save large amount of data. There is one method of NSArray and NSDictionary as writeToFile for saving data.

3. Sqlite : If your application deals with large amount of data with relationship then we should use sqlite. It's API is written in C language and embedded with our application so it is very fast. There are ORM for bringing gap between obj c app and sqlite like, FMDB,  Realm

4. Keychain : If you want to save highly sensitive and secure data like passwords and secret codes then there is a good news for you. Storing data in keychain is most secure way. To store data, I have taken library named SwiftKeyChainWrapper from cocoapods.

To Save data in Keychain : 

let saveSuccessful: Bool = KeychainWrapper.standard.set("Some String", forKey: "myKey")

To get data from Keychain :

let retrievedPassword: String? = KeychainWrapper.standard.string(forKey: "userPassword")

To remove data from keychain : 

let removeSuccessful: Bool = KeychainWrapper.standard.remove(key: "myKey")

5. Files : You can save data to any type of file. There are three type of folder like Document, Library, Tmp fo saving various type of file.

6. Core Data : Apple’s solution for persistence allows applications to persist data of any form and retrieve it. It  isn’t technically a database, although it usually stores its data in one (an SQLite DB). It’s not an object-relational mapper (ORM), though it can feel like one. It’s truly an object graph, allowing you to create, store, and retrieve objects that have attributes and relationships to other objects. Its simplicity and power allow you to persist data of any form, from basic data models to complex.

I have written for loop from 1 to 10. But when i=7 then then it comes out from loop. I have not written any specific keyword nor exception generated. How it is possible?

ANS :

for(int i=0, i<10:i++)
{
     if(i==6)
          {
              i=10;
           }
}

Here I am trying to iterate from 0 to 9. This loop is iterating till Condition i<10 matches.

Now in iterating, when loop comes i=6 I will make i=10.

So in next iteration, i<10 condition is mismatched. So it comes out from loop.

There is no any keyword used or any exception generated.

Bonus Tip : You can run swift code online here

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What is StackView? What is advantage and distribution type of stackview?

Ans : Stack View allows to layout views in a stack in either horizontal or vertical fashion. In Xcode-7, stackview is introduced.

Advantage : Stacks are containers that keep views aligned automatically.

Distribution Types :

Fill(Default) : When you place your controls inside a UIStackView with Fill set as the distribution, it will keep all but one of the controls at their natural size and stretch one of them to fill the space. It determines which control to stretch by noting which one has the lowest Content Hugging Priority (CHP).

Fill Equally : With this type, each control in a UIStackView will be of equal size. All of the space between the controls will be used up, if possible. I added a spacing of eight between the UITextFields, so again you could see the size of each one. With this type, the CHP does not matter, because each control is the same size.

Fill Proportionally : The UIStackView will ensure the controls maintain the same proportion relative to one another as your layout grows and shrinks. Unlike the previous two settings, the Fill Proportionally distribution needs the controls to have an intrinsic content size. The Fill and Fill Equally distribution tell their child controls how big they should be, but this one is the other way around (as long as there is enough space for all of your controls to be their natural size). The proportions for the images and labels are maintained for the different layout sizes.

Equal Spacing : This distribution type will maintain an equal spacing between each of the controls and will not resize the controls themselves.

Equal Centring :  It will equally space the centres of the controls. Space between every control is equal.

What is inout parameter in swift?

 Function parameters are constants by default. Trying to change the value of a function parameter from within the body of that function results in a compile-time error. This means that you can’t change the value of a parameter by mistake. If you want a function to modify a parameter’s value, and you want those changes to persist after the function call has ended, define that parameter as an in-out parameter instead.

func add(x: Int, y: Int) -> Int
{
x=7              error: Can not assign to value x, x is let constant
return(x+y)
}

func add(x: inout Int, y: Int) -> Int
{
x=7           
return(x+y)
}

Bonus Tip : You can run swift code online here

  

Collectionview or Scrollview or PageViewController, Which one is most preferable?

It depends on requirement. If too many pages or items, then Collectionview or pageviewcontroller is good option. Because Collectionview can reuse cell for display view, whereas PageViewController load only current page,previous page and next page in memory. As Scrollview loads all subviews in memory, it will consume too much memory.

Pageviewcontroller load multiple viewcontroller in memory. So preferable most in following order :

1. UICollectionview

2. UIPageViewController

3. UIScrollView

UITableview and UICollectionview

Similarity :
The way to setup both with cell registration, dequeing cells, specifying size and heights are pretty much the same.

Diffrence :
1. The collection view is much more robust in terms of layout features and other things like animations.

Tableiw is a simple list, which displays single-dimensional rows of data. It’s smooth because of cell reuse and other magic.

2. UICollectionView is the model for displaying multidimensional data . 

UITableViewhas a couple of styles of cells stacked one over the other. You should not try to bend it to do any other kind of things too much complex than that kind of layout.

3. UICollectionView is a much more powerful set of classes that allow modifying almost every aspect of how your data will appear on the screen, especially its layout but also other things.

Advantage of Collectionview :

The biggest advantage of using collection view is the ability to scroll horizontally.
 If you want multiple columns in your apps then UICollectionView is the best way to do it. 

It is possible to have multiple columns in table view as well but it gets really messy since you are dealing with and array to display data in a table view.

UICollectionView Supports complex layouts. Apple provides you with something called UICollectionViewDelegateFlowLayout which provides you the flow of left and right as well as up and down.

UICollectionView supports custom animations based on different layouts which again cannot be done in UITableView.

Disadvantage of Collectionview (Advantage of Tablvieview over) :

Major disadvantages of using UICollectionView is auto sizing of your cells. It takes a lot of trial and error to get auto sizing to work correctly. UITableView wins here as all you have to do is return UITableView automatic dimensions for the height of your row in each one of your cells.

Summary :
If you need more control over your layout, go with UICollectionView, If you need simple list go with UITableview.