Lazy Properties

Lazy store properties value not calculated until first accessed.
You can declare a lazy property with simply "lazy" keyword.
The closure associated to lazy property is executed only if you read the property.
Lazy properties works with class or struct.
Lazy store properties must be declare with var, because it's initial value might note be retrieved until after instance initialization complete.

Example : 

class LazyExample{

    var iCount = 0

    func testFunc() -> String{

        iCount += 1

        return "iCount Value \(iCount)"

    }

    lazy var tempLazy : String  = {

        print("access value")

        return testFunc()

    }()

}

let lazyObj = LazyExample()

print(lazyObj.tempLazy) ///Only single time value access

print(lazyObj.tempLazy)

print(lazyObj.tempLazy)

Output: 

access value

iCount Value 1

iCount Value 1

iCount Value 1

Another example :

class ViewController: UIViewController {

    lazy var buildVersion : String = {

        print("only once initialize after that get from session")

        print("every time not get from app bundle")

        return (Bundle.main.infoDictionary?["CFBundleVersion"]  as? String) ?? ""

    }()

    override func viewDidLoad() {

        super.viewDidLoad()

        

        print(buildVersion)

    }

    override func viewWillAppear(_ animated: Bool) {

        super.viewWillAppear(animated)

        print(buildVersion)

    }

    override func viewDidAppear(_ animated: Bool) {

        super.viewDidAppear(animated)

        /*

         this is simple buildVerison property

         that get session after once initialize

         */

        print(buildVersion)

    }

}

For more info about lazy properties  watch below video

Extensions

Extensions means add additional functionality to existing types.
Extensions add new functionality to existing class, struct, enum or protocols.
Extensions are similar to categories in Objective-C.

Extension syntax
Declare Extension with "extension" keyword

extension Type {

    ///Additional functionality add hear

}

An Extension can extends one or more protocols.

extension Type : Protocol{

    ///protocols method hear

}

Example

extension String{

    ///Trim string value

    func toTrim() -> String{

        return self.trimmingCharacters(in: .whitespacesAndNewlines)

    }

}

///Simply use like below

let strName = " use of string extension "

print(strName)

print(strName.toTrim())

Output : 

 use of string extension 

use of string extension

Another example

extension UIView {

    func defaultRadius() {

        self.layer.cornerRadius = self.frame.size.width / 2

    }

}

let parentView = UIView.init(frame: CGRect.init(x: 0, y: 0, width: 50, height: 50))

parentView.backgroundColor = UIColor.red

parentView.defaultRadius()

Output: 

UIView Extension

Computed Properties

Extension can add computed instance property and computed type property to existing type.

Example

extension Int{

    var squareRoot : Int {

        return self * self

    }

}

let iValue = 5

print(iValue.squareRoot)

Output : 

25

Methods

Extensions add new instance method and type method to existence type.

Example

extension Int{

    func repeatValue(value : () -> Void) {

        for _ in 0..<self{

            value()

        }

    }

}

5.repeatValue {

    print("hello world")

}

Output :

hello world

hello world

hello world

hello world

hello world

For more about Extensions watch below video