Swift - Extensions

Functionality of an existing class, structure or enumeration type can be added with the help of extensions. Type functionality can be added with extensions but overriding the functionality is not possible with extensions.
Swift Extension Functionalities −
Adding computed properties and computed type properties
Defining instance and type methods.
Providing new initializers.
Defining subscripts
Defining and using new nested types
Making an existing type conform to a protocol
Extensions are declared with the keyword 'extension'
extension SomeType {
   // new functionality can be added here
}

Existing type can also be added with extensions to make it as a protocol standard and its syntax is similar to that of classes or structures.
extension SomeType: SomeProtocol, AnotherProtocol {
   // protocol requirements is described here
}

Computed 'instance' and 'type' properties can also be extended with the help of extensions.
extension Int {
   var add: Int {return self + 100 }
   var sub: Int { return self - 10 }
   var mul: Int { return self * 10 }
   var div: Int { return self / 5 }
}

let addition = 3.add
print("Addition is \(addition)")

let subtraction = 120.sub
print("Subtraction is \(subtraction)")

let multiplication = 39.mul
print("Multiplication is \(multiplication)")

let division = 55.div
print("Division is \(division)")

let mix = 30.add + 34.sub
print("Mixed Type is \(mix)")
When we run the above program using playground, we get the following result −
Addition is 103
Subtraction is 110
Multiplication is 390
Division is 11
Mixed Type is 154

Swift 4 provides the flexibility to add new initializers to an existing type by extensions. The user can add their own custom types to extend the types already defined and additional initialization options are also possible. Extensions supports only init(). deinit() is not supported by the extensions.
struct sum {
   var num1 = 100, num2 = 200
}

struct diff {
   var no1 = 200, no2 = 100
}

struct mult {
   var a = sum()
   var b = diff()
}

let calc = mult()
print ("Inside mult block \(calc.a.num1, calc.a.num2)")
print("Inside mult block \(calc.b.no1, calc.b.no2)")

let memcalc = mult(a: sum(num1: 300, num2: 500),b: diff(no1: 300, no2: 100))
print("Inside mult block \(memcalc.a.num1, memcalc.a.num2)")
print("Inside mult block \(memcalc.b.no1, memcalc.b.no2)")

extension mult {
   init(x: sum, y: diff) {
      let X = x.num1 + x.num2
      let Y = y.no1 + y.no2
   }
}

let a = sum(num1: 100, num2: 200)
print("Inside Sum Block:\( a.num1, a.num2)")

let b = diff(no1: 200, no2: 100)
print("Inside Diff Block: \(b.no1, b.no2)")
When we run the above program using playground, we get the following result −
Inside mult block (100, 200)
Inside mult block (200, 100)
Inside mult block (300, 500)
Inside mult block (300, 100)
Inside Sum Block:(100, 200)
Inside Diff Block: (200, 100)

New instance methods and type methods can be added further to the subclass with the help of extensions.
extension Int {
   func topics(summation: () -> ()) {
      for _ in 0..<self {
         summation()
      }
   }
}

4.topics(summation: {
   print("Inside Extensions Block")
})

3.topics(summation: {
   print("Inside Type Casting Block")
})
When we run the above program using playground, we get the following result −
Inside Extensions Block
Inside Extensions Block
Inside Extensions Block
Inside Extensions Block
Inside Type Casting Block
Inside Type Casting Block
Inside Type Casting Block

topics() function takes argument of type '(summation: () → ())' to indicate the function does not take any arguments and it won't return any values. To call that function multiple number of times, for block is initialized and call to the method with topic() is initialized.
Instance methods can also be mutated when declared as extensions.
Structure and enumeration methods that modify self or its properties must mark the instance method as mutating, just like mutating methods from an original implementation.
extension Double {
   mutating func square() {
      let pi = 3.1415
      self = pi * self * self
   }
}

var Trial1 = 3.3
Trial1.square()
print("Area of circle is: \(Trial1)")

var Trial2 = 5.8
Trial2.square()
print("Area of circle is: \(Trial2)")

var Trial3 = 120.3
Trial3.square()
print("Area of circle is: \(Trial3)")
When we run the above program using playground, we get the following result −
Area of circle is: 34.210935
Area of circle is: 105.68006
Area of circle is: 45464.070735

Adding new subscripts to already declared instances can also be possible with extensions.
extension Int {
   subscript(var multtable: Int) -> Int {
      var no1 = 1
      while multtable > 0 {
         no1 *= 10
         --multtable
      }
      return (self / no1) % 10
   }
}

print(12[0])
print(7869[1])
print(786543[2])
When we run the above program using playground, we get the following result −
2
6
5

Nested types for class, structure and enumeration instances can also be extended with the help of extensions.
extension Int {
   enum calc {
      case add
      case sub
      case mult
      case div
      case anything
   }
   var print: calc {
      switch self {
         case 0:
            return .add
         case 1:
            return .sub
         case 2:
            return .mult
         case 3:
            return .div
         default:
            return .anything
      }
   }
}

func result(numb: [Int]) {
   for i in numb {
      switch i.print {
         case .add:
            print(" 10 ")
         case .sub:
            print(" 20 ")
         case .mult:
            print(" 30 ")
         case .div:
            print(" 40 ")
         default:
            print(" 50 ")
      }
   }
}
result(numb: [0, 1, 2, 3, 4, 7])
When we run the above program using playground, we get the following result −
10
20
30
40
50
50


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