Protocols

A protocol defines a blueprint of methods and properties that suit a particular task or piece of functionality. Actual implementation will be provided by other types: structures, classes, enumerations. So it is similar to the interface in Java and Kotlin.

protocol ProtocolA{
    var name : String {get}
    func greeting()
}

class MyClass: ProtocolA{
    // ...
    var name: String { return "your name" }   
    func greeting(){ print("Hello \(name)") }
}

A protocol can inherit one or more other protocols. Also you can combine protocols in typealias statement.

protocol MyProtocol: ProtocolA, ProtocolB {
   func extraMethod()
}

typealias MyProtocol = ProtocolA & ProtocolB

Protocol extension can be used for providing default implementation. Also you can add implementation to the specified type.

extension ProtocolA {
    var name: String { 
        return "Harry" 
    }
    func greeting(){ 
        print("Hello \(name)") 
    }
}

struct A {}
class B {}

extension A: ProtocolA {
    // ...
}
extension B: ProtocolA {
    // ...
}

Generics can restrict type in where clause. The following extension makes Array instances conform to the TextRepresentable protocol whenever they store elements of a type that conforms to TextRepresentable.

extension Array: TextRepresentable where Element: TextRepresentable {
    var textualDescription: String {
        let itemsAsText = self.map { $0.textualDescription }
        return "[" + itemsAsText.joined(separator: ", ") + "]"
    }
}

mutating modifier specifies that method will modify (or mutate) the instance it belongs to.

protocol Togglable {
    mutating func toggle()
}

enum OnOffSwitch: Togglable {
    case off, on
    mutating func toggle() {
        switch self {
        case .off:
            self = .on
        case .on:
            self = .off
        }
    }
}

Protocol conformance can be tested by is and as operators.

for item in items {
   if let it = item as? rectangle {
      print("Hello \(it.greeting())")
   } else {
      print("Name is unknown")
   }
}