Protocols

A protocol is a collection of methods, properties, and rules that a class, struct, or enum can adopt.

Syntax

A protocol is created with the protocol keyword:

protocol protocolName {
  // Protocol body
}

Note: Names are written in PascalCase.

For class, structure, or enumeration data types to adopt a protocol, it chains onto their definition and is separated with a colon:

class MyClass: MyProtocol, OtherProtocol, ... {
  // This can be a class, structure, or enumeration
}

Property Requirements

Property requirements are variables that indicate a type and the get/set keywords determine if that property is gettable and/or settable.

protocol MyProtocol {
  var getAndSet: Int { get set }
  var onlyGet: String { get }
}

Note: Gettable variables can be read and settable variables can be set or changed. Programmers sometimes only allow a variable to be gettable because they don’t want it to be mutated.

Data types that adopt a protocol, must conform to the properties and methods defined in it.

protocol Grammar {
  var alphabet: String { get }
}

struct Language: Grammar {
  var alphabet: String
}

let english = Language(alphabet: "abcdefghijklmnopqrstuvwxyz")

print(english.alphabet)
// Output: abcdefghijklmnopqrstuvwxyz

Built-in Protocols

Swift includes certain built-in protocols.

CaseIterable

CaseIterable has an allCases property. An enumeration can adopt this protocol to gain access to all its values.

enum MageAdvancementTree: CaseIterable {
  case novice, firstJob, secondJob, thirdJob, fourthJob
}
print("You will have \(MageAdvancementTree.allCases.count) different jobs as a Mage.")
// Output: You will have 5 different jobs as a Mage.