CODEWITHSUNDEEP
swift
Ygor Nascimento
Ygor Nascimento

Reputation: 3

Using Protocols Functions inside a Class

studying Protocols here and I'd like to know why that code does not print. Thanks in advance.

import Foundation

protocol PrintSomething {
    func printMyname(name: String)
}

class Display {
    var myProtocol: PrintSomething?

    func somethingToDisplay() {
        myProtocol?.printMyname(name: "Ygor")
    }
}

let display = Display()

display.somethingToDisplay()

Upvotes: 0

Views: 70

Answers (4)

lajosdeme
lajosdeme

Reputation: 2407

The documentation is pretty clear about what a protocol is:

A protocol defines a blueprint of methods, properties, and other requirements that suit a particular task or piece of functionality. The protocol can then be adopted by a class, structure, or enumeration to provide an actual implementation of those requirements. Any type that satisfies the requirements of a protocol is said to conform to that protocol.

For starters, you need an object to conform to your protocol which provides an actual implementation of the blueprint, because that is what a protocol is. Imagine you're building a house. You have the blueprint for it, but you still have to implement it in the real world. 

protocol PrintSomething {
    func printMyname(name: String)
}

class Display: PrintSomething {

    func printMyname(name: String) {
        print(name)
    }
}

let display = Display()
display.printMyname(name: "My name")

If you wish to understand protocols in Swift I strongly recommend you begin with the documentation and start practicing the examples.

Upvotes: 1

Asad Amodi
Asad Amodi

Reputation: 136

You can use your code like below 

protocol PrintSomething {
    //declaration
    func printMyname(name: String)
}

class Display: PrintSomething {
    //defination
    func printMyname(name: String) {
        print(name)
    }


     var myProtocol: PrintSomething?

    func somethingToDisplay() {
        myProtocol?.printMyname(name: "Ygor")
    }
}

And finally you can call it from any where like below 

let display = Display()

display.somethingToDisplay()

You can read more about Protocol here

Upvotes: 1

Procrastin8
Procrastin8

Reputation: 4503

Because you never instantiated an actual instance of PrintSomething. You have a reference to an optional which defaults to nil. Are you perhaps trying to have Display conform to the protocol? You need an actual type to conform to the protocol and then instantiate that to be able to call it.

struct Printer: PrintSomething {
  func printMyname(name: String) {
    print(name)
  }
}

class Display {
  var myProtocol: PrintSomething = Printer()

  func somethingToDisplay() {
      myProtocol?.printMyname(name: "Ygor")
  }
}

Now you have a real type (a struct) implementing the protocol, you're instantiating it so that the reference isn't nil, and then you're calling it.

You can do better, though, by having Display implement the protocol directly.

class Display: PrintSomething {
  func printMyname(name: String) {
    print(name)
  }
}

let display = Display()
display.printMyname(name: "whatever")

Upvotes: 1

Joakim Danielson
Joakim Danielson

Reputation: 51955

You have defined a property to be of a protocol type but you never assign something to that property and for that you need a concrete type, a struct or a class, to implement your protocol,

struct Example: PrintSomething {
    func printMyname(name: String) {
        print("This is my name: \(name)")
}

then you need to use it

let display = Display()
display.myProtocol = Example()

Now it will print when you call display.somethingToDisplay()

Upvotes: 0

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