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WWDC 2015 is where Protocol Oriented Programming (POP) is officially being preached.

It gave us a different perspective of programming, one that does NOT use class and inheritance.

Instead, POP use struct and protocol.

The Problem with Object Oriented Programming Language

The problem is that in practice, using class inheritance is at times not correct.

Or not flexible.

Because with inheritance, the only structure you can make is a hierarchy. You are limited by that.

But with protocols, you are essentially giving your object traits.

With protocol extension for the trait, you can give it a default trait.

It makes resuable code, much more resuable (:

To build a component, you just need to specify these individual traits, like how building blocks should be!

An Example

We look at an example of a login view controller, with a password text field.

class LoginViewController: UIViewController {
  let passwordValidator = PasswordValidator()

  @IBAction func loginButtonPressed() {
    if passwordValidator.isPasswordValid(passwordTextField.text!) {
      ...
    }
  }
}

The above use a decompostion pattern, with a PasswordValidator object.

This is nice and clean, as we put the validation logic in an object of it’s own.

Here is how you can improve it with POP, with a default implementation.

protocol ValidatesPassword {
  func isPasswordValid(password: String) -> Bool
}

extension ValidatesPassword {
  func isPasswordValid(password: String) -> Bool {
    // A default implementation
  }
}

The validation logic is still a separate object, but now in a protocol extension.

To use,

class LoginViewController: UIViewController, ValidatesPassword {
  @IBAction func loginButtonPressed() {
    if isPasswordValid(passwordTextField.text!) {
      ...
    }
  }
}

It is now even cleaner!

You can call isPasswordValid because of ValidatesPassword, the protocol which your view controller now has.