convenience-init.md

Convenience Init

Introduction

Welcome to the first lesson of Object Oriented Swift. You will learn how to become lazy. In other words, you become smarter and effective when it comes to initializing an object. In programming, getting lazy to write less code is in fact good for your teammates and users as long as it's readable and gets the job done. Bill Gates once mentioned, whenever there is hard work to be done, he assigns it to the laziest person as he or she is sure to find an easy way of doing it.

Problem

I'm lazy. Can we initialize quickly?

Create Class with Designated Init

Create a class called Human

class Human {
  var name: String
  init(name: String) {
    self.name = name
 }

  // Convenience init intializes the designated init method
  convenience init() {
    self.init(name: "Bob Lee")
  }
}

Convenience init initializes the designated init method by calling self.init.

Create Object

let lee = Human(name: "Bob Lee") // init(name: String)
let bob = Human() // Convenience init()

Initialize UIColor

Create an instance that stores UIColor.

import UIKit

let randomColor = UIColor(colorLiteralRed: 255/255, green: 80/255, blue: 85/255, alpha: 1)

Custom Convenience Init

Create our own convenience init that initializes the designated init by adding extension to UIColor.

extension UIColor {
 convenience init(r: Float, g: Float, b: Float) {
   self.init(colorLiteralRed: r/255, green: g/255, blue: b/255, alpha: 1)
 }
}

let myFavoriteColor = UIColor(r: 255, g: 80, b: 85)

Application

1. Quick initialization for open source projects
2. Custom convenience inits with UIKit

Source Code

2001_convenience_init.playground

Conclusion

You've learned how to become lazy and effective at the same time. Imagine, you are the owner of a library for your teammates. They no longer have to use the bloated designated init method. Instead, you may provide multiple convenience init methods so that their lives become easier. It is similar to how the Apple engineers have provided us with convenience init methods for UIColor. It's a win-win situation.

In the next lesson, you will learn how to fail initializations.