What is the difference between convenience init vs init in swift, explicit examples better

Question

I am having troubles to understand the difference between both, or the purpose of the convenience init.

Answer 1

Convenience initializers are used when you have some class with a lot of properties that makes it kind of "Painful" to always initialize wit with all that variables, so what you do with convenience initializer is that you just pass some of the variables to initialize the object, and assign the rest with a default value. There is a very good video on Ray Wenderlich website, not sure its free or not because I have a paid account. Here is an example where you can see that instead of initializing my object with all those variables Im just giving it a title.

struct Scene {
  var minutes = 0
}

class Movie {
  var title: String
  var author: String
  var date: Int
  var scenes: [Scene]

  init(title: String, author: String, date: Int) {
    self.title = title
    self.author = author
    self.date = date
    scenes = [Scene]()
  }

  convenience init(title:String) {
    self.init(title:title, author: "Unknown", date:2016)
  }

  func addPage(page: Scene) {
    scenes.append(page)
  }
}


var myMovie = Movie(title: "my title") // Using convenicence initializer
var otherMovie = Movie(title: "My Title", author: "My Author", date: 12) // Using a long normal initializer

Answer 2

the convenience init makes it optional to initialize a class with values.

Answer 3

So it comes in handy when you don't need to specify each and every property for a class. So for instance, if I want to create all adventures with starting HP value of 100, I would use this following convenience init and just add in a name. This is going to cut down on the code a lot.

class Adventure { 

// Instance Properties

    var name: String
    var hp: Int
    let maxHealth: Int = 100

    // Optionals

    var specialMove: String?

    init(name: String, hp: Int) {

        self.name = name
        self.hp = hp
    }

    convenience init(name: String){
        self.init(name: name, hp: 100)
    }
}

Answer 4

Standard init:

Designated initializers are the primary initializers for a class. A designated initializer fully initializes all properties introduced by that class and calls an appropriate superclass initializer to continue the initialization process up the superclass chain.

convenience init:

Convenience initializers are secondary, supporting initializers for a class. You can define a convenience initializer to call a designated initializer from the same class as the convenience initializer with some of the designated initializer’s parameters set to default values. You can also define a convenience initializer to create an instance of that class for a specific use case or input value type.

per the Swift Documentation

In a nutshell, this means that you can use a convenience initializer to make calling a designated initializer faster and more "convenient". So convenience initializers require the use of self.init instead of the super.init you might see in an override of a designated initializer.

pseudocode example:

init(param1, param2, param3, ... , paramN) {
     // code
}

// can call this initializer and only enter one parameter,
// set the rest as defaults
convenience init(myParamN) {
     self.init(defaultParam1, defaultParam2, defaultParam3, ... , myParamN)
}

I use these a lot when creating custom views and such that have long initializers with mainly defaults. The docs do a better job explaining than I can, check them out!

Answer 5

All answers are sounds good but, lets understand it with an simple example

class X{                                     
   var temp1
   init(a: Int){
        self.temp1 = a
       }

Now, we know a class can inherit another class, So

class Z: X{                                     
   var temp2
   init(a: Int, b: Int){
        self.temp2 = b
        super.init(a: a)
       }

Now, in this case while creating instance for class Z, you will have to provide both values 'a' and 'b'.

let z = Z(a: 1, b: 2)

But, what if you only want to pass b's value and want rest to take default value for others, then in that case you need to initialise other values a default value. But wait how?, for that U need to set it well before in the class only.

//This is inside the class Z, so consider it inside class Z's declaration
convenience init(b: Int){
    self.init(a: 0, b: b)
}
convenience init(){
    self.init(a: 0, b: 0)
}

And now, you can create class Z's instances with providing some, all or none values for the variables.

let z1 = Z(b: 2)
let z2 = Z()

Answer 6

It makes sense if your use case is to call an initializer in another initializer in the same class.

Try to do this in playground

class Player {
    let name: String
    let level: Int

    init(name: String, level: Int) {
        self.name = name
        self.level = level
    }
    
    init(name: String) {
        self.init(name: name, level: 0) //<- Call the initializer above?

        //Sorry you can't do that. How about adding a convenience keyword?
    }
}

Player(name:"LoseALot")

With convenience keyword

class Player {
    let name: String
    let level: Int

    init(name: String, level: Int) {
        self.name = name
        self.level = level
    }
    
    //Add the convenience keyword
    convenience init(name: String) {
        self.init(name: name, level: 0) //Yes! I am now allowed to call my fellow initializer!
    }
}