Initialization is hugely important in any modern language. This is not a full guide (Apple have provided a long guide, which is referenced below) but rather a shorter example-laden walk through initialization for Swift.
Difficulty: Beginner | Easy | Normal | Challenging This article has been developed using Xcode 12.2, and Swift 5.3
Some understanding of OO concepts Some idea of Structs and Classes in Swift
Initializers: Called to create a new instance of a type, be it a struct, enum or a class. Initializer delegation: Where an initializer calls other initializers to perform part of an instances initialization.
#Initializers
An initializer can easily be created within a struct (note that the internal and external names are usually the same in an initializer)
struct Address {
var firstLine : String
var secondLine : String
init(firstLine: String, secondLine: String) {
self.firstLine = firstLine
self.secondLine = secondLine
}
}
var testAddress = Address(firstLine: "FL", secondLine: "SL")However, Swift has a memberwize initializer (so long as you do not provide a single initializer).
struct AddressDefault {
var firstLine : String
var secondLine : String
}
var testDefaultAddress = AddressDefault(firstLine: "FL", secondLine: "SL")We can also call another initializer to perform the initialization (or part of it). This is called initializer delegation.
struct Bill {
var amount : Float
var billedTo : String
init(amount: Float, billedTo: String){
self.amount = amount
self.billedTo = billedTo
}
init(amount: Double, billedTo: String) {
self.init(amount: Float(amount), billedTo: billedTo)
}
}Designated initializers for a simple class
These are the main initializers used for a class, and they are required to have at least one. This generic example applies to any concrete class type.
class SimpleClass {
var simplePropertyString: String
init(simplePropertyString: String) {
self.simplePropertyString = simplePropertyString
}
}
var simpleClass = SimpleClass(simplePropertyString: "Test Property")Convenience initializers for a simple class Here the convenience init does not demand that we pass an ID value, and instead declares an id number (ignore the ridiculous logic in this example).
class User {
var name: String
var id: Int
init(name: String, id: Int) {
self.name = name
self.id = id
}
convenience init(name: String) {
let number = Int.random(in: 0 ..< 10)
self.init(name: name, id: number)
}
}
A convenience initializer must call a designated initializer
Convenience and Designated initializers for subclasses
This example ignores the complexity of Apple's documentation where multiple designated initializers are possible (which of course they are). The rule is simply: a designated init must call the designated init from the immediate superclass. However, the example of a registeredUser and User below the reasons behind this become clear. A registered user needs to have the variable previousVisits initialized before using the initializer in the superclass. Take a look below:
class User {
var name: String
var id: Int
init(name: String, id: Int) {
self.name = name
self.id = id
}
convenience init(name: String) {
let number = Int.random(in: 0 ..< 10)
self.init(name: name, id: number)
}
}
class registeredUser: User {
var previousVisits: Int
init(name: String, id: Int, previousVisits: Int) {
self.previousVisits = previousVisits
super.init(name: name, id: id)
}
convenience override init(name: String, id: Int) {
self.init(name: name, id: id, previousVisits: 0)
}
}A designated init must call the designated init from the immediate superclass
Failable Initializers This can apply to a class, a struct or an emum. Here the example shows that you only have 5 days in a working week (and names them).
enum WeekDay: String {
case monday, tuesday, wednesday, thursday, friday
init?(index: Int){
switch index {
case 0 : self = .monday
case 1 : self = .tuesday
case 2 : self = .wednesday
case 3 : self = .thursday
case 4 : self = .friday
default : return nil
}
}
}
let mDay = WeekDay.init(index: 0)
print (mDay) // Initialization.WeekDay.monday
let noDay = WeekDay.init(index: 9)
print (noDay) // nilThese initializers can fail. However the Swift book says: "You can override a failable initializer with a nonfailable initializer but not the other way round" So you can define a subclass for which initialization cannot fail, even though initialization of the superclass is allowed to fail.
class Policeman {
var name: String
var gunLicence = String()
// initialize with no licence
init (name: String) {
self.name = name
}
init?(name: String, licence: String)
{
if licence.isEmpty {return nil}
self.name = name
}
}
class ArmedPolice: Policeman {
override init(name: String) {
super.init(name: name)
self.gunLicence = "Arms"
}
override init(name: String, licence: String) {
// cannot override the failable init in policeman
super.init(name: name)
if licence.isEmpty {
self.gunLicence = "Arms"
} else {
self.gunLicence = licence
}
}
}Required Initializers A required initializer means that every subclass of the class must implement that initializer. Of course dogs are wild, apart from when they aren't.
class Dog {
var wild: Bool = true
var legs = 4
required init(wild: Bool) {
self.wild = wild
}
init() {
}
}
class Yorkie: Dog {
required init(wild: Bool) {
super.init(wild: wild)
}
}
var dogInstance = Dog()
var alsatian = Dog(wild: true)
var tommo = Yorkie(wild: false)References
For a full in-depth guide, there is no better place to go than Apple's docs, which is linked to below:
I hope this little guide has been of help to you!
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