Initialization
最后更新于：2022-04-01 02:48:06
## Initialization
Classes and structures must set all of their stored properties to an appropriate initial value by the time an instance of that class or structure is created. Stored properties cannot be left in an indeterminate state.
Swift provides an automatic external name for every parameter in an initializer if you don’t provide an external name yourself. This automatic external name is the same as the local name, as if you had written a hash symbol before every initialization parameter.
If you do not want to provide an external name for a parameter in an initializer, provide an underscore (_) as an explicit external name for that parameter to override the default behavior described above.
~~~
struct Color {
let red = 0.0, green = 0.0, blue = 0.0
init(red: Double, green: Double, blue: Double) {
self.red = red
self.green = green
self.blue = blue
}
}
let magenta = Color(red: 1.0, green: 0.0, blue: 1.0)
let veryGreen = Color(0.0, 1.0, 0.0)
// this reports a compile-time error - external names are required
~~~
You can modify the value of a constant property at any point during initialization, as long as it is set to a definite value by the time initialization finishes.
~~~
class SurveyQuestion {
let text: String
var response: String?
init(text: String) {
self.text = text
}
func ask() {
println(text)
}
}
let beetsQuestion = SurveyQuestion(text: "How about beets?")
beetsQuestion.ask()
// prints "How about beets?"
beetsQuestion.response = "I also like beets. (But not with cheese.)"
~~~
structure types automatically receive a memberwise initializer if they provide default values for all of their stored properties and do not define any of their own custom initializers.
~~~
struct Size {
var width = 0.0, height = 0.0
}
let twoByTwo = Size(width: 2.0, height: 2.0)
~~~
If you want your custom value type to be initializable with the default initializer and memberwise initializer, and also with your own custom initializers, write your custom initializers in an extension rather than as part of the value type’s original implementation.
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 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.
To simplify the relationships between designated and convenience initializers, Swift applies the following three rules for delegation calls between initializers:
Rule 1： Designated initializers must call a designated initializer from their immediate superclass.
Rule 2： Convenience initializers must call another initializer available in the same class.
Rule 3： Convenience initializers must ultimately end up calling a designated initializer.
A simple way to remember this is:
Designated initializers must always delegate up. Convenience initializers must always delegate across.
Class initialization in Swift is a two-phase process. In the first phase, each stored property is assigned an initial value by the class that introduced it. Once the initial state for every stored property has been determined, the second phase begins, and each class is given the opportunity to customize its stored properties further before the new instance is considered ready for use.
~~~
class Food {
var name: String
init(name: String) {
self.name = name
}
convenience init() {
self.init(name: "[Unnamed]")
}
}
let namedMeat = Food(name: "Bacon")
// namedMeat's name is “Bacon"
let mysteryMeat = Food()
// mysteryMeat's name is "[Unnamed]”
class RecipeIngredient: Food {
var quantity: Int
init(name: String, quantity: Int) {
self.quantity = quantity
super.init(name: name)
}
convenience init(name: String) {
self.init(name: name, quantity: 1)
}
}
let oneMysteryItem = RecipeIngredient()
let oneBacon = RecipeIngredient(name: "Bacon")
let sixEggs = RecipeIngredient(name: "Eggs", quantity: 6)
class ShoppingListItem: RecipeIngredient {
var purchased = false
var description: String {
var output = "\(quantity) x \(name.lowercaseString)"
output += purchased ? " ✔" : " ✘"
return output
}
}
var breakfastList = [
ShoppingListItem(),
ShoppingListItem(name: "Bacon"),
ShoppingListItem(name: "Eggs", quantity: 6),
]
breakfastList[0].name = "Orange juice"
breakfastList[0].purchased = true
for item in breakfastList {
println(item.description)
}
// 1 x orange juice ✔
// 1 x bacon ✘
// 6 x eggs ✘
~~~
~~~
class SomeClass {
let someProperty: SomeType = {
// create a default value for someProperty inside this closure
// someValue must be of the same type as SomeType
return someValue
}()
}
~~~
Note that the closure’s end curly brace is followed by an empty pair of parentheses. This tells Swift to execute the closure immediately.
~~~
struct Checkerboard {
let boardColors: Bool[] = {
var temporaryBoard = Bool[]()
var isBlack = false
for i in 1...10 {
for j in 1...10 {
temporaryBoard.append(isBlack)
isBlack = !isBlack
}
isBlack = !isBlack
}
return temporaryBoard
}()
func squareIsBlackAtRow(row: Int, column: Int) -> Bool {
return boardColors[(row * 10) + column]
}
}
let board = Checkerboard()
println(board.squareIsBlackAtRow(0, column: 1))
// prints "true"
println(board.squareIsBlackAtRow(9, column: 9))
// prints "false"
~~~