What does this mean: "You do not have to provide an explicit implementation of a required initializer if you can satisfy the requirement"? - swift

Question

I've found this note in the Swift documentation about initializers:
You do not have to provide an explicit implementation of a required initializer if you can satisfy the requirement with an inherited initializer.
What is an "explicit" implementation? What is an "implicit" one then?
What does "satisfy the requirement with an inherited initializer" mean precisely?
Could you give me a code example, in which I don't have to provide an explicit implementation of a required initializer?

Answer 1

Here's an example with an inline explanation:
protocol JSONInitializable { // Use Encoders, but just for example
init(fromJSON: String)
}
class Foo: JSONInitializable {
let x: Int
// "required" is necessary because this init is required for the
// conformance to JSONInitializable
required init(fromJSON json: String) {
//...
x = 123 //some value from the JSON
}
}
class Baz: Foo {
// `init(fromJSON json: String)` can be inherited,
// so it's implicitly defined for Baz, as well as Foo.
}
class Bar: Foo {
// The presence of this uninitialized constant `y` requires an
// a value in the declaration, or an initializer that sets it
let y: Int
// Since we didn't specify a value for `y` in its declaration,
// this initializer must be explicitly specified so as to initialize `y`.
// Doing so blocks the inheritance of `init(fromJSON json: String)` from
// the super class, and requires us to define it ourselves,
// in order to preserve conformance to `JSONInitializable`
required init(fromJSON json: String) {
//...
y = 0
super.init(fromJSON: json)
}
}

Answer 2

It is saying this: If you have initialized all your properties as you declare them, there's no need to write an initializer.
Normally, when you have properties declared and not set, you write an init() method and set them there, and if there's a required initializer in the parent class, you call
super.init(possible, arg: anotherArg)
Since you don't need to set anything, you don't need to write anything, and since there's no init in your class, the super call will happen automatically. (Of course, if the required init needs values passed in, you still need to supply them.) So how is that accomplished? (see below.) The bottom line is that most of the work is done for you.
However, once you write an init(), then you're taking away this automatic behavior, and you have to make sure that those "automatic" calls are explicitly made.
Finally, before I provide an example, I should address this statement:
"if you can satisfy the requirement with an inherited initializer"
If the parent doesn't have an initializer which takes no arguments, how will it get those arguments? In that case you need initialize your class using that required super's init with the proper arguments.
Here's an example of a view controller which is not set up in IB, I would normally create it using the super's required initializer, since I didn't write an initializer for my derived class:
let vc = MagicController(nibName: nil, bundle: nil)
import UIKit
import Dotzu
class MagicController: UIViewController, UIGestureRecognizerDelegate {
let action = #selector(MagicController.buttonTapped(_:))
let action2 = #selector(MagicController.secondButtonTapped(_:))
let mainAction = #selector(MagicController.mainButtonTapped(_:))
let defaultPalette = Palette.randomPalette()
var questions = [Question]()
var startTime = TimeInterval()
var endTime = TimeInterval()
var selectedTimeIndex = 0
var selectedTagIndex = 0
var selectedRatingIndex = 0
var selectedTag = "swift"
let timeSpanDropDown = DropDown()
let ratingDropDown = DropDown()
let tagDropDown = DropDown()
var pageNumber = 1
var savedIndex = 0
var savedPostId = -1
var quotaCount = -1
var isFirstTime = true
let queryFactory = Queries(client: APIClient())
var timeSpanButton = UIBarButtonItem()
var ratingButton = UIBarButtonItem()
var tagButton = UIBarButtonItem()
var dbButton = UIBarButtonItem() // 🗄
/// start the console log, configure the main view's buttons/actions and present the UI
override func viewDidLoad() {
super.viewDidLoad()
Dotzu.sharedManager.enable()
self.edgesForExtendedLayout = []
configureButtons(container: view)
view.setNeedsLayout()
}
///configure the buttons/actions, prepare and present the UI
/// The first time this is called it sets up the buttons and drop downs
/// cleanupDataSource() has no effect the first time it is called
/// - Parameter animated: passed to super
override func viewWillAppear(_ animated: Bool) {
super.viewWillAppear(animated)
if isFirstTime {
isFirstTime = false
initializeButtons()
setupDropDowns()
}
cleanupDataSource()
}
/// etc.
}
As you can see, I did not write an init for this view controller, even though a view controller has a required init. I initialized my view controller using an init() that isn't even in my code.