what is 'where self' in protocol extension - swift

Question

I saw so many examples with below format
extension Protocolname where Self: UIViewController
What is where Self in protocol extension. I couldn't find the documentation on this.

Answer 1

That syntax is: https://developer.apple.com/library/content/documentation/Swift/Conceptual/Swift_Programming_Language/Protocols.html#//apple_ref/doc/uid/TP40014097-CH25-ID521
Consider:
protocol Meh {
func doSomething()
}
// Extend protocol Meh, where `Self` is of type `UIViewController`
// func blah() will only exist for classes that inherit `UIViewController`.
// In fact, this entire extension only exists for `UIViewController` subclasses.
extension Meh where Self: UIViewController {
func blah() {
print("Blah")
}
func foo() {
print("Foo")
}
}
class Foo : UIViewController, Meh { //This compiles and since Foo is a `UIViewController` subclass, it has access to all of `Meh` extension functions and `Meh` itself. IE: `doSomething, blah, foo`.
func doSomething() {
print("Do Something")
}
}
class Obj : NSObject, Meh { //While this compiles, it won't have access to any of `Meh` extension functions. It only has access to `Meh.doSomething()`.
func doSomething() {
print("Do Something")
}
}
The below will give a compiler error because Obj doesn't have access to Meh extension functions.
let i = Obj()
i.blah()
But the below will work.
let j = Foo()
j.blah()
In other words, Meh.blah() is only available to classes that are of type UIViewController.

Answer 2

Here is an example which explains that what is the use of where self: UIViewController
protocol SBIdentifiable {
static var sbIdentifier: String { get }
}
extension SBIdentifiable where Self: UIViewController {
static var sbIdentifier: String {
return String(describing: self)
}
}
extension UIVieWcontroller: SBIdentifiable { }
class ViewController: UIViewController {
func loadView() {
/*Below line we are using the sbIdentifier which will return the
ViewController class name.
and same name we would mentioned inside ViewController
storyboard ID. So that we do not need to write the identifier everytime.
So here where Self: UIViewController means it will only conform the protocol of type UIViewController*/
let viewController = self.instantiateViewController(withIdentifier:
self.sbIdentifier) as? SomeBiewController
}
}

Answer 3

You can find the same example here: WWDC2015-408, (Highly recommend to watch it，it illustrates the reason)
And also, another similar example is Extensions with a Generic Where Clause
struct Stack<Element> {
var items = [Element]()
mutating func push(_ item: Element) {
items.append(item)
}
mutating func pop() -> Element {
return items.removeLast()
}
}
The where clause add a requirement to the extension, so that the extension adds the isTop(_:) method only when the items in the stack are equatable.
extension Stack where Element: Equatable {
func isTop(_ item: Element) -> Bool {
guard let topItem = items.last else {
return false
}
return topItem == item
}
}