Have been exploring this quiz:

var quiz = ("iOS", "Quiz") {
    didSet {
        quiz.1 = "Swift"
    }
}
quiz.0 = "Advanced"
// What's the value of quiz?

Answer is obvious. Value of quiz variable is ("Advanced", "Swift").

Solution: ("Advanced", "Swift")

But what the hell are they talking about “observer will only avoid being recursively called” and “property is set on self”:

In Swift 5, when setting a property from within its own didSet or willSet observer, the observer will only avoid being recursively called if the property is set on self (either implicitly or explicitly)

First, 1 isn’t a property but element of tuple, at least Swift reference manual calls it so. Second, it isn’t accessed with self, but with the name of the variable. Actually it isn’t possible to use self.1 in context of didSet here, compiler complains Use of unresolved identifier 'self'. I find it weird as you have to remember to change name of observed variable in didSet when you change name of variable quiz. Third, despite element 1 is accessed not with self, observer avoids running recursively.

What I have learned here, is that any variable could be observed, not only property. So this is valid:

var a = 10 {
    didSet {
        print(a)
    }
}

Again, a in snipped above could be accessed with variable name, not with self. I find it to ba a flow of the language.

Another quiz. I find this quiz question really useful for understanding protocol extensions:

protocol Drawing {
    func render()
}

extension Drawing {
    func circle() { print("protocol") }
    func render() { circle() }
}

class SVG: Drawing {
    func circle() { print("class") }
}

SVG().render()

// What's the output?

TODO: I am feeling like I have to reread about protocol extensions in Swift.

Exploring apple docs about protocol extensions found out that the first snippet in chapter Protocols of Swift language guide doesn’t compile. Asked Swift forums if I should report this.

Finally, I got quiz question with Drawing above. Extension of Drawing adds func circle() and this isn’t additional requirement! This is just part of implementation of protocol extensions. Protocol extension can’t extend protocols with new requirements! This means that func circle() from SVG conforming to protocol Drawing is just different function, it doesn’t implements protocol requirement or override something!

Accidentally found Appetize which allows to run demo of iOS apps in web. I should use it!