Hi ! Can anyone explain for me the mean of phase: "where Self == PeriodicTimelineSchedule". I mean, idk what is SELF , can the == operator?.

extension TimelineSchedule where Self == PeriodicTimelineSchedule {
    static var everyFiveSeconds: PeriodicTimelineSchedule {
        get { .init(from: .now, by: 5.0) }
    }
}

So there is a protocol called TimelineSchedule. Different data structures can conform to TimelineSchedule. So you could have a PeriodicTimelineSchedule or ExplicitTimelineSchedule or a EveryMinuteTimelineSchedule.

Self refers to the type of whatever it is you are extending.

This extension is saying "extend the TimelineSchedule protocol with a new static computed var property called everyFiveSeconds but only do it when the actual type is a PeriodicTimelineSchedule". In other words, it will not extend any other conforming type (like ExplicitTimelineSchedule or EveryMinuteTimelineSchedule).

extension TimelineSchedule where Self == PeriodicTimelineSchedule {
    static var everyFiveSeconds: PeriodicTimelineSchedule {
        get { .init(from: .now, by: 5.0) }
    }
}

let sched1: PeriodicTimelineSchedule = .everyFiveSeconds       //allowed
let sched2: EveryMinuteTimelineSchedule = .everyFiveSeconds    //error!
let sched3: ExplicitTimelineSchedule = .everyFiveSeconds       //error!

Difference between self and Self.

thanks ! I got this

If TimelineSchedule and PeriodicTimelineSchedule are both protocols, is there a reason that the extension is on TimelineSchedule with a requirement that the instance be a PeriodicTimelineSchedule, rather than make it an extension on PeriodicTimelineSchedule?

TimelineSchedule is a protocol.

PeriodicTimelineSchedule is a struct that conforms to the TimelineSchedule protocol.

That makes sense. For some reason, I thought that PeriodicTimelineSchedule was a protocol as well. My bad.