ObservingPantry.swift

final class Pantry {
    let jams = Observable([Jam(flavour: .apple)])
    
    func add(jam: Jam) {
        jams.value.append(jam)
    }
}

struct Jam {
    enum Flavour: String {
        case apple, orange
    }
    
    let flavour: Flavour
    
    init(flavour: Flavour) {
        self.flavour = flavour
    }
}

let pantry = Pantry()
print("Adding count and contents observers.")
let observer = pantry.jams.observe { (jams) in
    print("Pantry now has \(jams.count) jars of jam.")
}
pantry.jams.observe(initial: true) { (jams) in
    let contents = jams.map({ $0.flavour.rawValue }).joined(separator: ", ")
    print("Jams in pantry: \(contents)")
}
print("Adding jam to pantry.")
pantry.add(jam: Jam(flavour: .orange))

print("Removing count observer.")
pantry.jams.removeObserver(observer)
print("Adding jam to pantry.")
pantry.add(jam: Jam(flavour: .apple))

/*
Adding count and contents observers.
Jams in pantry: apple
Adding jam to pantry.
Pantry now has 2 jars of jam.
Jams in pantry: apple, orange
Removing count observer.
Adding jam to pantry.
Jams in pantry: apple, orange, apple
*/