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Let's say you've installed RxSwift via CocoaPods or whatever. And you can compile with import RxSwift at the top of your swift file.

Let's create a simple Observable with Observable.from simply listing a bunch of integers.

We'll subscribe it, looking at the next and error states. When we hit onNext we just print:

let _ = Observable.from([404, 808]).subscribe(
    onNext: {s in
        print(s)
    },
    onError: {e in
        if e is GatesOfHellOpen {
            print("It's a Saturday!")
        }
    })
}

The onError function is odd. First it's referencing an error. There's probably an easier and better way to do this with enums but I know little about that yet. They're defined class TheRapture : Error {} and class GatesOfHellOpen : Error {}.

If you want to send one down, change the first line for let _ = Observable.error(GatesOfHellOpen()).subscribe( for example.