Is there a preferred type for an Observable with no need for a value in Next events?

Question

I have an Observable which is only used for triggering flatMap/map. So I only ever need the Next event and never a value. I could use my own concept for such a trash value, but I'm wondering if there's an RxSwift convention for it.

Here's what I'm dealing with:

// I'd rather not have an Element type that someone might use let triggeringObservable: Observable<SomeSessionClass>   // ...  triggeringObservable.map { _ -> String in // The actual value is ignored     return SomeLibrary.username() // `username()` is only ready when `triggeringObservable` sends Next } 

In this example, triggeringObservable is rx_observer on some property in the library which will let us know that username() is ready to be called.

Answer 1

You can simply use an Observable<Void> for this purpose. Like so:

   let triggerObservable = Observable<Void>.just()      triggerObservable.subscribeNext() {         debugPrint("received notification!")     }.addDisposableTo(disposeBag) 

or in your example:

let triggeringObservable: Observable<Void>   // ...  triggeringObservable.map { Void -> String in // The actual value is ignored    return SomeLibrary.username()  } 

Answer 2

In Swift 4 (and probably earlier) you can pass an empty tuple as an argument to a method expecting a Void associated type.

var triggerObservable = PublishSubject<Void>() ... triggerObservable.onNext(()) // "()" empty tuple is accepted for a Void argument 

Or you can define an extension to overload onNext() with no args for the Void case:

extension ObserverType where E == Void {     public func onNext() {         onNext(())     } }  ... triggerObservable.onNext()