RxSwift: Return a new observable with an error

Question

I have a function that return a Bool Observable depending if it was ok or not.

func test() -> Observable<Bool> {
   if everythingIsOk {
      return just(true)
   }
   return just(false) <- how can i here return a custom error to retrieve what failed?
}

Solution 1

just<E>(element: E) -> Observable<E>

Returns an observable sequence that contains a single element. Instead, you should use something like that:

create<E>(subscribe: (AnyObserver<E>) -> Disposable) -> Observable<E>

Create method creates an observable sequence from a specified subscribe method implementation.

In your case:

private let realm = try! Realm()

func save(customObject: CustomObject) -> Observable<Bool> {
    return create({ observer -> Disposable in
        do {
            try self.realm.write {
                self.realm.add(customObject, update: true)
                observer.onNext(true)
                observer.onCompleted()
            }
        } catch {
            // .Error sequence will be automatically completed
            observer.onError(NSError(domai...)
        }

        // if realm.write is sync task(by default it is, as I know) you can actually return NopDisposable
        return NopDisposable.instance
        // otherwise you should cancel write transaction in AnonymousDisposable
    })
}

AnonymousDisposable is the action that’s called in case you want to get interrupted. Say you leave your view controller or the app needs to be done with the service and you don’t need to call this request any longer. It’s great for video uploads or something much larger. You can do request.cancel() which cleans up all the resources when you’re done with it. This gets called on either completion or error.

Solution 2

For creating observables there is create function. You can use it like this:

func test() -> Observable<Bool> {
   return create({ (observer) -> Disposable in
//      Some condition
      observer.onNext(true)
//      Some other condition
      observer.onNext(false)
//      Some other condition
      observer.onError(NSError(domain: "My domain", code: -1, userInfo: nil))
//      Some other condition
      observer.onCompleted()

      return AnonymousDisposable {
//         Dispose resources here
      }
//      If u have nothing to dipose use NopDisposable.instance


   })
}

Solution 3

Use a result enum as your observable value.

public enum Result<Value> {
    case success(Value)
    case failure(Error)
}

func test() -> Observable<Result<Bool>> {
   if everythingIsOk {
      return just(.success(true))
   }

   let error = ...
   return just(.failure(error))
}

Comments

[Godfather]

Whats the difference between observer.onNext(true) And just(true)

[Serg Dort]

In your simple example there is no difference, but in more complex, if for example you need to make some asynchronous work, like network request or fetch some data from data base you will not be able to do this. And just operator never fails it "just" posts the value , which you what to post

[Godfather]

I'm not sure why you put .onCompleted() in other condition, its supposed to be called always despite it completes successful or failed ?

[Serg Dort]

The meaning of .onCompleted() that you are done. Yeah my bad :( . Also it will call onComplete callback if it exists on subscriptions and (as well as onError) call dispose on Disposable

[Morgz]

Just want to add that in my experience it's best to put the onNext() etc. outside of the try block. The Realm write isn't actually persisted until it exists that closure.

[Ariel]

observables should send values of the specified type, or fail. mixing the 2 is a negative pattern to observables concept because you can receive an error either in the onNext block and the onError block.

[Tommy Sadiq Hinrichsen]

You are correct, but there are some cases where you don't want the subscription to end because of an error.

[Ariel]

than you can use materialize/dematerialize or catchError(...)/catchErrorJustReturn(...).

[Tommy Sadiq Hinrichsen]

catchError and catchErrorJustReturn will also send a completed event and thus ending the subscription.