Does the order of subscribeOn and observeOn matter?

Question

I'm a little bit confused about the order you can call the subscribeOn and observeOn methods on observables. I read a couple of posts and one guys says that it doesn't matter and just uses thing in his example and other people say it does matter. So here is my question:

For example:

self.remoteService.rxGetAllLanguages()
            .observeOn(MainScheduler.instance)
            .subscribeOn(ConcurrentDispatchQueueScheduler(globalConcurrentQueueQOS: .Background))
            .subscribe({ e in
                switch e {
                case .Next(let element):

                case .Error(let e):
                    DDLogError("Error in  \(e)")
                case .Completed:
                    DDLogDebug("Completed")
                }
                }
            ).addDisposableTo(self.disposeBag)

Is that the same as:

  self.remoteService.rxGetAllLanguages()
                    .subscribeOn(ConcurrentDispatchQueueScheduler(globalConcurrentQueueQOS: .Background))
                    .observeOn(MainScheduler.instance)
                    .subscribe({ e in
                        switch e {
                        case .Next(let element):

                        case .Error(let e):
                            DDLogError("Error in  \(e)")
                        case .Completed:
                            DDLogDebug("Completed")
                        }
                        }
                    ).addDisposableTo(self.disposeBag)

If I correctly understand the mechanisms they are different. The first one does all the work on the main thread and the second does all the work on another thread and then dispatches back to the main thread. But I'm nut sure so can someone clear this for me please?

Answer 1

Where you call subscribeOn() in a chain doesn't really matter. Where you call observeOn() does matter.

subscribeOn() tells the whole chain which thread to start processing on. You should only call it once per chain. If you call it again lower down the stream it will have no effect.

observeOn() causes all operations which happen below it to be executed on the specified scheduler. You can call it multiple times per stream to move between different threads.

Take the following example:

doSomethingRx()
    .subscribeOn(BackgroundScheduler)
    .doAnotherThing()
    .observeOn(ComputationScheduler)
    .doSomethingElse()
    .observeOn(MainScheduler)
    .subscribe(//...)

• The subscribeOn causes doSomethingRx to be called on the BackgroundScheduler.
• doAnotherThing will continue on BackgroundScheduler
• then observeOn switches the stream to the ComputationScheduler
• doSomethingElse will happen on the ComputationScheduler
• another observeOn switches the stream to the MainScheduler
• subscribe happens on the MainScheduler