What’s the difference between SubscribeOn and ObserveOn

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It helped me to understand this by thinking of SubscribeOn as setting the thread being “passed up” the chain and ObserveOn as setting the thread “passed down” the chain.

Subscriber thread "passed up" and Observer thread "passed down"

The code below uses named threads which you can play with.

Thread.CurrentThread.Name = "Main";

IScheduler thread1 = new NewThreadScheduler(x => new Thread(x) { Name = "Thread1" });
IScheduler thread2 = new NewThreadScheduler(x => new Thread(x) { Name = "Thread2" });

Observable.Create<int>(o =>
{
    Console.WriteLine("Subscribing on " + Thread.CurrentThread.Name);
    o.OnNext(1);
    return Disposable.Create(() => {});
})
.SubscribeOn(thread1)
.ObserveOn(thread2)
.Subscribe(x => Console.WriteLine("Observing '" + x + "' on " + Thread.CurrentThread.Name));

The output of the above is:

Subscribing on Thread1
Observing 1 on Thread2

It’s also interesting to see that when you comment out the SubscribeOn line, the output is:

Subscribing on Main
Observing 1 on Thread2

Because by default the subscription “passes up” whichever thread was running (Main here). Then the ObserveOn “passes down” Thread2.

If you instead comment out the ObserveOn line, the output is:

Subscribing on Thread1
Observing 1 on Thread1

Because we “pass up” the subscription on Thread1, and by default this same thread is “passed down” and used to run the observation.

In a GUI context, to keep things responsive you want the least amount of work done on the GUI thread but you need the subscription done on the GUI thread (to synchronise UI updates). So you want to .ObserveOn the GUI thread.

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