Quite often when working with Rx, you’ll end up with an observable of observables:
IObservable<IObservable<int>> for example. When you were learning Rx, you were probably told to use
SelectMany to flatten the observable:
obs1 .SelectMany(_ => Observable.FromAsync(() => DoWebCall())) ...
This flattens the
IObservable<int>. “Case closed” you might think, but here’s something to consider:
SelectMany will merge all the observables returned inside the operator. Let’s look a bit into that.
To better understand
SelectMany we’ll start by having a look at
I think the most common uses of
Merge is either as a static method on
Observable or as an operator taking an argument:
// As a static method Observable.Merge( obs1, obs2 ) ... // As an operator taking an argument obs1 .Merge(obs2) ...
Merge can also be used as an operator taking no arguments:
obs1 .Merge() ...
Merge only works on
IObservable<IObservable<T>>, or an observable of observables. Observables of observables are a bit hard to grasp, so here’s one way to think about it:
IObservable<T> is something that will happen some time in the future. That future might be 1ms away or 10 minutes, doesn’t matter when - you’ll get a signal when it happens. The signal will contain 1 value of type
IObservable<IObservable<T>> is something that will signal when something is scheduled to happen in the future. When that scheduled thing happens, you’ll get a signal (and that signal will contain 1 value of type
This is can very quickly become very complex, which is why merge these observable. Instead of being told that a new observable has been added, we automatically subscribe to the new observable and get notified about the result in our existing observable.
This is what
Merge does. You give it an
IObservable<IObservable<T>> and you’ll get back an
IObservable<T>. Whenever there’s a new inner observable,
Merge will automatically subscribe to it and pass Next and Error signals to the outer observable.
Merge in this manner, we can rewrite the first example like this:
// First example obs1 .SelectMany(_ => Observable.FromAsync(() => DoWebCall())) ... // Rewritten obs1 .Select(_ => Observable.FromAsync(() => DoWebCall())) .Merge() ...
You’ve just seen how
SelectMany basically works:
- Transform a signal to an observable (
- Merge that signal into the original/outer observable (
Imagine however that the observable that is returned inside the
SelectMany emits multiple signals. In some cases this is what you want, but when doing web requests you’re most likely only interested in the most recent request sent.
SelectMany will pass on any signal that is returned, regardless whether they are in order or not.
This is perhaps best explained visually. This shows an
Observable.Interval emitting a signal every 2 seconds (these signals are again transformed to strings to more easily distinguish them). That signal is transformed to a new
Observable.Interval which will emit signals half a second apart.
Notice how the signals from the different
Observable.Intervals mix with each other. That might not be what we want.
Switch also works on
IObservable<IObservable<T>>, but in contrast to
Merge, it switches to the most recent
IObservable<T> inside the
When a new
IObservable<T> is emitted by
Switch will unsubscribe to previous
IObservable<T> and subscribe to the new one. That way you won’t get any signals from the previous observable if it should happen to send a signal after you’ve received the new observable.
This example uses a
Select().Switch() combo. Notice how we only receive signals from the most recent
The last operator I’m going to cover is
Concat also only allows one inner observable produce values, but it will wait for the previous to finish before it subscribes to the new one. When a complete signal is sent by the previous observable,
Concat will subscribe to the next and let that finish before subscribing to the next one.
Let’s look at a gif:
Each observable emits 4 signals, and when that’s done
Concat let’s the next observable emit it’s 4 values.
The tool I used for these examples is Linqpad. It’s an excellent tool if you’re doing any kind .Net development.