The doOn*() operators

In the previous chapter, we've used the map() operator a couple of times to just print to the console what's happening inside our Observable chains. However, this isn't very convenient. The map() operator always needs to return a value that is passed further down the chain, and it can catch only onNext signals.

That's why RxPHP has a couple of operators with the common prefix doOn*:

• doOnNext(), doOnError(), doOnCompleted(): Each of these operators takes a callable as a parameter that is executed when they receive their respective signal
• doOnEach(): This operator takes an instance of ObserverInterface as a parameter and executes its handlers for each signal

So these operators are very similar to the methods subscribeCallback() and subscribe(). The biggest advantage is in the way doOn* operators work internally. They never modify the value going through and just execute our callables, which is ideal for quickly...