rxcpp - why don't all observers' on_next function get called when an observable emits a value

Question

I'm trying to understand how to use rxcpp, my impression was that when an observable emits a value, all observers who are subscribed will get notified by having their their on_next() methods called, passing them the emitted value.

This is not the case with the following example:

auto eventloop = rxcpp::observe_on_event_loop();

printf("Start task\n");

auto values = rxcpp::observable<>::interval(std::chrono::seconds(2)).map(
        [](int i){
            printf("Observable sending: %d\n", i);
            return i;
        }
);

values.
    subscribe_on(eventloop).
    take(2).
    as_blocking().
    subscribe(
        [](int v){printf("#1 onNext: %d\n", v);},
        [](){printf("#1 onCompleted\n");});

values.
    subscribe_on(eventloop).
    take(2).
    as_blocking().
    subscribe(
        [](int v){printf("#2 onNext: %d\n", v);},
        [](){printf("#2 onCompleted\n");});

printf("Finish task\n");

I expected the output to be something like:

Start task
Observable sending: 1
#1 onNext: 1
#2 onNext: 1
Observable sending: 2
#1 onNext: 2
#1 onCompleted
#2 onNext: 2
#2 onCompleted
Finish task

i.e. on_next being called on all subscribed observers when the new value comes through.

Instead, the output is actually:

Start task
Observable sending: 1
#1 onNext: 1
Observable sending: 2
#1 onNext: 2
#1 onCompleted
Observable sending: 1
#2 onNext: 1
Observable sending: 2
#2 onNext: 2
#2 onCompleted
Finish task

Using as_blocking() will prevent the second subscribe from starting until the fist completes — Kirk Shoop, Sep 25 '16 at 21:59

score 2 · Accepted Answer · answered Sep 25 '16 at 19:18

This is the classic hot vs. cold behavior.

A hot observable will do as you expect. Interval is a cold observable so each subscribe produces an independent set of values.

The publish operator will take a single cold observable and share it as a hot observable.

In this case it would be.

auto sharedvalues = values.publish().ref_count();

Then use sharedvalues instead of values in the subscribe expressions.

A search for hot vs cold observables will find extensive discussion of this topic.

rxcpp - why don't all observers' on_next function get called when an observable emits a value

1 Answers1