Combine a list of Observables and wait until all completed

You can use flatMap in case you have dynamic tasks composition. Something like this:

public Observable<Boolean> whenAll(List<Observable<Boolean>> tasks) {
    return Observable.from(tasks)
            //execute in parallel
            .flatMap(task -> task.observeOn(Schedulers.computation()))
            //wait, until all task are executed
            //be aware, all your observable should emit onComplete event
            //otherwise you will wait forever
            .toList()
            //could implement more intelligent logic. eg. check that everything is successful
            .map(results -> true);
}

Another good example of parallel execution

Note: I do not really know your requirements for error handling. For example, what to do if only one task fails. I think you should verify this scenario.

It sounds like you're looking for the Zip operator.

There are a few different ways of using it, so let's look at an example. Say we have a few simple observables of different types:

Observable<Integer> obs1 = Observable.just(1);
Observable<String> obs2 = Observable.just("Blah");
Observable<Boolean> obs3 = Observable.just(true);

The simplest way to wait for them all is something like this:

Observable.zip(obs1, obs2, obs3, (Integer i, String s, Boolean b) -> i + " " + s + " " + b)
.subscribe(str -> System.out.println(str));

Note that in the zip function, the parameters have concrete types that correspond to the types of the observables being zipped.

Zipping a list of observables is also possible, either directly:

List<Observable<?>> obsList = Arrays.asList(obs1, obs2, obs3);

Observable.zip(obsList, (i) -> i[0] + " " + i[1] + " " + i[2])
.subscribe(str -> System.out.println(str));

...or by wrapping the list into an Observable<Observable<?>>:

Observable<Observable<?>> obsObs = Observable.from(obsList);

Observable.zip(obsObs, (i) -> i[0] + " " + i[1] + " " + i[2])
.subscribe(str -> System.out.println(str));

However, in both of these cases, the zip function can only accept a single Object[] parameter since the types of the observables in the list are not known in advance as well as their number. This means that that the zip function would have to check the number of parameters and cast them accordingly.

Regardless, all of the above examples will eventually print 1 Blah true

EDIT: When using Zip, make sure that the Observables being zipped all emit the same number of items. In the above examples all three observables emitted a single item. If we were to change them to something like this:

Observable<Integer> obs1 = Observable.from(new Integer[]{1,2,3}); //Emits three items
Observable<String> obs2 = Observable.from(new String[]{"Blah","Hello"}); //Emits two items
Observable<Boolean> obs3 = Observable.from(new Boolean[]{true,true}); //Emits two items

Then 1, Blah, True and 2, Hello, True would be the only items passed into the zip function(s). The item 3would never be zipped since the other observables have completed.