将2个RxJava Observables转换为1个地图

Question

I have following scenario. I'm getting data from server using Retrofit2. I want to download Collection<Account> of Account entities, and for each of those accounts to download Collection<Project> of Project entities. At the end I want to get Map<Account, Collection<Project>> accountsWithProjects .

I have following methods:

1. Observable<Collection<Account>> requestAllAccounts()
2. Observable<Collection<Project>> requestProjectsInfoForTempoAccount(Account accunt)

Question: Is it possible to zip or create somehow foreach iteration of these two reactive streams and collect all the items to the map?

I can also get the collection of projects for account synchronously if it helps, however can't do it on main thread, since developing for Android and the result must be observed on main thread.

Answer 1

There you have it:

requestAllAccounts().flatMap(new Func1<Collection<Account>, Observable<Account>>() {
        @Override
        public Observable<Account> call(Collection<Account> accountCollection) {
            return Observable.from(accountCollection);
        }
    }).flatMap(new Func1<Account, Observable<Pair<Account, Collection<Project>>>>() {
        @Override
        public Observable<Pair<Account, Collection<Project>>> call(Account account) {
            return Observable.just(account).zipWith(requestProjectsInfoForTempoAccount(account),
                    new Func2<Account, Collection<Project>, Pair<Account, Collection<Project>>>() {
                        @Override
                        public Pair<Account, Collection<Project>> call(Account account, Collection<Project> projects) {
                            return new Pair<Account, Collection<Project>>(account, projects);
                        }
                    });
        }
    }).toList()
            .flatMap(new Func1<List<Pair<Account, Collection<Project>>>, Observable<Map<Account, Collection<Project>>>>() {
                @Override
                public Observable<Map<Account, Collection<Project>>> call(List<Pair<Account, Collection<Project>>> pairs) {
                    Map<Account, Collection<Project>> map = new HashMap<Account, Collection<Project>>();
                    for (Pair<Account, Collection<Project>> pair : pairs) {
                        map.put(pair.first, pair.second);
                    }
                    return Observable.just(map);
                }
            });

What is going on here?

1. Get all accounts.
2. Split list into single accounts
3. Invoke searching for projects on single account and return the Pair<Account, Collection<Project>>
4. Merge all objects
5. Transform List<Pair<Account, Collection<Project>>> to Map<Account, Collection<Project>>

Answer 2

Here's a better way:

requestAllAccounts()
.flatMap(account -> requestProjectsInfoForTempoAccount(account),
   (acccount, projects) -> Pair::of)
.toMap(Pair::key, Pair::value);

(expansion to Java 7 left as an excercise for the reader)