RxJava - 将列表和请求转换为地图

Question

I have a list of "Parent" objects and I want to get a "Child" list for each of them. I want as result a map of Parent and Child list. So the result is

Map<Parent, List<Child>> result = new HashMap<>();

My sample Parent list:

List<Parent> parents = new ArrayList<>();
parents.add(new Parent(1, "Parent1"));
parents.add(new Parent(2, "Parent2"));
parents.add(new Parent(3, "Parent3"));
parents.add(new Parent(4, "Parent4"));
parents.add(new Parent(5, "Parent5"));

I want to iterate them and ask children one by one

 @GET("api/childs/{parentId}")
 Observable<Response<List<Child>>> getChilds(@Path("parentId") int parentId);

What is the best RX structure for this?

Thank you, Robert

Answer 1

Observable.from(parents)
    .concatMap(p -> getChilds(p.getId())
        .map(Response::getValue)
        .map(children -> Pair.of(p, children)))
    .toMap(Pair::getLeft, Pair::getRight)

Where Pair is a simple wrapper of two value left and right .

Answer 2

The implementation that I thought of is along these lines:

Iterate through the parents, and for each try to form a Map.Entry with the children by using flatMap() . Then you can collect them into a Map . The code looks like this:

public static void main(String[] args) {

    List<Parent> parents = new ArrayList<>();
    parents.add(new Parent(1, "Parent1"));
    parents.add(new Parent(2, "Parent2"));
    parents.add(new Parent(3, "Parent3"));
    parents.add(new Parent(4, "Parent4"));
    parents.add(new Parent(5, "Parent5"));

    Single<Map<Parent, List<Child>>> result = Observable.fromIterable(parents)
            .flatMap(getMapEntries())
            .toMap(keySelector(), valueSelector());
}

private static Function<Parent, Observable<Map.Entry<Parent, List<Child>>>> getMapEntries() {
    return new Function<Parent, Observable<Map.Entry<Parent, List<Child>>>>() {
        @Override
        public Observable<Map.Entry<Parent, List<Child>>> apply(Parent parent) throws Exception {
            return getChilds(parent.getId())
                    .map(extractResponse())
                    .map(createMapEntry(parent));
        }
    };
}

private static Function<Response<List<Child>>, List<Child>> extractResponse() {
    return new Function<Response<List<Child>>, List<Child>>() {

        @Override
        public List<Child> apply(Response<List<Child>> listResponse) throws Exception {
            return listResponse.body();
        }
    };
}

private static Function<List<Child>, Map.Entry<Parent, List<Child>>> createMapEntry(Parent parent) {
    return new Function<List<Child>, Map.Entry<Parent, List<Child>>>() {
        @Override
        public Map.Entry<Parent, List<Child>> apply(List<Child> children) throws Exception {
            return new AbstractMap.SimpleImmutableEntry<>(parent, children);
        }
    };
}

private static Function<Map.Entry<Parent, List<Child>>, Parent> keySelector() {
    return new Function<Map.Entry<Parent, List<Child>>, Parent>() {
        @Override
        public Parent apply(Map.Entry<Parent, List<Child>> parentListEntry) throws Exception {
            return parentListEntry.getKey();
        }
    };
}

private static Function<Map.Entry<Parent, List<Child>>, List<Child>> valueSelector() {
    return new Function<Map.Entry<Parent, List<Child>>, List<Child>>() {
        @Override
        public List<Child> apply(Map.Entry<Parent, List<Child>> parentListEntry) throws Exception {
            return parentListEntry.getValue();
        }
    };
}

However, you should also handle errors for your Response . You can do it in extractResponse() , by throwing an exception for instance.

Btw, the plural for child is children .