Paginate Observable results without recursion - RxJava

Question

I\'ve got a pretty standard API pagination problem which you can handle with some simple recursion. Here\'s a fabricated example:

public Observable

        

Answer 1

Another approach is to use token stream: get data for initial token, push next token to stream once fresh remote data is obtained, and resubscribe until token is empty

 public Observable paging() {

        Subject tokenStream = BehaviorSubject.create().toSerialized();

        tokenStream.onNext(Token.startToken());

        Observable dataStream =
                Observable.defer(() -> tokenStream.first().flatMap(this::remoteData))
                        .doOnNext(window -> tokenStream.onNext(window.getToken()))
                        .repeatWhen(completed -> completed.flatMap(__ -> tokenStream).takeWhile(Token::hasMore));

        return dataStream;
    }

The result is

Window{next token=Token{key='1'}, data='data for token: Token{key=''}'}
Window{next token=Token{key='2'}, data='data for token: Token{key='1'}'}
Window{next token=Token{key='3'}, data='data for token: Token{key='2'}'}
Window{next token=Token{key='4'}, data='data for token: Token{key='3'}'}
Window{next token=Token{key='5'}, data='data for token: Token{key='4'}'}
Window{next token=Token{key='6'}, data='data for token: Token{key='5'}'}
Window{next token=Token{key='7'}, data='data for token: Token{key='6'}'}
Window{next token=Token{key='8'}, data='data for token: Token{key='7'}'}
Window{next token=Token{key='9'}, data='data for token: Token{key='8'}'}
Window{next token=Token{key='10'}, data='data for token: Token{key='9'}'}

Copy pastable sample

public class RxPaging {

    public Observable paging() {

        Subject tokenStream = BehaviorSubject.create().toSerialized();

        tokenStream.onNext(Token.startToken());

        Observable dataStream =
                Observable.defer(() -> tokenStream.first().flatMap(this::remoteData))
                        .doOnNext(window -> tokenStream.onNext(window.getToken()))
                        .repeatWhen(completed -> completed.flatMap(__ -> tokenStream).takeWhile(Token::hasMore));

        return dataStream;
    }

    private Observable remoteData(Token token) {
        /*limit number of pages*/
        int page = page(token);
        Token nextToken = page < 10
                ? nextPageToken(token)
                : Token.endToken();

        return Observable
                .just(new Window(nextToken, "data for token: " + token))
                .delay(100, TimeUnit.MILLISECONDS);
    }

    private int page(Token token) {
        String key = token.getKey();
        return key.isEmpty() ? 0 : Integer.parseInt(key);
    }

    private Token nextPageToken(Token token) {
        String tokenKey = token.getKey();
        return tokenKey.isEmpty() ? new Token("1") : nextToken(tokenKey);
    }

    private Token nextToken(String tokenKey) {
        return new Token(String.valueOf(Integer.parseInt(tokenKey) + 1));
    }

    public static class Token {
        private final String key;

        private Token(String key) {
            this.key = key;
        }

        public static Token endToken() {
            return startToken();
        }

        public static Token startToken() {
            return new Token("");
        }

        public String getKey() {
            return key;
        }

        public boolean hasMore() {
            return !key.isEmpty();
        }

        @Override
        public String toString() {
            return "Token{" +
                    "key='" + key + '\'' +
                    '}';
        }
    }


    public static class Window {
        private final Token token;
        private final String data;

        public Window(Token token, String data) {
            this.token = token;
            this.data = data;
        }

        public Token getToken() {
            return token;
        }

        public String getData() {
            return data;
        }

        @Override
        public String toString() {
            return "Window{" +
                    "next token=" + token +
                    ", data='" + data + '\'' +
                    '}';
        }
    }

    @Test
    public void testPaging() throws Exception {
        paging().toBlocking().subscribe(System.out::println);
    }
}