Bridging the gap between callbacks and RxJava

At times, you may have to mix your RxJava code with a callback-based API. For example, service proxy interfaces can only be defined with callbacks, but the implementation uses the Vert.x Rxified API.

In this case, the io.vertx.reactivex.SingleHelper.toObserver class can adapt a Handler<AsyncResult<T>> to an RxJava SingleObserver<T>:

@Override
public WikiDatabaseService fetchAllPagesData(Handler<AsyncResult<List<JsonObject>>> resultHandler) { (1)
  dbClient.rxQuery(sqlQueries.get(SqlQuery.ALL_PAGES_DATA))
    .map(ResultSet::getRows)
    .subscribe(SingleHelper.toObserver(resultHandler));  (2)
  return this;
}

1. fetchAllPagesData is an asynchronous service proxy operation, defined with a Handler<AsyncResult<List<JsonObject>>> callback.

2. The toObserver method adapts the resultHandler to a SingleObserver<List<JsonObject>>, so that the handler is invoked when the list of rows is emitted.