Android Room with RxJava: get data from Room with the loop

Question

I am new for using RxJava and Room. What I trying to do is run a for loop to get data from database. The for loop iterate from first day of month to the last day of month.
Here is the Dao for this query.

@Query("SELECT SUM(duration) FROM xxx WHERE timeStamp >= :start and timeStamp <= :end and userId = :userId")
    Flowable<Integer> getDuration(String userId, long start, long end);

And Here is how i using RxJava to get the result.

Calendar day1 = Calendar.getInstance();
Calendar day2 = Calendar.getInstance();
int maxLoopIndex = day1.getActualMaximum(Calendar.DAY_OF_MONTH);
day1.setFirstDayOfWeek(Calendar.MONDAY);
day2.setFirstDayOfWeek(Calendar.MONDAY);
day1.set(Calendar.DATE, day1.getActualMinimum(Calendar.DATE));
day2.set(Calendar.DATE, day2.getActualMinimum(Calendar.DATE));
day1.set(Calendar.HOUR_OF_DAY, 0);
day1.set(Calendar.MINUTE, 0);
day1.set(Calendar.SECOND, 0);
day1.set(Calendar.MILLISECOND, 0);
day2.set(Calendar.HOUR_OF_DAY, 23);
day2.set(Calendar.MINUTE, 59);
day2.set(Calendar.SECOND, 59);
day2.set(Calendar.MILLISECOND, 999);
ArrayList<Pair<Long, Long>> maxDayCount = new ArrayList<>();

//Get all the timeStamp in a month, where maxDayCount can be 30, 31, 28, 29. 
for (int i = 0; i < maxDayCount; i++) {
         Pair<Long, Long> P = Pair.create(day1.getTimeInMillis(), day2.getTimeInMillis());
         pairArrayList.add(P);
         day1.add(Calendar.DATE, 1);
         day2.add(Calendar.DATE, 1);
}

// Using Flowable.formIterable to run through the list and get the data from room
Flowable.fromIterable(pairArrayList)
                    .flatMap(new Function<Pair<Long, Long>, Flowable<Integer>>() {
                        @Override
                        public Flowable<Integer> apply(@NonNull Pair<Long, Long> date) throws Exception {
                            return roomdb.Dao().getDuration(
                                    User.getCurUser().getId(), date.first, date.second
                            );
                        }
                    })
                    .subscribeOn(Schedulers.io())
                    .observeOn(AndroidSchedulers.mainThread())
                    .subscribe(new Consumer<Integer>() {
                        @Override
                        public void accept(@NonNull Integer source) throws Exception {
                            Log.d(TAG, "Duration: "+source);
                            // I want to get the index of pairArrayList to store the duration in 
                            // corresponding array
                        }
                    });

However in subscribe I can get the result return by room however I can not get which index is run in pairArrayList. Is there any way I can get the index? Furthermore is there any better way to get data from room with the loop?

Answer 1

Let's begin with the final structure. It should contain the day of month and duration:

class DayDuration {

    public Integer day;
    public Long duration;

    public DayDuration(Integer day, Long duration) {
        this.day = day;
        this.duration = duration;
    }

    @Override
    public boolean equals(Object o) { /* implementation */ }

    @Override
    public int hashCode() { /* implementation */ }
}

Creation of final Flowable what emits requested items might look like the following code. I have used ThreetenBP library to handle date/time operations because Android Calendar API is pure hell. Recommend you do the same:

class SO64870062 {

    private Flowable<Long> getDuration(String userId, long start, long end) {
        return Flowable.fromCallable(() -> start); // mock data
    }

    @NotNull
    private Flowable<LocalDate> getDaysInMonth(YearMonth yearMonth) { // (1)
        LocalDate start = LocalDate.of(yearMonth.getYear(), yearMonth.getMonthValue(), 1);
        LocalDate end = start.with(TemporalAdjusters.lastDayOfMonth());

        return Flowable.create(emitter -> {
            LocalDate current = start;

            while (!current.isAfter(end)) { // (2)
                emitter.onNext(current);
                current = current.plusDays(1);
            }

            emitter.onComplete();
        }, BackpressureStrategy.BUFFER);
    }

    @NotNull
    private Flowable<DayDuration> getDurationForDay(String userId, LocalDate localDate) {
        long startDayMillis = localDate.atStartOfDay().atZone(ZoneOffset.UTC) // (3)
                .toInstant()
                .toEpochMilli();

        long endDayMillis = localDate.atTime(LocalTime.MAX).atZone(ZoneOffset.UTC)
                .toInstant()
                .toEpochMilli();

        return getDuration(userId, startDayMillis, endDayMillis) // (4)
                .map(duration -> new DayDuration(localDate.getDayOfMonth(), duration));
    }

    public Flowable<DayDuration> getDayDurations(String userId, YearMonth yearMonth) {
        return getDaysInMonth(yearMonth)
                .flatMap(localDate -> getDurationForDay(userId, localDate));
    }
}

Important and interesting parts:

1. Function getDaysInMonth() creates Flowable what emits all days of requested month.
2. Iteration from start (first day of a month) to end (last day of a month) date and emitting all of the days.
3. Make sure you set the zone you use within timestamps in your database. I have used UTC for simplicity.
4. Combine duration from a database with the current date.

Last but not least, let's check if it works correctly:

public class SO64870062Test {

    @Test
    public void whenDaysRequestedForApril2020ThenEmitted() {
        SO64870062 tested = new SO64870062();

        TestSubscriber<DayDuration> testSubscriber = tested
                .getDayDurations("userId", YearMonth.of(2020, 11))
                .test();

        testSubscriber.assertValueCount(30);
        testSubscriber.assertValueAt(1, new DayDuration(2, 1604275200000L));
        testSubscriber.assertComplete();
    }
}