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Source: Medium, danlew, Medium and Medium and GitHub, GitHub

Answer: Install RxAndroid, optional if you have to deal with Parse instal RxParse

Sample easy eaxmple to create a task that count the number of days ago:

• Create CountDaysAgoTask.java:

public class CountDayAgoTask {
    private Observable<String> mCountDayObservable;

    public CountDayAgoTask(Date date){
        setupObservable(date);
    }

    private void setupObservable(final Date date){
        mCountDayObservable = Observable
                .fromCallable(new Callable<String>() { //Callable only emit one value and only emit once there's a subscription to it
                    @Override
                    public String call() throws Exception {
                        return yourFunctionToCount(date);
                    }
                });
    }

    public Observable<String> getCountDayObservable() {
        return mCountDayObservable;
    }
}

Then in whatever class or Activity you need to receive the result:

public void setDaysAgoInBackground(final Date date) {
    if (date != null) {
        CountDayAgoTask countDayAgoTask = new CountDayAgoTask(date);
        Observable<String> countDayObservable = countDayAgoTask.getCountDayObservable();
        mCountDaySub = countDayObservable
                .subscribeOn(Schedulers.io())  //these two line is to make the task run in a different thread
                .observeOn(AndroidSchedulers.mainThread())
                .subscribe(new Subscriber<String>() { //this is where the subscription begin
                    @Override
                    public void onCompleted() {
                        if (mCountDaySub != null && !mCountDaySub.isUnsubscribed()) { //either do this here on your Activity's onStop
                            mCountDaySub.unsubscribe();
                        }
                    }

                    @Override
                    public void onError(Throwable e) {
                        e.printStackTrace();
                    }

                    @Override
                    public void onNext(String s) {
                        yourTextView.setText(s);
                    }
                });
    }
}