Detect when the RecyclerView reaches the bottom in Observable manner

MyRxRecyclerView

package com.saber.ekip.views.rxRecyclerView;

import android.support.annotation.CheckResult;
import android.support.annotation.NonNull;
import android.support.v7.widget.RecyclerView;

import io.reactivex.Observable;

import static com.saber.ekip.views.rxRecyclerView.Preconditions.checkNotNull;

/**
 * Created by sabersolooki on 1/5/18.
 */

public class MyRxRecyclerView {
    @CheckResult
    @NonNull
    public static Observable<Integer> pagingChanges(@NonNull RecyclerView view) {
        checkNotNull(view, "view == null");
        return new RecyclerViewPagingObservable(view);
    }
}

RecyclerViewPagingObservable

package com.saber.ekip.views.rxRecyclerView;

import android.support.v7.widget.LinearLayoutManager;
import android.support.v7.widget.RecyclerView;
import android.util.Log;

import com.google.android.flexbox.FlexboxLayoutManager;

import java.util.HashMap;
import java.util.Map;

import io.reactivex.Observable;
import io.reactivex.Observer;
import io.reactivex.android.MainThreadDisposable;

import static com.saber.ekip.views.rxRecyclerView.Preconditions.checkMainThread;

/**
 * Created by sabersolooki on 1/5/18.
 */

final public class RecyclerViewPagingObservable extends Observable<Integer>{
    private final RecyclerView view;

    RecyclerViewPagingObservable(RecyclerView view) {
        this.view = view;
    }

    @Override
    protected void subscribeActual(Observer<? super Integer> observer) {
        if (!checkMainThread(observer)) {
            return;
        }
        Listener listener = new Listener(view, observer);
        observer.onSubscribe(listener);
        view.addOnScrollListener(listener.scrollListener);

        if (view.getAdapter() != null) {
            view.getAdapter().registerAdapterDataObserver(new RecyclerView.AdapterDataObserver() {
                @Override
                public void onChanged() {
                    super.onChanged();
                    listener.calculatedPage.clear();
                    listener.counter = 1;
                }
            });
        }
    }

    final class Listener extends MainThreadDisposable {
        private final RecyclerView recyclerView;
        private final RecyclerView.OnScrollListener scrollListener;
        private int pastVisibleItems, visibleItemCount, totalItemCount;
        private int counter = 1;
        private Map<Integer, Integer> calculatedPage = new HashMap<>();

        Listener(RecyclerView recyclerView, final Observer<? super Integer> observer) {
            this.recyclerView = recyclerView;
            final RecyclerView.LayoutManager layoutManager = this.recyclerView.getLayoutManager();
            this.scrollListener = new RecyclerView.OnScrollListener() {
                @Override public void onScrollStateChanged(RecyclerView recyclerView, int newState) {
                    try {
                        visibleItemCount = layoutManager.getChildCount();
                        totalItemCount = layoutManager.getItemCount();
                        if (layoutManager instanceof LinearLayoutManager)
                            pastVisibleItems = ((LinearLayoutManager) layoutManager).findFirstVisibleItemPosition();
                        else if (layoutManager instanceof FlexboxLayoutManager)
                            pastVisibleItems = ((FlexboxLayoutManager) layoutManager).findFirstVisibleItemPosition();
                        else
                            Log.w("RxRecyclerView", "Your layout manager is not appropriate for paging.");

                    }
                    catch (NullPointerException e) {
                        throw new NullPointerException("Layout manager is null.You should set manager before set observable");
                    }

                    if (!calculatedPage.containsKey(totalItemCount)) {
                        if ((visibleItemCount + pastVisibleItems) >= totalItemCount) {
                            calculatedPage.put(totalItemCount, counter);
                            counter++;
                            if (!isDisposed()) {
                                observer.onNext(counter);
                            }
                        }
                    }
                }
            };
        }

        @Override protected void onDispose() {
            recyclerView.removeOnScrollListener(scrollListener);
        }
    }
}