Example for fetching data from remote or local using RxJava

MultiSourcesTest.java

package net.lamtran.rxjava2;

import org.junit.After;
import org.junit.Before;
import org.junit.Test;

import java.util.List;

import rx.observers.TestSubscriber;

import static org.hamcrest.MatcherAssert.assertThat;
import static org.hamcrest.core.IsEqual.equalTo;

/**
 * Created by lam on 9/23/16.
 */

public class MultiSourcesTest {

  StoreProvider mStoreProvider;

  @After
  public void reset() throws Exception {
    mStoreProvider.reset();
    mStoreProvider = null;
  }

  @Before
  public void setup() throws Exception {
    mStoreProvider = new StoreProvider();
  }

  @Test
  public void testGetStoresFirstFromApi() throws Exception {
    System.out.println("Test getting store with data coming from api");
    TestSubscriber<List<Store>> testSubscriber = new TestSubscriber<>();
    mStoreProvider.fetchStores().subscribe(testSubscriber);
    testSubscriber.assertNoErrors();
    testSubscriber.assertCompleted();
    assertThat(testSubscriber.getOnNextEvents().size(), equalTo(1));
    List<Store> stores = testSubscriber.getOnNextEvents().get(0);
    assertThat(stores.size(), equalTo(10));
    assertThat(StoreProvider.sStores.size(), equalTo(10));
  }

  @Test
  public void testGetStoresFirstFromLocal() throws Exception {
    System.out.println("Test getting store with data coming from local");
    StoreProvider.sStores = mStoreProvider.getStores();
    TestSubscriber<List<Store>> testSubscriber = new TestSubscriber<>();
    mStoreProvider.fetchStores().subscribe(testSubscriber);
    testSubscriber.assertNoErrors();
    testSubscriber.assertCompleted();
    assertThat(testSubscriber.getOnNextEvents().size(), equalTo(1));
    List<Store> stores = testSubscriber.getOnNextEvents().get(0);
    assertThat(stores.size(), equalTo(10));
    assertThat(StoreProvider.sStores.size(), equalTo(10));
  }
}

Store.java

package net.lamtran.rxjava2;

/**
 * Created by lam on 9/23/16.
 */

public class Store {

  private final String address;

  private final int id;

  public Store(int id, String address) {
    this.id = id;
    this.address = address;
  }

  public String getAddress() {
    return address;
  }

  public int getId() {
    return id;
  }
}

StoreProvider.java

package net.lamtran.rxjava2;

import java.util.ArrayList;
import java.util.List;

import rx.Observable;

/**
 * Created by lam on 9/23/16.
 */

public class StoreProvider {

  public static List<Store> sStores;

  public Observable<List<Store>> fetchStores() {
    return Observable.concat(fetchFromLocal(), fetchFromApi())
        .first(stores -> stores != null && !stores.isEmpty());
  }

  public List<Store> getStores() {
    List<Store> stores = new ArrayList<>(10);
    for (int i = 0; i < 10; i++) {
      stores.add(new Store(i, "address " + i));
    }
    return stores;
  }

  public void reset() {
    sStores = null;
  }

  private Observable<List<Store>> fetchFromApi() {
    return Observable.defer(() -> {
      System.out.println("- fetching from api");
      List<Store> stores = getStores();
      return Observable.just(stores);
    }).doOnNext(stores -> {
      System.out.println("- saving to local");
      sStores = stores;
    });
  }

  private Observable<List<Store>> fetchFromLocal() {
    return Observable.defer(() -> {
      System.out.println("- fetching from local");
      return Observable.just(sStores);
    });
  }
}