Introducing ReactiveAirplaneMode

2017-08-15 Android, Open source, RxJava No comments

I’m continuing Rxfication of the Android. Recently I released brand new library called ReactiveAirplaneMode. As you may guess, it allows listening Airplane mode on Android device with RxJava observables. A usual I’ve hidden all implementation details, BroadcastReceivers and rest of the Android related stuff behind RxJava abstraction layer, so API is really simple. Just take a look on that:

ReactiveAirplaneMode.create()
    .observe(context)
    .subscribeOn(Schedulers.io())
    .observeOn(AndroidSchedulers.mainThread())
    .subscribe(isOn -> textView.setText(String.format("Airplane mode on: %s", isOn.toString())));

In the code above subscriber will be notified only when airplane mode changes.

If you want to read airplane mode and then listen to it, you can use the following method:

ReactiveAirplaneMode.create()
    .getAndObserve(context)
    .subscribeOn(Schedulers.io())
    .observeOn(AndroidSchedulers.mainThread())
    .subscribe(isOn -> textView.setText(String.format("Airplane mode on: %s", isOn.toString())));

If you want to check airplane mode only once, you can use get(context) method, which returns Single<Boolean> value:

ReactiveAirplaneMode.create()
    .get(context)
    .subscribeOn(Schedulers.io())
    .observeOn(AndroidSchedulers.mainThread())
    .subscribe(isOn -> textView.setText(String.format("Airplane mode on: %s", isOn.toString())));

If you want to check airplane mode only once without using Reactive Streams, just call isAirplaneModeOn(context) method:

boolean isOn = ReactiveAirplaneMode.create().isAirplaneModeOn(context);

You can add this library to your project via Gradle:

dependencies {
  compile 'com.github.pwittchen:reactiveairplanemode:0.0.1'
}

If you want to know more details, see sample app, documentation & tests, check repository with the source code at: https://github.com/pwittchen/ReactiveAirplaneMode.

Using Tmux plugins with Tpm

2017-08-07 Linux, Tmux No comments

Recently, I decided to organize my Unix dotfiles in a better way. I had a few custom scripts I used in my Tmux bottom bar. I kept these scripts in .scripts directory and during installation or upgrade of my personal configuration, install.sh script was copying them from .scripts directory to /usr/local/bin/ directory. I wanted to make this configuration more solid and consistent, so I decided to transform these scripts into tmux plugins managed by tpm. I was already using a few plugins like:

In my Tmux bottom bar, I display battery level, uptime, CPU, RAM, IP number and song currently played on Spotify. Previously I just used scripts copied to /usr/local/bin/ and configuration looked like that:

set -g status-right "↑ #(showUptime) ⇅ #(showCpuUsage) ☰ #(showRamUsage) ∴ #(showIp) ↯ #{showBatteryLevel} ⧖ #(date '+%a, %b %d, %H:%M') "

I created the following plugins to replace these scripts:

In order to use Tmux plugins, we need to install Tmux Plugin Manager:

git clone https://github.com/tmux-plugins/tpm ~/.tmux/plugins/tpm

and initialize it at the bottom of our .tmux.conf file:

run '~/.tmux/plugins/tpm/tpm'

After that, it’s good to reload shell (source ~/.zshrc) and Tmux config (tmux source-file ~/.tmux.conf)

Next, we can add our plugins to .tmux.conf file:

set -g @plugin 'tmux-plugins/tmux-sidebar'
set -g @plugin 'tmux-plugins/tmux-copycat'
set -g @plugin 'tmux-plugins/tmux-pain-control'
set -g @plugin 'tmux-plugins/tmux-urlview'
set -g @plugin 'pwittchen/tmux-plugin-battery'
set -g @plugin 'pwittchen/tmux-plugin-uptime'
set -g @plugin 'pwittchen/tmux-plugin-cpu'
set -g @plugin 'pwittchen/tmux-plugin-ram'
set -g @plugin 'pwittchen/tmux-plugin-ip'
set -g @plugin 'pwittchen/tmux-plugin-spotify'

When we are in Tmux, we can install plugins by pressing prefix + I to install plugins. In my case, prefix = Ctrl+b.
After that, we can hit Enter and we’re ready to go!

Now, I could update my .tmux.conf with the variables defined by my plugins:

set -g status-right " 🔉 #{spotify_song} ↑ #{uptime} ⇅ #{cpu} ☰ #{ram} ∴ #{ip} ↯ #{battery_level} ⧖ #(date '+%a, %b %d, %H:%M') "

After this operation, I could remove custom scripts from my dotfiles and desired functionality is delivered via plugins. Moreover, anyone can install these plugins via tpm without messing with custom scripts!

screenshot from tmux after configuration

Right now, my plugins are in kind of messy state and they don’t work perfectly across all operating systems (e.g. there are problems on macOS), but they’re usable under Linux Ubuntu 16.04 LTS and it’s a good beginning for organizing mess created by the custom scripts.

That’s it! I have plans to publish another article describing how to write your custom Tmux plugin, which can be managed via tpm.

Releasing ReactiveNetwork v. 0.11.0 with Walled Garden AKA Great Firewall support

2017-08-06 Android, Java, Open source No comments

In the latest release of ReactiveNetwork library, I focused on Walled Garden AKA Great Firewall support during checking Internet connectivity. There are countries with limited Internet access like China and in such cases, pinging commonly known host like www.google.com may have different results than in other countries because it may be blocked. We may get false positive results because users will generally have an access to the Internet, but they don’t have access only to several websites. To solve that problem, I created WalledGardenInternetObservingStrategy and made it default strategy for checking Internet connectivity inside the library. Of course, you can still use SocketInternetObservingStrategy if you want to.

Detailed release notes are as follows:

RxJava1.x

  • added WalledGardenInternetObservingStrategy – fixes #116
  • made WalledGardenInternetObservingStrategy a default strategy for checking Internet connectivity
  • added documentation for NetworkObservingStrategy – solves #197
  • added documentation for InternetObservingStrategy – solves #198
  • bumped Kotlin version to 1.1.3-2
  • bumped Gradle Android Tools version to 2.3.3
  • bumped Retrolambda to 3.7.0

RxJava2.x

  • added WalledGardenInternetObservingStrategy – fixes #116
  • made WalledGardenInternetObservingStrategy a default strategy for checking Internet connectivity
  • added documentation for NetworkObservingStrategy – solves #197
  • added documentation for InternetObservingStrategy – solves #198
  • fixed package name in AndroidManifest.xml file – solves #195
  • bumped RxJava2 version to 2.1.2
  • bumped Kotlin version to 1.1.3-2
  • bumped Gradle Android Tools version to 2.3.3
  • bumped Retrolambda to 3.7.0
  • increased code coverage with unit tests

Repository address: https://github.com/pwittchen/ReactiveNetwork.

Happy coding!

Releasing ReactiveNetwork v. 0.10.0

2017-07-20 Android, Open source, RxJava No comments

I’ve recently released ReactiveNetwork library v. 0.10.0 for RxJava1.x and RxJava2.x.
ReactiveNetwork is an Android library listening network connection state and Internet connectivity with RxJava Observables, which I’m developing for approximately 2 years now.

In this version, I’ve done a few bug fixes and added new features for RxJava2.x version.

Below, you can find the release notes:

Release for RxJava1.x

  • bumped RxJava1 version to 1.3.0
  • bumped test dependencies
  • created Code of Conduct
  • updated Kotlin version in sample apps
  • added retrolambda to the sample Java app – issue #163
  • fixed behavior of network observing in disconnected state – issue #159

Release for RxJava2.x

  • bumped RxJava2 version to 2.1.1
  • bumped test dependencies
  • created Code of Conduct
  • updated unit tests
  • updated Kotlin version in sample apps
  • added retrolambda to the sample Java app – issue #163
  • fixed behavior of network observing in disconnected state – issue #159
  • added the following methods to ReactiveNetwork class:
    • Single<Boolean> checkInternetConnectivity()
    • Single<Boolean> checkInternetConnectivity(InternetObservingStrategy strategy)
    • Single<Boolean> checkInternetConnectivity(String host, int port, int timeoutInMs)
    • Single<Boolean> checkInternetConnectivity(String host, int port,
      int timeoutInMs, ErrorHandler errorHandler)
    • Single<Boolean> checkInternetConnectivity(InternetObservingStrategy strategy,
      String host, int port, int timeoutInMs,
      ErrorHandler errorHandler)

You can add it to your project via Gradle:

RxJava1.x:

dependencies {
  compile 'com.github.pwittchen:reactivenetwork:0.10.0'
}

RxJava2.x:

dependencies {
  compile 'com.github.pwittchen:reactivenetwork-rx2:0.10.0'
}

Now, in RxJava2.x version, we have the possibility to check Internet connectivity once without any pooling with new Single type. It may be helpful in the specific use-cases when we’re focusing on smaller battery usage, a smaller amount of data being sent over the network and lower number of network connections.

I’m planning to publish more real life usage examples of this library in the future articles on this blog.

I have plans for a few updates in the next version. If you’re interested in this project or you’re using it, please stay tuned and keep an eye on it at GitHub.

Release of prefser v. 2.1.0 with RxJava2 support

2017-06-19 Android, Java, Open source, RxJava 2 comments

I’ve recently released new version of prefser library for Android. In case you don’t know, it’s a wrapper for Android SharedPreferences with object serialization and RxJava Observables. This version has the new artifact, which has codebase migrated to RxJava2.x. As usual, I kept backward compatibility with RxJava1.x.

You can find more details about the project at https://github.com/pwittchen/prefser.

If you want to use it in your mobile project, you need the following dependencies in the build.gradle file:

dependencies {
  compile 'com.github.pwittchen:prefser-rx2:2.1.0'
  compile 'io.reactivex:rxandroid:2.0.1'
}

Short release notes can be found at https://github.com/pwittchen/prefser/releases.

This update was requested by at least two developers on GitHub and it’s my second most popular project, so I hope you’ll find it useful if you’re in the process of migrating from RxJava1.x to RxJava2.x. I still have 4 remaining RxJava1.x libraries waiting for the upgrade. If you want to perform any updates via Pull Requests, you’re more than welcome.

DSP 2017 – Summary

2017-05-31 DSP2017 2 comments

Introduction

Some time ago, I’ve decided to participate in “Get Noticed 2017” (“Daj się poznać” in Polish) competition. It’s competition for Polish programmers in which people have to create a blog if they don’t have it (luckily, I could skip that step) and write at least 2 articles per week from the beginning of the March til the end of May, which is exactly today (for 3 months). Posts had to be categorized on the blog. You can find all my posts related to this competition in DSP2017 category. Besides this, competitors had to create their own open-source projects (or one project) and contribute to them during the competition. I already had my projects, so I kept resolving planned issues within them. Nevertheless, I created new, small projects. I treated that competition as a challenge and possibility to try something new. Moreover, it was some kind of motivation for doing more stuff. I already had a blog and OS projects, so I thought it will be easy for me and I’ll just keep doing what I already do. I was a little bit wrong.

Blogging

I’ve written 28 articles on this blog in DSP2017 category (including this one). Writing two articles per week is a lot. Previously I was writing approximately 1 article per month or sometimes more when I wanted to. Now I was writing 8 articles per month, so It’s 800% more often than usual. It forced me to be a little bit more organized & creative. I also created a backlog of article topics and a backlog of the software projects (big & small) I wanted to create & eventually, describe.

In the chart below, you can see how the number of the articles increased when I started participating in the competition.

Moreover, I received more comments than usual. The peak in 2015 you see on the picture is an article about Test coverage report for Android application. For some reason, people found it interesting and had a lot of questions.

I observed slightly more visitors on the blog, but not very huge. There was a peak in the beginning of the March, but later it decreased to a bit higher level than it was to the starting point. Below you can see statistics from the Google Analytics from the last 90 days. The drops on the plots are on the weekends. It shows people usually visit my website during the work week. Interesting information is the fact that drop of the visitors in the beginning of the April on the weekend was smaller than usual because I was speaking at the conference, so probably people googled me these days.

Projects

During the DSP2017, I created the following new projects:

  • spotify-cli-linux – a command-line interface to Spotify on Linux written in Python
  • tmux-auto-pane – a tiny tool for creating pre-defined tile layouts in tmux on linux with xdotool written in Bash
  • YaaS Java SDK – Hybris as a Service Java SDK for microservice proxy

During the DSP2017, I updated the following projects:

  • ReactiveBeacons – Android library scanning BLE beacons nearby with RxJava (an update included migration to RxJava2)
  • ReactiveNetwork – Android library listening network connection state and Internet connectivity with RxJava Observables (an update included migration to RxJava2)
  • prefser – Wrapper for Android SharedPreferences with object serialization and RxJava Observables
  • swipe – detects swipe events on Android (with RxJava or listener)
  • pkup – semi-automated generating of PKUP report (tiny tool for formal reporting stuff at my work)

I also updated my dotfiles and a few on my public knowledge sources I store on GitHub, but they are not counted as a “regular projects”

Conferences

During this competition I also gave a talk during Kariera IT Conference in Katowice, Poland titled How to make open-source projects, which people want to use. I was invited to this conference as a speaker, what was very flattering. I also received nice, valuable feedback from the conference organizers. Besides that, I also participated in the J On The Beach conference in Malaga, Spain as an attendee. Unfortunately, I haven’t found time to write an article summarizing this event. I can tell you it was really great. Besides the conference itself, location, weather & food were perfect! I also had to write two articles on the blog for the given week in advance, because I haven’t taken my laptop on this trip.

Summary

To wrap up, participating in this competition was quite challenging and harder than I thought it would be. Nevertheless, it helped me to be consistent and more organized. I also boosted my creativity and ability to produce more articles as well as writing and language skills (English is not my native language). It was hard but fun. The challenge is accomplished! There are over 1000 participants, so I treat this event rather as a challenge for myself than competing with others. Moreover, it’s hard to measure and compare such work among different people. Nevertheless, I’m looking for results anyway :-).

Update

Woohoo! I was on the 14th place in the second round of the competition! Results are here: http://uczestnicy.dajsiepoznac.pl/finalisci. Unfortunately, there were better competitors in the final round. Congrats to organizer, winners and all the people who participated in this competition & meetup in Warsaw. It was a really fun experience! 🙂

You can also check my tweet from the meeting in MS Warsaw:

New reactive data types in RxJava2

2017-05-31 DSP2017, Java, RxJava No comments

Introduction

I’m still exploring reactive programming world and RxJava library. Recently, I’ve migrated a few of my open-source libraries from RxJava1 to RxJava2 and written yet another project in RxJava2 from the beginning. Nevertheless, I’m still learning this library and its concept. It’s very wide topic. In RxJava1 we simply had one reactive data type called Observable. In RxJava2, we have more data types like Observable, Flowable, Single, Maybe & Completable. In this article, I’ll briefly explain their purpose and tell you when to use which. The general idea behind these types is code semantics. We should tell consumer of our code, what he or she can expect from our API. Introducing more reactive data types can increase readability and stability of our code base.

Observable

Observable is basically the same Reactive type, we had in RxJava1. It doesn’t have backpressure support.

We should use Observable, when:

  • our data source emits less than 1000 items, so there’s practically no chance of occurring OutOfMemoryException
  • we are working with GUI events, which usually don’t occurs very often and don’t have to be backpressured
  • we are working with synchronous code on legacy JVM like Java 1.6 and we want to have streams features like in Java 8

Observable

Flowable

Flowable type has very similar semantics to Observable. We can operate on Flowable streams with map, flatmap, filter, etc. in the same way as on the Observable type. The main difference is backpressure support.

We should use Flowable when we are:

  • dealing with 10k+ elements in a stream
  • dealing with frequent events (e.g. sensors readings)
  • reading/parsing files from disk
  • reading values from database through JDBC
  • using network/streaming I/O
  • reading/writing to many blocking or pull-based data sources

To learn more, read note about Observable vs. Flowable on wiki of RxJava2 on GitHub.

Single

Single reactive type has been redesigned from scratch in RxJava 2. It’s designed to handle just one event in an asynchronous manner. Good application of this type is single HTTP request when we expect just one response or error and nothing else. It can emit on onSuccess (single value) or onError event (error).

Single

Maybe

Maybe represents a deferred computation and emission of a maybe value or exception. Maybe is a wrapper around an operation/event that may have either:

  • A single result
  • Error
  • No result

Just take a look at the scheme.

The interface of the main consumer of this type have the following methods: onSuccess, onError, onComplete. Conceptually, Maybe is a union of Single and Completable providing the means to capture an emission pattern where there could be 0 or 1 item or an error signaled by some reactive source.

Maybe

Completable

Completable type can be used when we have an Observable that we don’t care about the value resulted from the operation (result is void). It handles only onComplete and onError events. Conceptually, Maybe is a union of Single and Completable providing the means to capture an emission pattern where there could be 0 or 1 item or an error signalled by some reactive source.

Read more about Maybe type on RxJava wiki.

Summary

As we can see, RxJava2 gives us new types, which can help explain our intentions more clearly. We can adjust concrete type to the specific situation. In addition, we can use backpressure for the data sources, which emit a lot of elements to make our projects more robust and stable. Last, but not least RxJava2 is compatible with Reactive Streams API, which is going to be part of the Java 9 specification.

References

Introducing YaaS Java SDK

2017-05-28 DSP2017, Hybris, Java, Open source, RxJava, YaaS No comments

Introduction

In my company, there’s a concept of so-called “innovation day”. I have the possibility to “use” 1 innovation day per 2 development sprints. Last year, I used only 1 day due to the tight release schedule and a lot of work. Now, we are right after release, so I had time to take innovation day once again. I’ve decided to create YaaS Java SDK. If you don’t know what the YaaS is, check out my previous article about Basic usage of YaaS proxy for the microservice. In a few words, it’s a proxy for the microservices with authorization & monitoring capabilities, which allows using other services available on the YaaS market. SDK created by me is really simple, was created in a short period of time and does not cover all features of the YaaS. This SDK allows performing authorized requests to the microservices hidden behind YaaS proxy.

Tech stack used for this project is as follows:

For unit testing I used:

Quick start

I wanted to make this SDK as simple as possible so the user can add YaaS integration to the Java application within just a few lines of code.

YaaSProject project = new YaaSProject.Builder()
    .withClientId("YOUR_CLIENT_ID")
    .withClientSecret("YOUR_CLIENT_SECRET")
    .withOrganization("YOUR_ORGANIZATION")
    .withService("YOUR_SERVICE")
    .withVersion("v1")
    .withZone(Zone.EU)
    .build();

Client client = new YaaS(project);

client.get("path/to/your/endpoint")
    .subscribe(response -> System.out.println(response.body().string()));

As you can see, it looks really simple and straightforward. In the code snippet above, we’ve done the following thigs:

  1. Defined YaaS Project with YaaS service
  2. Created YaaS Client
  3. Performed HTTP GET request to the endpoint of the microservice asynchronously
  4. Received and printed body of the HTTP response from the microservice on the current thread as a String

All of that was done with Single type from RxJava2, which wraps Response type from OkHttp. We have a reactive stream of HTTP response here and we can do with it whatever RxJava2 offers us. Like filtering, mapping, throttling, combining it with other stream and so on.

For more information, visit repository of the project at: https://github.com/pwittchen/yaas-java-sdk.

Future plans

I have the following plans related to this project, which may be realized when I’ll have time:

  • Add more unit tests (I didn’t have enough time to cover all cases)
  • Add continuous integration
  • Integrate YaaS with SAP Hybris Backoffice or SAP Hybris Core Platform through this SDK (PoC)
  • YaaS Android SDK (copy YaaS Java SDK, downgrade it to Java 7 & optionally migrate to Kotlin and create sample mobile app)
  • Optionally, add more features to YaaS Java SDK
  • Optionally, deploy an artifact to Maven Central repository
  • Optionally, create SDKs for different programming languages (especially those I don’t know well or I don’t know at all – just to learn them)

Links

Interesting links related to this article:

Releasing Prefser v. 2.0.7

2017-05-28 Android, DSP2017, Java, Open source No comments

I’ve recently released new version of Prefser. It’s a wrapper for Android SharedPreferences with object serialization and RxJava Observables.
The new version number is 2.0.7.

In this release, I performed mostly internal work not related to the external library API. Nevertheless, it’s important for the library development in the future.

The following things were done:

  • updated dependencies
  • updated Gradle configuration
  • migrated unit tests to Robolectric
  • started executing unit tests on Travis CI
  • added integration with codecov.io and coverage report
  • extracted code related to accessors from the Prefser class (refactoring library internals)

Organizational work is done and now I’m ready for migration to RxJava2 in this project on a separate branch. I want to keep backward compatibility with RxJava1 as in my other projects. This update is planned for version 2.1.0.

Stay tuned!

Joining lists of RxJava Observables

2017-05-15 DSP2017, Java, RxJava No comments

In RxJava we have a few operators for joining Observables. The most common are:

Take a look at the documentation in these links. It has interactive marble diagrams showing how the operators work on the streams. You can move marbles along the lines and see how the output stream changes. It really helps to understand how it works.

Code snippets in this article are based on RxJava 2.1.0 with JUnit 4.12 and Google Truth 0.32 for unit tests.

Let’s say, we have the following Observables:

public Observable<String> emitNumbers() {
  return Observable.fromArray("1", "2", "3", "4").delay(1, TimeUnit.SECONDS);
}

public Observable<String> emitLetters() {
  return Observable.fromArray("a", "b", "c", "d");
}

We can merge them in the different ways.

Concat

Concat operator emits the emissions from two or more Observables without interleaving them.

We can perform the following operation:

public Observable<String> concatStreams() {
  return Observable.concat(emitNumbers(), emitLetters());
}

The easiest way to verify, how this operator works, is to create exploratory unit test as follows:

@Test
public void shouldConcatStreams() {
  // given
  Observable<String> observable = playground.concatStreams();
  List<String> expectedValues = Arrays.asList("1","2","3","4","a","b","c","d");
  List<String> joinedValues = new ArrayList<>();

  // when
  observable.blockingSubscribe(s -> joinedValues.add(s));

  // then
  assertThat(joinedValues).isEqualTo(expectedValues);
}

This operation can be represented graphically as well.

         1 --- 2 --- 3 --- 4
                  |
         a --- b --- c --- d
                  |
                  |
                concat
                  |
                 \|/
1 -- 2 -- 3 -- 4 --- a -- b -- c -- d  

As we can see one stream is appended to another regardless of the execution time of both streams.

Merge

Merge operator combines multiple Observables into one by merging their emissions.

Here we have a similar story, but changed operator:

public Observable<String> mergeStreams() {
  return Observable.merge(emitNumbers(), emitLetters());
}

We are writing another unit test:

@Test
public void shouldMergeStreams() {
  // given
  Observable<String> observable = playground.mergeStreams();
  List<String> expectedValues = Arrays.asList("a","b","c","d","1","2","3","4");
  List<String> joinedValues = new ArrayList<>();

  // when
  observable.blockingSubscribe(s -> joinedValues.add(s));

  // then
  assertThat(joinedValues).isEqualTo(expectedValues);
}

Merge operation should look like that:

         1 --- 2 --- 3 --- 4
                  |
         a --- b --- c --- d
                  |
                  |
                merge
                  |
                 \|/
a -- b -- c -- d --- 1 -- 2 -- 3 -- 4

This operator doesn’t synchronize the streams and merges them as values are emitted. Numbers are emitted later than letters, so letters are placed in the beginning of the output stream. Try to manipulate marble on the interactive diagram on the reactivex.io website to see how it should work.

Zip

The last operator, I’d like to discuss in this article is “Zip” operator. Zip combines the emissions of multiple Observables together via a specified function and emit single items for each combination based on the results of this function.

In simple words, it waits until many observables are emitted and then combines them into a pair (or triple Observable, etc. in the case or more Observables).

Now, we need to create a function, which will transform our streams and return combined stream.

public Observable<String> zipStreams() {
  return Observable.zip(emitNumbers(), emitLetters(),
      (s1, s2) -> String.format("(%s,%s)", s1, s2));
}

Next, we can verify it with test as usual:

@Test
public void shouldZipStreams() {
  // given
  Observable<String> observable = playground.zipStreams();
  List<String> expectedValues = Arrays.asList("(1,a)","(2,b)","(3,c)","(4,d)");
  List<String> joinedValues = new ArrayList<>();

  // when
  observable.blockingSubscribe(s -> joinedValues.add(s));

  // then
  assertThat(joinedValues).isEqualTo(expectedValues);
}

and it can be represented graphically like that:

        1 --- 2 --- 3 --- 4
                 |
        a --- b --- c --- d
                 |
                 |
                zip
                 |
                \|/
 (1,a) -- (2,b) --- (3,c) -- (4,d)

Now, we have pairs of merged streams.

Summary

Of course, RxJava is complicated library and these methods are not covering all possibilities of merging and combining the Observable streams. Neverhteless, examples in this article are quite basic and may help you to understand how mentioned operators work. After that we can apply the best operator to appropriate situation.


Reference thread on StackOverflow: http://stackoverflow.com/questions/28843318/android-rxjava-joining-lists