Overpasser - open source, fluid Java interface to OpenStreetMap data

Get the code at: https://github.com/zsoltk/overpasser

Provides an easy way to fetch POIs and other OpenStreetMap data. No more query string forging by hand!

Comes with a Retrofit adapter and an Android sample to get you up and running immediately:

Open source Conway's Game of Life for Android with editable rules

Source: https://github.com/zsoltk/GameOfLife

You can use the codebase to implement your own cellular automata, as it was intentionally designed to be easy: implement your own rules (maybe color mapping for custom states) and you're good to go.

Even if you're not familiar with cellular automata, both playing with the app and discovering the solutions behind it can be fun.

You can also edit the rules in the app to have some pretty interesting results instantly:

Some Gradle tricks for Android

Just my two cents on my favorite gradle tips & tricks.

Some twists of using Google Maps in your Android app - Part 2: Adding a Floating Action Button and fighting BadParcelableException

Part 1: Smart follow

If you want to add a custom control over your custom MapFragment, there's actually more than one way to do that. Either you can add your control in the layout of your enclosing Activity, or you can override the onCreateView method in your Fragment and add the view there.

Because the previous will add an element that is visible on top of all your Fragments displayed in your Activity (which sometimes can be useful, too), now I'm gonna go with the latter:

@Override
public View onCreateView(LayoutInflater inflater, ViewGroup parent, Bundle savedInstanceState) {
    View view = super.onCreateView(inflater, parent, savedInstanceState);
    TouchableWrapper touchableWrapper = new TouchableWrapper(getActivity(), this);
    touchableWrapper.addView(view);

    View controls = inflater.inflate(R.layout.map_overlay_controls, null);
    touchableWrapper.addView(controls);

    return touchableWrapper;
}

Here I'm inflating a TouchableWrapper (see Part 1 to see why) and adding a custom layout on top of that, but you could just skip those lines and call view.addView(controls) directly. In this case I'm adding a Floating Action Button from https://github.com/clans/FloatingActionButton among other things.

Now the problem might not appear immediately - unless of course your mommy have taught you to always use the Don't keep activities flag in the Developer options when you were a little toddler, as she should have.

The next time your app comes back after having been put to rest by the OS and tries to restore its state, there's a good chance that it will crash and burn with a BadParcelableException:

java.lang.RuntimeException: Unable to start activity ComponentInfo{...}: android.os.BadParcelableException: ClassNotFoundException when unmarshalling: com.github.clans.fab.FloatingActionButton$ProgressSavedState
            at android.app.ActivityThread.performLaunchActivity(ActivityThread.java:2314)
            at android.app.ActivityThread.handleLaunchActivity(ActivityThread.java:2388)
            at android.app.ActivityThread.access$800(ActivityThread.java:148)
            at android.app.ActivityThread$H.handleMessage(ActivityThread.java:1292)
            at android.os.Handler.dispatchMessage(Handler.java:102)
            at android.os.Looper.loop(Looper.java:135)
            at android.app.ActivityThread.main(ActivityThread.java:5312)
            at java.lang.reflect.Method.invoke(Native Method)
            at java.lang.reflect.Method.invoke(Method.java:372)
            at com.android.internal.os.ZygoteInit$MethodAndArgsCaller.run(ZygoteInit.java:901)
            at com.android.internal.os.ZygoteInit.main(ZygoteInit.java:696)
     Caused by: android.os.BadParcelableException: ClassNotFoundException when unmarshalling: com.github.clans.fab.FloatingActionButton$ProgressSavedState

So what is the problem here and how can you solve it?

Some twists of using Google Maps in your Android app - Part 1: Smart follow

Following the user's location only when he hasn't panned the map away (as otherwise the view will animate back to the current location every now and then, providing quite a frustrating experience) is a commonly used functionality in applications with maps (say, tracking your route and such). I kinda feel there should be an out-of-the-box method for this in Google Maps SDK, but whatever, here you go if you want to implement it.

Android Vision API

Back in the day if you wanted face detection, you had to code it yourself. How much easier and simpler it is now, with the new Vision API on Android:

It even tells you how much the detected face is smiling! Awesome.

Arjunu.com has a great tutorial on how to integrate it into your project, check it out.

Also, you can check out the official samples on GitHub.

On a slightly off-topic sidenote (using face detection, but not the Vision API), I just saw Boo! on Android Experiments yesterday: 

Little creatures gather on the screen. As soon as they see your face, they flee in panic. Too cute to not include it here :)

Understanding Reactive Operators

RxMarbles helps you understand rx operators the visual way. Drag them marbles!

A list of libraries that aid reactive development on Android

As RxJava is gaining popularity among Android developers, I felt like compiling a list from libraries that offer a reactive API:

https://github.com/zsoltk/RxAndroidLibs

Learning RxJava / RxAndroid

If you only heard about it, but didn't know where to start, you'll find these links useful:

1. NotRxJava guide for lazy folks
   Yaroslav Heriatovych explores step by step how going async will first lead you to a callback hell, and how by applying the rx approach you can get back to a clean solution without losing your sanity. Also: cats!
2. The introduction to Reactive Programming you've been missing
   Reading this article from André Staltz just might flip the switch to have that "Aha!" moment you've been waiting for. Even though the sample code is for JS, you will find it useful for your RxJava adventures. Try to implement the same project in Android as a great exercise!
3. Grokking RxJava, Part 1: The Basics
   A great four part step by step tutorial by Dan Lew (see also part 2, part 3, part 4). Also make sure you watch this video by him: Reactive Extensions: Beyond the Basics

Update: Some more links:

1. RxMarbles
   An absolutely great, interactive site to learn about rx operators the visual way
2. Party tricks with RxJava, RxAndroid & Retrolambda:
   There are some really useful code snippets to get you started in no time

Also check out A list of libraries that aid reactive development on Android .