RexWeather is a sample Android Studio project demonstrating the use of Retrofit and RxJava to interact with web services. Link to the blog post.

Retrofit is a REST client for Android and Java. It allows you to turn a REST API into a Java interface by using annotations to describe the HTTP requests. It can then generate an implementation of the interface for you. This means that you can go from GET /users/{userId}/posts to webService.fetchUserPosts(userId) in a few lines of code. Retrofit is very easy to use and it integrates well with RxJava.

RxJava is a Java implementation of Rx, the Reactive Extensions library from the .NET world. It allows you to compose asynchronous and event-based programs in a declarative manner. Let's say that you want to implement something like this:

• Start observing our current location. When a location change happens, in parallel,
  • Send a web service request A
  • Send a web service request B
    • Using the result from B, send a web service request C
• When the results for both A and C are back, update the UI

RxJava allows you to write out your program pretty much as above. It abstracts out concerns about things like threading, synchronization, thread-safety, concurrent data structures, and non-blocking I/O. You can tell RxJava to observe the location changes and perform the web service requests in a background thread, and pass you the final results in the UI thread. If any exceptions are thrown at any point in the chain of events, you get told about it in one convenient place. You're not drowning in a spaghetti of onSuccess and onError callbacks. You're building pipelines.

The app uses OpenWeatherMap to fetch the current weather as well as the seven day forecast.

The current weather can be obtained by making a call similar to the following:

http://api.openweathermap.org/data/2.5/weather?lat=-31.9522&lon=115.8589&units=metric

The returned JSON looks like this:

{
  "coord": {
    "lon": 115.86,
    "lat": -31.95
  },
  "sys": {
    "message": 0.1843,
    "country": "AU",
    "sunrise": 1404602233,
    "sunset": 1404638729
  },
  "weather": [
    {
      "id": 802,
      "main": "Clouds",
      "description": "scattered clouds",
      "icon": "03n"
    }
  ],
  "base": "cmc stations",
  "main": {
    "temp": 13,
    "pressure": 1015,
    "humidity": 71,
    "temp_min": 13,
    "temp_max": 13
  },
  "wind": {
    "speed": 3.1,
    "deg": 300
  },
  "rain": {
    "3h": 0
  },
  "clouds": {
    "all": 40
  },
  "dt": 1404657000,
  "id": 6692202,
  "name": "South Perth",
  "cod": 200
}

The 7 day forecast can be obtained like so:

http://api.openweathermap.org/data/2.5/forecast/daily?lat=-31.9522&lon=115.8589&mode=json&units=metric&cnt=7

And the returned JSON looks like this:

{
  "cod": "200",
  "message": 0.0094,
  "city": {
    "id": 2063523,
    "name": "South Perth",
    "coord": {
      "lon": 115.833328,
      "lat": -31.933331
    },
    "country": "AU",
    "population": 0
  },
  "cnt": 7,
  "list": [
    {
      "dt": 1404619200,
      "temp": {
        "day": 13,
        "min": 13,
        "max": 13.31,
        "night": 13.31,
        "eve": 13,
        "morn": 13
      },
      "pressure": 1016.99,
      "humidity": 100,
      "weather": [
        {
          "id": 501,
          "main": "Rain",
          "description": "moderate rain",
          "icon": "10d"
        }
      ],
      "speed": 7.25,
      "deg": 225,
      "clouds": 92,
      "rain": 4
    },
  ]

  // Six more entries ...

}