by Ahmed Rizwan

RxAndroid and Retrofit 2.0

Ok, so this isn’t new or anything, but I thought let’s just make a simple tutorial with the new Retrofit 2.0. This should give us a starting point.

This isn’t a tutorial for RxAndroid. If you don’t know much about RxAndroid, you should first check this out.

Let’s get into it then. Here are the things you’ll need before we start:

  1. RxAndroid and Retrofit
  2. Gson (I’ll be using Gson, you can use other parsers as well)
  3. An Internet connection!

So after you’ve added the dependencies, your gradle files should look something like this (ignore the Retrolambda plugin, I just added it for code conciseness because… Lambdas! *_*):

Now you might be wondering, what is Retrofit exactly? Well, Retrofit is an HTTP Client, but it’s type-safe. That means you can transform an HTTP API into a Java Interface. This makes it ridiculously convenient to interact with the API.

The Starting Example

In the example, I’m going to use the OpenWeather API and I’ll keep it as simple as possible. I’ll just get the weather forecast for today in three steps.

Step 1: Generate Java Model (Pojo) classes from JSON

Here’s the API URL:

http://api.openweathermap.org/data/2.5/weather?q=London

And the response returned when you call it is:

{ “coord”: { “lon”: -0.13, “lat”: 51.51 }, “weather”: [ { “id”: 521, “main”: “Rain”, “description”: “shower rain”, “icon”: “09d” } ], “base”: “cmc stations”, “main”: { “temp”: 289.49, “pressure”: 993, “humidity”: 67, “temp_min”: 285.93, “temp_max”: 291.15 }, “wind”: { “speed”: 8.7, “deg”: 210, “gust”: 14.4 }, “rain”: { “1h”: 1.02 }, “clouds”: { “all”: 40 }, “dt”: 1442242382, “sys”: { “type”: 1, “id”: 5091, “message”: 0.0052, “country”: “GB”, “sunrise”: 1442208848, “sunset”: 1442254609 }, “id”: 2643743, “name”: “London”, “cod”: 200}

Looks pretty messy, right? Well, don’t worry, just go to this awesome website and paste in the JSON. It’ll come out looking like this:

If the JSON has to many model classes to be generated, the easier thing to do is generate a Jar and download it, extract it, and then add the files. Otherwise just click on Preview, and copy-paste the classes you need. Do remove the @Generated(“org.jsonschema2pojo”) from each model class, as this annotation isn’t recognized by Android by default.

I downloaded the Jar because there are a lot of classes. And also because I’m lazy. :)

After extracting and adding the classes, now the project tree looks something like this:

So far, so good!

Just an evil raccoon being evil.

Step 2: Create Retrofit Interface for your API calls

For Retrofit, you have to create an interface for the endpoints of your API.

When creating the interface, you should ask yourself: what exactly is the meaning of life? And secondly: what information do I need from the API?

For me, the answer to both questions is WeatherData (the top-most object). Now let’s examine the URL:

http://api.openweathermap.org/data/2.5/weather?q=London

There’s a query at the very end, so I’ll do this:

Note: in Retrofit 2.0, the endpoint path string should NOT start with “/” 
@GET("/weather?") --> [email protected]("weather?")  --> correct

Now as parameter, I’ll send the query value using the @Query annotation.

As you’ll notice, I’m returning the Observable of WeatherData. That’s Rx right there!

Step 3: Create the Retrofit Adapter and WeatherService instance

Now in our Activity, we have to create a Retrofit adapter using the Base URL, along with some other info. Once built, we can initiate an object of WeatherService interface, then call the method.

Take a look at this delicious code:

So the question here is what the hell are those addCallAdapter and addConverterFactory methods doing there?

Well in order for our calls to return type Observable, we have to set the call adapter to RxJavaCallAdapter.

And addConverFactory is there to tell Retrofit which sort of converter I want it to use for serializing the JSON. I prefer the GSON converter. There are other converters available too.

So, for these two you need to add their dependencies to your gradle:

compile 'com.squareup.retrofit2:adapter-rxjava:2.0.2'compile 'com.squareup.retrofit2:converter-gson:2.0.0'

Now run the code, and voila! It’ll log the weather description.

A Different Example

So yeah… Another example! Because why not? This time, I’ll try out the GitHub API. Again just 3 steps.

First, the API call URL:

https://api.github.com/users/ahmedrizwan

and the response:

{    "login": "ahmedrizwan",    "id": 4357275,    "avatar_url": "https://avatars.githubusercontent.com/u/4357275?v=3",    "gravatar_id": "",    "url": "https://api.github.com/users/ahmedrizwan",    "html_url": "https://github.com/ahmedrizwan",    "followers_url": "https://api.github.com/users/ahmedrizwan/followers",    "following_url": "https://api.github.com/users/ahmedrizwan/following{/other_user}",    "gists_url": "https://api.github.com/users/ahmedrizwan/gists{/gist_id}",    "starred_url": "https://api.github.com/users/ahmedrizwan/starred{/owner}{/repo}",    "subscriptions_url": "https://api.github.com/users/ahmedrizwan/subscriptions",    "organizations_url": "https://api.github.com/users/ahmedrizwan/orgs",    "repos_url": "https://api.github.com/users/ahmedrizwan/repos",    "events_url": "https://api.github.com/users/ahmedrizwan/events{/privacy}",    "received_events_url": "https://api.github.com/users/ahmedrizwan/received_events",    "type": "User",    "site_admin": false,    "name": "ahmed",    "company": null,    "blog": "https://medium.com/@ahmedrizwan",    "location": null,    "email": "[email protected]",    "hireable": true,    "bio": null,    "public_repos": 19,    "public_gists": 0,    "followers": 25,    "following": 16,    "created_at": "2013-05-06T18:32:59Z",    "updated_at": "2016-07-08T11:29:26Z"}

Again, the JSON looks pretty messy.

Step 1: Generate the Pojos (plain old Java objects)

I copied the response and pasted it here (yet again):

And clicked on Preview.

Then I copied the Github class into my Project. *music intensifies*

Step 2: Create Interface for Endpoints

Now examine the API URL. And I mean, really examine it. You’ll see the endpoint starts from users and ends with the username.

https://api.github.com/users/ahmedrizwan

So the interface I created looks like this:

I created a call method, and used @Path annotation to replace the value of {username} in the EndPoint string dynamically.

Step 3: Create adapter and instance of GithubService

Here’s the beautiful code for doing just that:

You’ll notice I’m mapping the user object to String (you can do that with Rx. So Its output becomes:

So this is it. Although the article doesn’t cover everything Retrofit and RxAndroid can do (of course), I hope it will get you off to a good start.

Happy coding!