Using Retrofit in Android

Using Retrofit is quite simple and straightforward.

First of all you need to add retrofit to your project, as example with Gradle build sytem.

compile 'com.squareup.retrofit:retrofit:1.7.1' |

another way you can download .jar and place it to your libs folder.

Then you need to define interfaces that will be used by Retrofit to make API calls to your REST endpoints. For example for users:

public interface YourUsersApi {   //You can use rx.java for sophisticated composition of requests    @GET("/users/{user}")   public Observable<SomeUserModel> fetchUser(@Path("user") String user);   //or you can just get your model if you use json api   @GET("/users/{user}")   public SomeUserModel fetchUser(@Path("user") String user);   //or if there are some special cases you can process your response manually    @GET("/users/{user}")   public Response fetchUser(@Path("user") String user);}

Ok. Now you have defined your API interface an you can try to use it.

To start you need to create an instance of RestAdapter and set base url of your API back-end. It's also quite simple:

RestAdapter restAdapter = new RestAdapter.Builder()   .setEndpoint("https://yourserveraddress.com")    .build();YourUsersApi yourUsersApi = restAdapter.create(YourUsersApi.class);

Here Retrofit will read your information from interface and under the hood it will create RestHandler according to meta-info your provided which actually will perform HTTP requests.

Then under the hood, once response is received, in case of json api your data will be transformed to your model using Gson library so you should be aware of that fact that limitations that are present in Gson are actually there in Retrofit.

To extend/override process of serialisers/deserialisation your response data to your models you might want to provide your custom serialisers/deserialisers to retrofit.

Here you need to implement Converter interface and implement 2 methods fromBody() and toBody().

Here is example:

public class SomeCustomRetrofitConverter implements Converter {    private GsonBuilder gb;    public SomeCustomRetrofitConverter() {        gb = new GsonBuilder();        //register your cursom custom type serialisers/deserialisers if needed        gb.registerTypeAdapter(SomeCutsomType.class, new SomeCutsomTypeDeserializer());    }    public static final String ENCODING = "UTF-8";    @Override    public Object fromBody(TypedInput body, Type type) throws ConversionException {        String charset = "UTF-8";        if (body.mimeType() != null) {            charset = MimeUtil.parseCharset(body.mimeType());        }        InputStreamReader isr = null;        try {           isr = new InputStreamReader(body.in(), charset);           Gson gson = gb.create();           return gson.fromJson(isr, type);        } catch (IOException e) {            throw new ConversionException(e);        } catch (JsonParseException e) {            throw new ConversionException(e);        } finally {            if (isr != null) {                   try {                      isr.close();                   } catch (IOException ignored) {                }            }        }    }    @Override    public TypedOutput toBody(Object object) {        try {            Gson gson = gb.create();            return new JsonTypedOutput(gson.toJson(object).getBytes(ENCODING), ENCODING);        } catch (UnsupportedEncodingException e) {            throw new AssertionError(e);        }     }    private static class JsonTypedOutput implements TypedOutput {        private final byte[] jsonBytes;        private final String mimeType;        JsonTypedOutput(byte[] jsonBytes, String encode) {            this.jsonBytes = jsonBytes;            this.mimeType = "application/json; charset=" + encode;        }        @Override        public String fileName() {            return null;        }       @Override       public String mimeType() {           return mimeType;       }       @Override       public long length() {          return jsonBytes.length;       }       @Override       public void writeTo(OutputStream out) throws IOException {           out.write(jsonBytes);       }    } }

And now you need to enable your custom adapters, if it was needed by using setConverter() on building RestAdapter

Ok. Now you are aware how you can get your data from server to your Android application. But you need somehow mange your data and invoke REST call in right place. There I would suggest to use android Service or AsyncTask or loader or rx.java that would query your data on background thread in order to not block your UI.

So now you can find the most appropriate place to call

SomeUserModel yourUser = yourUsersApi.fetchUser("someUsers")

to fetch your remote data.

I have just used retrofit for a couple of weeks and at first I found it hard to use in my application. I would like to share to you the easiest way to use retrofit in you application. And then later on if you already have a good grasp in retrofit you can enhance your codes(separating your ui from api and use callbacks) and maybe get some techniques from the post above.

In your app you have Login,Activity for list of task,and activity to view detailed task.

First thing is you need to add retrofit in your app and theres 2 ways, follow @artemis post above.

Retrofit uses interface as your API. So, create an interface class.

public interface MyApi{/*LOGIN*/@GET("/api_reciever/login") //your login function in your apipublic void login(@Query("username") String username,@Query("password") String password,Callback<String> calback); //this is for your login, and you can used String as response or you can use a POJO, retrofit is very rubust to convert JSON to POJO/*GET LIST*/@GET("/api_reciever/getlist") //a function in your api to get all the listpublic void getTaskList(@Query("user_uuid") String user_uuid,Callback<ArrayList<Task>> callback); //this is an example of response POJO - make sure your variable name is the same with your json tagging/*GET LIST*/@GET("/api_reciever/getlistdetails") //a function in your api to get all the listpublic void getTaskDetail(@Query("task_uuid") String task_uuid,Callback<Task> callback);   //this is an example of response POJO - make sure your variable name is the same with your json tagging}

Create another interface class to hold all your BASE ADDRESS of your api

public interface Constants{   public String URL = "www.yoururl.com"}

In your Login activity create a method to handle the retrofit

private void myLogin(String username,String password){RestAdapter restAdapter = new RestAdapter.Builder()    .setEndpoint(Constants.URL)  //call your base url    .build();MyApi mylogin = restAdapter.create(MyApi.class); //this is how retrofit create your apimylogin.login(username,password,new Callback<String>() {        @Override        public void success(String s, Response response) {            //process your response if login successfull you can call Intent and launch your main activity        }        @Override        public void failure(RetrofitError retrofitError) {            retrofitError.printStackTrace(); //to see if you have errors        }    });}

In your MainActivityList

private void myList(String user_uuid){RestAdapter restAdapter = new RestAdapter.Builder()    .setEndpoint(Constants.URL)  //call your base url    .build();MyApi mytask = restAdapter.create(MyApi.class); //this is how retrofit create your apimytask.getTaskDetail(user_uuid,new Callback<Task>>() {        @Override        public void success(ArrayList<Task> list, Response response) {            //process your response if successful load the list in your listview adapter        }        @Override        public void failure(RetrofitError retrofitError) {            retrofitError.printStackTrace(); //to see if you have errors        }    });}

In your Detailed List

private void myDetailed(String task_uuid){RestAdapter restAdapter = new RestAdapter.Builder()    .setEndpoint(Constants.URL)  //call your base url    .build();MyApi mytask = restAdapter.create(MyApi.class); //this is how retrofit create your apimytask.getTaskList(task_uuid,new Callback<Task>() {        @Override        public void success(Task task, Response response) {            //process your response if successful do what you want in your task        }        @Override        public void failure(RetrofitError retrofitError) {            retrofitError.printStackTrace(); //to see if you have errors        }    });}

Hope this would help you though its really the simplest way to use retrofit.

Take a look at this , excellent blog on using Retrofit in conjunction with Otto, both libraries are from Square.

http://www.mdswanson.com/blog/2014/04/07/durable-android-rest-clients.html

The basic idea is that you will hold a reference to a "repository" object in your Application class. This object will have methods that "subscribe" to rest api event requests. When one is received it will make the proper Retrofit call, and then "post" the response, which can then be "subscribed" to by another component (such as the activity that made the request).

Once you have this all setup properly, accessing data via your rest api becomes very easy. For example, making are request for data would look something like this :

    mBus.post(new GetMicropostsRequest(mUserId));

and consuming the data would look something like this:

@Subscribepublic void onGetUserProfileResponse(GetUserProfileResponse event) {    mView.setUserIcon("http://www.gravatar.com/avatar/" + event.getGravatar_id());    mView.setUserName(event.getName());}

It takes a little bit of upfront effort, but in the end it becomes "trivial" to access anything you need from our backend via Rest.