Networking with the Volley Library

Overview

Volley is a library that makes networking for Android apps easier and most importantly, faster. Volley Library was announced by Ficus Kirkpatrick at Google I/O '13! It was first used by the Play Store team in Play Store Application and then they released it as an Open Source Library.

Why Volley?

• Volley can pretty much do everything with that has to do with Networking in Android.
• Volley automatically schedules all network requests such as fetching responses for image from web.
• Volley provides transparent disk and memory caching.
• Volley provides powerful cancellation request API for canceling a single request or you can set blocks of requests to cancel.
• Volley provides powerful customization abilities.
• Volley provides debugging and tracing tools.

Setup Volley

Adding Volley to our app/build.gradle file:

dependencies {
    compile 'com.android.volley:volley:1.0.0'
}

And add the internet permission in AndroidManifest.xml:

<?xml version="1.0" encoding="utf-8"?>
<manifest xmlns:android="http://schemas.android.com/apk/res/android"
    package="com.example.simplenetworking"
    android:versionCode="1"
    android:versionName="1.0" >
 
   <!-- Add permissions here -->
   <uses-permission android:name="android.permission.INTERNET" /> 
   <uses-permission android:name="android.permission.ACCESS_NETWORK_STATE" />

</manifest>

How to use Volley?

Volley has two classes that you will have to deal with:

1. RequestQueue - Requests are queued up here to be executed
2. Request (and any extension of it) - Constructing an network request

A Request object comes in three major types:

• JsonObjectRequest — To send and receive JSON Object from the server
• JsonArrayRequest — To receive JSON Array from the server
• ImageRequest - To receive an image from the server
• StringRequest — To retrieve response body as String (ideally if you intend to parse the response by yourself)

Constructing a RequestQueue

All requests in Volley are placed in a queue first and then processed, here is how you will be creating a request queue:

public MainActivity extends Activity {
	private RequestQueue mRequestQueue;

	protected void onCreate(Bundle savedInstanceState) {
		super.onCreate(savedInstanceState);
		setContentView(R.layout.main_screen_layout);
		// ...
		mRequestQueue = Volley.newRequestQueue(this);
	}
}

Creating a Singleton Queue

See this guide for creating a singleton to use for sending requests.

Accessing JSON Data

After this step you are ready to create your Request objects which represents a desired request to be executed. Then we add that request onto the queue.

public class MainActivity extends Activity {
	private RequestQueue mRequestQueue;

        // ...

	private void fetchJsonResponse() {
		// Pass second argument as "null" for GET requests
		JsonObjectRequest req = new JsonObjectRequest(Request.Method.GET, "http://ip.jsontest.com/", null,
		    new Response.Listener<JSONObject>() {
		        @Override
		        public void onResponse(JSONObject response) {
		            try {
				String result = "Your IP Address is " + response.getString("ip");
				Toast.makeText(MainActivity.this, result, Toast.LENGTH_SHORT).show();
			    } catch (JSONException e) {
			        e.printStackTrace();
			    }
	                }
		    }, new Response.ErrorListener() {
			@Override
			public void onErrorResponse(VolleyError error) {
				VolleyLog.e("Error: ", error.getMessage());
			}
		});

		/* Add your Requests to the RequestQueue to execute */
		mRequestQueue.add(req);
	}
}

And that will execute the request to the server and respond back with the result as specified in the Response.Listener callback. For a more detailed look at Volley, check out this volley tutorial.

Canceling Requests

You can tag a request with:

StringRequest stringRequest = ...;
RequestQueue mRequestQueue = ...;

// Set the tag on the request.
stringRequest.setTag(TAG);

// Add the request to the RequestQueue.
mRequestQueue.add(stringRequest);

You can now cancel all requests with this tag using the cancelAll on the request queue:

@Override
protected void onStop() {
    super.onStop();
    if (mRequestQueue != null) {
        mRequestQueue.cancelAll(TAG);
    }
}

References

• https://smaspe.github.io/2013/06/03/volley-part1.html
• http://java.dzone.com/articles/android-%E2%80%93-volley-library
• http://arnab.ch/blog/2013/08/asynchronous-http-requests-in-android-using-volley/
• http://files.evancharlton.com/volley-docs/