How to call Functions from Android

Twilio Functions are a perfect fit for mobile app developers. You can focus on writing your app, and let Twilio host and run the server code you need.

You don't need a special Twilio SDK or library to call Twilio Functions from your mobile app—your Function will respond to a normal HTTP call. We'll use Google's Volley HTTP Networking library with this example, but you can use other HTTP libraries for Java or Android if you would like.

In this guide, we'll show you how to set up a Twilio Function, call it from a web browser, and then call that function from an Android application. Our function will return a joke as a string. You could extend it to make it choose a random joke from a list, or by category. We'll keep it brief, and just return a hard coded string.

Let's start by creating a Function and giving it the path of /joke. Be sure to set the visibility of this Function to public, to avoid any hurdles when making your HTTP calls:

Create and host a Function

In order to run any of the following examples, you will first need to create a Function into which you can paste the example code. You can create a Function using the Twilio Console or the Serverless Toolkit as explained below:

ConsoleServerless Toolkit

If you prefer a UI-driven approach, creating and deploying a Function can be done entirely using the Twilio Console and the following steps:

Log in to the Twilio Console and navigate to the Functions tab. If you need an account, you can sign up for a free Twilio account here!
Functions are contained within Services. Create a Service by clicking the Create Service button and providing a name such as test-function.
Once you've been redirected to the new Service, click the Add + button and select Add Function from the dropdown.
This will create a new Protected Function for you with the option to rename it. The name of the file will be path it is accessed from.
Copy any one of the example code snippets from this page that you want to experiment with, and paste the code into your newly created Function. You can quickly switch examples by using the dropdown menu of the code rail.
Click Save to save your Function's contents.
Click Deploy All to build and deploy the Function. After a short delay, your Function will be accessible from: https://<service-name>-<random-characters>-<optional-domain-suffix>.twil.io/<function-path>
For example: test-function-3548.twil.io/hello-world.

The Serverless Toolkit enables you with local development, project deployment, and other functionality via the Twilio CLI. To get up and running with these examples using Serverless Toolkit, follow this process:

From the CLI, run twilio serverless:init <your-service-name> --empty to bootstrap your local environment.
Navigate into your new project directory using cd <your-service-name>
In the /functions directory, create a new JavaScript file that is named respective to the purpose of the Function. For example, sms-reply.protected.js for a Protected Function intended to handle incoming SMS.
Populate the file using the code example of your choice and save. Note A Function can only export a single handler. You will want to create separate files if you want to run and/or deploy multiple examples at once.

Once your Function(s) code is written and saved, you can test it either by running it locally (and optionally tunneling requests to it via a tool like ngrok), or by deploying the Function and executing against the deployed url(s).

Run your Function in local development

Run twilio serverless:start from your CLI to start the project locally. The Function(s) in your project will be accessible from http://localhost:3000/sms-reply

If you want to test a Function as a Twilio webhook, run: twilio phone-numbers:update <your Twilio phone number> --sms-url "http://localhost:3000/sms-reply"
This will automatically generate an ngrok tunnel from Twilio to your locally running Function, so you can start sending texts to it. You can apply the same process but with the voice-url flag instead if you want to test with Twilio Voice.
If your code does not connect to Twilio Voice/Messages as a webhook, you can start your dev server and start an ngrok tunnel in the same command with the ngrok flag. For example: twilio serverless:start --ngrok=""

Deploy your Function

To deploy your Function and have access to live url(s), run twilio serverless:deploy from your CLI. This will deploy your Function(s) to Twilio under a development environment by default, where they can be accessed from:

https://<service-name>-<random-characters>-dev.twil.io/<function-path>

For example: https://incoming-sms-examples-3421-dev.twil.io/sms-reply

Your Function is now ready to be invoked by HTTP requests, set as the webhook of a Twilio phone number, invoked by a Twilio Studio Run Function Widget, and more!

With the Function created, we'll need to edit the boilerplate code that is generated for the Function—by default, it comes with some code to return TwiML. We're only going to return a joke. And it's a bad joke.

Return a Joke with a Twilio Function

1exports.handler = (context, event, callback) => {
2  const joke = 'How many apples grow on a tree? They all do!';
3  return callback(null, joke);
4};

Copy the above code into the Twilio Functions code editor. Please, change the joke to something better. Press the Save button to save that code, and click Deploy All to deploy your Function.

Call a Twilio Function from the Web

To call your new Function from the web, get the Function's URL by clicking the Copy URL icon next to the path, and then paste that URL into any web browser (you don't have to be authenticated with Twilio). You'll get a text response containing whatever you return from your Function!

Call a Twilio Function from Android

We are going to use Google's open source Volley library to call our Twilio Function. Volley is a great choice, and provides built-in request classes for retrieving strings, JSON objects, and JSON Arrays.

Volley is not part of the Android SDK. You will need to include the Volley library in your build.gradle file as a dependency, like this:

1dependecies {
2    ...
3    compile 'com.android.volley:volley:1.0.0'
4    ...
5}

If you are using Android Studio, be sure to Sync your gradle file after this edit.

Don't forget to also put the INTERNET permission request into your AndroidManifest.xml file as well, or you will get an exception when you make an HTTP request.

<uses-permission android:name="android.permission.INTERNET"/>

The Volley StringRequest constructor takes the HTTP method used (GET in our case), the URL to retrieve, and two listeners—one for a successful response, and one if there is an error. The onResponse and onErrorResponse methods will be on the main (UI) thread, so you can modify the user interface. In our case, we are just going to log the responses to the console.

Call a Twilio Function from Android

1String url = "https://yourdomain.twil.io/joke";
2StringRequest request = new StringRequest(Request.Method.GET,
3    url,
4    new Response.Listener<String>() {
5        @Override
6        public void onResponse(String response) {
7            Log.d("APP", response);
8        }
9    },
10    new Response.ErrorListener() {
11        @Override
12        public void onErrorResponse(VolleyError error) {
13            Log.d("APP", error.getLocalizedMessage());
14        }
15    }
16);
17Volley.newRequestQueue(context).add(request);

Return JSON from a Twilio Function

Our previous example Function returned plain text. You can also return JSON from a Twilio Function, by passing a JavaScript object or array to the callback function. For instance, we can create another Twilio Function to return a list of jokes, along with an id and a favorite count. Create a new Function with a path of /jokes.

A Twilio Function that Returns a JSON Array

1exports.handler = (context, event, callback) => {
2  const knockKnock = { id: 1, text: 'Knock, knock', favorited: 37 };
3  const chicken = {
4    id: 2,
5    text: 'Why did the chicken cross the road?',
6    favorited: 12,
7  };
8  const jokes = [knockKnock, chicken];
9  return callback(null, jokes);
10};

Parse JSON from a Twilio Function

From Android, we call this Function the same way that we did our first Function (don't forget to change the path to /jokes). We can use Volley's JsonArrayRequest object similarly to how we used StringRequest before.

Call a Twilio Function that returns JSON from Android

1String url = "https://yourdomain.twil.io/jokes";
2JsonArrayRequest request = new JsonArrayRequest(url,
3        new Response.Listener<JSONArray>() {
4
5            @Override
6            public void onResponse(JSONArray response) {
7                Log.d("APP", response.toString());
8            }
9        },
10        new Response.ErrorListener() {
11            @Override
12            public void onErrorResponse(VolleyError error) {
13                Log.d("APP", error.getLocalizedMessage());
14            }
15        }
16);
17Volley.newRequestQueue(context).add(request);

You've now seen how to run Node.js code as a Twilio Function, and how your mobile application can use this as a serverless backend to provide data for your application.

Where to go next? You could extend the Function to choose a random joke from that array. You can also use Twilio functionality from inside your Function, for instance to send an SMS, or to return an access token for Video, Chat, or Sync. Check out the Programmable SMS Quickstart for Twilio Functions and Programmable Voice Quickstart for Twilio Functions for more quick introductions to these key features of Functions.