• Intro: a Computational Thinking Project
• Server
  • What we've done
  • Execution requirements
  • Setup
• Client
  • What we've done
  • Execution requirements
  • Setup
  • Interaction API's
• Try it yourself!
  • Demo 1: Messaging
  • Demo 2: Bouncing ball
• About us
• License

The Shared Snap! project wants to combine Berkeley's Snap! learning tool with a custom RESTful Web Server in order to allow microworlds intercommunication.

Given the absence of this feature in the Snap! environment, we decided to tackle this challenge: we think that introducing communication between microworlds residing in different browser windows (or even in different machines) could be very fun and engaging, allowing for lots of new possibilities. Already existing Snap! official libraries allow to make RESTful requests and to interact with JSON objects, therefore the choice of a RESTful Web Server came up naturally.

The image below shows the sequence diagram for the data exchanged between the Client actor (the one who wants to communicate with another actor remotely), the Walkie Talkie actor, and the Web Server.

First of all, we thought of modeling the connection as something to do in order to allow the sending and reception of messages to and from other users connected to the same server.

We decided to introduce at least a simple authentication system that assign an unique Token to every user at the moment of the connection to the server. That token must be included by the client in every subsequent request inside the Authorization header. To send a message, it's mandatory to be connected to the server, providing your authentication token along with the recipient's nickname.

The custom Web Server has been developed using Kotlin, VertX and MongoDB, implementing handlers for a set of previously defined RESTful API which documentation is available on SwaggerHub.

• Java 8 must be installed;

• a MongoDB daemon must be running on localhost:10000.

It is possible to download the source code and the executable jar directly from the project releases page.

You can modify the configuration file config.yaml according to your needs: you can set the host and port of the server and the MongoDB connection_string. By default they are set respectively to: localhost, 10000 and mongodb://localhost:27017/snapdb.

In order to run the Web Service you must execute the following command:

java -jar shared-snap-1.0-all.jar

N.B. The server has been currently designed to use the http protocol while Snap website uses the https protocol. Running everything locally arises no problems but, if at least one client resides on a different machine than the server, it will be necessary to take one of the following measures:

• the Web Service must be configured to use the https protocol since browsers block mixed content by default;

• alternatively, permissions for mixed content must be manually enabled for your browser.

We started the client development by importing two official Snap! libraries and modifying them in order to obtain a better error handling capabilities:

• we extended Web Services Access (HTTPS) in order to make it possibile to read both the body and the http response code. This way, our Walkie Talkie actor can be able to always know and report back the reason of an eventual request failure;

• we extended Deal with JSON data in order to better manage the transformation error that may arise from a JSON formatted string to Snap's list conversion.

From there, we added an abstraction layer through a set of new Snap! blocks, useful for the interaction with our custom Web server. Finally, we added an additional abstraction layer with the purpose of hiding technicalities such as the polling and authentication system behind the Walkie Talkie actor, allowing the use of the application even by less experienced users.

• The server must be running;

• it's strongly recommended to use the Chrome browser (or Chromium derivates) to avoid problems with CORS requests.

To use Shared Snap! you need to import first our custom blocks and then the Walkie Talkie actor. It's possible to choose between the following methods:

• clone the repository and import to your Snap project the shared snap blocks.xml and walkie talkie.xml files placed inside the client directory;
• alternatively, just downloads the single files and import them to your Snap project: custom blocks, walkie talkie actor.

The Walkie Talkie actor offers simple a API for connecting, disconnecting, sending and receiving messages.

• 
  Connect as <your nickname> to the server with the provided address. This block will fail if another user is already connected to the same server with the same nickname.

• 
  Disconnect your user from the server; as a result your nickname will become available again in that server.

• 
  Send a message specifying the JSON content and the recepient username.

• 
  To receive a remote message you must use the when I receive block with the "any message" parameter selected and then filter the message for the remote message type.
  Note that every time our Walkie Talkie actor receives a message intended for our user (sent to our nickname) it broadcasts a JSON message with the fields type set to "remote message", sender and content where the content corresponds to the actual JSON object that was sent to us by the sender.

{
    "type": "remote message",
    "sender": "...",
    "content": { ... }
}

• Using the when I receive block it's also possible to observe many informational messages that come from the Walkie Talkie as listed in the image below.
  

To showcase our project functionalities we decided to implement two public demos.
The first one is like an hello world where a text message is exchanged from a sender to a receiver.
The second shows how a ball can bounce from one microworld to another.

To run the first demo follow these steps:

1. start the server - see the server setup

2. open both the following links: receiver, sender

3. click the green flag on the Receiver

4. click the green flag on the Sender

To run the second demo follow these steps:

1. start the server - see the server setup

2. open both the following links: left screen, right screen

3. on the right screen hit the c button to connect to the server (the circle inside the walkie talkie will light up from gray to green)

4. on the left screen hit the c button to connect to the server (the circle inside the walkie talkie will light up from gray to green)

The ball will appear at the center of the left screen and will start moving from one browser window to the other.

When the ball passes from one world to another the message exchanged between the two contains: the vertical position coordinate, the module and the angle of the velocity vector.

{
    "y": ...,
    "velocityModule": ...,
    "velocityAngle": ...
}

Course reference:

• https://www.unibo.it/it/didattica/insegnamenti/insegnamento/2018/430273

Team members:

• [email protected]
• [email protected]

Shared Snap! is licensed under the Affero General Public License.