Example Code
Products and Environment
This section reflects the products and operating system used to create the example.To download NI software, including the products shown below, visit ni.com/downloads.
- LabVIEW G
Programming Language
Description
Earlier this year I posted about the myRIO Giant Tetris project I worked on which ended up appearing at NI Week in Texas and NI Days in the UK this year. One of the coolest features of this was the ability to control the game using a web browser from a computer or mobile device. The technology for doing this was WebSockets which allows you to create and maintain an open WebSocket connection (using TCP/IP) and send data with low-latency and overhead between a modern web-browser and a server (in this case LabVIEW RT).
I have finally been able to tidy up the code and offer it to the community to play with!
Here's what I originally wrote about WebSockets for the Giant Tetris:
WebSockets.
WebSockets is a web technology that allows a browser and a server to communicate in a way similar to TCP/IP. The browser sends a WebSocket upgrade request (very similar to a HTTP request), the server sends a HTTP response with specific headers and then you can send/receive data through the open connection. To implement this in LabVIEW I used the TCP/IP VIs and some additional VIs for performing the HTTP handshaking and framing the websockets data. In my case, the browser is communicating directly with the myRIO (or it could be other RT targets or PCs) with no intermediate server.
I've been waiting for an excuse use WebSockets as a tech demo for my work - we have used LabVIEW Remote Front Panels in the past for communicating with LabVIEW (a cRIO). Although it was easy to setup, it requires the LabVIEW run-time engine to be installed and only works with certain browsers (Internet Explorer). Having done quite a lot of web development, I know that there is an absolutely vast pool of resources for using a web browser for displaying data and doing asynchronous communications using JavaScript and I'd like to leverage that to create great looking user interfaces (using HTML and CSS) that anyone can view/use with any web browser - including mobile devices (and especially tablets - iPad etc.)! WebSockets fulfils the requirement of having data update in real-time and allow the user to press buttons & enter values.
The library I have produced acts as a wrapper for the TCP Read/Write VIs and includes a handshaking VI for reading and responding to an incoming handshake request from the browser.
API - Server
Here's an overview of the API for the LabVIEW Side:
Do Handshake: Performs the WebSocket handshaking after a connection is made - reads the headers from the HTTP request and replies with the appropriate WebSocket key to open the connection.
Do Client Handshake: This VI allows you to initiate a connection acting as a WebSocket client, communicating to an existing WebSocket server (either implementing in LabVIEW or otherwise). It sends the necessary HTTP header to start a WebSocket connection and waits for the reply from the server.
Get Header: I store the header name/value pairs as variant attributes, you can use this VI to retrieve a header value.
Read: Wrapper for the TCP Read primitive that splits out the data from the WebSocket frame. The returned Opcode is as per the WebSocket standard.
Write: Wrapper for the TCP Write primitive that packages the data into a WebSocket frame and sends to the connection.
Close: Sends the WebSocket close Opcode to the browser. According to the WebSockets spec, both sides of the connection are supposed to send the close Opcode before terminating the TCP connection so this VI doesn't close the TCP connection.
As it's just a wrapper for TCP/IP functions with some maths/string manipulation (for hashing the WebSocket key) it works on RT and Windows targets.
API - Client
For writing your javascript for establishing the connection and sending/receiving data from the server I suggest you check out the wealth of tutorials and information available on the internet. The included example (see below) has the basics and I wrote a little more about the Javascript side at the link for the myRIO Giant Tetris which includes some sample code.
Example
The included example VI, when run, acts as a server for an incoming WebSocket connection, displays the string data received from the connected browser and allows you to send string data back to the browser. The included index.html file in the installed package acts as a client to connect and display the string data:
Known Limitations:
- Currently only supports sending/receiving string data even though the WebSocket spec supports binary data transfer. I figured that this wouldn't be a huge problem as I figured the most likely use case is using strings or json-encoded data and if you used binary you would need to be able to do the conversion between javascript and LabVIEW formats yourself.
- It hasn't been tested for sending frames with a long data segment. The WebSocket spec packages the header differently depending on data length and I haven't fully tested that these are correctly prepared for longer data segments.
Usage / Disclaimer:
I've published this in an attempt that it will be useful for the NI community. The package/source is released under the MIT License. The source code and releases are available on GitHub: https://github.com/MediaMongrels-Ltd/LabVIEW-WebSockets-API
If you find anything wrong with it then I hope you'll let me know in the comments so it can be improved and built upon.
If you use it, it would be great if you credit me (Sam Sharp - MediaMongrels Ltd - sam@mediamongrels.com - http://www.mediamongrels.com) but I'm not going to demand it. 
Installation/Usage:
The VI package is available/installed through the LabVIEW Tools Network VIPM repository at the following link: vipm://mediamongrels_ltd_lib_websockets_api
The VI Package is for >=LV2013. You can probably save for a previous version - I don't think it uses anything specific to LV2013 but I was using it with a myRIO hence LV2013 (plus LV2013 has those neat JSON encode/decode which is great for converting between LV and Javascript data types!).
Example code from the Example Code Exchange in the NI Community is licensed with the MIT license.
