Create message converter

Build a simple message converter for transforming custom GPS messages to match the foxglove.LocationFix schema, for visualization in Foxglove's Map panel.

Setting up

In a Terminal window, cd into the directory where your source code will live and run the following command:

npm init foxglove-extension@latest myExtensionName

This uses create-foxglove-extension to create a myExtensionName directory containing source code for an example message converter.

Registering a converter

The index.ts file in your project's src folder is the entry point for your extension source code. It must export an activate function that accepts a single extensionContext argument of type ExtensionContext.

To register a message converter, we call registerMessageConverter on the extensionContext argument with three arguments: the source schema type, the destination schema type, and the actual converter function.

import { MessageEvent } from "@foxglove/extension";

export function activate(extensionContext: ExtensionContext) {
  extensionContext.registerMessageConverter({
    fromSchemaName: "...",
    toSchemaName: "...",
    converter: (inputMessage: MyInputType, messageEvent: MessageEvent<MyInputType>) => {
      // ...
    },
  });
}

The converter function takes two arguments – the input topic message, and the full message event. The message event can be used to access other relevant pieces of information – like the message's publishTime, receiveTime, and topic name.

Writing the converter

Let's assume that our data contains GPS messages of type sensors.MyGps, which contain a lat and lon field.

type MyGps = {
  lat: number;
  lon: number;
};

To visualize GPS coordinates, the Map panel requires messages in the foxglove.LocationFix format. In short, our converter needs to turn MyGps messages into messages that adhere to the foxglove.LocationFix schema.

First, specify the from schema (sensors.MyGps) and to schema (foxglove.LocationFix), to inform Foxglove that our registered converter will turn sensors.MyGps messages into foxglove.LocationFix messages.

Then, write the converter function. In our example, we'll re-map the lat and lon fields to the latitude and longitude fields that the foxglove.LocationFix schema expects:

export function activate(extensionContext: ExtensionContext) {
  extensionContext.registerMessageConverter({
    fromSchemaName: "sensors.MyGps",
    toSchemaName: "foxglove.LocationFix",
    converter: (myGps: MyGps, messageEvent: MessageEvent<MyGps>) => {
      return {
        latitude: myGps.lat,
        longitude: myGps.lon,
      };
    },
  });
}

Testing

Once we've packaged and installed our extension, load any data source containing sensors.MyGps messages in Foxglove and visualize them with the Map panel.

NOTE: Message converters run on-demand only when the topic is subscribed to by a panel.