webthing

Implementation of an HTTP Web Thing.

Installation

webthing can be installed via npm, as such:

$ npm install webthing

Example

In this example we will set up a dimmable light and a humidity sensor (both using fake data, of course). Both working examples can be found in here.

Dimmable Light

Imagine you have a dimmable Light that you want to expose via the web of things API. The Light can be turned on/off and the brightness can be set from 0% to 100%. Besides the name, description, and type, a dimmableLight is required to expose two properties:

on: the state of the light, whether it is turned on or off
- Setting this property via a PUT {"on": true/false} call to the REST API toggles the light.
level: the brightness level of the light from 0-100%
- Setting this property via a PUT call to the REST API sets the brightness level of this light.

First we create a new Thing:

const light = new Thing('My Lamp', 'dimmableLight', 'A web connected lamp');

Now we can add the required properties.

The on property reports and sets the on/off state of the light. For this, we need to have a Value Object which holds the actual state and also how to turn the light on/off. For our purposes, we just want to log the new state if the light is switched on/off.

light.addProperty(
  new Property(
    light,
    'on',
    new Value(true, (v) => console.log('On-State is now', v)),
    {
      type: 'boolean',
      description: 'Whether the lamp is turned on',
    }));

The level property reports the brightness level of the light and sets the level. Like before, instead of actually setting the level of a light, we just log the level to std::out.

light.addProperty(
  new Property(
    light,
    'level',
    new Value(50, l => console.log('New light level is', l)),
    {
      type: 'number',
      description: 'The level of light from 0-100',
      minimum: 0,
      maximum: 100,
    }));

Now we can add our newly created thing to the server and start it:

// If adding more than one thing, use MultipleThings() with a name.
// In the single thing case, the thing's name will be broadcast.
const server = new WebThingServer(SingleThing(light), 8888);

process.on('SIGINT', () => {
  server.stop();
  process.exit();
});

server.start();

This will start the server, making the light available via the WoT REST API and announcing it as a discoverable resource on your local network via mDNS.

Sensor

Let's now also connect a humidity sensor to the server we set up for our light.

A multiLevelSensor (a sensor that can also return a level instead of just true/false) has two required properties (besides the name, type, and optional description): on and level. We want to monitor those properties and get notified if the value changes.

First we create a new Thing:

const sensor = new Thing('My Humidity Sensor',
                         'multiLevelSensor',
                         'A web connected humidity sensor');

Then we create and add the appropriate properties:

on: tells us whether the sensor is on (i.e. high), or off (i.e. low)

sensor.addProperty(
  new Property(
    sensor,
    'on',
    new Value(true),
    {
      type: 'boolean',
      description: 'Whether the sensor is on',
    }));

level: tells us what the sensor is actually reading

Contrary to the light, the value cannot be set via an API call, as it wouldn't make much sense, to SET what a sensor is reading. Therefore, we are utilizing a readOnly Value by omitting the valueForwarder parameter.

Map<String, Object> levelDescription = new HashMap<>();
levelDescription.put('type', 'number');
levelDescription.put('description', 'The current humidity in %');
levelDescription.put('unit', '%');

const level = new Value(0.0);

sensor.addProperty(
  new Property(
    sensor,
    'level',
    level,
    {
      type: 'number',
      description: 'The current humidity in %',
      unit: '%',
    }));

Now we have a sensor that constantly reports 0%. To make it usable, we need a thread or some kind of input when the sensor has a new reading available. For this purpose we start a thread that queries the physical sensor every few seconds. For our purposes, it just calls a fake method.

// Poll the sensor reading every 3 seconds
setInterval(() => {
  // Update the underlying value, which in turn notifies all listeners
  level.notifyOfExternalUpdate(readFromGPIO());
}, 3000);

This will update our Value object with the sensor readings via the this.level.notifyOfExternalUpdate(readFromGPIO()); call. The Value object now notifies the property and the thing that the value has changed, which in turn notifies all websocket listeners.

addmao/webthing-node

webthing

Installation

Example

Dimmable Light

Sensor