webthing 0.15.0

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 Light 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.

• brightness: 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:

light = Thing(
    'urn:dev:ops:my-lamp-1234',
    'My Lamp',
    ['OnOffSwitch', 'Light'],
    '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 a method 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.add_property(
    Property(
        light,
        'on',
        Value(True, lambda v: print('On-State is now', v)),
        metadata={
            '@type': 'OnOffProperty',
            'title': 'On/Off',
            'type': 'boolean',
            'description': 'Whether the lamp is turned on',
        }))

The brightness 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.

light.add_property(
    Property(
        light,
        'brightness',
        Value(50, lambda v: print('Brightness is now', v)),
        metadata={
            '@type': 'BrightnessProperty',
            'title': 'Brightness',
            'type': 'number',
            'description': 'The level of light from 0-100',
            'minimum': 0,
            'maximum': 100,
            'unit': 'percent',
        }))

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.
server = WebThingServer(SingleThing(light), port=8888)

try:
    server.start()
except KeyboardInterrupt:
    server.stop()

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 returns a level instead of just on/off) has one required property (besides the name, type, and optional description): level. We want to monitor this property and get notified if the value changes.

First we create a new Thing:

sensor = Thing(
    'urn:dev:ops:my-humidity-sensor-1234',
    'My Humidity Sensor',
     ['MultiLevelSensor'],
     'A web connected humidity sensor'
)

Then we create and add the appropriate property:

• 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 creating a readOnly property.

    level = Value(0.0);
    
    sensor.add_property(
        Property(
            sensor,
            'level',
            level,
            metadata={
                '@type': 'LevelProperty',
                'title': 'Humidity',
                'type': 'number',
                'description': 'The current humidity in %',
                'minimum': 0,
                'maximum': 100,
                'unit': 'percent',
                'readOnly': True,
            }))

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.

self.sensor_update_task = \
    get_event_loop().create_task(self.update_level())

async def update_level(self):
    try:
        while True:
            await sleep(3)
            new_level = self.read_from_gpio()
            logging.debug('setting new humidity level: %s', new_level)
            self.level.notify_of_external_update(new_level)
    except CancelledError:
        pass

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