Scenic.Sensor

Scenic.Sensor is a combination pub/sub server and data cache for sensors. It is intended to be the interface between sensors and Scenic scenes, although it has no dependencies on Scenic and can be used in other applications.

Installation

Scenic.Sensor can be installed by adding :scenic_sensor to your list of dependencies in mix.exs:

def deps do
  [
    {:scenic_sensor, "~> 0.7.0"}
  ]
end

Startup

In order to use Scenic.Sensor, you must first add it to your supervision tree. It should be ordered in the tree so that it has a chance to initialize before other processes start making calls to it.

  def start(_type, _args) do
    import Supervisor.Spec, warn: false

    opts = [strategy: :one_for_one, name: ScenicExample]
    children = [
      {Scenic.Sensor, nil},
      ...
    ]
    Supervisor.start_link(children, strategy: :one_for_one)
  end

Why Scenic.Sensor

Sensors and scenes often need to communicate, but tend to operate on different timelines.

Some sensors update fairly slowly or don't behave well when asked to get data at random times by multiple clients. Scenic.Sensor lets you create a GenServer that collects data from a sensor in a well-behaved manner, yet is able to serve that data on demand or by subscription to many clients.

Registering Sensors

Before a process can start publishing data from a sensor, it must register that sensor with Scenic.Sensor. This both prevents other processes from stepping on that data and alerts any subscribing processes that the sensor is coming online.

  Scenic.Sensor.register( :sensor_id, version, description )

The :sensor_id parameter must be an atom that names the sensor. Subscribers will look for data from this sensor through that id.

The version and description paramters are bitstrings that describe this sensor. Scenic.Sensor itself does not process these values, but passes them to the listeners when the sensor comes online or when the sensors are listed.

Sensors can also unregister if they are no longer available.

  Scenic.Sensor.unregister( :sensor_id )

Simply exiting the sensor does also cleans up its registration.

Publishing Data

When a sensor process publishes data, two things happen. First, that data is cached in an :ets table so that future requests for that data from scenes happen quickly and don't need to bother the sensor. Second, any processes that have subscribed to that sensor are sent a message containing the new data.

  Scenic.Sensor.publish( :sensor_id, sensor_value )

The :sensor_id parameter must be an atom that was previously registered by calling process.

The sensor_value parameter can be anything that makes sense for the sensor.

Subscribing to a sensor

Scenes (or any other process) can subscribe to a sensor. They will receive messages when the sensor updates its data, comes online, or goes away.

  Scenic.Sensor.subscribe( :sensor_id )

The :sensor_id parameter is the atom registered for the sensor.

The subscribing process will then start receiving messages that can be handled with handle_info/2

event	message
data	{:sensor, :data, {:sensor_id, data, timestamp}}
registered	{:sensor, :registered, {:sensor_id, version, description}}
unregistered	{:sensor, :unregistered, :sensor_id}

Scenes can also unsubscribe if they are no longer interested in updates.

  Scenic.Sensor.unsubscribe( :sensor_id )

Other functions

Any process can get data from a sensor on demand, whether or not it is a subscriber.

  Scenic.Sensor.get( :sensor_id )
  >> {:ok, data}

Any process can list the currently registered sensors

  Scenic.Sensor.list()
  >> [{:sensor_id, version, description, pid}]