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After setting up your workspace for the development of a driver, you will have a project with the following structure (with names of packages and files set to your choices):
Lifecycle
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The class MyDriver is the starting point of your driver implementation and contains all the callback methods called during the driver lifecycle:
The constructor MyDriver(String id) is called only once when the drivers are instantiated during the startup phase of Sfera. No logic should be added here. The id parameter is set to the ID assigned to the driver instance in the configuration. It can be retrieved calling the getId() method.
After creation, the driver instance will follow this lifecycle:
- onInit()
This method is called when starting the driver. Here you should read the driver configuration (config parameter) which may contain parameters such as the IP address of the field system, a user account name, or any other info needed for the system integration.
Then you may initialize the components and data structures of your driver and try to establish a connection with the field system.
After a successful initialization your implementation should return true to proceed to the loop() method.
If the initialization fails (e.g. cannot connect to the field system), return false. The system will call onQuit() and then try the initialization again.
- loop()
After a successful initialization, this method is called continuously as long as it returns true. Here you can poll the field system for state changes or wait for push notifications from it.
Between consecutive loop calls there is no delay, so make sure to release CPU by means of thread sleeping or by executing non-busy waiting operations, to avoid having your driver consume all the CPU resources.
If something goes wrong during your loop, simply return false: the driver will quit and restart from initialization.
- quit
While your driver is running (i.e. while executing onInit() or loop()), the system may want to stop it (e.g. because of a user command). This is accomplished by sending an interrupt to the driver’s thread. As you can see these callbacks can throw InterruptedExceptions, this is because in your implementation you should avoid catching them and let your driver have a graceful shutdown.
After interruption onQuit() is called and the driver will remain in a quitted state until restarted by a start command.
- onQuit()
Here you should cleanup and release your resources to ensure a graceful shutdown. After the termination of this method, the driver should be able to restart as if it was just instantiated.
There is another callback method that you can optionally override:
This method is called when a change in the driver configuration file has been detected.
If not overridden, the default implementation will quit and restart the driver.
Try out the lifecycle of your driver: add a configuration file called mydriver.yml in the “config/drivers” directory with the following content:
where the value of “type” is the fully-classified name of your driver class and “param” is just a test parameter that we are going to read.
Use the following implementation for your driver:
Now start Sfera (menu Run > Run History > Sfera-MyDriver) and check the consolle. You will see your driver being initialized and then it will start looping.
To stop Sfera type sys quit in the console and hit Enter; you will see that your driver will be quitted before shutdown.
Events
To generate events from your driver you should instantiate Event objects and post them to the system Bus.
As you can see in the above project structure, in the sub-package .events there is a MyDriverEvent interface that extends the Event interface.
This is a simple tag interface that should be implemented by all your event classes so that applications can easily register to all of your driver events.
Create a different class for any type of event your driver is going to generate and let them extend the standard event classes provided:
- BooleanEvent: for events that can be represented by a boolean value (true or false); for instance, the “on” state of a light.
- NumberEvent: for events with numerical value, e.g. a temperature read by a thermostat.
- StringEvent: for events whose value needs a textual representation, e.g. the name of the currently playing track of an audio system
If none of the above classes fits your event, extend the BaseEvent class which is the simplest implementation of the Event interface.
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Events are uniquely identified by an ID, this means that you could virtually have a single event class and instantiate it with different IDs. It is anyhow recommended to have several classes (or even interfaces) to group your events into different categories so that applications can more easily register to a subset of your driver’s events.
Here are some examples of event classes for your driver:
Now you can post your event from your driver class to the system Bus:
This will result in triggering the events:
If you want your event to be posted only if its value changed from the last time it has been posted, use:
As you can see, you shall always pass a reference to your driver instance (this) as the source node of the events so that applications handling them can get this reference and possibly call methods to issue commands for your driver.
State events
Sfera will take care of generating some events for your driver that represent the current state in the life-cycle of the driver.
These events will have ID <driver_id>.driverState and the following String values:
- init: when the driver is being initialized, i.e. just before onInit() is called
- running: when the driver has been successfully initialized and the loop() cycle is starting
- quit: when the driver is about to get quitted, i.e. just before onQuit() is called
These events objects will implement the DriverStateEvent tag interface and, if the driver includes a general interface for the driver’s events using the name convention <driver_package>.events.<driver_class>Event described above (e.g. the interface com.example.sfera.drivers.mydriver.events.MyDriverEvent), they will dynamically implement this interface too.
Commands
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Your driver class should expose public methods that applications can call to issue commands to the field system or to perform any other operation on your driver.
For instance, you could add these methods:
Note that when writing scripts logic the script engine will treat your getXY() and setXY(value) methods as properties too:
The setLight() method can be redesigned to allow a syntax that resembles the light events we saw above:
to this end, simply create a method light(int num) which returns an object that has a public setOn(boolean val) method.
Your driver is now ready to be deployed. Use maven to build your jar plugin and test it in a Sfera installation dropping it in the “plugins” directory.
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Persistent data
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If your driver needs to persist some data (a database, a property file, an image, …) use as root directories the paths returned by:
- getDriverGlobalDataDir(): for data accessible to every instance of your driver
- getDriverInstanceDataDir(): for data relative to a single instance of your driver