Attributes are variables that control the behavior of the component. They
are stored in the ids and they can be changed dynamically by the codels
during execution. Genom provides a way to set and get attributes through the
chosen middleware.
An "attribute" declaration creates a service that can either set or get one or several attibutes.
Attributes can have a number of properties:
A string that describes the functionality of the service.
A "validate" codel can be declared to check the validity of
input data and throw an exception in case of a problem. The validate
codel is called before updating the value in the ids. The codel can
throw an exception if the new value is not acceptable. In this case, the
value of the corresponding ids member is not changed. If the codel
returns no error, the value of the ids member is updated.
This would declare an attribute for changing the speed of a moving thing:
component foo {
ids { double velocity };
exception invalid_speed;
attribute SetSpeed(in velocity = 0.0: "Thing velocity")
{
doc "Change the velocity of a moving thing";
validate check_velocity(local in velocity);
throw invalid_speed;
};
};In this example SetSpeed is a setter for the velocity variable (in
keyword). It is possible to specify a default value ("= 0.0") and a small
documentation string that is used by interactive applications to guide the
user.
This example is of course not functional. The velocity value should
be used by other codels and services as the reference value for actually
controlling the moving thing.