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Events

Events are discrete actions that change the dynamic state of an AICA application. They are handled and executed by the Dynamic State Engine. For this reason, they are sometimes referred to as "state events".

Event types​

The following events can be used to dynamically update the state of a running application:

  • Load or unload a component, controller or hardware interface
  • Trigger a lifecycle transition (for example, configure, activate or deactivate) on a lifecycle component
  • Start or stop a controller
  • Set the value of a parameter on a loaded component or controller
  • Call a service on a component

Triggering events​

Events can be triggered externally by a user through interactions with the Developer Interface UI or via the REST API. They can also be triggered internally by the Dynamic State Engine according to the application description as a result of component predicates and conditions.

Predicates​

Predicates are logical statements about a component that evaluate to true or false and are used to indicate key component states. AICA components broadcast their predicates to a global channel in a predicate message containing the component name, the predicate name, and the current value (true or false) of the predicate.

While the term "predicate" has several formal definitions in grammar, logic and mathematics, at AICA the grammatical definition is used when naming predicates.

Components define predicates depending on their function. A component that calculates if a given input state is within some parameterized limits might have a predicate "is in bounds".

This definition of predicates allows application descriptions to trigger events in a very declarative way. For example:

When component A is in bounds, load component B

Here, component A is the source of the predicate, is in bounds is the predicate itself, and load component B is the event that is triggered when the predicate is true.

note

Events are only triggered on the rising edge of a predicate value.

Conditions​

Predicate statements can be manipulated with logical operators in conditional statements to create more advanced event triggers.

When component B is not in bounds, do ...

When component A is active and component B is not in bounds, do ...

AICA conditions support the NOT, AND, OR and XOR operators (also known as the "not", "all", "any", and "one of" operators, respectively).

note

Events are only triggered on the rising edge of a conditional value.