OnCommand
A command function of an AggregateRoot: it takes the command, or a Command of it, and decides the events. A function named onCommand needs no annotation, but only the annotation declares produces.
Its return type says what it decides, in this order of preference:
the event it produces (
CartClosed), orE?when the command may change nothing (null is a no-op), a sealed type of its events, or a list of one event type; it rejects by throwing:reject(code, message),require(wow.command.invalid) orcheck(wow.command.refused). The compiler reads the events from the type, and the kernel fails a command whose function produces another (wow.command.undeclared-event).Decision, for a return type that cannot name its events (two kinds of event without a common sealed type, a domain event and a lifecycle event), with produces listing them; the kernel holds the function to that list. Without produces, the Manifest lists no events for it, and the kernel does not check them.
Java has no DecideScope receiver: a Java function returns its event, or null, and rejects by throwing RejectedException, or returns a Decision.
Parameters
the events of the aggregate the function can decide, which the Manifest lists as the command's produces, for a function whose return type does not say: a Decision, a Mono<Decision>, Any. Each must be an event of the aggregate. A function whose return type already names its events (an event, a sealed type, a list of them) may not declare them again. Empty, the default, declares nothing: produces stays unknown.
the commands the function may schedule with DecideScope.schedule, by their classes: a command class of this context or of another (from that context's api), or a sealed type, which stands for its concrete subclasses. The Manifest lists them as the aggregate's schedules, by full type name (a schema reference for another context's). The compiler cannot read the calls in a function's body, so they are declared; empty, the default, declares none.
whether the command is decided on a deleted aggregate too, as RestoreAggregate is: a command that restores it, producing AggregateRestored beside its own events, or one a deleted aggregate still takes. Every other command is refused on a deleted aggregate. Such a command produces AggregateRestored and its aggregate's own events alone. False by default.
A command changes the aggregate's lifecycle by producing the lifecycle event beside its own events, in one commit: CloseCart(reason) produces CartClosed(reason) and AggregateDeleted; the kernel then marks it deleted, as DeleteAggregate does. Deciding the same event twice changes nothing more; a command that would change nothing returns Decision.NoOp.