Delayed Event

A delayed event is the ability for the statechart to send an event after a certain period of time. Delayed events can be used to simulate delayed transitions.


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A delayed event is specified using the following syntax:

<send event="my_event" delay="5s"/>

The delay attribute specifies a CSS timing value. The statechart implementation will raise the specified event after the amount of time. It is possible to cancel such a delayed event. In order to do so, an identifier needs to be added to the <send>

<send id="my_timeout" event="my_event" delay="5s"/>

To cancel, use the identifier in a <cancel> element:

<cancel sendid="my_timeout"/>

These operations are typically done in entry or exit handlers, but can also be used in transitions.

Simulating delayed transitions

Delayed transitions are not part of Statechart XML. Instead, you have to specify to send a delayed event, and a normal transition that handles that event.

For a simple delayed transition from some_state to some_other_state after 5 seconds, you need:

  • An entry handler that send a “fake” event my_event with a 5 second delay
  • An exit handler that cancels the sending of the event (in case the state is exited before 5 seconds)
  • A transition that handles the “fake” event by transitioning to the some_other_state.

Care must be taken to ensure statechart-wide unique values for the event and id, because there is no “local” scope.

This is the result:

<state id="some_state">
    <send id="my_timeout" event="my_event"  delay="5s"/>
    <cancel sendid="my_timeout"/>
  <transition event="my_event" target="some_other_state" />