MATLAB Object-Oriented Programming

Determine If Event Has Listeners

In this section...

“Do Listeners Exist for This Event?” on page 11-37

“Why Test for Listeners” on page 11-37

“Coding Patterns” on page 11-37

“Listeners in Heterogeneous Arrays” on page 11-38

Do Listeners Exist for This Event?

Use the event.hasListener function to determine if a specific event has listeners.

event.hasListener accepts an array of event source objects and an event name as

input arguments. It returns an array of logical values indicating if listeners exist for the

specified event on each object in the array.

NoteWhen called, event.hasListener must have NotifyAccess for the event. That

is, call event.hasListener in a context in which you can call notify for the event in

question.

Why Test for Listeners

Use event.hasListener to avoid sending event notifications when there are no

listeners for the event. For example, if creating custom event data consumes significant

resources, or if events are triggered repeatedly, use event.hasListener to test for

listeners before performing these steps.

Coding Patterns

• Conditionalize the creation of event data and the call to notify using
  event.hasListener. For an object array a, determine if there are listeners before
  creating event data and triggering the event:

if any(event.hasListener(a,'NameOfEvent'))

evt = MyCustomEventData(...);

notify(a,'NameOfEvent',evt)

end

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