`event.hasListener`

Tests whether an event of an object has listeners.

Category: Programming > Classes

Syntax

tf = event.hasListener(obj, 'EventName')

Returns true if at least one listener is attached to the event of obj.

Description

A class that triggers events can use this test to skip building expensive event data when nobody is listening. Listeners created with addlistener or listener count as long as they exist and are enabled, and deleting them makes the answer false again.

Input arguments

obj: A handle object.
'EventName': Name of an event of the object's class.

Output arguments

tf: Logical scalar.

Example

Before and after adding a listener

m = containers.Map();
event.hasListener(m, 'ObjectBeingDestroyed')
lh = addlistener(m, 'ObjectBeingDestroyed', @(s, e) disp('gone'));
event.hasListener(m, 'ObjectBeingDestroyed')
delete(lh)
event.hasListener(m, 'ObjectBeingDestroyed')

Output

ans = logical
   0
ans = logical
   1
ans = logical
   0

See also

addlistener listener notify events