MATLAB Object-Oriented Programming

• The order in which listeners execute is not defined.


• Listeners should not modify the event data object passed to the listener callback,
  because other listeners are passed this same handle object.

Default Event Data

Events provide information to listener callbacks by passing an event data argument to the

callback function. By default, MATLAB passes an event.EventData object to the

listener callback. This object has two properties:

• EventName — The event name as defined in the class event block


• Source — The object that is the source of the event

MATLAB passes the source object to the listener callback in the required event data

argument. Use the source object to access any of the object's public properties from

within your listener callback function.

Customize Event Data

You can create a subclass of the event.EventData class to provide additional

information to listener callback functions. The subclass would define properties to contain

the additional data and provide a method to construct the derived event data object so it

can be passed to the notify method.

“Define Event-Specific Data” on page 11-26 provides an example showing how to

customize this data.

Events Only in Handle Classes

You can define events only in handle classes. This restriction exists because a value class

is visible only in a single MATLAB workspace so no callback or listener can have access to

the object that triggered the event. The callback could have access to a copy of the object.

However, accessing a copy is not useful because the callback cannot access the current

state of the object that triggered the event or effect any changes in that object.

“Comparison of Handle and Value Classes” on page 7-2 provides general information on

handle classes.

“Events and Listeners Syntax” on page 11-22 shows the syntax for defining a handle

class and events.

11 Events — Sending and Responding to Messages