Events represent changes or actions that occur within objects. For example,
Modification of class data
Execution of a method
Querying or setting a property value
Destruction of an object
Basically, any activity that you can detect programmatically can generate an event and communicate information to other objects.
MATLAB® classes define a process that communicates the occurrence of events to other objects that need to respond to the events. The event model works this way:
A handle class declares a name used to represent an event. Name Events
After creating an object of the event-declaring class, attach listener to that object. Control Listener Lifecycle
Listeners execute a callback function when notified that the event has occurred. Specifying Listener Callbacks
You can bind listeners to the lifecycle of the object that defines the event, or limit listeners to the existence and scope of the listener object. Control Listener Lifecycle
The following diagram illustrates the event model.
There are certain limitations to the use of events:
The event source cannot guarantee that listeners exist when the triggering the event.
A listener cannot prevent other listeners from being notified that the event occurred.
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.
Events provide information to listener callbacks by passing
an event data argument to the callback function. By default, MATLAB passes
to the listener callback. This object has two properties:
EventName — The event
name as defined in the class
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. This enables you to access any of the object's public properties from within your listener callback function.
You can create a subclass of the
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
Define Event-Specific Data provides an example showing how to customize this data.
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 generally 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 provides general information on handle classes.
Events and Listeners Syntax shows the syntax for defining a handle class and events.
There are four predefined events related to properties:
PreSet — Triggered just
before the property value is set, before calling its set access method
PostSet — Triggered just
after the property value is set
PreGet — Triggered just
before a property value query is serviced, before calling its get
PostGet — Triggered just
after returning the property value to the query
These events are predefined and do not need to be listed in
When a property event occurs, the callback is passed an
This object has three properties:
EventName — The name of
the event described by this data object
Source — The source object
whose class defines the event described by the data object
AffectedObject — The object
whose property is the source for this event (that is,
the object whose property was either accessed or modified).
You can define your own property-change event data by subclassing
event.EventData class. Note that the
is a sealed subclass of
See Listen for Changes to Property Values for a description of the process for creating property listeners.
See The PostSet Event Listener for an example.
See Property Access Methods for information on methods that control access to property values.
Listeners encapsulate the response to an event. Listener objects
belong to the
which is a handle class that defines the following properties:
Source — Handle or array
of handles of the object that generated the event
EventName — Name of the
Callback — Function to execute
when an enabled listener receives event notification
Enabled — Callback function
executes only when
See Enable and Disable Listeners for
Recursive — Allow listener
to trigger the same event that caused execution of the callback.
default. If the callback triggers the event for which it is defined
as the callback, the listener cannot execute recursively. Therefore,
false if the
callback must trigger its own event. Setting the
true can create a situation where infinite recursion
reaches the recursion limit and triggers an error.
Control Listener Lifecycle provides more specific information.