Events and Listeners — Concepts

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The Event Model Default Event Data Events Only in Handle Classes Property-Set and Query Events Listeners

The Event Model

Events represent changes or actions that occur within class instances. 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. Naming Events

• After creating an instance of the event-declaring class, you can attach listener objects to it. Ways to Create Listeners

• A call to a class method broadcasts a notice of the event to listeners. The class user determines when to trigger the event. Triggering Events

• Listeners execute a callback function when notified that the event has occurred. Defining Listener Callback Functions

• 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. Ways to Create Listeners

The following diagram illustrates the event model.

Default Event Data

Events provide information to listener callbacks by passing an event data argument to the callback function. By default, MATLAB passes anevent.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. This enables you to access any of the object's public properties from within your listener callback function.

Customizing 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.

Defining Event-Specific Data 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 generally useful because the callback cannot access the current state of the object that triggered the event or effect any changes in that object.

Comparing Handle and Value Classes provides general information on handle classes.

Events and Listeners — Syntax and Techniques shows the syntax for defining a handle class and events.

Property-Set and Query 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 access method

• PostGet — Triggered just after returning the property value to the query

These events are predefined and do not need to be listed in the class events block.

When a property event occurs, the callback is passed an event.PropertyEvent object. 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, AffectedObject contains the object whose property was either accessed or modified).

You can define your own property-change event data by subclassing the event.EventData class. Note that the event.PropertyEventclass is a sealed subclass of event.EventData.

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

Listeners encapsulate the response to an event. Listener objects belong to the event.listener class, 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 event

• Callback — Function to execute when an enabled listener receives event notification

• Enabled — Callback function executes only when Enabled is true. See Enabling and Disabling the Listeners for an example.

• Recursive — Allow listener to cause the same event that triggered the execution of the callback

  Recursive is true by default. It is possible to create a situation where infinite recursion reaches the recursion limit and eventually triggers an error. If you set Recursive to false, the listener cannot execute recursively if the callback triggers its own event.

Ways to Create Listeners provides more specific information.