MATLAB®

Listening for Changes to Property Values

Creating Property Listeners

You can listen to the predeclared property events (named: PreSet, PostSet, PreGet, and PostGet) by creating a listener for those named events:

• Set the SetObservable property attribute of the property to true

• Define a callback function

• Create a property listener by including the name of the property as well as the event in the call to addlistener (see Add a Listener to the Property)

• Optionally subclass event.data to create a specialized event data object to pass to the callback function

The following code segments show how to create a PostSet property listener for a property called Limits.

Set Property Attributes to Enable Property Events

In the properties block, set the Limits property SetObservable to true:

properties (SetObservable = true) % Can define property listeners
   Limits = [-2*pi 2*pi];
   ...
end

Define a Callback Function for the Property Event

Define a callback function, passing the required arguments. For example, assume the handlePropertyEvents function is a method of the class whose property is being set. This method requires three arguments:

• The first argument (obj) is the class instance containing the property that triggers the PostSet event.

• The second argument is the event source (the Limits property in this case).

• The third argument is the event.PropertyEvent object.

function handlePropertyEvents(obj,src,evnt)
   switch src.Name % switch on the property name
      case 'Limits'
         obj.calculateData(obj.Limits) % Call class method passing new value for property
   ...
end

The property (obj.Limits) has the new value because this is a PostSet event. Note that src is a meta.property object for the Limits property.

Obtaining Information About Classes with Meta-Classes provides more information about the meta.property class.

Add a Listener to the Property

The addlistener method enables you to attach a listener to a property without storing the listener object as a persistent variable. For a property event, use the four-argument version of addlistener:

lh = addlistener(obj,'Limits','PostSet',@(src,evnt)handlePropertyEvents(obj,src,evnt));

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Defining Events and Listeners — Syntax and Techniques

Example — Using Events to Update Graphs