Observe Changes to Property Values

This example shows how to listen for changes to a property value. This example uses:

• PostSet event predefined by MATLAB^®

• SetObservable property attribute to enable triggering the property PostSet event.

• addlistener handle class method to create the listener

classdef PropLis < handle
   % Define a property that is SetObservable
   properties (SetObservable)
      ObservedProp = 1
   end
   methods
      function attachListener(obj)
         %Attach a listener to a PropListener object
         addlistener(obj,'ObservedProp','PostSet',@PropLis.propChange);
      end
   end
   methods (Static)
      function propChange(metaProp,eventData)
         % Callback for PostSet event
         % Inputs: matlab.metadata.Property object, event.PropertyEvent
         h = eventData.AffectedObject;
         propName = metaProp.Name;
         disp(['The ',propName,' property has changed.'])
         disp(['The new value is: ',num2str(h.ObservedProp)])
         disp(['Its default value is: ',num2str(metaProp.DefaultValue)])
      end
   end
end

The PropLis class uses an ordinary method (attachListener) to add the listener for the ObservedProp property. If the PropLis class defines a constructor, the constructor can contain the call to addlistener.

The listener callback is a static method (propChange). MATLAB passes two arguments when calling this function:

• metaProp — a matlab.metadata.Property object for ObservedProp

• eventData — an event.PropertyEvent object contain event-specific data.

These arguments provide information about the property and the event.

Use the PropLis class by creating an instance and calling its attachListener method:

plObj = PropLis;
plObj.ObservedProp

ans =

     1

plObj.attachListener
plObj.ObservedProp = 2;

The ObservedProp property has changed.
The new value is: 2
Its default value is: 1