Callback Execution

When Callbacks Execute

Listeners execute their callback function when notified that the event has occurred. Listeners are passive observers in the sense that errors in the execution of a listener callback do not prevent the execution of other listeners responding to the same event, or execution of the function that triggered the event.

Callback function execution continues until the function completes. If an error occurs in a callback function, execution stops and control returns to the calling function. Then any remaining listener callback functions execute.

Listener Order of Execution

The order in which listeners callback functions execute after the firing of an event is undefined. However, all listener callbacks execute synchronously with the event firing.

The handle class notify method calls all listeners before returning execution to the function that called notify.

Callbacks That Call `notify`

Do not modify and reuse or copy and reuse the event data object that you pass to notify, which is then passed to the listener callback.

Listener callbacks can call notify to trigger events, including the same event that invoked the callback. When a function calls notify, MATLAB^® sets the property values of the event data object that is passed to callback functions. To ensure that these properties have appropriate values for subsequently called callbacks, always create a new event data object if you call notify with custom event data.

Manage Callback Errors

If you want to control how your program responds to errors, use a try/catch statement in your listener callback function to handle errors.

Invoke Functions from Function Handles

When you create a function handle inside a class method, the context of the method determines the context in which the function executes. This context gives the function access to private and protected methods that are accessible to that class.

For example, the UpdateEvt class defines an event named Update and a listener for that event. The listener callback is the private method evtCb.

classdef UpdateEvt < handle
   events
      Update
   end
   methods
      function obj = UpdateEvt
         addlistener(obj,'Update',@evtCb);
      end
   end
   methods (Access = private)
      function obj = evtCb(obj,varargin)
         disp('Updated Event Triggered')
      end
   end
end

Private methods are normally accessible only by class methods. However, because the function handle is created in a class method, notify can execute the callback from outside of the class:

a = UpdateEvt;
a.notify('Update')

Updated Event Triggered