Some Matlab versions crash when using listener

Dear Forum,

I've been playing around with listeners in Matlab, i.e. implementation of a weak object reference by using listeners. It appears that my code works fine when using release 2016a or 2015b. It, however, crashes on 2014a, 2015a and 2015a/sp. (Technically it is really not crash but rather a freeze, which does not make a difference to me, since I have to kill the program and restart).

I consider that my implementation is an unfortunate pick and by accident it works with the latest releases of Matlab. I also consider that there is bug. In both cases I kindly ask for a workaround in order to enable my program for any Matlab version.

My code looks like this:

First, the object managed by the weak reference:

classdef ManagedObject < handle
    %MANAGEDOBJECT Summary of this class goes here
    %   Detailed explanation goes here

      properties
          listenToRuntimeQuery;
      end    

      methods
          function setupConnection(obj, weakReference)

              % setup listener
              for i=1:length(weakReference)

                  % get weak reference
                  wRef        = weakReference(i);

                  % get instance of object
                  cObj        = obj(i);
                  callback    = @cObj.getObject;

                  % create listener connection
                  obj(i).listenToRuntimeQuery = event.listener(weakReference(i), 'notification',...
                      @(src, evnt) WeakReference.respond(src, evnt, callback));                
              end                        
          end
          function delete(obj)

              % delete listener
              delete(obj.listenToRuntimeQuery);

              % notify            
              WeakReference.notifyOnDelete();

          end
          function obj = getObject(obj)
              % Helper callback to request object from weak reference
          end
      end
  end

Second, the implmentation of the weak reference:

classdef WeakReference < handle
    %SMARTPOINTER Summary of this class goes here
    %   Detailed explanation goes here

      events
          notification;
      end

      properties
          reference;
      end    

      % Constructor
      methods
          function obj = WeakReference()
          end
      end

      methods
          function queryRuntime(obj)

              % notify
              notify(obj, 'notification');

          end
      end

      methods(Static)        
          function respond(src, evnt, subject)

              %disp('responding');

              % store reference temporarily
              src.reference = subject();            
          end 
      end

      methods        
          function reference = getReference(obj)

              % query runtime
              obj.queryRuntime();            

              % return reference
              reference = obj.reference();

              % clear buffer
              obj.reference = [];
          end 
      end

      methods (Static)
          function notifyOnDelete()
              disp('Object has been cleared');
          end
      end
  end

Third, my sample script which will hang on second execution at least in 2014a, 2015a and 2015a/sp1.

% clear all
clear classes

% test settings
N = 10000;

% clear test objects
clear('weakRef')
clear('testObject')

% create object array
testObject(N) = ManagedObject();

% create weak listener
weakRef(N) = WeakReference();

% create smartpointer
testObject.setupConnection(weakRef);

% get reference
for i=1:N
   weakRef(i).getReference();
end

Can someone help?

Thanks, Daniel

Answer 1

[Philip Borghesani]

MATLAB is not crashing just taking a very long time to complete. Try it with smaller values of N and look how fast the time increases with increases of N. Most of the time is being spent in the destruction sequence called by clear.

You have bumped into a documented weakness in how anonymous functions are implemented in versions prior to R2015b. When an anonymous function is created in older versions of MATLAB all variables in the current workspace are captured by the function.

In your code SetupConnection is causing many many cross links of the full object array and local variables. The solution is to create the listener callback in a separate function:

        function setupConnection(obj, weakReference)
              % setup listener
              for i=1:length(weakReference)
                  obj(i).listenToRuntimeQuery=  event.listener(weakReference(i), 'notification',...
                      MakeListenerCallback(obj(i))); 
              end                        
          end

%I placed this function after the end block of the ManagedObject class
function listener=MakeListenerCallback(obj)
  getfn=@() obj.getObject;
  listener=@(src, evnt) WeakReference.respond(src, evnt, getfn);
end

There are other improvements that can be made to the code. I suggest adding a clear statement to the end of the script and play with any changes observing performance effects using the profiler.

Answer 2

[Darim]

Philip, thanks a lot for the code snippet and the explanation! This solves my problem and makes things clear now.

Regarding the profiler result, I have to admit that I was happy at the point where my code was running with 2016a. Therefore there are some unneccesary function calls in my example. But anyhow, I think that most of the time is spent at listener construction and notification, which cannot be improved very much. What do you think?