Help with OutputFcn on lsqcurvefit

Hi everyone,

I have to make a very large fitting with 'lsqcurvefit', so the run will take a lot of time. I am thinking about designing an output function that writes on a file the solutions found after every successful iteration, so if the run is aborted, I can view the last successful solution adquired.

For that, I suppose the output function must say "is resnorm (n+1) < resnorm (n), open a file and write 'x'", where n is the iteration number. But I don't know how to call 'resnorm' values on my output function and write this up. Can you please give an idea about how to begin?

Thanks !!