Returns the worker object for loading records for this record box.
        
        :return     <XOrbLookupWorker>

def worker(self):
        """
        Returns the worker object for loading records for this record box.
        
        :return     <XOrbLookupWorker>
        """
        if self._worker is None:
            self._worker = XOrbLookupWorker(self.isThreadEnabled())
            self._worker.setBatchSize(self._batchSize)
            self._worker.setBatched(not self.isThreadEnabled())
            
            # connect the worker
            self.loadRequested.connect(self._worker.loadRecords)
            self._worker.loadingStarted.connect(self.markLoadingStarted)
            self._worker.loadingFinished.connect(self.markLoadingFinished)
            self._worker.loadedRecords.connect(self.addRecordsFromThread)
        
        return self._worker