code: function on(event, listener) {
      // call from main process always.
  
      // add listener to common event emitter for main process.
      cee.on(event, function(id, data, ipcEvent) {
        listener(data, ipcEvent)
          .then(function(result) {
            cee.emit(getSuccessEventName(event, id), result);
          })
          .catch(function(result) {
            cee.emit(getFailureEventName(event, id), result);
          });
      });
    }
docs: listen event.
  
  @param {String} event event name.
  @param {Function} listener listener function.