code: function(target, dd, e, dragData) {
          if (this.dropOK) {
              var value = dragData.field.getValue();
              target.record.set(target.fieldName, value);
              this.onCellDrop(target.fieldName, value);
              return true;
          }
      }
docs: Process the drop event if we have previously ascertained that a drop is OK.