code: function (cell, bottom, ignoreRange) {
              try {
                  var cellInfo = this.getCellInfo(cell),
                      nextRowIndex, nextColIndex;
                  var len = this.selectedTds.length && !ignoreRange,
                      range = this.cellsRange;
                  //末行或者末列没有后置单元格
                  if ((!bottom && (cellInfo.rowIndex == 0)) || (bottom && (!len ? (cellInfo.rowIndex + cellInfo.rowSpan > this.rowsNum - 1) : (range.endRowIndex == this.rowsNum - 1)))) return null;
  
                  nextRowIndex = !bottom ? ( !len ? cellInfo.rowIndex - 1 : range.beginRowIndex - 1)
                      : ( !len ? (cellInfo.rowIndex + cellInfo.rowSpan) : range.endRowIndex + 1);
                  nextColIndex = !len ? cellInfo.colIndex : range.beginColIndex;
                  return this.getCell(this.indexTable[nextRowIndex][nextColIndex].rowIndex, this.indexTable[nextRowIndex][nextColIndex].cellIndex);
              } catch (e) {
                  showError(e);
              }
          }
docs: 获取视觉上的后置单元格
  @param cell
  @param bottom