PhpSpreadsheet导出Excel文件空白问题解决

在一个项目中集成了PhpSpreadsheet，试图将平台的部分数据导出为Excel，为此专门封装了一个Office服务组件。但是好巧不巧导出的Excel文件竟然是空白的。如下图所示：

Tp-admin框架中Office组件源码参考：https://gitee.com/zkii_admin/Tp-admin/blob/master/extend/service/OfficeService.php

其中有个导出Excel文件的方法是这样写的。

/**
* 导出Excel2
* @param $list Array 数据列
* @param $filename String 文件名
* @param $title String 标题
* @param $keys Array 字段与说明
*/
public function toExcelWithHead($list,$filename,$title,$keys){
$sheet = new Spreadsheet();
$begin = 1;
$filename = empty($filename) ? time().".xlsx" : $filename.".xlsx";
$keymap = array_column($keys,'notes');
$filedmap = array_column($keys,'fields');
$col_num = sizeof($keymap);
if(!empty($title)){
$sheet->getProperties()->setTitle($title);
$sheet->getActiveSheet()->setCellValue('A1',$title);
$sheet->getActiveSheet()->getStyle('A1')->getAlignment()->setHorizontal(\PhpOffice\PhpSpreadsheet\Style\Alignment::HORIZONTAL_CENTER);
$sheet->getActiveSheet()->mergeCells('A1:'.self::$cellKey[$col_num-1].'1');
$begin +=1;
}
if(!empty($keymap)){
for($i=0;$i<$col_num;$i++){
$sheet->getActiveSheet()->getStyle(self::$cellKey[$i].$begin)->getAlignment()->setHorizontal(\PhpOffice\PhpSpreadsheet\Style\Alignment::HORIZONTAL_CENTER);
$sheet->setActiveSheetIndex(0)->setCellValue(self::$cellKey[$i].$begin,mb_convert_encoding($keymap[$i],'utf-8'));
}
$begin +=1;
}
$leng = sizeof($list);
for($row = 0;$row < $leng;$row++){
$j = $row+$begin;
for($t=0;$t<$col_num;$t++){
$sheet->getActiveSheet()->setCellValueByColumnAndRow($t+1,$j,mb_convert_encoding($list[$row][$filedmap[$t]],'utf-8'));
}
}
ob_end_clean();
header('Content-Type: application/vnd.openxmlformats-officedocument.spreadsheetml.sheet');
header('Content-Disposition: attachment; filename="' . $filename . '"');
header('Cache-Control: max-age=0');
$writer = IOFactory::createWriter($sheet, 'Xlsx');
$writer->save('php://output');
}