输入框只能输入数字，并启用纯数字键盘

import 'package:flutter/material.dart';
import 'package:flutter/services.dart';
 TextField(
          inputFormatters: [
            WhitelistingTextInputFormatter(RegExp("[a-zA-Z]")),//只允许输入字母
          ],
   ),
    TextField(
              keyboardType: TextInputType.number,
          inputFormatters: [WhitelistingTextInputFormatter.digitsOnly],//只允许输入数字
   ),
    TextField(
                          inputFormatters: [
                            WhitelistingTextInputFormatter(RegExp("[0-9.]")),//只允许输入小数
                          ],
    ）

参考：
Flutter之基础Widget之TextField
https://juejin.im/post/5cc2c880e51d453fdf6834b4