使用inputFormatters属性。以下代码将textFormField的长度限制为42：

new TextFormField(
  inputFormatters: [
    new LengthLimitingTextInputFormatter(42),
  ],
);

**更新：**首先导入程序包。import 'package:flutter/services.dart';

使用maxLength

TextFormField(
  controller: _deskripsiController,
  onFieldSubmitted: (String value){
        setState(() {
          _deskripsiController.text = value;
        });
  },
  decoration: const InputDecoration(
        border: const UnderlineInputBorder(),
        labelText: "Deskripsi",
  ),
  maxLength: 8,
)

最好的方法是inputFormatters选项：

inputFormatters: [
   new LengthLimitingTextInputFormatter(11),// for mobile 
],

除了hahnsaja应答之外，在你添加maxLength之后，counter会出现在TextFormField小部件的底部。为了能够隐藏它，把它添加到InputDecoration中：

counterText: '',
counterStyle: TextStyle(fontSize: 0),

示例：

TextFormField(
  controller: _deskripsiController,
  onFieldSubmitted: (String value){
        setState(() {
          _deskripsiController.text = value;
        });
  },
  decoration: const InputDecoration(
        border: const UnderlineInputBorder(),
        labelText: "Deskripsi",
        counterText: '',
        counterStyle: TextStyle(fontSize: 0),
  ),
  maxLength: 8,
)

尝试使用以下代码

TextFormField(inputFormatters: [LengthLimitingTextInputFormatter(10)])