如何限制Flutter中文本字段的大小？

我必须在RSproute提到的内容上添加一个附加代码段。完整的代码在这里：

TextEditingController _controller = new TextEditingController();String text = ""; // empty string to carry what was there before it onChangedint maxLength = ......new TextField(    controller: _controller,    onChange: (String newVal) {        if(newVal.length <= maxLength){ text = newVal;        }else{ _controller.value = new TextEditingValue(     text: text,     selection: new TextSelection(         baseOffset: maxLength,         extentOffset: maxLength,         affinity: TextAffinity.downstream,         isDirectional: false     ),     composing: new TextRange(         start: 0, end: maxLength     ) ); _controller.text = text;        }     });