flutter自定义TextInputFormatter，一个中

class IgnoreOtherCn2Formatter extends TextInputFormatter{

  final int maxLength;
  IgnoreOtherCn2Formatter({this.maxLength = -1});

  int count;
  var _regExp=r"^[\u4E00-\u9FA5A-Za-z0-9_]+$";
  @override
  TextEditingValue formatEditUpdate(TextEditingValue oldValue, TextEditingValue newValue) {

    int newValueLength = newValue.text.length;
    int count = 0;
    int oldValueLength = oldValue.text.length;
    if(newValueLength == 0){
      return newValue;
    }
    if(maxLength >0){
      for (int i = 0; i < newValueLength; i++) {
        if (newValue.text.codeUnitAt(i) > 122) {
          ///中文字符按照2个计算
          count++;
        }
        if(i >0 && count +i > maxLength-1){
          String text =newValue.text.substring(0,i);
          LogUtil.v("formatEditUpdate 0 maxLength：$maxLength i:$i   count:$count  text:$text  newValueLength:$newValueLength  oldValueLength:$oldValueLength oldValue:${oldValue.text}   newValue:${newValue.text}   ",tag: "IgnoreOtherFormatterCn");
          return newValue.copyWith(text:text,composing:TextRange.empty,selection: TextSelection.fromPosition(
              TextPosition(offset: i,affinity: TextAffinity.downstream)));
        }
      }
    }
    LogUtil.v("formatEditUpdate maxLength：$maxLength count:$count  newValueLength:$newValueLength  oldValueLength:$oldValueLength oldValue:${oldValue.text}   newValue:${newValue.text}   ",tag: "IgnoreOtherFormatterCn");
    if(newValueLength>0 && RegExp(_regExp).firstMatch(newValue.text)!=null){
      if(newValueLength +count<= maxLength){
        return newValue;
      }
    }
    return oldValue;
  }
}