TextField(
                 controller: TextEditingController.fromValue(TextEditingValue(
                                  text: _txtController.text,
                                  selection: TextSelection.fromPosition(
                                      TextPosition(affinity: TextAffinity.downstream, offset:_txtController.text.length)))),
                 decoration: InputDecoration(
                           border: InputBorder.none,
                  ),
                 style: TextStyle(fontSize: 16),
                  inputFormatters: [
                                LengthLimitingTextInputFormatter(15),
                   ]
)

     final int selfMaxLength;
     final Function maxLengthCallBack;
     final TextEditingController myController;

    @override 
    void initState() {
    ……
    myController =  TextEditingController.fromValue(TextEditingValue(
                                  text: _txtController.text,
                                  selection: TextSelection.fromPosition(
                                      TextPosition(affinity: TextAffinity.downstream, offset:_txtController.text.length))));
    if (widget.controller.text.length == widget.selfMaxLength) {
       hasAlreadyMaxLength = true;
       lastMaxContent = widget.controller.text;
    } else {
      hasAlreadyMaxLength = false;
      lastMaxContent = "";
          }
    }
 
    @override
    Widget build(BuildContext context) {
          return Container(
                child: TextField(
                 controller: myController,
                 decoration: InputDecoration(
                           border: InputBorder.none,
                  ),
                 style: TextStyle(fontSize: 16),
                  inputFormatters: [
                                LengthLimitingTextInputFormatter(15),
                   ]，
                   onChanged: (String value) {
                    _actionMaxLengthState(value);
                  }
                 )
          )
    }

    void _resetSelection(String newText) {
     var sRunes = newText.runes;
     String result;
     for (int i = 0; i < sRunes.length; i++) {
         if (String.fromCharCodes(sRunes, 0, sRunes.length - i).length <= widget.selfMaxLength) {
         result = String.fromCharCodes(sRunes, 0, sRunes.length - i);
         break;
      }
    }
    TextSelection temp = myController.value.selection.copyWith(
      baseOffset: result.length,
      extentOffset: result.length,
    );
    TextRange fixRange = widget.controller.value.composing;
    if (widget.controller.value.composing.end > result.length) {
      fixRange = TextRange(start: fixRange.start, end: result.length);
    } 
    
    //composing is used iOS inputMethod
    myController.value = TextEditingValue(text: result, selection: temp, composing: fixRange);
    lastMaxContent = result;
    }

    void _initOldDataSelection(String newText) {
          TextSelection actualSelection = myController.value.selection;
          actualSelection = myController.value.selection.copyWith(
                  baseOffset: lastMaxContent.length,
                  extentOffset: lastMaxContent.length,
           );
    //ios: When TextRange is not -1, the next update will replace all the variable values ​​of start and end directly. When you are sure that the content is unchanged, please change them to -1
    TextRange fixRange = TextRange(start: -1, end: -1);
          myController.value = TextEditingValue(text: lastMaxContent, selection: actualSelection, composing: fixRange);
    }

    void _actionMaxLengthState(String newText) {
    if (newText.length >= widget.selfMaxLength) {
      if (maxLengthCallBack != null) maxLengthCallBack();
      if (hasAlreadyMaxLength) {
        _initOldDataSelection(newText);
      } else {
        hasAlreadyMaxLength = true;
        _resetSelection(newText);
      }
    } else {
      //For the controller's text setting, you must change the value or set the text directly, and the selection is the default value of -1.
      widget.controller.value = TextEditingValue(text: newText, selection: widget.controller.value.selection, composing: widget.controller.value.composing);
      hasAlreadyMaxLength = false;
      lastMaxContent = "";
      }
    }