label_textfield_add_events.dart

@override
void initState() {
  ...
  _focusNode.addListener(() {
    if (!_focusNode.hasFocus) {
      setState(() {
        if (txtController.text.isEmpty) {
          isShowHintText = true;
        } else {
          isShowHintText = false;
        }
      });
    }
  });
  super.initState();
}

...
TextField(
  focusNode: _focusNode,
  style: widget.txtStyle,
  textAlign: TextAlign.left,
  controller: txtController,
  decoration: InputDecoration(
    border: (widget.showBorder) ? UnderlineInputBorder() : InputBorder.none,
    hintText: (isShowHintText) ? widget.hintText : '',
  ),
  onTap: () => _onTap(),
  onChanged: (value) => _onChanged(value),
  onEditingComplete: () => _onComplete(),
);
...

void _onTap() {
  setState(() {
    isShowHintText = false;
  });
}

void _onChanged(String value) {
  if (widget.onTxtChanged != null) {
    widget.onTxtChanged(value);
  }
  setState(() {});
}

void _onComplete() {
  setState(() {
    if (txtController.text.isEmpty) {
      isShowHintText = true;
    } else {
      isShowHintText = false;
    }
  });
}