How to update state inside addPostFrameCallback?-CodePudding

I'm trying to check the number of columns a Wrap widget creates on screen, I wrote this code:

late double _charsWidth;
 late int _numberOfColumns;

  @override
  void initState() {
    //_scrollController.jumpTo(0);
    _charsWidth = widget.characters.length * 100;
    WidgetsBinding.instance?.addPostFrameCallback((_) {
      final RenderBox renderBox =
          _containerKey.currentContext?.findRenderObject() as RenderBox;
          print('_charsWidth: $_charsWidth');
          print('renderBox.size.width: ${renderBox.size.width}');
          setState(() {
            _numberOfColumns = (renderBox.size.width / _charsWidth).ceil();
          });
          
       super.initState();
    print(_numberOfColumns); // error
    });

But I'm getting an error LateInitializationError: Field '_numberOfColumns@45197325' has not been initialized. with print(_numberOfColumns).

What's the correct way of updating the state here?

CodePudding user response：

remove the late keyword, avoid using late keyword before null-able variable

 double? _charsWidth;
 int? _numberOfColumns;