Why this function is null?-CodePudding

Im need that function for callback, but it gives a null,

int? _handleIncrement(int? variable) {
    if (variable == null) {
      variable = 0;
    } else if (variable < 5) {
      variable  ;
    } else {
      variable;
    }
  

  void _incrementVariable(int? variable) {
    
        variable = _handleIncrement(variable);
  }
  
  int? val;
  _incrementVariable(val);

I need it in a button

This is the button

class ExpTile extends StatelessWidget {
  final String name;
  final void Function(int?) onIncrement;

  final List<Widget> children;
  final int? variable;

  ExpTile(
      {required this.name,
      this.children = const <Widget>[],
      required this.onIncrement,
      required this.variable,
      key})
      : super(key: key);

  @override
  Widget build(BuildContext context) {
    return Card(
      child: ExpansionTile(
          title: ElevatedButton(
            onPressed: () {
              onIncrement(variable);
            },
            child: Row(
              children: [Text(name), Text(variable.toString())],
            ),
          ),
          children: children),
    );
  }

This is the way I call the widget in the father page

ExpTile(
              name: "Defense",
              variable: defense,
              onIncrement: _incrementVariable,
              children: [])

I'm trying different things in dartpad, but I don´t know why the logic doesn't work. always return me the initial value of defense. I'm reading different things about dart functions and callbacks but I dont know where is the problem. Thanks

CodePudding user response：

First, you have to define variable in the parent Widget, if you want to see changes there

class ParentWidget extends StatefulWidget {
  const ParentWidget({super.key, required this.title});
  final String title;

  @override
  State<ParentWidget> createState() => _ParentWidgetState();
}

class _ParentWidgetState extends State<ParentWidget> {
  int? variable; //here


  @override
  Widget build(BuildContext context) {
    //your widgets...
              ExpTile(
              name: "Defense",
              variable: variable,
              onIncrement: _incrementVariable,
              children: [])
     //the rest of your widgets...
  }
}

Then you have to define you increment function as a function which changes your variable's value, and redraw the widget.

  void _incrementVariable() {
        setState(() {
          if (variable == null) {
            variable = 0;
          } else if (variable! < 5) {
            variable = variable!   1;
          } 
        });
  }

and then call the function (In the ExpTile widget) this way:

       onPressed: () {
          widget.onIncrement();
        },