const widget in Flutter is not allowing me to pass arguments in constructor-CodePudding

I am trying to define a parameter as callback in a StatefulWidget widget and using that widget as const child widget in another StatefulWidget widget. But it is not allowing me to pass any argument with const child widget.

Here is my code snippet:

Container(
  child: const LeftSidePanel(
                  checking: (value) 
                  {
                  
                  },
                ),
         ),

Here is the class of child widget used in above code:

class LeftSidePanel extends StatefulWidget {
  final Function(bool?) checking;
  @override
  State<StatefulWidget> createState() {
    // TODO: implement createState
    return LeftPanelWidgetState();
  }

  const LeftSidePanel({
    required this.checking,
  });
}

In the first code snippet, it is giving error saying "A value of type 'Null' can't be assigned to a parameter of type 'dynamic Function(bool?)' in a const constructor. (Documentation) Try using a subtype, or removing the keyword 'const'."

Can anyone help me with this issue. How can I solve this.

Thanks in advance

CodePudding user response：

Remove the const parameter before LeftSidePanel:

import 'package:flutter/material.dart';

class LeftSidePanel extends StatefulWidget {
  final Function(bool?) checking;

  const LeftSidePanel({Key? key, required this.checking}) : super(key: key);

  @override
  State<LeftSidePanel> createState() => _LeftSidePanelState();
}

class _LeftSidePanelState extends State<LeftSidePanel> {
  @override
  Widget build(BuildContext context) {
    return Container();
  }
}

class Test extends StatelessWidget {
  const Test({Key? key}) : super(key: key);

  @override
  Widget build(BuildContext context) {
    return Container(
      child: LeftSidePanel(
        checking: (value) {},
      ),
    );
  }
}

That's because the code of your Function is not an const, so LeftSidePanel can't be a const.

CodePudding user response：

Removing const will probably solve your issue. I tried running your code and removing it solved it. To answer why, I guess const doesn't allow callbacks or methods.

class MyHomePage extends StatefulWidget {
  const MyHomePage({Key? key, required this.title}) : super(key: key);

  final String title;

  @override
  State<MyHomePage> createState() => _MyHomePageState();
}

class _MyHomePageState extends State<MyHomePage> {
  @override
  Widget build(BuildContext context) {
    return Scaffold(
      appBar: AppBar(
        title: Text(widget.title),
      ),
      body: Center(
        child: LeftSidePanel(
          checking: (value) {},
        ),
      ),
    );
  }
}

class LeftSidePanel extends StatefulWidget {
  final Function(bool?) checking;
  const LeftSidePanel({Key? key, required this.checking}) : super(key: key);

  @override
  State<LeftSidePanel> createState() => _LeftSidePanelState();
}

class _LeftSidePanelState extends State<LeftSidePanel> {
  @override
  Widget build(BuildContext context) {
    return Container();
  }
}

CodePudding user response：

You are passing a callback that is not const (Invalid constant value) and therefore, you should just remove const that will work for you

           Container(
              child: LeftSidePanel(
                checking: (value) {},
              ),
           );

CodePudding user response：

As far as I know only top level functions or static functions are considered const values in dart. Thus the lambda you are passing in for checking is not a const value. Therefore you cannot use the const version of the constructor. To solve this you could either not use the const version of the constructor by omitting the const keyword or pass a top level function to checking.