Flutter/Dart: Create/assign dynamic List-CodePudding

A list "options" is generated from json file as below and no issue: For simplicity, some parts were removed

Future getidea(String subject) async {

  List ideaList = [];
  for (var idea in ideaListTemp) {
    List options = [];
    options.add(idea["A"]);
    options.add(idea["B"]);
    options.add(idea["C"]);

    ideaList.add(ideaItem(
      idea["ideaText"],
      idea["ideaMedia"],
      idea[options],
    ));
  }
  return (ideaList);
}

class ideaItem {
  final String ideaText;
  final String ideaMedia;
  List? options;

  ideaItem(
    this.ideaText,
    this.ideaMedia,
    this.options,
  );
}

However, when I use "options" to create a Widget, this error occur. error: The argument type 'List?' can't be assigned to the parameter type 'List'.

  Widget build(BuildContext context) {
    return Expanded(
      child: SingleChildScrollView(
        padding: const EdgeInsets.all(_horizontalMargin),
        physics: const AlwaysScrollableScrollPhysics(),
        child: Column(
          children: [
            _buildOptions(widget.fromJSON.options), // this will have the error message
          ],
        ),
      ),
    );
  }
  
  Widget _buildOptions(List option) {
  List<Widget> listElevatedButton = [];
  for (var i = 0; i < option.length; i  ) {
    //print(allOptions[i]);
    listElevatedButton.add(
      Builder(
        builder: (context) => Padding(
          padding: const EdgeInsets.all(8.0),
          child: ElevatedButton(
            onPressed: () {
              print('Use the update function');
            },
            child: Text(
              option[i],
            ),
          ),
        ),
      ),
    );
  }
  return Column(children: listElevatedButton);
}

What is the best practice to solve the issue?

CodePudding user response：

The variable options is being used carelessly, You need to make sure that the options has value before using it, To do this you can put a null check like so:

_buildOptions(widget.fromJSON.options) -> _buildOptions(widget.fromJSON.options ?? [])

This will give an empty list to _buildOptions function as long as options is empty and It won't build any widgets in _buildOptions function.

I hope you understand the concept here.

CodePudding user response：

Your properties 'options' has type (optional) ?List, which mean it can contain List or null value. And you try use 'options' like argument in '_buildOptions' function, which need type List, not (optional) ?List.

Rewrite code like this:

Column(
  children: [
    _buildOptions(widget.fromJSON.options ?? []),
  ],
),

If the "widget.fromJSON.options" value is null, the _buildOptions will have the empty List, otherwise, it will have the value of the "widget.fromJSON.options" value.

Good luck ;)