Flutter/Dart：创建/分配动态列表

Question

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?

Answer 1

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.

Answer 2

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;)