Flutter nested ListView.separated

Question

I'd like to have a ListView of columns that could scroll horizontally and in each of those columns a ListView of rows that could scroll vertically.

Here's the code that I wrote:

@override
Widget build(BuildContext context) {
  return Center(
    child: Column(
      mainAxisAlignment: MainAxisAlignment.start,
      mainAxisSize: MainAxisSize.min,
      children: [
        Text(
          "AppName",
          textAlign: TextAlign.center,
        ),
        Expanded(
          //This is the first ListView, the itemBuilder returns a Column
          child: ListView.separated(
            scrollDirection: Axis.horizontal,
            controller: scrollController,
            itemCount: filesProvider.directories.length,
            itemBuilder: (BuildContext context, int index) {
              return buildColumn(context, index, filesProvider);
            },
            separatorBuilder: (BuildContext context, int index) {
              return SizedBox(
                width: 30,
              );
            },
          ),
        ),
      ],
    ),
  );
}

Column buildColumn(BuildContext context, int index, FilesProvider provider) {
  return Column(
    children: [
      Text(
        "Column "+index,
        textAlign: TextAlign.center,
      ),
      Expanded(
        //This is the second ListView, the itemBuilder returns a Row
        child: ListView.separated(
          scrollDirection: Axis.vertical,
          controller: scrollController,
          itemCount: (provider.dirNames[index].length / colonnePerTipo).round(),
          itemBuilder: (BuildContext context, int rowIndex) {
            return buildRow(context, rowIndex, provider);
          },
          separatorBuilder: (BuildContext context, int rowIndex) {
            return SizedBox(
              width: 10,
            );
          },
        ),
      ),
    ],
  );
}

Row buildRow(BuildContext context, int index, FilesProvider provider) {
  List<Widget> sizedBoxes = [];

  for (int i = 0; i < buttonForRow; i++) {
    sizedBoxes.add(
      SizedBox(
        width: 20,
        child: ElevatedButton(
          child: Text("Button"),
        ),
      ),
    );
  }

  return Row(
    mainAxisAlignment: MainAxisAlignment.spaceEvenly,
    children: sizedBoxes,
  );
}

It seems fine to me but when I run it I receive this error:

======== Exception caught by rendering library =====================================================
The following assertion was thrown during paint():
RenderBox was not laid out: RenderRepaintBoundary#29de6 relayoutBoundary=up2 NEEDS-PAINT
'package:flutter/src/rendering/box.dart':
Failed assertion: line 1929 pos 12: 'hasSize'

I don't understand what the problem could be. Can someone help me?

Can you simplify the code?

Md. Yeasin Sheikh
– Md. Yeasin Sheikh

2022-02-26 17:23:05 +00:00
Commented Feb 26, 2022 at 17:23 — Md. Yeasin Sheikh
– Md. Yeasin Sheikh, Commented Feb 26, 2022 at 17:23

Prateek Jain · Accepted Answer · 2022-02-26 17:53:54Z

You are getting this error because you have wrapped your widget in Expanded widget and hence your ListView.separated widgets viewports are getting unbounded height and width. ListView.separated doesn't know how much space to take.

try wrapping both your ListView.separated widgets in SizedBox instead of Expanded to give them height and width constraints

try this code

    @override
  Widget build(BuildContext context) {
    return Scaffold(
      body: Center(
        child: Column(
          mainAxisAlignment: MainAxisAlignment.start,
          mainAxisSize: MainAxisSize.min,
          children: [
            const Text(
              "AppName",
              textAlign: TextAlign.center,
            ),
            SizedBox(
              height: 300,
              //This is the first ListView, the itemBuilder returns a Column
              child: ListView.separated(
                scrollDirection: Axis.horizontal,
                itemCount: 10,
                itemBuilder: (BuildContext context, int index) {
                  return buildColumn(context, index);
                },
                separatorBuilder: (BuildContext context, int index) {
                  return const SizedBox(
                    width: 30,
                  );
                },
              ),
            ),
          ],
        ),
      ),
    );
  }

  Column buildColumn(BuildContext context, int index) {
    return Column(
      children: [
        Text(
          "Column " + index.toString(),
          textAlign: TextAlign.center,
        ),
        SizedBox(
          height: 200,
          width: 180,
          //This is the second ListView, the itemBuilder returns a Row
          child: ListView.separated(
            scrollDirection: Axis.vertical,
            itemCount: 5,
            itemBuilder: (BuildContext context, int rowIndex) {
              return buildRow(context, rowIndex);
            },
            separatorBuilder: (BuildContext context, int rowIndex) {
              return const SizedBox(
                height: 10,
              );
            },
          ),
        ),
      ],
    );
  }

  Row buildRow(BuildContext context, int index) {
    List<Widget> sizedBoxes = [];

    for (int i = 0; i < 3; i++) {
      sizedBoxes.add(
        SizedBox(
          width: 60,
          child: ElevatedButton(
            onPressed: () {  },
            child: const Text("cta"),
          ),
        ),
      );
    }

    return Row(
      mainAxisAlignment: MainAxisAlignment.spaceEvenly,
      children: sizedBoxes,
    );
  }

screenshot

This solution solved the problem, thanks. I also made some experiment and discovered that the first ListView could be wrapped by an Expanded so that it could occupy all the available space but the inner one has to be wrapped in something else, as the SizedBox, with dimensions. Plus, the inner one needs to have both width and height so that it know how much space use for the List.

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