why I got 'A value of type 'List<dynamic>' can't be returned from the meth-CodePudding

Hello everyone I'm trying to create a list contains values in .txt file and then chunk it into sublists. But when I try to return the chunk list I got this error:

A value of type 'List' can't be returned from the method 'localPath' because it has a return type of 'Future<List>

this is my code :

  Future<List<int>> localPath() async {
    final textasset = "assets/112936-bluetooth.txt";
    final text = await rootBundle.loadString(textasset);
    final bytes =
        text.split(',').map((s) => s.trim()).map((s) => int.parse(s)).toList();
    print(bytes);
    final chunks=[];
    int chunkSize = 20;
    for (int i = 0; i < bytes.length; i  = chunkSize) {
      chunks.add(bytes.sublist(
          i, i   chunkSize > bytes.length ? bytes.length : i   chunkSize));
      print(chunks);
    }
    return chunks;
  }

this is a screenshot

Thanks in advance for your help

CodePudding user response：

What you are doing at below line

chunks.add(bytes.sublist(i, i   chunkSize > bytes.length ? bytes.length : i   chunkSize));

is, you are adding a List<int>(which is the sublist of bytes) inside a List named chunks.

Which makes chunks dataType List<List<int>>

and your return type for the function localPath() is Future<List<int>> which is incorrect. It should be Future<List<List<int>>>.

Solution 1.

Future<List<List<int>>> localPath() async {
  const textasset = "assets/112936-bluetooth.txt";
  final text = await rootBundle.loadString(textasset);
  final bytes = text.split(',').map((s) => s.trim()).map((s) => int.parse(s)).toList();
  print(bytes);
  final List<List<int>> chunks = [];
  int chunkSize = 20;
  for (int i = 0; i < bytes.length; i  = chunkSize) {
    chunks.add(bytes.sublist(i, i   chunkSize > bytes.length ? bytes.length : i   chunkSize));
    print(chunks);
  }
  return chunks;
}

Solution 2.

If required you can try

chunks.addAll(bytes.sublist(i, i   chunkSize > bytes.length ? bytes.length : i   chunkSize));

it will merge all the sublists into chunks and you wont need to change above things.

Future<List<int>> localPath() async {
  const textasset = "assets/112936-bluetooth.txt";
  final text = await rootBundle.loadString(textasset);
  final bytes = text.split(',').map((s) => s.trim()).map((s) => int.parse(s)).toList();
  print(bytes);
  final List<int> chunks = [];
  int chunkSize = 20;
  for (int i = 0; i < bytes.length; i  = chunkSize) {
    chunks.addAll(bytes.sublist(i, i   chunkSize > bytes.length ? bytes.length : i   chunkSize));
    print(chunks);
  }
  return chunks;
}

CodePudding user response：

You are mixing up an asynchronous function with a function that returns a Future. Right now, you are using both, but you might not want to do that.

It is sufficient to mark your function as async and return a List<int>. So you could change your function to

List<int> localPath() async {
  // Rest of your code goes here...
  // As you marked your function with async, you can use await here
  return chunks;
}

CodePudding user response：

You can include the dataType on chunks like

  final List<int> chunks = [];

Use chunks.addAll instead of .add to return List<int>.

Future<List<int>> localPath() async {
  final textasset = "assets/112936-bluetooth.txt";
  final text = await rootBundle.loadString(textasset);
  final bytes =
      text.split(',').map((s) => s.trim()).map((s) => int.parse(s)).toList();
  print(bytes);
  final List<int> chunks = [];
  int chunkSize = 20;
  for (int i = 0; i < bytes.length; i  = chunkSize) {
    chunks.addAll(bytes.sublist(
        i, i   chunkSize > bytes.length ? bytes.length : i   chunkSize));
    print(chunks);
  }
  return chunks;
}