Elements of array are fixed-length while array itself is not

Question

var data = List();
snapshot.data.documents.forEach((doc) => data.add(doc.data)); //my example data is one doc
data.add({"x": "y"}); //works
data[0]["bar"].add({"foo": "bar"}); //doesn't

My data looks like this:

  data = [
    {
      "foo": "",
      "bar": [
        {"foo": "bar"},
        {"foo": "bar"},
        {"foo": "bar"},
      ],
    },
  ];

When I do the second array modification, it gives me this error:

Unhandled Exception: Unsupported operation: Cannot add to a fixed-length list

I have tried doing this with normal data that is not fetched from firebase, and it works as expected. I also tried using List.from to populate the array but it didn't work either.

jamesdlin · Accepted Answer · 2020-08-13 00:42:44Z

2

snapshot.data.documents[0]['bar'] apparently is a fixed-length (i.e., non-growable) List. You can't make a non-growable List growable. You instead need to create a growable copy:

data[0]["bar"] = data[0]["bar"].toList(); // Create a copy.
data[0]["bar"].add({"foo": "bar"}); // Now this should work.

Alternatively:

data[0]["bar"] = [...data[0]["bar"], {"foo": "bar"}];

However, while the latter approach is more compact, it is less clear that the copy is explicitly desired.

answered Aug 13, 2020 at 0:42

jamesdlin

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1 Comment

Nicholas Ficara Over a year ago

Thanks. This was the issue. It works now. Is there any other way of fixing it by forcing the data to be growable upon adding it to the array?

Amon C · Accepted Answer · 2020-08-12 23:12:57Z

0

Do like this:

var data = List();
var data2= List();

snapshot.data.documents.forEach((doc) => data.add(doc.data)); //my example data is one doc
data.add({"x": "y"}); //works

data2=data[0]["bar"];
data2.add({"foo": "bar"});

answered Aug 12, 2020 at 23:12

Amon C

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1 Comment

jamesdlin Over a year ago

data2 = List() is pointless if you're just going to reassign it later, and adding the data2 variable won't change a fixed-length List to a growable one.

Alfonso Ramirez Zarate · Accepted Answer · 2020-08-12 23:13:10Z

0

The problem is add method is used over an object type. You need to add a cast before using add method:

  (data[0]["bar"] as List<Map<String, String>>).add({"foo": "bar"});

answered Aug 12, 2020 at 23:13

Alfonso Ramirez Zarate

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1 Comment

jamesdlin Over a year ago

I'm pretty sure the static type is dynamic. Otherwise there would probably be a different error about there not being an add method. The error message clearly indicates that the problem is that the OP is trying to modify a fixed-length List, not a growable one.

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