flutter Text exception - expression is not a valid compile-time constant

Question

I'm trying to understand the List example in flutter_gallery. My approach is to simplify the `` code by refactoring into (my project).

I'm seeing a breaking change in this commit

I/flutter (14712): 'file:///Users/hong/Flutter/github/flutter_gallery_material_list/lib/main.dart': error: line 54:
I/flutter (14712): expression is not a valid compile-time constant
I/flutter (14712):     const Text t = const Text(text);

The source code starting line 53 is:

  MergeSemantics _mergeSemanticsOf(String text, _MaterialListType listType) {
    const Text t = const Text(text);
    //const Text t = const Text('we want a variable here');
    return new MergeSemantics(
      child: new ListTile(
          dense: true,
          title: t,
          trailing: new Radio<_MaterialListType>(
            value: listType,
            groupValue: _itemType,
            onChanged: changeItemType,
          )),
    );
  }

I can only make it compile with something like: const Text t = const Text('we want a variable here');.

I understand what the exception says, but wonder if there is a way to pass a variable to Text().

This is the exception popup (in a red box) in VSCode:

And this is the exception on an Android phone (Samsung S7)

A search on Stackoverflow shows this, which looks not related to my question.

Answer 1

Constants (i.e. const) in Dart are compile-time, that is, they must not rely on the runtime of your application in anyway, and can only be simple side-effect free constructor invocations (i.e. const constructors) or literals like strings, numbers, and lists/maps.

For example, this is a compile-time string:

const version = 'v1.0.0';

And I could use it below:

const Text(version)

Dart supports limited expressions as well, as compile-time constants:

const Text('My version is: $version')

However in your example, text is not a compile-time constant.

Lets view this through a simpler example, called showMyName:

Widget showMyName(String name) => const Text(name);

This will get an identical error to what you saw, because we're trying to create a compile-time constant Text from a runtime provided value (the argument name). Of course, we don't need Text to be a compile-time constant. You can simply use new:

Widget showMyName(String name) => new Text(name);

In future versions of Dart (with --preview-dart-2), you can omit new:

Widget showMyName(String name) => Text(name);