[SOLVED] What does :final name mean in Dart language

What does :final name mean in Dart language

I'm playing with riverpod in flutter, and I got this example

AsyncError(:final error) => Text('Error: $error'),

It works, but I'm trying to understand what does :final error mean? If I remove the : it throws an error Object patterns can only use named fields.

Can you please provide a better example when is required to use it?

Trying to understand a certain syntax in Dart language

Solution

This is the ability to destructure a class instance field. To use the fields of an object at the time of destructuring, you can use the syntax Object(:[type] field) (conditionally). Here are more examples:

AsyncValue? value;
final result = switch (value) {
  AsyncError(:final error) => '$error',
  AsyncError(:var error, :StackTrace stackTrace) => '$error $stackTrace',
  AsyncError(:var error, stackTrace: StackTrace st) => '$error $st',
  AsyncError<int>(:final int value) => '$value',
  AsyncError(:Object error) => '$error',
  AsyncValue<int>(hasValue: true) => 'hasValue is true',
  AsyncValue<int>(:var hasValue) when hasValue == false => '$hasValue',
  _ => 'default',

The example itself doesn't make sense here, I only showed the syntax of field extraction. Specify : (or just inside the object's parentheses) and use the IDE prompts to see the fields available for destructuring.

Use a custom field when destructuring if necessary (as with stackTrace: StackTrace st).

You can also immediately start mapping fields to value, as in the AsyncValue.hasValue example. Use the when syntax if you need this field later.

There is a very large syntax, I recommend viewing the information using the links below: