hashCode generator enhancement

[mzdm]

When the built-in generator (ALT + Insert on Windows) is used for example in model classes, we can use it for generating ==, hashCode and toString() methods.

Let's say we have this model class:

class Example {
  const Example({
    this.category,
    this.id,
    this.isFavorite,
    this.name,
    this.price,
    this.startDate,
    this.expirationDate,
  });

  final Category category;
  final int id;
  final bool isFavorite;
  final String name;
  final int price;
  final DateTime startDate;
  final DateTime expirationDate;
}

Generated hashCode method for this example is:

@override
  int get hashCode =>
      category.hashCode ^
      id.hashCode ^
      isFavorite.hashCode ^
      name.hashCode ^
      price.hashCode ^
      startDate.hashCode ^
      expirationDate.hashCode;

Instead, I think it would be better if it was generating this
using hashValues
(if it would have more than 1 argument and less than 20)

@override
  int get hashCode => hashValues(category, id, isFavorite, name, price, startDate, expirationDate);

[mzdm]

Additional details:
combining hashes with XOR operation may cause issues (see felangel/equatable#39).

class Point {
  final int x = 3;
  final int y = 4;
}

and

class Point {
  final int x = 4;
  final int y = 3;
}

will hash to the same value.

hashValues method uses Jenkins hash function but to use it the import 'dart:ui' is needed.

however ...

there's now some activity hinting that a method that combines hashes is coming from ui package to main sdk core so it can be used without importing.
related: dart-lang/sdk#11617 dart-lang/sdk#25217

[pq]

/fyi @scheglov @bwilkerson

[bwilkerson]

Yes, and when the new static methods in Object land, we should definitely update the generator to make use of them. Given how soon they will likely land, I don't think it's worth changing it to use hashValues.

[stevemessick]

@mzdm it is actually acceptable to have two non-equal objects that hash to the same value, although always better if they don't.

Closing since the is an issue for the Dart SDK.

[bwilkerson]

Actually, I believe that the generator is implemented in IntelliJ, though it's probably in the Dart plugin. (On the Mac it's command-M.) It uses a dialog to query which fields to include in the comparisons and that isn't something that server-based tools can do.

@alexander-doroshko

[alexander-doroshko]

True, items from the Generate popup (hashCode, ==(), toString(), Override Methods, Implement Methods) are implemented in the Dart plugin.

[stevemessick]

https://youtrack.jetbrains.com/issue/IDEA-249561

[lukepighetti]

How do we determine when Object.hashAll will be available?