Code generator for widget equality

[DaveShuckerow]

It'd be really nice if we had a code generation for equality between widgets, like for example:

@equalableFields
class MyWidget extends StatefulWidget {
  final Foo myFoo;
  final Bar myBar;
  ...
}

would generate the following:

class MyWidget extends StatefulWidget {
  final Foo myFoo;
  final Bar myBar;
  ...
  bool hasEqualFields(MyWidget other) => myFoo == other.myFoo && myBar == other.myBar;
}

[Hixie]

cc @eseidel

We have requested this from our upstream: https://github.com/dart-lang/sdk/issues/26703

[Hixie]

I'm going to close this because in our experience checking equality of widgets would be expensive since you'd be walking a tree. But please follow the aforementioned Dart bug if you want to implement this anyway in your code.

[iLoveDocs]

@Hixie Does this also mean that there is no benefit of using const in widgets like

const SizedBox(width: 2)

vs

SizedBox(width: 2)

in terms of performance?

[Hixie]

const is great for performance because it makes the nodes equal with a pointer test, which is very cheap, and bypasses any need to walk any trees (i.e. it's O(1)).
operator == on a node with a child means potentially traversing the same size tree as the check would be avoiding in the first place, except you have to do it at every level, so you end up O(N^2) which is more expensive than just doing the work in the first place (O(N)).

[github-actions]

This thread has been automatically locked since there has not been any recent activity after it was closed. If you are still experiencing a similar issue, please open a new bug, including the output of flutter doctor -v and a minimal reproduction of the issue.