Document why Widget.operator== is marked as non-virtual

Why is Widget.operator== marked as non-virtual?

Trying to override == on a widget triggers a warning, but there's no explanation on why (neither in the warning nor in the sources), and is not immediately obvious as to why we shouldn't do so.

@Hixie

We should probably document this.

I'm not sure why you'd want to override it though. Widgets are expected to be immutable. Even though Dart can't really guarantee immutability of e.g. a child list, the framework breaks if you mutate a property of the widget.

The default implementations of equals (i.e. identical) and hashCode (i.e. identityHashCode) should be good enough for that, and implementing it otherwise could slow things down when the framework goes to compare widgets in the tree.

/cc @Hixie @goderbauer who probably have more insight into this.

Flutter optimize rebuilds around the == operator.

Overriding == allows filtering rebuilds in situations that would otherwise rebuild for no reason.

It's mostly for leaf nodes though.

Can you give an example?

class Leaf extends StatelessWidget {
  const Leaf({Key key, this.label}) : super(key: key);

  final String label;

  @override
  bool operator ==(Object other) {
    return other is Leaf && other.key == key && other.label == label;
  }

  @override
  int get hashCode => hashValues(key, label);

  @override
  Widget build(BuildContext context) {
    return Text(label);
  }
}

This makes the widget behaves like a const instance (don't rebuild if didn't change), but works with dynamic values.

Some context for why this was done is in the linked from the PR that added the @nonVirtual annotation: #46900

@goderbauer

@goderbauer - I did look through that and saw a few places where @Hixie said not to do it, but never why.

It was likely about non-leaf widgets.

If a widget has a child, and that child too overrides ==, then we could end up effectively comparing the entire widget tree.

But I don't see any issue with leaf widgets though.

Do we have any benchmarks for the leaf case? I wouldn't be surprised if the mere existence of that operator== made everything more expensive (since it turns a single-dispatch into a dynamic dispatch for every single widget comparison ever).

I think if we don't want developers to implement Widget.==, we should just use identical instead

That could be confusing - someone expects the framework to be using their overridden ==, but it's actually just using identical and returning an unexpected result...

Regardless of what choice we make here (keeping nonVirtual, using identity) we won't be able to get away with documenting it extensively. equality as a concept is too heavily overloaded

I meant "without" documenting it extensively

I am pretty sure there are cases when Widget.operator== overriding is useful.
Consider an example: https://dartpad.dev/c20abd67d2cde74263ba1b49b368e062
There is a dynamic list of Items displayed in ListView. Overriding the operator== allows to avoid rebuilding all ItemViews in the viewport on Item addition, only a new one is built (or none if it happens outside the viewport).
This reduces worst frame build time from 25ms to less than 16ms on my device in profiling mode.

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Document why `Widget.operator==` is marked as non-virtual #49490

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Document why `Widget.operator==` is marked as non-virtual #49490