Filtering Consumer updates

[aloisdeniel]

Having a way to filter updates at the consumer level may be useful to fine-grain builds.

For example, if we have a ChangeNotifier model with multiples properties,notifyListeners would be called each time a value of a property changed. If we have a Consumer for that model that only depends on one of the properties, it is updated even when an other property changes.

We could have a bool shouldBuild(T previousValue, T newValue) argument for Consumer for that purpose.

[rrousselGit]

How would that be implemented? From my understanding there's no way for widgets to cancel a rebuild

[rrousselGit]

I'm going to close this, as it's not doable.

Not only do widgets cannot cancel a rebuild, but there's also a bigger flaw:

In 99% of the situations with ChangeNotifier, previousValue & newValue are actually the same.

[fantasy525]

@rrousselGit I also need this.

notifyListeners would be called each time a value of a property changed. If we have a Consumer for that model that only depends on one of the properties, it is updated even when an other property changes.

for best performance,I don't think it need to rebuild the entire screen and also I don't want to write a ChangeNotifier for every property.I just need write a ChangeNotifier with many properties.

[fantasy525]

@aloisdeniel do you have any ideas?

[rrousselGit]

That's simply not possible with mutable objects like ChangeNotifier. You need to use an immutable architecture

[fantasy525]

@rrousselGit can you provide me some examples?and your example is too simple.sorry. i'm confused

[fantasy525]

I'm going to close this, as it's not doable.

Not only do widgets cannot cancel a rebuild, but there's also a bigger flaw:

In 99% of the situations with ChangeNotifier, previousValue & newValue are actually the same.

I think you can save last value,like Int ,String value.

[rrousselGit]

From a discussion on gitter:

Most of the time, the worry you have is just premature optimisation.

But if you really need that optimisation, you can ProxyProvider

class ManyMutableThings with ChangeNotifier {
  String foo;
  int bar;
}

class ImmutableCopyOfTheFewFieldsYouNeed {
  ImmutableCopyOfTheFewFieldsYouNeed({this.foo});

  final String foo;
}

Widget build(BuildContext context) {
  return ProxyProvider<ManyMutableThings, ImmutableCopyOfTheFewFieldsYouNeed>(
    builder: (_, model, __) => ImmutableCopyOfTheFewFieldsYouNeed(foo: model.foo);
    updateShouldNotify: (a, b) => a.foo == b.foo,
    child: Consumer<ImmutableCopyOfTheFewFieldsYouNeed>(
      builder: (_, data) {
        return Text(data.foo);
      },
  );
}