onSet should have function signature void Function(Thing)

[deadsoul44]

I am trying to use built_value with MobX. I will add new thing to the ObservableStream whenever it is rebuilt. So, onSet should have function signature void Function(Thing)

    thingController = StreamController<Thing>();
    thingStream = ObservableStream(thingController.stream);
    thingBuilder = thing.toBuilder();
    thingBuilder.onSet = thingController.add; // this does not work. A value of type 'void Function(dynamic)' can't be assigned to a variable of type 'void Function()'

I am aware of the use case in this issue: #589

There can be a solution to cover both situations or change the function definition because we can throttle rebuilds.

[davidmorgan]

Maybe the _finalizeBuilder hook can do what you need?

It's static rather than per instance, but the builder is passed in, and you could build it to get the new instance.

https://github.com/google/built_value.dart/blob/master/end_to_end_test/lib/values.dart#L593

[deadsoul44]

I am not sure how to do it with _finalizeBuilder. It is a private method and somehow I have to pass in a StreamController instance to add the new built instance.

[deadsoul44]

Changing onSet function signature from void Function() to void Function(Thing) is a breaking change but it also covers the case in #589 .

void Function(BuiltValue Thing) {
   // do nothing with Thing
   // call your zero argument function
   build();
}

[davidmorgan]

Do you want a new StreamController for every instance? If so, you could make it a field on the class, and call it in the class constructor...

[deadsoul44]

No, I will have a single instance of ObservableStream in a MobX store. Everytime, a model class is rebuilt, it should be added to the streamController. So, I want to set streamController.add function to model class instance onSet function like in the code snippet above which is actually inside a MobX store.

[davidmorgan]

If there is a single stream, you can use _finalizeBuilder, just make the stream a static field somewhere so you don't need to pass it in.

[deadsoul44]

It doesn't work because I have to add the rebuilt instance to the streamController but it ends up with infinite loop of _finalizeBuilder and b.build():

  static void _finalizeBuilder(LeagueBuilder b) {
    NotificationsStore_.streamController.add(b.build()));
  }

Another problem is that testing can be problematic.

Moreover, I had to convert private MobX store to public MobX store:

class NotificationsStore = NotificationsStore_ with _$NotificationsStore;

// ignore: camel_case_types
abstract class NotificationsStore_ extends BaseStore with Store {

My purpose is to decouple model classes and stores completely. I want to create a package to add reactivity to BuiltValue fields in MobX stores with only an annotation:

// a regular reactive int field in a MobX store:
@observable
int myInt;

// possible annotation with a package
@observable_built_value
MyBuiltValueObject myBuiltValueObject;

[davidmorgan]

I think it should work to use a static bool flag to stop the infinite loops, e.g.:

static bool _inFinalizer = false;

static void _finalizeBuilder(LeagueBuilder b) {
  if (!_inFinalizer) {
    _inFinalizer = true;
    NotificationsStore_.streamController.add(b.build()));
    _inFinalizer = false;
  }
}

[deadsoul44]

What is holding you back from changing function signature other than being a breaking change which will impact limited number of users who can quickly migrate :)

[davidmorgan]

Doesn't onSet fire too often? It fires on every individual change to the builder, which is more often than when you call build.

[deadsoul44]

This problem also exists with the current state. Changing function signature will not affect the number of UI rebuilds.

Anyway, I think it is my job to care about frequent UI rebuilds and I can easily throttle UI rebuilds with stream or other means of transformation.

Moreover, people who are using MobX with json_serializable are currently already annotating every field as observable in the model class. It has the same effect and they don't even care about the frequency of UI rebuilds.