Disallow adding non-factory constructors

[chikamichi]

Hi,

I have some code available on GitHub showcasing a weird issue.

Context: refactoring a working app to leverage DDD, with proper SoC between:

• features (what the end-user does)
  • which leverage widgets (what the end-user sees and can interact with)
    • which leverage widgets controllers (indirection sublayer in the Presentation layer)
      • which (in this POC) are Riverpod async providers for modddels (in the Domain layer), with eventually remote data being fetched through a DataMapper layer

---

I got a union of modddels (source) named MapLayer, with three unioned "variants": MapLayer.raster, MapLayer.vector and MapLayer.geojson. I applied your insights from #4 and got it working… until I try to get a modddel instance.

On this specific call I get the following exception: UnsupportedError (Unsupported operation: It seems like you constructed your class using MyClass._(), or you tried to access an instance member from within the annotated class.)

My understanding is that it would only ever be raised only if trying to call MapLayer._() or an instance method such as map on a raw MapLayer instance, but it’s not what I’m doing here: I’m instantiating through the factory MapLayer.raster.

What am I missing?

Thank you!

[CodingSoot]

Hi,

The problem is in your Percentage modddel :

class Percentage extends SingleValueObject<InvalidPercentage, ValidPercentage>
    with _$Percentage {
  Percentage._();

  factory Percentage(double value) {
    return _$Percentage._create(
      value: value,
    );
  }
  
  /// ⚠️ The problem is here
  Percentage.maxed() {
    Percentage(1.0);
  }

  // ...
}

Here, you're declaring a named constructor Percentage.maxed. Calling it is the same as calling Percentage._() : it creates a blank unusable instance of Percentage, and that's why the error is thrown. In the constructor body, you properly create a Percentage instance, but you don't use it.

What you want to achieve is not possible, as normally you would need to use a named factory constructor, something like this :

factory Percentage.maxed() {
   return Percentage(1.0);
}

However, named factory constructors are reserved for creating unions of modddels. What I would suggest is to instead use a static method or getter :

static Percentage maxed() => Percentage(1.0);

// or
static Percentage get maxed => Percentage(1.0);

I think I should completely disallow adding non-factory constructors to a modddel class, as I can't see any valid usecase for them.

[CodingSoot]

@chikamichi Let me know if this fixed your issue

[chikamichi]

Thank you very much! You were right.

I was in the process of uncovering the exact stacktrace for I had a feeling MapLayer was not the actual culprit… you saved me some time again ;)

I think I should completely disallow adding non-factory constructors to a modddel class, as I can't see any valid usecase for them.

Agreed… until someone finds a legit use-case ;) Maybe just some warning/insight in the doc?

[CodingSoot]

Agreed… until someone finds a legit use-case ;) Maybe just some warning/insight in the doc?

I've decided to completely disallow non-factory constructors, as they serve no practical purpose and could potentially confuse newcomers.