Support component inheritance

[f-mon]

At current time Angular Dart doesn't support component hineritance of @component and @input and other metadata. This very important feature has been added in angular 2.3.0, are there any plan to support this in angular dart?

[daniel-v]

Might I ask what is this blocked by? This would be a neat feature.

[TedSander]

We need to use resolved ASTs for codegen to enable this. The blocker is currently being worked on.

[matanlurey]

@f-mon @daniel-v This will be in 3.0.0, though I can't say if it will be immediate or not.

[matanlurey]

Pushed to 4.0. We have all the work ready to support this, but are still testing this doesn't cause regressions or build performance issues so we don't want to block 3.0. Should be coming soon.

[matanlurey]

This is now planned for the 4.0 release.

[matanlurey]

If anyone would like to start an RFC on how this should work (or refer to TypeScript's implementation) that would be awesome. There is a chance this might slip from 4.0 but I'd personally love to see it.

[Lisenish]

I would like to participate in this.
Also, I want to note it would be great to implement @component annotation inheritance not only @input like on TS Angular, as we can see in this issue angular/angular#7968 this is a highly requested feature but looks like guys from TS version doesn't agree with the community proposal. So it can be the small killer feature of AngularDart vs AngularTS =).

[leonsenft]

We plan to support inheritance for all property annotations in 4.0.0.

• @ContentChild
• @ContentChildren
• @HostBinding
• @HostListener
• @Input
• @Output
• @ViewChild
• @ViewChildren

Derived components will inherit their ancestor's property annotations, and may choose to override their implementations. If a derived component re-declares a named annotation on a different property, we'll treat this similarly to how duplicate annotations in a component currently behave.

We won't be adding support for inheritance of class annotations yet.

• @Component
• @Directive
• @Pipe

[leonsenft]

Thank you for your support and interest @Lisenish. If you have any feedback on our current plans I'd be glad to hear it. If you feel strongly about inheritable class annotations, please feel to provide some details on how you'd like that to work.

There's no one clear way to inherit class annotations. Since we haven't seen much demand for it yet, there's not much sense in committing to one arbitrary solution.

[zoechi]

We won't be adding support for inheritance of class annotations yet.

I assume this just means annotations of sub and superclass aren't "merged", but need to repeated with all parameters on the subclass, right?

[matanlurey]

It means the top-level @Component|Directive|Pipe is not inherited, only individual annotations on fields. For example, this would be inherited:

class AComp {
  @Input()
  String a;
}

// Still has an @Input() for a.
class BComp extends AComp {}

[Lisenish]

@leonsenft Good to hear, thanks!
About annotation inheritance, this is mostly about reusing parent component template and providers. I have the case where I need child component that differs from the parent only in a selector and some business logic. Now I have to duplicate component template in child component
I understand that class annotation inheritance isn't very transparent and why you are trying to avoid this feature. But maybe we can figure out how to deal with this duplication?

[leonsenft]

@Lisenish
If your desire is to inherit the template and providers as-is, you can already do so without duplication by using templateUrl and a predefined list of providers.

const baseProviders = const [...];

@Component(
  selector: 'base',
  templateUrl: 'base_component.html',
  providers: baseProviders,
)
class BaseComponent {
  ...
}

@Component(
  selector: 'derived',
  templateUrl: 'base_component.html', // reuse supertype template URL
  providers: baseProviders, // reuse supertype providers
)
class DerivedComponent extends BaseComponent {
  ...
}

Yes, you still have to specify the templateUrl and providers argument in the component annotation, but this is a tiny, constant amount of work per component.

My concern with your proposed solution is it's all or nothing. You either inherit the base metadata as-is, or replace it entirely. A solution where the derived component can interact in some way with its inherited metadata is much more compelling.

Notice that all of the metadata we're initially supporting annotates a component method or property. Methods and property accessors can reference their inherited implementations via super and extend on them if desired.

No parallel to this concept exists for annotation arguments. For example, a @Component annotation on a derived component could not reference the template specified in the @Component annotation of the base component.

Just to be clear, we're not ruling out any particular implementation yet. We want to be sure that if and when we implement this, we get it right.

I'd like to reiterate my earlier point that we just haven't seen much of a demand for inheritable templates/styles/providers and how particular implementations could solve problems in ways the framework currently struggles to. If you have a compelling example, we'd be happy to see a new issue with a detailed proposal.

[zoechi]

Polymer dropped template inheritance before 1.0 where the subclass can extend the template, because it was too complicated.

[Lisenish]

@leonsenft Thanks for the detailed answer and clarification!
Yeah, I was thinking about that solution that you proposed too after I had wrote comment.
Actually, I changed my mind I would agree with you, looks like it's pretty difficult to implement and say how it should work while profit is minimal.
For now, we are waiting Input and View/ContentChild inheritance, this is must have for sure!

[matanlurey]

@leonsenft your latest commits landed into master - do you want to mark this done?

[leonsenft]

@matanlurey Not yet; only very limited support is landed. Currently components only support inheritance from ancestor components and directives. I have a pending change which extends inheritance to all ancestors, so you can annotate regular classes with metadata.