[ffigen] Obj-C protocols: static getter can't be accessed through an instance?

[mhernz]

Maybe this is a silly question. I'm calling a method in Objective-C that returns an NSArray of objects conforming to the following protocol:

https://github.com/maplibre/maplibre-native/blob/main/platform/darwin/src/MLNFeature.h

Then I want to access @property (nonatomic, copy) id identifier on those objects, but I get a linter error:

    final query = _mapView
        .visibleFeaturesAtPoint$1(
          point.toCGPoint(),
          inStyleLayersWithIdentifiers: NSSet.of(layerIds?.map((s) => s.toNSString()) ?? []),
        );
    final features = query.map(MLNFeature.castFrom);
    return features.map((f) => RenderedFeature(
      id: f.identifier,
         // ^ The static getter 'identifier' can't be accessed through an instance.
         //   Try using the class 'MLNFeature' to access the getter.
    )).toList();

And the static member MLNFeature.identifier is of type ObjCProtocolMethod<ObjCObjectBase? Function()> which I cannot figure out how to call on a particular object.

Could somebody walk me through how to access this property?

[mhernz]

pubspec.yaml:

https://gist.github.com/mhernz/9eda846b5ce2e2a15a23b245ff9e16b8

The relevant output in maplibre_ffi.dart:

https://gist.github.com/mhernz/9a0943499636797010e1b35f8a220b73

[liamappelbe]

The generated protocols objects aren't designed to be interacted with as instances. They're just designed to:

1. Allow the protocol to be implemented from Dart, using the implement method etc. The static identifier member is an object representing that method, used during the protocol implementation process.
2. Act as a sentinel type to mark the ObjC interfaces that implement the protocol.

I can't really add support for the sort of method calling you want under the current design, since the protocol's namespace would get super cluttered, and collisions would be likely.

It would probably be a better design to split the implementation stuff and method objects into a separate implementation helper class:

• Foo: Sentinel type for protocol Foo, but also has all the infra to allow methods to be called directly on protocol instances.
• Foo$Builder: Contains the implement stuff and method objects.

@mhernz For now, as a work around, you can write an empty ObjC interface that implements that protocol. That interface's generated wrapper will have all the protocol's methods. Then you can cast your protocol instances to the wrapper interface using .castFrom().

[mhernz]

@liamappelbe Ah, that makes sense. Maybe a quick note could be added to https://pub.dev/packages/ffigen to prevent confused users like me :)

The empty interface workaround worked like a charm, thanks for your response!