feat(ios): forward mapViewImpl prop so RNMBXMapViewFactory is usable on iOS

[SamuelBrucksch]

Description

Makes the existing RNMBXMapViewFactory hook reachable on iOS.

RNMBXMapViewFactory.register(_:factory:) is public API, and RNMBXMapView.createMapView() already prefers a factory-built MapView when mapViewImpl is set:

func createMapView() -> MapView {
  if let mapViewImpl = mapViewImpl, let mapViewInstance = createAndAddMapViewImpl(mapViewImpl, self) {
    _mapView = mapViewInstance
  } else {
    _mapView = MapView(frame: self.bounds, mapInitOptions: MapInitOptions())
    ...

But nothing ever assigned RNMBXMapView.mapViewImpl on iOS, so that branch was unreachable from JS and every mapViewImpl value fell through to the default MapView. The prop is declared in src/specs/RNMBXMapViewNativeComponent.ts and on the JS component, and it is forwarded on Android (RNMBXMapViewManager.setMapViewImpl) — this only brings iOS in line, so it's a missing-functionality fix rather than a behaviour change.

The assignment is placed before [_view didSetProps:@[]], since that is where createMapView() runs.

Why this is useful

The factory is the only way to control MapInitOptions from an app, which matters for things the props don't cover — in our case MapOptions.pixelRatio. MapboxMaps defaults it to UIScreen.main.nativeScale and sizes the Metal drawable as bounds * pixelRatio, so a CarPlay map renders at the phone's scale (3.0) on a head unit that only has ~1.88 px/pt — ~2.5x the pixels per frame, discarded by the compositor. With this prop forwarded, the app can register a factory that builds the MapView with the car screen's pixel ratio; no library change beyond this is needed.

Checklist

• I've read CONTRIBUTING.md
• I updated the doc/other generated code with running yarn generate in the root folder
  • no generated output is affected: the prop already exists in the specs and is marked @private in MapView.tsx, so it is not part of the generated docs
• I have tested the new feature on /example app.
  • In V11 mode/ios
  • In New Architecture mode/ios
  • In V11 mode/android
  • In New Architecture mode/android
• I added/updated a sample - if a new feature was implemented (/example)

How it was verified

Not via /example — verified in a production app (RN 0.86, New Architecture, iOS, @rnmapbox/maps 10.3.2 with this change applied through patch-package):

• before: a MapView with mapViewImpl="…" silently used the default MapView; the registered factory closure was never invoked (and createAndAddMapViewImpl's "No mapview factory registered" error never fired either, since mapViewImpl was nil at that point).

• after: the factory closure runs for every CarPlay surface and the returned MapView is the one used. Log line from the factory, on a real head unit connection:

  CarMirrorMap | impl=abrpCarMirror:headUnit:3 pixelRatio=1.8836 screenSource=carPlayScene
  carScreen=(scale: 3.0, nativeScale: 3.0, bounds: (1019.3, 573.3), nativeBounds: (1920.0, 1080.0))
  

  and the Metal drawable then matches the head unit's resolution instead of ~1.6x it.

Happy to add an /example scene for it if you'd like — it needs a factory registration in the example app's AppDelegate since the hook is native-only, so I left that out of this PR unless you want it.

---

🤖 Prepared by Devin on behalf of @SamuelBrucksch.

[SamuelBrucksch]

Some history I dug up while checking for duplicates — this is closer to a regression than a gap that was never filled:

• feat(ios): mapViewImpl so native code can register custom mapView fac… #3275 (merged 2023-12-15) introduced RNMBXMapViewFactory, the mapViewImpl prop in src/specs, the RNMBXMapView.mapViewImpl property and the createMapView() branch, plus the Android forwarding.
• chore: mapViewImpl support #3317 (merged 2024-01-14) wired it up on iOS for the old architecture, with RCT_EXPORT_VIEW_PROPERTY(mapViewImpl, NSString) in ios/RNMBX/RNMBXMapViewManager.m.

ios/RNMBX/RNMBXMapViewManager.m no longer exists on main (paper view managers went away with the old-architecture support), and RNMBXMapViewComponentView.mm never carried the equivalent line — so the iOS side of the feature has been dead since then, while Android kept working through RNMBXMapViewManager.setMapViewImpl. This PR just restores what #3317 did, in the Fabric component view.

Possibly related: #2728 ("Map aspect ratio broken inside CarPlay on iOS", also reported for an external monitor) was closed as not-planned for lack of resources. That class of problem comes from the map being initialised with the main screen's scale — MapboxMaps defaults MapOptions.pixelRatio to UIScreen.main.nativeScale and hard-wires the Metal drawable to it — so with this prop forwarded, apps rendering onto a secondary screen can correct it themselves through a factory without any further library change. Not claiming it as a fix for that issue, just noting the connection.

For completeness: no open issue tracks this. I searched the repo for mapViewImpl, MapViewFactory, MapInitOptions, pixelRatio and CarPlay; the only other hits are the two merged PRs above.