slot has no effect on iOS with New Architecture (Fabric) enabled — RNMBXSetCommonLayerPropsWithoutSourceID never reads it

[tomsisbos]

Environment

• @rnmapbox/maps: 10.3.5 (also reproduced on 10.2.10)
• Mapbox Maps SDK (iOS): 11.23.1
• React Native: 0.81.5
• Platform: iOS only (not reproduced on Android)
• New Architecture (Fabric): enabled

Description

slot has no effect whatsoever on any layer on iOS with the New Architecture enabled — every combination of "top" / "middle" / "bottom" / unset produces a pixel-identical result, and the layer always ends up positioned as if unslotted.

Root cause: on Fabric, prop updates go through RNMBXSetCommonLayerPropsWithoutSourceID in ios/RNMBX/RNMBXFabricHelpers.h, which explicitly reads and applies id, sourceID, filter, aboveLayerID, belowLayerID, layerIndex, reactStyle, maxZoomLevel, minZoomLevel — but never reads slot, even though slot is declared in the codegen prop spec (e.g. src/specs/RNMBXRasterLayerNativeComponent.ts) and is correctly read/applied on the Swift side (RNMBXLayer.swift's setBaseOptions) once it reaches it. It just never reaches it on Fabric.

The old-architecture path doesn't go through this Fabric helper, so this only affects apps with newArchEnabled: true. PointAnnotationManager's own Fabric component view wires slot correctly (added in #4209/#4240) — it's specifically this shared layer helper that's missing it.

Reproduction

Any layer with slot set, e.g.:

<RasterLayer id="my-layer" sourceID="my-source" slot="bottom" style={{...}} />

on a style that imports Mapbox Standard, with New Architecture enabled — the layer renders as if slot were never set (typically ends up above line/road layers and below symbol/label layers, i.e. treated as unslotted).

Fix

Patch attached (also runnable via patch-package) — adds a compile-time check (slot isn't declared on every layer's Props struct — Sky, Background and FillExtrusion don't have it) and applies _view.slot when present:

diff --git a/node_modules/@rnmapbox/maps/ios/RNMBX/RNMBXFabricHelpers.h b/node_modules/@rnmapbox/maps/ios/RNMBX/RNMBXFabricHelpers.h
index 974ee24..17de6d1 100644
--- a/node_modules/@rnmapbox/maps/ios/RNMBX/RNMBXFabricHelpers.h
+++ b/node_modules/@rnmapbox/maps/ios/RNMBX/RNMBXFabricHelpers.h
@@ -1,9 +1,21 @@
 #import <React/RCTConversions.h>
 #import <folly/dynamic.h>
 #import <react/renderer/components/rnmapbox_maps_specs/Props.h>
+#import <type_traits>
 
 #import "rnmapbox_maps-Swift.pre.h"
 
+// Not every layer's codegen Props struct declares `slot` (Sky, Background and
+// FillExtrusion don't), but RNMBXSetCommonLayerPropsWithoutSourceID is one
+// template instantiated for all of them — an unconditional `newProps.slot`
+// fails to compile for those three. This detects the member at compile time
+// so the same function works for both.
+template <typename T, typename = void>
+struct RNMBXPropsHasSlot : std::false_type {};
+
+template <typename T>
+struct RNMBXPropsHasSlot<T, std::void_t<decltype(std::declval<T>().slot)>> : std::true_type {};
+
 // copied from RCTFollyConvert
 static id RNMBXConvertFollyDynamicToId(const folly::dynamic &dyn)
 {
@@ -105,6 +117,17 @@ void RNMBXSetCommonLayerPropsWithoutSourceID(const T& newProps, RNMBXLayer *_vie
     if (minZoomLevel != nil) {
         _view.minZoomLevel = minZoomLevel;
     }
+    // Upstream gap: `slot` is declared in the codegen spec and read on the old
+    // architecture, but this Fabric prop-application helper never reads it, so
+    // `slot` silently never reaches the layer on the New Architecture.
+    if constexpr (RNMBXPropsHasSlot<T>::value) {
+        id slot = RNMBXConvertFollyDynamicToId(newProps.slot);
+        if (slot != nil) {
+            _view.slot = slot;
+        }
+    }
 }
 
 template <typename T>

Confirmed working on-device (iOS, physical device) after applying this patch and rebuilding: slot="bottom" correctly positions a raster layer below Standard's line/symbol layers, which it did not before the patch regardless of the slot value passed.

[github-actions]

Lint failed 😭

Please fix the errors in your code example - More info.:

error: Parsing error: Unexpected token } at example.jsx:2:74:
  1 | 
> 2 | <RasterLayer id="my-layer" sourceID="my-source" slot="bottom" style={{...}} />
    |                                                                          ^
  3 | 


1 error found.```