Support defining a custom style layer from iOS Swift (#3154)

platform/darwin/app/CustomStyleLayerExample.m

@@ -72,7 +72,7 @@
 }
 
 - (void)didMoveToMapView:(MLNMapView *)mapView {
-    MLNBackendResource resource = [mapView backendResource];
+    MLNBackendResource* resource = [mapView backendResource];
     
     NSString *shaderSource = @
 "    #include <metal_stdlib>\n"
@@ -140,7 +140,7 @@
     id<MTLRenderCommandEncoder> renderEncoder = self.renderEncoder;
     if(renderEncoder != nil)
     {
-        MLNBackendResource resource = [mapView backendResource];
+        MLNBackendResource* resource = [mapView backendResource];
         
         vector_uint2 _viewportSize;
         _viewportSize.x = resource.mtkView.drawableSize.width;

platform/darwin/bazel/files.bzl

@@ -161,6 +161,7 @@ MLN_DARWIN_PRIVATE_HEADERS = [
 
 MLN_DARWIN_PUBLIC_OBJCPP_SOURCE = [
     "src/MLNAttributionInfo.mm",
+    "src/MLNBackendResource.mm",
     "src/MLNComputedShapeSource.mm",
     "src/MLNCustomStyleLayer.mm",
     "src/MLNDefaultStyle.mm",

platform/darwin/src/MLNBackendResource.h

@@ -2,16 +2,28 @@
 
 #import <MetalKit/MetalKit.h>
 
-typedef struct {
-    MTKView *mtkView;
-    id<MTLDevice> device;
-    MTLRenderPassDescriptor *renderPassDescriptor;
-    id<MTLCommandBuffer> commandBuffer;
-} MLNBackendResource;
+@interface MLNBackendResource : NSObject
+
+@property (nonatomic, strong) MTKView *mtkView;
+@property (nonatomic, strong) id<MTLDevice> device;
+@property (nonatomic, strong) MTLRenderPassDescriptor *renderPassDescriptor;
+@property (nonatomic, strong) id<MTLCommandBuffer> commandBuffer;
+
+- (instancetype)initWithMTKView:(MTKView *)mtkView
+                         device:(id<MTLDevice>)device
+           renderPassDescriptor:(MTLRenderPassDescriptor *)renderPassDescriptor
+                  commandBuffer:(id<MTLCommandBuffer>)commandBuffer;
+
+@end
 
 #else
 
-typedef struct {
-} MLNBackendResource;
+#import <Foundation/Foundation.h>
+#import "MLNFoundation.h"
+
+MLN_EXPORT
+@interface MLNBackendResource : NSObject
+
+@end
 
 #endif

platform/darwin/src/MLNBackendResource.mm

@@ -0,0 +1,36 @@
+#if MLN_RENDER_BACKEND_METAL
+
+#import "MLNBackendResource.h"
+
+@implementation MLNBackendResource
+
+- (instancetype)initWithMTKView:(MTKView *)mtkView
+                         device:(id<MTLDevice>)device
+           renderPassDescriptor:(MTLRenderPassDescriptor *)renderPassDescriptor
+                  commandBuffer:(id<MTLCommandBuffer>)commandBuffer {
+    self = [super init];
+    if (self) {
+        _mtkView = mtkView;
+        _device = device;
+        _renderPassDescriptor = renderPassDescriptor;
+        _commandBuffer = commandBuffer;
+    }
+    return self;
+}
+
+@end
+
+#else
+
+#import "MLNBackendResource.h"
+
+@implementation MLNBackendResource
+
+- (instancetype)init {
+    self = [super init];
+    return self;
+}
+
+@end
+
+#endif