Metal backend resource for MacOS fixes (#4020)

Co-authored-by: pre-commit-ci[bot] <66853113+pre-commit-ci[bot]@users.noreply.github.com>

platform/macos/src/MLNMapView+Impl.h

@@ -2,6 +2,8 @@
 #import <mbgl/map/map_observer.hpp>
 #import <mbgl/util/image.hpp>
 
+#import "MLNBackendResource.h"
+
 @class MLNMapView;
 
 typedef struct _CGLContextObject* CGLContextObj;
@@ -20,6 +22,11 @@ class MLNMapViewImpl : public mbgl::MapObserver {
 
   virtual CGLContextObj getCGLContextObj() { return nullptr; }
 
+#if MLN_RENDER_BACKEND_METAL
+  // Returns the backend resource for Metal rendering in custom layers
+  virtual MLNBackendResource* getObject() { return nullptr; }
+#endif
+
   // Called by the view delegate when it's time to render.
   void render();
 

platform/macos/src/MLNMapView+Metal.h

@@ -36,6 +36,7 @@ class MLNMapViewMetalImpl final : public MLNMapViewImpl,
   mbgl::gfx::RendererBackend& getRendererBackend() override { return *this; }
 
   mbgl::PremultipliedImage readStillImage() override;
+  MLNBackendResource* getObject() override;
 
  private:
   bool presentsWithTransaction = false;

platform/macos/src/MLNMapView+Metal.mm

@@ -44,8 +44,10 @@ public:
             commandQueue = [mtlView.device newCommandQueue];
         }
 
-        commandBuffer = [commandQueue commandBuffer];
-        commandBufferPtr = NS::RetainPtr((__bridge MTL::CommandBuffer*)commandBuffer);
+        if (!commandBuffer) {
+            commandBuffer = [commandQueue commandBuffer];
+            commandBufferPtr = NS::RetainPtr((__bridge MTL::CommandBuffer*)commandBuffer);
+        }
     }
 
     const mbgl::mtl::RendererBackend& getBackend() const override { return backend; }
@@ -158,3 +160,12 @@ mbgl::PremultipliedImage MLNMapViewMetalImpl::readStillImage() {
     // return readFramebuffer(mapView.framebufferSize); // TODO: RendererBackend::readFramebuffer
     return {};
 }
+
+MLNBackendResource* MLNMapViewMetalImpl::getObject() {
+    auto& resource = getResource<MLNMapViewMetalRenderableResource>();
+
+    return [[MLNBackendResource alloc] initWithMTKView:resource.mtlView
+                                                device:resource.mtlView.device
+                                  renderPassDescriptor:resource.mtlView.currentRenderPassDescriptor
+                                         commandBuffer:resource.commandBuffer];
+}

platform/macos/src/MLNMapView.h

@@ -2,6 +2,7 @@
 #import <CoreLocation/CoreLocation.h>
 #import <QuartzCore/QuartzCore.h>
 
+#import "MLNBackendResource.h"
 #import "MLNFoundation.h"
 #import "MLNGeometry.h"
 #import "MLNMapOptions.h"
@@ -1336,6 +1337,16 @@ around the returned camera object if it were set as the receiver’s camera.
  */
 - (void)clearActionJournalLog;
 
+#if MLN_RENDER_BACKEND_METAL
+/**
+ Returns the Metal backend resource for use in custom style layers.
+
+ This property provides access to the Metal device, command buffer, and other
+ resources needed for custom Metal rendering in MLNCustomStyleLayer subclasses.
+ */
+- (MLNBackendResource *)backendResource;
+#endif
+
 @end
 
 NS_ASSUME_NONNULL_END

platform/macos/src/MLNMapView.mm

@@ -48,6 +48,7 @@
 #import <unordered_set>
 
 #import "MLNMapView+Impl.h"
+#import "MLNBackendResource.h"
 #import "NSBundle+MLNAdditions.h"
 #import "NSDate+MLNAdditions.h"
 #import "NSProcessInfo+MLNAdditions.h"
@@ -3314,4 +3315,10 @@ public:
     actionJournal->clearLog();
 }
 
+#if MLN_RENDER_BACKEND_METAL
+- (MLNBackendResource *)backendResource {
+    return _mbglView->getObject();
+}
+#endif
+
 @end