Changes to the custom Metal layer to allow more flexible rendering (#4446)

Co-authored-by: pre-commit-ci[bot] <66853113+pre-commit-ci[bot]@users.noreply.github.com>
Co-authored-by: copilot-swe-agent[bot] <198982749+Copilot@users.noreply.github.com>
Co-authored-by: louwers <649392+louwers@users.noreply.github.com>
Co-authored-by: Bart Louwers <bart@emeel.net>

platform/darwin/app/CustomStyleLayerExample.m

@@ -135,9 +135,18 @@
 
 }
 
+- (void)preDrawInMapView:(MLNMapView *)mapView withContext:(MLNStyleLayerDrawingContext)context {
+    id<MTLCommandBuffer> commandBuffer = self.commandBuffer;
+
+    id<MTLBlitCommandEncoder> blitEncoder = [commandBuffer blitCommandEncoder];
+    [blitEncoder endEncoding];
+}
+
 - (void)drawInMapView:(MLNMapView *)mapView withContext:(MLNStyleLayerDrawingContext)context {
     // Use the supplied render command encoder to encode commands
+    id<MTLCommandBuffer> commandBuffer = self.commandBuffer;
     id<MTLRenderCommandEncoder> renderEncoder = self.renderEncoder;
+    MTLRenderPassDescriptor *renderPassDesc = self.renderPassDesc;
     if(renderEncoder != nil)
     {
         MLNBackendResource* resource = [mapView backendResource];

platform/darwin/src/MLNCustomStyleLayer.h

@@ -83,6 +83,18 @@ MLN_EXPORT
 #pragma clang diagnostic pop
 
 #if MLN_RENDER_BACKEND_METAL
+/// The Metal command buffer is used to generate render and other encoders..
+///
+/// This property is only valid when using the Metal-based rendering backend.
+/// If the OpenGL backend is in use, this property will be `nil`.
+@property (nonatomic, weak) id<MTLCommandBuffer> commandBuffer;
+
+/// The Metal render pass descriptor Metal draw commands.
+///
+/// This property is only valid when using the Metal-based rendering backend.
+/// If the OpenGL backend is in use, this property will be `nil`.
+@property (nonatomic, weak) MTLRenderPassDescriptor *renderPassDesc;
+
 /// The Metal render command encoder used for issuing Metal draw commands.
 ///
 /// This property is only valid when using the Metal-based rendering backend.
@@ -124,6 +136,19 @@ MLN_EXPORT
 /// - Parameter mapView: The map view from whose style the layer is about to be removed.
 - (void)willMoveFromMapView:(MLNMapView *)mapView;
 
+/// Called right before  the layer needs to draw a new frame in a map view.
+///
+/// Override this method in a subclass to do any pre-drawing of the layer’s content. The default
+/// implementation of this method does nothing.
+///
+/// This call exists to do things with the rendering context before MapLibre Native adds its
+/// own rendering commands.
+///
+/// - Parameters:
+///   - mapView: The map view to which the layer draws.
+///   - context: A context structure with information defining the frame to draw.
+- (void)preDrawInMapView:(MLNMapView *)mapView withContext:(MLNStyleLayerDrawingContext)context;
+
 /// Called each time the layer needs to draw a new frame in a map view.
 ///
 /// Override this method in a subclass to draw the layer’s content. The default

platform/darwin/src/MLNCustomStyleLayer.mm

@@ -10,6 +10,7 @@
 #import <MetalKit/MetalKit.h>
 #endif
 
+#include <mbgl/gfx/context.hpp>
 #include <mbgl/math/wrap.hpp>
 #include <mbgl/style/layers/custom_layer.hpp>
 
@@ -72,6 +73,9 @@ class MLNCustomLayerHost;
 - (void)willMoveFromMapView:(MLNMapView *)mapView {
 }
 
+- (void)preDrawInMapView:(MLNMapView *)mapView withContext:(MLNStyleLayerDrawingContext)context {
+}
+
 - (void)drawInMapView:(MLNMapView *)mapView withContext:(MLNStyleLayerDrawingContext)context {
 }
 
@@ -99,10 +103,47 @@ public:
     }
   }
 
+  void preRender(const mbgl::gfx::Context &context,
+                 const mbgl::style::CustomLayerRenderParameters &parameters) override {
+    if (!layer) return;
+
+#if MLN_RENDER_BACKEND_METAL
+    auto renderPassDesc =
+        static_cast<const mbgl::style::mtl::CustomLayerRenderParameters &>(parameters)
+            .renderPassDesc.get();
+    MTL::CommandBuffer *cmdPtr =
+        static_cast<const mbgl::style::mtl::CustomLayerRenderParameters &>(parameters)
+            .commandBuffer.get();
+    id<MTLCommandBuffer> commandBuffer = (__bridge id<MTLCommandBuffer>)cmdPtr;
+    layer.commandBuffer = commandBuffer;
+    layer.renderPassDesc = (__bridge MTLRenderPassDescriptor *)renderPassDesc;
+
+#endif
+
+    MLNStyleLayerDrawingContext drawingContext = {
+        .size = CGSizeMake(parameters.width, parameters.height),
+        .centerCoordinate = CLLocationCoordinate2DMake(parameters.latitude, parameters.longitude),
+        .zoomLevel = parameters.zoom,
+        .direction = mbgl::util::wrap(parameters.bearing, 0., 360.),
+        .pitch = static_cast<CGFloat>(parameters.pitch),
+        .fieldOfView = static_cast<CGFloat>(parameters.fieldOfView),
+        .projectionMatrix = MLNMatrix4Make(parameters.projectionMatrix),
+        .nearClippedProjectionMatrix = MLNMatrix4Make(parameters.nearClippedProjectionMatrix)};
+
+    if (layer.mapView) {
+      [layer preDrawInMapView:layer.mapView withContext:drawingContext];
+    }
+  }
+
   void render(const mbgl::style::CustomLayerRenderParameters &parameters) override {
     if (!layer) return;
 
 #if MLN_RENDER_BACKEND_METAL
+    MTL::CommandBuffer *cmdPtr =
+        static_cast<const mbgl::style::mtl::CustomLayerRenderParameters &>(parameters)
+            .commandBuffer.get();
+    id<MTLCommandBuffer> commandBuffer = (__bridge id<MTLCommandBuffer>)cmdPtr;
+    layer.commandBuffer = commandBuffer;
     MTL::RenderCommandEncoder *ptr =
         static_cast<const mbgl::style::mtl::CustomLayerRenderParameters &>(parameters)
             .encoder.get();