TestMetalContext: Use ARC-managed Metal types (flutter/engine#56717)

Previously, we could not include any Objective-C types in test_metal_context.h, since that file was transitively included in pure C++ translation units. All users have been refactored into backend-specific files, and all Metal-related files are Objective-C++ files.

We now use Metal types directly in the header, without the workarounds.

Issue: flutter#158998
Issue: flutter#137801

[C++, Objective-C, Java style guides]: https://github.com/flutter/engine/blob/main/CONTRIBUTING.md#style

engine/src/flutter/shell/platform/embedder/tests/embedder_test_context_metal.mm

@@ -19,8 +19,9 @@
   renderer_config_.type = FlutterRendererType::kMetal;
   renderer_config_.metal = {
       .struct_size = sizeof(FlutterMetalRendererConfig),
-      .device = metal_context_->GetMetalDevice(),
-      .present_command_queue = metal_context_->GetMetalCommandQueue(),
+      .device = (__bridge FlutterMetalDeviceHandle)metal_context_->GetMetalDevice(),
+      .present_command_queue =
+          (__bridge FlutterMetalCommandQueueHandle)metal_context_->GetMetalCommandQueue(),
       .get_next_drawable_callback =
           [](void* user_data, const FlutterFrameInfo* frame_info) {
             return reinterpret_cast<EmbedderTestContextMetal*>(user_data)->GetNextDrawable(

engine/src/flutter/testing/test_metal_context.h

@@ -9,6 +9,8 @@
 #include <memory>
 #include <mutex>
 
+#include <Metal/Metal.h>
+
 #include "third_party/skia/include/gpu/ganesh/GrDirectContext.h"
 #include "third_party/skia/include/ports/SkCFObject.h"
 
@@ -27,9 +29,9 @@ class TestMetalContext {
 
   ~TestMetalContext();
 
-  void* GetMetalDevice() const;
+  id<MTLDevice> GetMetalDevice() const;
 
-  void* GetMetalCommandQueue() const;
+  id<MTLCommandQueue> GetMetalCommandQueue() const;
 
   sk_sp<GrDirectContext> GetSkiaContext() const;
 
@@ -41,7 +43,8 @@ class TestMetalContext {
   TextureInfo GetTextureInfo(int64_t texture_id);
 
  private:
-  std::unique_ptr<MetalObjCFields> metal_;
+  id<MTLDevice> device_;
+  id<MTLCommandQueue> command_queue_;
   sk_sp<GrDirectContext> skia_context_;
   std::mutex textures_mutex_;
   int64_t texture_id_ctr_ = 1;                 // guarded by textures_mutex

engine/src/flutter/testing/test_metal_context.mm

@@ -17,12 +17,6 @@
 
 namespace flutter::testing {
 
-// TOOD(cbracken): https://github.com/flutter/flutter/issues/157942
-struct MetalObjCFields {
-  id<MTLDevice> device;
-  id<MTLCommandQueue> command_queue;
-};
-
 TestMetalContext::TestMetalContext() {
   id<MTLDevice> device = MTLCreateSystemDefaultDevice();
   if (!device) {
@@ -48,22 +42,21 @@
     FML_LOG(ERROR) << "Could not create the GrDirectContext from the Metal Device "
                       "and command queue.";
   }
-
-  metal_ = std::make_unique<MetalObjCFields>(MetalObjCFields{device, command_queue});
+  device_ = device;
+  command_queue_ = command_queue;
 }
 
 TestMetalContext::~TestMetalContext() {
   std::scoped_lock lock(textures_mutex_);
   textures_.clear();
-  metal_.reset();
 }
 
-void* TestMetalContext::GetMetalDevice() const {
-  return metal_ ? (__bridge void*)metal_->device : nil;
+id<MTLDevice> TestMetalContext::GetMetalDevice() const {
+  return device_;
 }
 
-void* TestMetalContext::GetMetalCommandQueue() const {
-  return metal_ ? (__bridge void*)metal_->command_queue : nil;
+id<MTLCommandQueue> TestMetalContext::GetMetalCommandQueue() const {
+  return command_queue_;
 }
 
 sk_sp<GrDirectContext> TestMetalContext::GetSkiaContext() const {
@@ -88,12 +81,12 @@
     return {.texture_id = -1, .texture = nullptr};
   }
 
-  if (!metal_) {
+  if (!device_) {
     FML_CHECK(false) << "Invalid Metal device.";
     return {.texture_id = -1, .texture = nullptr};
   }
 
-  id<MTLTexture> texture = [metal_->device newTextureWithDescriptor:texture_descriptor];
+  id<MTLTexture> texture = [device_ newTextureWithDescriptor:texture_descriptor];
   if (!texture) {
     FML_CHECK(false) << "Could not create texture from texture descriptor.";
     return {.texture_id = -1, .texture = nullptr};