macos: MetalView, render an MTKView

macos/Sources/Helpers/MetalView.swift

@@ -0,0 +1,33 @@
+import SwiftUI
+import MetalKit
+
+/// Implements the logic for a metal view used by MetalView.
+protocol MetalViewRenderer: MTKViewDelegate {
+    init(metalView: MTKView)
+}
+
+/// Renders an MTKView with the given renderer class.
+struct MetalView<R: MetalViewRenderer>: View {
+    @State private var metalView = MTKView()
+    @State private var renderer: R?
+
+    var body: some View {
+        MetalViewRepresentable(metalView: $metalView)
+          .onAppear { renderer = R(metalView: metalView) }
+    }
+}
+
+fileprivate struct MetalViewRepresentable: NSViewRepresentable {
+    @Binding var metalView: MTKView
+
+    func makeNSView(context: Context) -> some NSView {
+        metalView
+    }
+    
+    func updateNSView(_ view: NSViewType, context: Context) {
+        updateMetalView()
+    }
+
+    func updateMetalView() {
+    }
+}