Support SwiftUI snapshots on macOS with NSImage

Sources/SnapshotTesting/Snapshotting/SwiftUIView.swift

@@ -28,7 +28,7 @@ extension Snapshotting where Value: SwiftUI.View, Format == UIImage {
   /// - Parameters:
   ///   - drawHierarchyInKeyWindow: Utilize the simulator's key window in order to render `UIAppearance` and `UIVisualEffect`s. This option requires a host application for your tests and will _not_ work for framework test targets.
   ///   - precision: The percentage of pixels that must match.
-  ///   - size: A view size override.
+  ///   - layout: A view size override.
   ///   - traits: A trait collection override.
   public static func image(
     drawHierarchyInKeyWindow: Bool = false,
@@ -80,4 +80,59 @@ extension Snapshotting where Value: SwiftUI.View, Format == UIImage {
   }
 }
 #endif
+
+#if os(macOS)
+@available(macOS 11.0, *)
+extension Snapshotting where Value: SwiftUI.View, Format == NSImage {
+
+  /// A snapshot strategy for comparing SwiftUI Views based on pixel equality.
+  public static var image: Snapshotting {
+    return .image()
+  }
+
+  /// A snapshot strategy for comparing SwiftUI Views based on pixel equality.
+  ///
+  /// - Parameters:
+  ///   - precision: The percentage of pixels that must match.
+  ///   - layout: A view size override.
+  ///   - appearance: A light/dark mode override.
+  public static func image(
+    precision: Float = 1,
+    layout: SwiftUISnapshotLayout = .sizeThatFits,
+    appearance: NSAppearance? = NSAppearance(named: .aqua)
+  ) -> Snapshotting {
+    let size: CGSize?
+
+    switch layout {
+    case .sizeThatFits:
+      size = nil
+    case let .fixed(width: width, height: height):
+      size = .init(width: width, height: height)
+    }
+    return SimplySnapshotting.image(precision: precision).asyncPullback { swiftUIView in
+        let controller = NSHostingController(rootView: swiftUIView)
+        let view = controller.view
+        view.appearance = appearance
+
+        let initialSize = view.frame.size
+        view.frame.size = size ?? controller.sizeThatFits(in: .zero)
+        guard view.frame.width > 0, view.frame.height > 0 else {
+          fatalError("View not renderable to image at size \(view.frame.size)")
+        }
+
+        return view.snapshot ?? Async { callback in
+        addImagesForRenderedViews(view).sequence().run { views in
+          let bitmapRep = view.bitmapImageRepForCachingDisplay(in: view.bounds)!
+          view.cacheDisplay(in: view.bounds, to: bitmapRep)
+          let image = NSImage(size: view.bounds.size)
+          image.addRepresentation(bitmapRep)
+          callback(image)
+          views.forEach { $0.removeFromSuperview() }
+          view.frame.size = initialSize
+        }
+      }
+    }
+  }
+}
+#endif
 #endif

Tests/SnapshotTestingTests/SnapshotTestingTests.swift

@@ -1071,6 +1071,27 @@ final class SnapshotTestingTests: XCTestCase {
     #endif
   }
 
+  @available(macOS 11.0, *)
+  func testSwiftUIView_macOS() {
+    #if os(macOS)
+    struct MyView: SwiftUI.View {
+      var body: some SwiftUI.View {
+        HStack {
+          Image(systemName: "checkmark.circle.fill")
+            Text("Checked").fixedSize()
+        }
+        .padding(5)
+        .background(RoundedRectangle(cornerRadius: 5.0).fill(Color.blue))
+        .padding(10)
+      }
+    }
+
+    let view = MyView().background(Color.yellow)
+
+    assertSnapshot(matching: view, as: .image)
+    #endif
+  }
+
   @available(*, deprecated)
   func testIsRecordingProxy() {
     SnapshotTesting.record = true