Add SwiftPM based example (#24)

Author: Kyle-Ye

.gitignore

@@ -5,6 +5,9 @@ xcuserdata/
 DerivedData/
 .swiftpm/configuration/registries.json
 .swiftpm/xcode/package.xcworkspace/contents.xcworkspacedata
+Example/.swiftpm/xcode/package.xcworkspace/contents.xcworkspacedata
 .netrc
 /.build-*
-/Benchmarks/.build
+/Benchmarks/.build
+/Example/.build
+Example/Package.resolved

Example/Package.swift

@@ -0,0 +1,64 @@
+// swift-tools-version: 5.9
+// The swift-tools-version declares the minimum version of Swift required to build this package.
+
+import Foundation
+import PackageDescription
+
+let exampleTarget = Target.executableTarget(
+    name: "Example",
+    dependencies: [
+        .product(name: "OpenSwiftUI", package: "OpenSwiftUI"),
+    ],
+    path: "Example",
+    sources: ["ExampleApp.swift", "ContentView.swift"]
+)
+
+let package = Package(
+    name: "Example",
+    platforms: [
+        .macOS(.v10_15),
+    ],
+    products: [.executable(name: "Example", targets: ["Example"])],
+    dependencies: [
+        .package(path: "../"),
+        .package(url: "https://github.com/OpenSwiftUIProject/OpenGraph", branch: "main"),
+    ],
+    targets: [
+        exampleTarget,
+    ]
+)
+
+func envEnable(_ key: String, default defaultValue: Bool = false) -> Bool {
+    guard let value = ProcessInfo.processInfo.environment[key] else {
+        return defaultValue
+    }
+    if value == "1" {
+        return true
+    } else if value == "0" {
+        return false
+    } else {
+        return defaultValue
+    }
+}
+
+#if os(macOS)
+let attributeGraphCondition = envEnable("OPENGRAPH_ATTRIBUTEGRAPH", default: true)
+#else
+let attributeGraphCondition = envEnable("OPENGRAPH_ATTRIBUTEGRAPH")
+#endif
+
+extension Target {
+    func addAGSettings() {
+        // FIXME: Weird SwiftPM behavior for binary Target. Otherwize we'll get the following error message
+        // "could not determine executable path for bundle 'AttributeGraph.framework'"
+        dependencies.append(.product(name: "AttributeGraph", package: "OpenGraph"))
+
+        var swiftSettings = swiftSettings ?? []
+        swiftSettings.append(.define("OPENGRAPH_ATTRIBUTEGRAPH"))
+        self.swiftSettings = swiftSettings
+    }
+}
+
+if attributeGraphCondition {
+    exampleTarget.addAGSettings()
+}

Sources/OpenSwiftUI/AppStructure/Scenes/TODO/WindowGroup.swift

@@ -6,7 +6,6 @@
 //  Lastest Version: iOS 15.5
 //  Status: WIP
 
-#if canImport(Darwin)
 @frozen
 public struct LocalizedStringKey {}
 
@@ -58,4 +57,3 @@ public struct WindowGroup<Content>: Scene where Content: View {
 
     var id: String?
 }
-#endif