Create DefaultBackend which adapts to match the current platform (wil…

…l use for examples once AppKitBackend is ready)

Package.swift

@@ -55,6 +55,19 @@ var swift510Products: [Product] = []
     ]
 #endif
 
+let defaultBackend: String
+if let backend = ProcessInfo.processInfo.environment["SCUI_DEFAULT_BACKEND"] {
+    defaultBackend = backend
+} else {
+    #if os(macOS)
+        defaultBackend = "AppKitBackend"
+    #elseif os(Windows)
+        defaultBackend = "WinUIBackend"
+    #else
+        defaultBackend = "GtkBackend"
+    #endif
+}
+
 let package = Package(
     name: "swift-cross-ui",
     platforms: [.macOS(.v10_15)],
@@ -109,6 +122,12 @@ let package = Package(
             name: "SwiftCrossUITests",
             dependencies: ["SwiftCrossUI"]
         ),
+        .target(
+            name: "DefaultBackend",
+            dependencies: [
+                .target(name: defaultBackend)
+            ]
+        ),
         .target(name: "AppKitBackend", dependencies: ["SwiftCrossUI"]),
         .target(
             name: "QtBackend",

Sources/DefaultBackend/DefaultBackend.swift

@@ -0,0 +1,21 @@
+#if canImport(AppKitBackend)
+    import AppKitBackend
+    public typealias DefaultBackend = AppKitBackend
+#elseif canImport(GtkBackend)
+    import GtkBackend
+    public typealias DefaultBackend = GtkBackend
+#elseif canImport(QtBackend)
+    import QtBackend
+    public typealias DefaultBackend = QtBackend
+#elseif canImport(WinUIBackend)
+    import WinUIBackend
+    public typealias DefaultBackend = WinUIBackend
+#elseif canImport(CursesBackend)
+    import CursesBackend
+    public typealias DefaultBackend = CursesBackend
+#elseif canImport(LVGLBackend)
+    import LVGLBackend
+    public typealias DefaultBackend = LVGLBackend
+#else
+    #error("Unknown backend selected")
+#endif