Add NativeView.rawView (#4)

Sources/Shaft/Backend/SDLMetalView.swift

@@ -62,6 +62,10 @@
                 drawable.present()
             }
         }
+
+        public override var rawView: UnsafeMutableRawPointer? {
+            sdlMetalView
+        }
     }
 
 #endif

Sources/Shaft/Backend/SDLView.swift

@@ -175,6 +175,10 @@ public class SDLView: NativeView {
             SDL_SetWindowTitle(sdlWindow, newValue)
         }
     }
+
+    public var rawView: UnsafeMutableRawPointer? {
+        nil
+    }
 }
 
 // typedef bool (SDLCALL * SDL_EventFilter) (void *userdata, SDL_Event * event);

Sources/Shaft/Core/Backend.swift

@@ -63,6 +63,19 @@ public protocol NativeView: AnyObject {
 
     /// Getting/Setting the title of the view if possible.
     var title: String { get set }
+
+    /// A raw pointer to the underlying view object that this NativeView wraps.
+    ///
+    /// - On MacOS, this is an pointer to a `NSView`.
+    /// - On iOS/tvOS, this is a pointer to a `UIView`.
+    ///
+    /// This is an optional property, and it is not guaranteed to be implemented
+    /// on all platforms.
+    var rawView: UnsafeMutableRawPointer? { get }
+}
+
+extension NativeView {
+    public var rawView: UnsafeMutableRawPointer? { nil }
 }
 
 #if canImport(AppKit)

Sources/Shaft/Widgets/Binding/WidgetsBinding.swift

@@ -167,14 +167,18 @@ public class WidgetsBinding {
 /// against the previous widget tree and any differences are applied to the
 /// underlying render tree, similar to what happens when a [StatefulWidget]
 /// rebuilds after calling [State.setState].
-public func runApp(_ app: Widget) {
+///
+/// An additional optional parameter `view` can be passed to specify the
+/// `NativeView` to use for rendering the app. If not provided, a new
+/// `NativeView` will be created.
+public func runApp(_ app: Widget, view: NativeView? = nil) {
     runPlainApp(
         DefaultApp { app }
     )
 }
 
-public func runPlainApp(_ app: Widget) {
-    guard let view = backend.createView() else {
+public func runPlainApp(_ app: Widget, view: NativeView? = nil) {
+    guard let view = view ?? backend.createView() else {
         fatalError("Failed to create view")
     }