Let windowing.dart compile for the web

flutter_window.dart imports dart:ffi and the platform window
controllers, so importing package:nativeapi_flutter/windowing.dart
still broke a web build after the nativeapi web mirror. windowing.dart
now exports a web version if (dart.library.js_interop) whose
nativeWindowOf returns null, as documented for a controller with no
window handle. The contentInset extension is plain Dart and moves to
its own file, shared by both.

Author: lijy91

bindings/dart/nativeapi_flutter/CHANGELOG.md

@@ -1,5 +1,9 @@
 ## 0.4.1
 
+* `package:nativeapi_flutter/windowing.dart` compiles for the web too
+  (leanflutter/tray_manager#108): there `nativeWindowOf` and `nativeWindow`
+  return null, since no native window stands behind a controller.
+
 * `DragToMoveArea` follows the macOS title-bar double-click preference,
   including Minimize and None, through the native window action.
 

bindings/dart/nativeapi_flutter/lib/src/windowing/flutter_window.dart

@@ -1,7 +1,6 @@
 // ignore_for_file: invalid_use_of_internal_member, implementation_imports
 
 import 'dart:ffi' as ffi;
-import 'dart:ui' show Offset;
 
 import 'package:flutter/src/widgets/_window.dart' as fw;
 import 'package:flutter/src/widgets/_window_linux.dart' as fw_linux;
@@ -10,8 +9,6 @@ import 'package:flutter/src/widgets/_window_win32.dart' as fw_win32;
 
 import 'package:nativeapi/nativeapi.dart' show Window;
 
-import '../conversions.dart';
-
 /// The native window behind a Flutter window controller, as a nativeapi
 /// [Window].
 ///
@@ -44,14 +41,3 @@ extension FlutterWindowControllerNativeWindow on fw.BaseWindowController {
   /// Shorthand for [nativeWindowOf].
   Window? get nativeWindow => nativeWindowOf(this);
 }
-
-extension NativeWindowGeometry on Window {
-  /// Offset from the window's outer frame to its content area (title bar and
-  /// left border), in logical pixels.
-  ///
-  /// `WindowDragSession` anchors on the frame while Flutter reports positions
-  /// inside the content, so an anchor derived from a pointer position needs
-  /// this added.
-  Offset get contentInset =>
-      contentBounds.toRect().topLeft - bounds.toRect().topLeft;
-}

bindings/dart/nativeapi_flutter/lib/src/windowing/flutter_window_web.dart

@@ -0,0 +1,17 @@
+// ignore_for_file: invalid_use_of_internal_member, implementation_imports
+
+import 'package:flutter/src/widgets/_window.dart' as fw;
+
+import 'package:nativeapi/nativeapi.dart' show Window;
+
+/// The native window behind a Flutter window controller, as a nativeapi
+/// [Window].
+///
+/// Always null on the web: there is no native window behind a controller, and
+/// the platform implementations (and `dart:ffi`) do not exist there.
+Window? nativeWindowOf(fw.BaseWindowController controller) => null;
+
+extension FlutterWindowControllerNativeWindow on fw.BaseWindowController {
+  /// Shorthand for [nativeWindowOf].
+  Window? get nativeWindow => nativeWindowOf(this);
+}

bindings/dart/nativeapi_flutter/lib/src/windowing/window_geometry.dart

@@ -0,0 +1,16 @@
+import 'dart:ui' show Offset;
+
+import 'package:nativeapi/nativeapi.dart' show Window;
+
+import '../conversions.dart';
+
+extension NativeWindowGeometry on Window {
+  /// Offset from the window's outer frame to its content area (title bar and
+  /// left border), in logical pixels.
+  ///
+  /// `WindowDragSession` anchors on the frame while Flutter reports positions
+  /// inside the content, so an anchor derived from a pointer position needs
+  /// this added.
+  Offset get contentInset =>
+      contentBounds.toRect().topLeft - bounds.toRect().topLeft;
+}

bindings/dart/nativeapi_flutter/lib/windowing.dart

@@ -8,6 +8,11 @@
 /// The stable channel does not offer `flutter config --enable-windowing`: an app
 /// sets `isWindowingEnabled = true` (from
 /// `package:flutter/src/foundation/_features.dart`) before its binding starts.
+///
+/// On the web, where there are no native windows, [nativeWindowOf] returns
+/// null, so an app that also targets the web can still import this library.
 library;
 
-export 'src/windowing/flutter_window.dart';
+export 'src/windowing/flutter_window.dart'
+    if (dart.library.js_interop) 'src/windowing/flutter_window_web.dart';
+export 'src/windowing/window_geometry.dart';