[camera] Use Pigeon for Windows C++->Dart (flutter#8001)

Replaces direct method channel usage with Pigeon for C++-to-Dart communication. On the Dart side, this re-uses the same structure used in other platform implementations.

Part of http://github.com/flutter/flutter/issues/117905

Author: stuartmorgan-g

packages/camera/camera_windows/CHANGELOG.md

@@ -1,5 +1,6 @@
-## NEXT
+## 0.2.5+1
 
+* Updates C++ to Dart communication to use Pigeon.
 * Updates minimum supported SDK version to Flutter 3.19/Dart 3.3.
 
 ## 0.2.5

packages/camera/camera_windows/lib/camera_windows.dart

@@ -27,8 +27,11 @@ class CameraWindows extends CameraPlatform {
   /// Interface for calling host-side code.
   final CameraApi _hostApi;
 
-  /// Camera specific method channels to allow communicating with specific cameras.
-  final Map<int, MethodChannel> _cameraChannels = <int, MethodChannel>{};
+  /// The per-camera handlers for messages that should be rebroadcast to
+  /// clients as [CameraEvent]s.
+  @visibleForTesting
+  final Map<int, HostCameraMessageHandler> hostCameraHandlers =
+      <int, HostCameraMessageHandler>{};
 
   // The stream to receive frames from the native code.
   StreamSubscription<dynamic>? _platformImageStreamSubscription;
@@ -105,15 +108,8 @@ class CameraWindows extends CameraPlatform {
     int cameraId, {
     ImageFormatGroup imageFormatGroup = ImageFormatGroup.unknown,
   }) async {
-    /// Creates channel for camera events.
-    _cameraChannels.putIfAbsent(cameraId, () {
-      final MethodChannel channel =
-          MethodChannel('plugins.flutter.io/camera_windows/camera$cameraId');
-      channel.setMethodCallHandler(
-        (MethodCall call) => handleCameraMethodCall(call, cameraId),
-      );
-      return channel;
-    });
+    hostCameraHandlers.putIfAbsent(cameraId,
+        () => HostCameraMessageHandler(cameraId, cameraEventStreamController));
 
     final PlatformSize reply;
     try {
@@ -140,11 +136,7 @@ class CameraWindows extends CameraPlatform {
     await _hostApi.dispose(cameraId);
 
     // Destroy method channel after camera is disposed to be able to handle last messages.
-    if (_cameraChannels.containsKey(cameraId)) {
-      final MethodChannel? cameraChannel = _cameraChannels[cameraId];
-      cameraChannel?.setMethodCallHandler(null);
-      _cameraChannels.remove(cameraId);
-    }
+    hostCameraHandlers.remove(cameraId)?.dispose();
   }
 
   @override
@@ -398,33 +390,6 @@ class CameraWindows extends CameraPlatform {
     return Texture(textureId: cameraId);
   }
 
-  /// Converts messages received from the native platform into camera events.
-  ///
-  /// This is only exposed for test purposes. It shouldn't be used by clients
-  /// of the plugin as it may break or change at any time.
-  @visibleForTesting
-  Future<dynamic> handleCameraMethodCall(MethodCall call, int cameraId) async {
-    switch (call.method) {
-      case 'camera_closing':
-        cameraEventStreamController.add(
-          CameraClosingEvent(
-            cameraId,
-          ),
-        );
-      case 'error':
-        final Map<String, Object?> arguments =
-            (call.arguments as Map<Object?, Object?>).cast<String, Object?>();
-        cameraEventStreamController.add(
-          CameraErrorEvent(
-            cameraId,
-            arguments['description']! as String,
-          ),
-        );
-      default:
-        throw UnimplementedError();
-    }
-  }
-
   /// Returns a [MediaSettings]'s Pigeon representation.
   PlatformMediaSettings _pigeonMediaSettings(MediaSettings? settings) {
     return PlatformMediaSettings(
@@ -467,3 +432,35 @@ class CameraWindows extends CameraPlatform {
     return PlatformResolutionPreset.max;
   }
 }
+
+/// Callback handler for camera-level events from the platform host.
+@visibleForTesting
+class HostCameraMessageHandler implements CameraEventApi {
+  /// Creates a new handler that listens for events from camera [cameraId], and
+  /// broadcasts them to [streamController].
+  HostCameraMessageHandler(this.cameraId, this.streamController) {
+    CameraEventApi.setUp(this, messageChannelSuffix: cameraId.toString());
+  }
+
+  /// Removes the handler for native messages.
+  void dispose() {
+    CameraEventApi.setUp(null, messageChannelSuffix: cameraId.toString());
+  }
+
+  /// The camera ID this handler listens for events from.
+  final int cameraId;
+
+  /// The controller used to broadcast camera events coming from the
+  /// host platform.
+  final StreamController<CameraEvent> streamController;
+
+  @override
+  void error(String message) {
+    streamController.add(CameraErrorEvent(cameraId, message));
+  }
+
+  @override
+  void cameraClosing() {
+    streamController.add(CameraClosingEvent(cameraId));
+  }
+}