[macos] Add reply to binary messenger (flutter/engine#9953)

* Add reply to binary messenger

* Address comments

* Formatting

* Address comments

engine/src/flutter/shell/platform/darwin/common/framework/Headers/FlutterBinaryMessenger.h

@@ -61,10 +61,7 @@ FLUTTER_EXPORT
  */
 - (void)sendOnChannel:(NSString*)channel
               message:(NSData* _Nullable)message
-          binaryReply:(FlutterBinaryReply _Nullable)callback
-    // TODO: Add macOS support for replies once
-    // https://github.com/flutter/flutter/issues/18852 is fixed.
-    API_UNAVAILABLE(macos);
+          binaryReply:(FlutterBinaryReply _Nullable)callback;
 
 /**
  * Registers a message handler for incoming binary messages from the Flutter side

engine/src/flutter/shell/platform/darwin/common/framework/Headers/FlutterChannels.h

@@ -103,11 +103,7 @@ FLUTTER_EXPORT
  * @param message The message. Must be supported by the codec of this channel.
  * @param callback A callback to be invoked with the message reply from Flutter.
  */
-- (void)sendMessage:(id _Nullable)message
-              reply:(FlutterReply _Nullable)callback
-    // TODO: Add macOS support for replies once
-    // https://github.com/flutter/flutter/issues/18852 is fixed.
-    API_UNAVAILABLE(macos);
+- (void)sendMessage:(id _Nullable)message reply:(FlutterReply _Nullable)callback;
 
 /**
  * Registers a message handler with this channel.

engine/src/flutter/shell/platform/darwin/common/framework/Source/FlutterCodecs.mm

@@ -73,7 +73,7 @@
 }
 
 - (id)decode:(NSData*)message {
-  if (message == nil)
+  if (message.length == 0)
     return nil;
   BOOL isSimpleValue = NO;
   id decoded = nil;

engine/src/flutter/shell/platform/darwin/macos/framework/Source/FLEEngine.mm

@@ -345,6 +345,49 @@ static void OnPlatformMessage(const FlutterPlatformMessage* message, FLEEngine*
   }
 }
 
+- (void)sendOnChannel:(NSString*)channel
+              message:(NSData* _Nullable)message
+          binaryReply:(FlutterBinaryReply _Nullable)callback {
+  struct Captures {
+    FlutterBinaryReply reply;
+  };
+  auto captures = std::make_unique<Captures>();
+  captures->reply = callback;
+  auto message_reply = [](const uint8_t* data, size_t data_size, void* user_data) {
+    auto captures = reinterpret_cast<Captures*>(user_data);
+    NSData* reply_data = [NSData dataWithBytes:(void*)data length:data_size];
+    captures->reply(reply_data);
+    delete captures;
+  };
+
+  FlutterPlatformMessageResponseHandle* response_handle = nullptr;
+  FlutterEngineResult result = FlutterPlatformMessageCreateResponseHandle(
+      _engine, message_reply, captures.get(), &response_handle);
+  if (result != kSuccess) {
+    NSLog(@"Failed to create a FlutterPlatformMessageResponseHandle");
+    return;
+  }
+  captures.release();
+
+  FlutterPlatformMessage platformMessage = {
+      .struct_size = sizeof(FlutterPlatformMessage),
+      .channel = [channel UTF8String],
+      .message = static_cast<const uint8_t*>(message.bytes),
+      .message_size = message.length,
+      .response_handle = response_handle,
+  };
+
+  result = FlutterEngineSendPlatformMessage(_engine, &platformMessage);
+  if (result != kSuccess) {
+    NSLog(@"Failed to send message to Flutter engine on channel '%@' (%d).", channel, result);
+  }
+
+  result = FlutterPlatformMessageReleaseResponseHandle(_engine, response_handle);
+  if (result != kSuccess) {
+    NSLog(@"Failed to release the response handle");
+  };
+}
+
 - (void)setMessageHandlerOnChannel:(nonnull NSString*)channel
               binaryMessageHandler:(nullable FlutterBinaryMessageHandler)handler {
   _messageHandlers[channel] = [handler copy];