void addJSONMessageHandler(String channel, Future callback(json))

Register a callback for receiving JSON messages from the host application.

Messages received from the host application are decoded as JSON before being passed to callback. The result of the callback is encoded as JSON before being returned to the host application.

Source 

static void addJSONMessageHandler(String channel, Future<dynamic> callback(dynamic json)) {
  PlatformMessages.setJSONMessageHandler(channel, callback);
}