void setStringMessageHandler(String channel, Future<String> handler(String message))

Set a callback for receiving string messages from the platform.

The given callback will replace the currently registered callback (if any).

Source 

static void setStringMessageHandler(String channel, Future<String> handler(String message)) {
  setBinaryMessageHandler(channel, (ByteData message) async {
    return _encodeUTF8(await handler(_decodeUTF8(message)));
  });
}