Question

我看到的每个示例都与下面的代码几乎相同。出于某种原因，这对我不起作用。问题是当我尝试分配 final MqttPublishMessage message 时，它说 c[0].payload 的类型为“MqttMessage”，不能分配给“MqttPublishMessage”类型的变量。

MqttServerClient client = new MqttServerClient(serverUri, clientId);

client.updates.listen((List<MqttReceivedMessage<MqttMessage>> c){
    final MqttPublishMessage message = c[0].payload; // Cannot assign type
    final payload = MqttPublishPayload.bytesToStringAsString(message.payload.message);

    print('Received message:$payload from topic: ${c[0].topic}>');
});

我试过铸造也不起作用。我已经完全复制了代码，但似乎没有任何效果。这是怎么回事？

Answer 1

这是提供的完整示例here

client.updates!.listen((List<MqttReceivedMessage<MqttMessage?>>? c) {
    final recMess = c![0].payload as MqttPublishMessage;
    final pt =
        MqttPublishPayload.bytesToStringAsString(recMess.payload.message!);

    /// The above may seem a little convoluted for users only interested in the
    /// payload, some users however may be interested in the received publish message,
    /// lets not constrain ourselves yet until the package has been in the wild
    /// for a while.
    /// The payload is a byte buffer, this will be specific to the topic
    print(
        'EXAMPLE::Change notification:: topic is <${c[0].topic}>, payload is <-- $pt -->');
    print('');
  });

正如你所看到的，他正在创建没有类型的变量，并且正在进行重铸：

final recMess = c![0].payload as MqttPublishMessage;

如果这不起作用，我会在客户端设置中查找一些错误，或者您接收的数据可能与您期望接收的数据不完全相同。

无法将“MqttMessage”类型的值分配给“MqttPublishMessage”类型的变量

1 个答案: