copy method
复制一个对象
Implementation
MqttData copy({MqttMessageType? messageType, String? data}) {
return MqttData(
topic: topic,
receive: receive,
sender: sender,
createdTime: createdTime,
timeout: timeout,
messageNumber: messageNumber,
secretKey: secretKey,
messageType: messageType ?? this.messageType,
dataLength: dataLength,
data: data ?? this.data,
checkCode: checkCode);
}