订阅主题
问题
您想订阅 MQTT 主题上的消息。
解决方案
使用 MQTT 输入 节点订阅代理并接收发布到匹配主题的消息。
示例
[{"id":"8024cb4.98c5238","type":"mqtt in","z":"eda2a949.74ea98","name":"","topic":"sensors/#","qos":"2","broker":"61de5090.0f5d9","x":240,"y":180,"wires":[["15d727dd.33e808"]]},{"id":"15d727dd.33e808","type":"debug","z":"eda2a949.74ea98","name":"","active":true,"console":"false","complete":"false","x":390,"y":180,"wires":[]},{"id":"61de5090.0f5d9","type":"mqtt-broker","z":"","broker":"localhost","port":"1883","clientid":"","usetls":false,"compatmode":true,"keepalive":"60","cleansession":true,"willTopic":"","willQos":"0","willPayload":"","birthTopic":"","birthQos":"0","birthPayload":""}]
讨论
MQTT 输入 节点必须硬编码要使用的主题过滤器 - 它不能动态更改。
一种可能的解决方法是将主题设置为环境变量，例如 $(MY_TOPIC)。当 Node-RED 运行时启动时，它会将环境变量的值替换到该节点的属性中。这确实允许更改主题，尽管这样做确实需要重新启动 Node-RED 才能获取环境变量的更改。
您还可以使用 MQTT 通配符，+ 用于单个主题级别或 # 用于多个级别。这允许您使用单个节点接收多个主题。消息将从节点发送，其中 msg.topic 设置为实际接收到的主题。
