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订阅命令并执行操作

收到消息时切换继电器

订阅命令并执行操作 是 CoddyKit 上的免费 Arduino & IoT Academy 课时。这是第 4 节课，共 4 节。你可以在下方免费阅读本课时的完整内容 — 然后在浏览器中使用内置代码编辑器和全天候 AI 导师进行实践。这是 Arduino & IoT Academy 学习路径的一部分，你的进度在网页和 CoddyKit 应用中同步。 Arduino & IoT Academy 课程共包含 4 节课。

本课时的部分内容尚未翻译，以英文显示。

Listening for Messages

Publishing sends data out; subscribing lets your device receive it. This is how the cloud sends commands back to hardware.

Subscribe to a Topic

Tell the broker which topic you want with subscribe. From then on, matching messages are pushed straight to your device.

client.subscribe("home/livingroom/light");

Messages Arrive in a Callback

Incoming messages trigger a callback function you write. The broker calls it every time a subscribed topic gets a message.

The Callback Signature

Your callback receives the topic, the payload bytes, and a length. You register it once during setup.

void onMessage(char* topic, byte* payload, unsigned int len) {
}

Register the Callback

Hook your function up with setCallback before connecting. The library then routes every message to it.

client.setCallback(onMessage);

Read the Payload Safely

The payload is raw bytes, not a finished string. Build a String by looping over its length so you never read past the end.

String msg;
for (int i = 0; i < len; i++)
  msg += (char)payload[i];

Decide What It Means

Compare the message to known commands. A simple if check on the text is enough to map ON and OFF to actions.

if (msg == "ON") digitalWrite(RELAY, HIGH);

Act on the Command

Now do something real: flip a relay, move a servo, or change a light. This closes the loop from cloud to physical world.

Check the Topic Too

If you subscribe to several topics, inspect the topic argument first. That tells you which device the command targets.

Re-Subscribe After Reconnect

A dropped link forgets your subscriptions. After every reconnect, call subscribe again or you will hear silence.

Keep Callbacks Short

Do quick work inside the callback and return fast. Long jobs there can stall the MQTT loop and miss new messages.

Quick Check

After a WiFi drop your device reconnects but stops responding to commands. What did the reconnect break?

Recap

You learned to subscribe, handle messages in a callback, read the payload, act on commands, and re-subscribe after reconnects. 🎉

常见问题解答

「订阅命令并执行操作」课时是免费的吗？

是的 — 「订阅命令并执行操作」的完整文本可在网页上免费阅读。要进行交互式练习（内置代码编辑器和全天候 AI 导师）并解锁 Arduino & IoT Academy 课程的其余内容，请升级到 CoddyKit PRO。 Arduino & IoT Academy 课程共包含 4 节课。

「订阅命令并执行操作」这节课中我会学到什么？

收到消息时切换继电器你通过在浏览器中直接运行的动手代码来练习 Arduino & IoT Academy，全天候 AI 导师会在你学习这节课的过程中回答你的问题。

学习 Arduino & IoT Academy 需要有经验吗？

无需任何先前经验。CoddyKit 上的 Arduino & IoT Academy 课程适合初学者到高级学习者，你可以从这里开始或从头开始，按照自己的节奏学习。这是第 4 节课，共 4 节。

「订阅命令并执行操作」课时需要多长时间？

大多数 CoddyKit 课程大约需要 5–10 分钟。每节课都很精短且互动，所以你能稳步进步，并在网页和应用中从离开的地方继续。

我能在这节 Arduino & IoT Academy 课中编写并运行代码吗？

能。每节 Arduino & IoT Academy 课都包含内置代码编辑器，你可以在浏览器中直接编写并运行真实代码，并获得即时 AI 反馈 — 无需本地设置。

此课程中的所有课时

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