esp8266实现mqtt消息订阅

要使用ESP8266实现MQTT消息订阅，需要完成以下步骤：

1. 安装MQTT库

在Arduino IDE中，打开“库管理器”，搜索并安装“PubSubClient”库。

2. 连接WiFi和MQTT服务器

使用ESP8266连接WiFi和MQTT服务器。在Arduino IDE中，打开“文件”->“示例”->“PubSubClient”->“mqtt_esp8266”，根据自己的WiFi和MQTT服务器信息修改以下代码：

const char* ssid = "your_SSID";
const char* password = "your_PASSWORD";
const char* mqtt_server = "your_MQTT_SERVER";
const char* mqtt_user = "your_MQTT_USER";
const char* mqtt_password = "your_MQTT_PASSWORD";

3. 订阅消息

在`setup()`函数中，使用`client.subscribe()`函数订阅MQTT消息。例如，订阅名为“test”的主题：

void setup() {
  // ...

  client.setServer(mqtt_server, 1883);
  client.setCallback(callback);
  while (!client.connected()) {
    if (client.connect("ESP8266Client", mqtt_user, mqtt_password )) {
      Serial.println("connected");
      client.subscribe("test");
    } else {
      Serial.print("failed, rc=");
      Serial.print(client.state());
      Serial.println(" try again in 5 seconds");
      delay(5000);
    }
  }
}

4. 处理消息

在`callback()`函数中，处理接收到的MQTT消息。例如，将消息打印到串口：

void callback(char* topic, byte* payload, unsigned int length) {
  Serial.print("Message arrived [");
  Serial.print(topic);
  Serial.print("] ");
  for (int i = 0; i < length; i++) {
    Serial.print((char)payload[i]);
  }
  Serial.println();
}

完整代码示例：

#include <ESP8266WiFi.h>
#include <PubSubClient.h>

const char* ssid = "your_SSID";
const char* password = "your_PASSWORD";
const char* mqtt_server = "your_MQTT_SERVER";
const char* mqtt_user = "your_MQTT_USER";
const char* mqtt_password = "your_MQTT_PASSWORD";

WiFiClient espClient;
PubSubClient client(espClient);

void setup() {
  Serial.begin(115200);
  WiFi.begin(ssid, password);

  while (WiFi.status() != WL_CONNECTED) {
    delay(1000);
    Serial.println("Connecting to WiFi...");
  }

  Serial.println("WiFi connected");
  Serial.println("IP address: ");
  Serial.println(WiFi.localIP());

  client.setServer(mqtt_server, 1883);
  client.setCallback(callback);

  while (!client.connected()) {
    if (client.connect("ESP8266Client", mqtt_user, mqtt_password )) {
      Serial.println("connected");
      client.subscribe("test");
    } else {
      Serial.print("failed, rc=");
      Serial.print(client.state());
      Serial.println(" try again in 5 seconds");
      delay(5000);
    }
  }
}

void loop() {
  if (!client.connected()) {
    reconnect();
  }
  client.loop();
}

void callback(char* topic, byte* payload, unsigned int length) {
  Serial.print("Message arrived [");
  Serial.print(topic);
  Serial.print("] ");
  for (int i = 0; i < length; i++) {
    Serial.print((char)payload[i]);
  }
  Serial.println();
}

void reconnect() {
  while (!client.connected()) {
    Serial.print("Attempting MQTT connection...");
    if (client.connect("ESP8266Client", mqtt_user, mqtt_password )) {
      Serial.println("connected");
      client.subscribe("test");
    } else {
      Serial.print("failed, rc=");
      Serial.print(client.state());
      Serial.println(" try again in 5 seconds");
      delay(5000);
    }
  }
}