写一个esp8266连接MQTT服务器的代码

好的，以下是一个简单的ESP8266连接MQTT服务器的代码示例，你可以根据自己的需求进行修改：

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

// WiFi网络信息
const char* ssid = "your_SSID";
const char* password = "your_PASSWORD";

// MQTT服务器信息
const char* mqtt_server = "your_MQTT_server";
const char* mqtt_username = "your_MQTT_username";
const char* mqtt_password = "your_MQTT_password";
const int mqtt_port = 1883;

WiFiClient espClient;
PubSubClient client(espClient);

void setup() {
  // 启动串口
  Serial.begin(115200);

  // 连接WiFi网络
  WiFi.begin(ssid, password);
  while (WiFi.status() != WL_CONNECTED) {
    delay(1000);
    Serial.println("Connecting to WiFi...");
  }
  Serial.println("Connected to WiFi");

  // 连接MQTT服务器
  client.setServer(mqtt_server, mqtt_port);
  client.setCallback(callback);

  while (!client.connected()) {
    Serial.println("Connecting to MQTT server...");
    if (client.connect("ESP8266Client", mqtt_username, mqtt_password )) {
      Serial.println("Connected to MQTT server");
      client.subscribe("your_topic");
    } else {
      Serial.print("Failed, rc=");
      Serial.print(client.state());
      Serial.println("Retrying 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 in topic: ");
  Serial.println(topic);

  Serial.print("Message:");
  for (int i = 0; i < length; i++) {
    Serial.print((char)payload[i]);
  }
  Serial.println();
}

void reconnect() {
  while (!client.connected()) {
    Serial.println("Reconnecting to MQTT server...");
    if (client.connect("ESP8266Client", mqtt_username, mqtt_password )) {
      Serial.println("Connected to MQTT server");
      client.subscribe("your_topic");
    } else {
      Serial.print("Failed, rc=");
      Serial.print(client.state());
      Serial.println("Retrying in 5 seconds...");
      delay(5000);
    }
  }
}

在代码中，你需要替换以下部分：

- `your_SSID`和`your_PASSWORD` 替换为你的WiFi网络名称和密码；
- `your_MQTT_server`、`your_MQTT_username`和`your_MQTT_password`替换为你的MQTT服务器信息；
- `your_topic`替换为你的MQTT主题。

记得在你的Arduino IDE中安装`PubSubClient`库，这是一个MQTT客户端库。