帮我用PubSubClient写个完整的MQTT例程

好的，我可以为您提供一个简单的MQTT例程，使用PubSubClient库来连接到MQTT代理服务器并发布和订阅主题。以下是完整的代码：

#include <WiFi.h>
#include <PubSubClient.h>

// WiFi连接信息
const char* ssid = "your_SSID";
const char* password = "your_PASSWORD";

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

WiFiClient espClient;
PubSubClient client(espClient);

void setup_wifi() {
  delay(10);
  Serial.println();
  Serial.print("Connecting to ");
  Serial.println(ssid);
  WiFi.begin(ssid, password);
  while (WiFi.status() != WL_CONNECTED) {
    delay(500);
    Serial.print(".");
  }
  Serial.println("");
  Serial.println("WiFi connected");
  Serial.println("IP address: ");
  Serial.println(WiFi.localIP());
}

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("Connecting to MQTT Broker...");
    if (client.connect("ESP32Client", mqtt_username, mqtt_password )) {
      Serial.println("connected");
      client.subscribe("testTopic");
    } else {
      Serial.print("failed, rc=");
      Serial.print(client.state());
      Serial.println(" try again in 5 seconds");
      delay(5000);
    }
  }
}

void setup() {
  Serial.begin(9600);
  setup_wifi();
  client.setServer(mqtt_server, mqtt_port);
  client.setCallback(callback);
}

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

  char msg[50];
  sprintf(msg, "Hello from ESP32");
  client.publish("testTopic", msg);
  delay(5000);
}

在使用此代码之前，请将代码中的以下值替换为您的WiFi和MQTT服务器信息：

- your_SSID：您的WiFi网络名称
- your_PASSWORD：您的WiFi网络密码
- your_MQTT_server_address：您的MQTT代理服务器地址
- mqtt_username：MQTT代理服务器的用户名
- mqtt_password：MQTT代理服务器的密码

此代码连接到您的WiFi网络和MQTT代理服务器，并发布名为"testTopic"的消息。当收到该主题的消息时，它将调用回调函数并将消息打印到串行监视器中。

请注意，您需要安装PubSubClient库才能编译此代码。