Sever.on监听的事件何时回调

这次的demo和之前用的相同的代码，但是控制台打印的不一样，因此写一个笔记记录下在测试连接通信以及测试 on 方法时，发现的一些现象。

先给出broker的代码：

js 复制代码

const mosca = require("mosca");

const MqttServer = new mosca.Server({
  port: 1883,
  id: "ZZK", //固定我们服务器的ID
});

// 监听客户端连接事件
MqttServer.on("clientConnected", (client) => {
  console.log(`Client connected: ${client.id}`);
});

// 监听客户端断开事件
MqttServer.on("clientDisconnected", (client) => {
  console.log(`Client disconnected: ${client.id}`);
});

// 监听 MQTT 消息发布事件
MqttServer.on("published", (packet, client) => {
  console.log(
    `Published: topic: ${packet.topic} || message: ${packet.payload.toString()}`
  );
});

// 监听 Mosca 服务器就绪事件
MqttServer.on("ready", () => {
  console.log("Mosca server is up and running");
});

在这里，我们主要监听了3个事件，客户端的连接和断开、以及broker发送消息

client_test1 代码：

js 复制代码

const mqtt = require("mqtt");

const url = "mqtt://localhost:1883";
const options = {
  clientId: "test1"
};

// 连接到本地 MQTT 服务器
const client = mqtt.connect(url, options);

// 监听连接事件
client.on("connect", () => {
  console.log("Connected to MQTT server");

  // 订阅一个主题(client_test2会发送这个主题的消息)
  client.subscribe("test2/topic", (err) => {
    if (!err) {
      console.log("test1 订阅test2/topic成功");
    }
  });
 //这个主题是 broker 默认给出的，ZZK 代表我们服务器的id，# 为通配符，其余的字段固定
  client.subscribe("$SYS/ZZK/new/#", (err) => {
    if (!err) {
      console.log("test1 订阅$SYS/ZZK/new/#成功");
    }
  });
});

// 监听消息到达事件
client.on("message", (topic, message) => {
  console.log(`Received message on topic ${topic}: ${message.toString()}`);
});

client_test2 代码：

js 复制代码

const mqtt = require("mqtt");

const url = "mqtt://localhost:1883";
const options = {
  clientId: "test2",
};

// 连接到本地 MQTT 服务器
const client = mqtt.connect(url, options);

client.on("connect", () => {
  // 订阅一个主题(这里主要是来测试 broker是否会发送消息)
  client.subscribe("test1/topic", (err) => {
    if (!err) {
      console.log("test2 订阅test1/topic成功");
    }
  });

  console.log("Connected to Mosca server");
  setInterval(() => {
    client.publish("test2/topic", "This is test2 publish");
    console.log("Message sent to test2/topic");
  }, 10000); // 10 秒
});

至此，我们的 broker、client1、client2 就编写好了。

主要的逻辑如下：

client1 订阅两个主题（test2/topic 与 $SYS/ZZK/new/#）

client2 订阅一个主题（test1/topic），并每个10s 发送主题为 test2/topic 的消息

client1 控制台会打印接收到的所有消息，格式为Received message on topic 主题：消息

broker会打印其发出去的消息，格式为Published: topic: 主题|| message: 消息

我们的测试流程如下：首先启动broker，然后启动client1，再启动client2。当client2发布了几条消息后，关闭它，然后再启动它。在broker控制台的打印：

bash 复制代码

Mosca server is up and running
Client connected: test1
Published: topic: $SYS/ZZK/new/clients || message: test1
Published: topic: $SYS/ZZK/new/subscribes || message: {"clientId":"test1","topic":"test2/topic"}   
Published: topic: $SYS/ZZK/new/subscribes || message: {"clientId":"test1","topic":"$SYS/ZZK/new/#"}

Client connected: test2
Published: topic: $SYS/ZZK/new/clients || message: test2
Published: topic: $SYS/ZZK/new/subscribes || message: {"clientId":"test2","topic":"test1/topic"}
Published: topic: test2/topic || message: This is test2 publish

Client disconnected: test2
Published: topic: $SYS/ZZK/new/unsubscribes || message: {"clientId":"test2","topic":"test1/topic"}
Published: topic: $SYS/ZZK/disconnect/clients || message: test2

Client connected: test2
Published: topic: $SYS/ZZK/new/clients || message: test2
Published: topic: $SYS/ZZK/new/subscribes || message: {"clientId":"test2","topic":"test1/topic"}
Published: topic: test2/topic || message: This is test2 publish

Client disconnected: test2
Published: topic: $SYS/ZZK/new/unsubscribes || message: {"clientId":"test2","topic":"test1/topic"}
Published: topic: $SYS/ZZK/disconnect/clients || message: test2

Client disconnected: test1
Published: topic: $SYS/ZZK/new/unsubscribes || message: {"clientId":"test1","topic":"test2/topic"}   
Published: topic: $SYS/ZZK/new/unsubscribes || message: {"clientId":"test1","topic":"$SYS/ZZK/new/#"}
Published: topic: $SYS/ZZK/disconnect/clients || message: test1

我们可以看到：

当有client连接broker的时候，broker会自动发送一条topic为$SYS/brokerID/new/clients的message: clientId。而如果客户端client_X订阅了某个主题topic_x。那么broker还会自动发送一条topic为$SYS/brokerID/new/subscribes的message: {"clientId":"client_X","topic":"topic_x"}。取消订阅也是如此。以上是broker自动发送的。
当client向broker发送消息的时候，broker会自动将其转发（这个是必然的）。
当client取消订阅时，broker会自动发送一条topic为$SYS/brokerID/new/unsubscribes的message, 与订阅时的格式一样，就是主题换了。同理，当client下线时也是如此。

这些就自动发送的消息就可以帮助我们知道client的一些行为，从而进行一些处理。

为了验证是否是由broker发送的，我们的client_test1的代码因此就订阅了$SYS/ZZK/new/#主题，同时通过将client_test2上线下线查看clien_test1控制台的输出：

bash 复制代码

Connected to MQTT server
Received message on topic $SYS/ZZK/new/subscribes: {"clientId":"test1","topic":"test2/topic"}   
test1 订阅test2/topic成功
Received message on topic $SYS/ZZK/new/subscribes: {"clientId":"test1","topic":"$SYS/ZZK/new/#"}
test1 订阅$SYS/ZZK/new/#成功

Received message on topic $SYS/ZZK/new/clients: test2
Received message on topic $SYS/ZZK/new/subscribes: {"clientId":"test2","topic":"test1/topic"}
Received message on topic test2/topic: This is test2 publish
Received message on topic $SYS/ZZK/new/unsubscribes: {"clientId":"test2","topic":"test1/topic"}

Received message on topic $SYS/ZZK/new/clients: test2
Received message on topic $SYS/ZZK/new/subscribes: {"clientId":"test2","topic":"test1/topic"}
Received message on topic test2/topic: This is test2 publish
Received message on topic $SYS/ZZK/new/unsubscribes: {"clientId":"test2","topic":"test1/topic"}

从这里我们可以看到，client_test1不仅仅接收到了client_test2发送的topic为test2/topic的消息：This is test2 publish,还接收到了 topic为 $SYS/ZZK/new/subscribes、$SYS/ZZK/new/clients的消息。这些就是broker自动发送的。

因此我们可以知道，server.on(Event) 方法是当broker去执行这些event的时候才会触发。且由于broker自带的一些回调函数，除了会执行我们编写的回调函数外，还会相应的自动去发送一些特定主题的消息。

搭建一个简单的MQTT物联网平台笔记（二）：测试连接

Sever.on监听的事件何时回调