Paho 从 on_message 回调更新用户数据

Question

基本上，我希望我的用户数据充当全局标志，在

on_message()

回调的连续执行之间读取和更新。我编写并运行了以下脚本：

import paho.mqtt.client as mqtt
MQTT_SERVER = "broker.hivemq.com"
MQTT_TOPIC = "IvanHu"

# The callback for when the client receives a CONNACK response from the server.
def on_connect(client, userdata, flags, rc):
    print("Connected with result code "+str(rc))
    client.subscribe(MQTT_TOPIC)
    
# The callback for when a PUBLISH message is received from the server.
def on_message(client, userdata, msg):
    print(userdata, end = '')
    print(" message received.")
    userdata += 1
    user_data_set(client_userdata)

client_userdata = 1

client = mqtt.Client(userdata=client_userdata)
client.on_connect = on_connect
client.on_message = on_message
client.connect(MQTT_SERVER, 1883, 60)

client.loop_forever()

然后我对主题 IvanHu 发表了 4 次。我希望我的 python paho 脚本的输出如下：

Connected with result code 0
1 message received.
2 message received.
3 message received.
4 message received.

然而，这就是我得到的：

Connected with result code 0
1 message received.
1 message received.
1 message received.
1 message received.

显然，变量 client_userdata 在连续的 on_message() 调用中从未更新。更新此变量的正确方法是什么？

Answer 1

在连接之前，您必须通过

client.user_data_set

为用户数据提供初始值，然后您可以在

on_message

回调中更新用户数据。

import paho.mqtt.client as mqtt
MQTT_SERVER = "broker.hivemq.com"
MQTT_TOPIC = "IvanHu"

# The callback for when the client receives a CONNACK response from the server.
def on_connect(client, userdata, flags, rc):
    print("Connected with result code "+str(rc))
    client.subscribe(MQTT_TOPIC)
    
# The callback for when a PUBLISH message is received from the server.
def on_message(client, userdata, msg):
    print(userdata, end = '')
    print(" message received.")
    user_data_set(userdata + 1)    # <-- Update userdata

client_userdata = 1

client = mqtt.Client(userdata=client_userdata)
client.on_connect = on_connect
client.on_message = on_message
client.user_data_set(1)    # <-- Initialize
client.connect(MQTT_SERVER, 1883, 60)

client.loop_forever()

Answer 2

您似乎错过了文档中有关更新

user_data

对象的部分

稍后可能会使用 user_data_set() 函数进行更新。

取自文档此处

这意味着您需要将新值传递给

set_userdata()

，因此应如下所示：

# The callback for when a PUBLISH message is received from the server.
def on_message(client, userdata, msg):
    print(userdata, end = '')
    print(" message received.")
    userdata += 1
    client.user_data_set(userdata)

Answer 3

我无法找到使用用户数据方法来满足我的要求的方法，但我可以使用全局变量方法来做到这一点。这是工作脚本：

import paho.mqtt.client as mqtt
MQTT_SERVER = "broker.hivemq.com"
MQTT_TOPIC = "IvanHu"

# The callback for when the client receives a CONNACK response from the server.
def on_connect(client, userdata, flags, rc):
    print("Connected with result code "+str(rc))
    client.subscribe(MQTT_TOPIC)
    
# The callback for when a PUBLISH message is received from the server.
def on_message(client, userdata, msg):
    global client_userdata
    print(client_userdata, end = '')
    print(" message received.")
    client_userdata += 1

client_userdata = 1

client = mqtt.Client()
client.on_connect = on_connect
client.on_message = on_message
client.connect(MQTT_SERVER, 1883, 60)

client.loop_forever()

输出：

Connected with result code 0
1 message received.
2 message received.
3 message received.
4 message received.

“全局变量”方法不使用发送到 MQTT 代理的用户数据，而是“本地”递增变量的值。它在本地计算收到“on_message”回调的次数。 添加打印以显示从 MQTT 代理收到的“userdata”变量将显示其值始终为 none，因为从未在 MQTT 客户端定义中定义或在创建后设置。

Answer 4

您实际上应该使用一些引用类型，例如 dict 等。

# Once you create the client, set some dict as user data
client.user_data_set({"counter": 0})

# When you get on_message, you can then edit just the key value, so the reference will take care of the data handling
def on_message(client, userdata, msg):
   userdata["counter"] += 1

在您的示例中，用户数据是数字，因此一旦您增加它，引用就会被覆盖，因此用户数据仅在本地使用。