参数: handle – A valid client handle from a successful call to MQTTClient_create(). context – A pointer to any application-specific context. The the <i>context</i> pointer is passed to each of the callback functions to provide access to the context information in the callback. cl – A pointer to an MQTTClient_connectionLost() callback function. You can set this to NULL if your application doesn't handle disconnections. ma – A pointer to an MQTTClient_messageArrived() callback function. This callback function must be set when you call MQTTClient_setCallbacks(), as otherwise there would be nowhere to deliver any incoming messages. dc – A pointer to an MQTTClient_deliveryComplete() callback function. You can set this to NULL if your application publishes synchronously or if you do not want to check for successful delivery. 返回: ::MQTTCLIENT_SUCCESS if the callbacks were correctly set, ::MQTTCLIENT_FAILURE if an error occurred.

这个函数的参数包括：handle——一个由MQTTClient_create()函数成功调用后返回的有效客户端句柄；context——一个指向任何应用程序特定上下文的指针。context指针将传递给每个回调函数，以便在回调函数中访问上下文信息。cl——一个指向MQTTClient_connectionLost()回调函数的指针。如果你的应用程序不处理断开连接，可以将其设置为NULL。ma——一个指向MQTTClient_messageArrived()回调函数的指针。在调用MQTTClient_setCallbacks()函数时，必须设置此回调函数，否则无法传递任何传入的消息。dc——一个指向MQTTClient_deliveryComplete()回调函数的指针。如果你的应用程序同步发布或不想检查成功发布，则可以将其设置为NULL。 返回值：如果回调函数正确设置，则返回MQTTCLIENT_SUCCESS，如果发生错误，则返回MQTTCLIENT_FAILURE。

相关问题

`java.util.Date` from String 2021-07-28 21:36:34: not a valid representation

The error message you are seeing suggests that the string you are trying to parse into a `java.util.Date` object is not in a valid format.

By default, `java.util.Date` expects the input string to be in the format of "EEE MMM dd HH:mm:ss zzz yyyy". If the input string does not match this format, you will get a `ParseException`.

To parse a string in a different format, you can use a `SimpleDateFormat` object to specify the input format. Here's an example:

String dateString = "2021-07-28 21:36:34";
SimpleDateFormat dateFormat = new SimpleDateFormat("yyyy-MM-dd HH:mm:ss");
Date date = dateFormat.parse(dateString);

This code creates a `SimpleDateFormat` object with the format "yyyy-MM-dd HH:mm:ss", which matches the format of the input string. It then calls the `parse` method on the `dateFormat` object to convert the string into a `Date` object.

Note that the `parse` method can also throw a `ParseException` if the input string does not match the specified format. You may want to handle this exception to provide a better error message to the user.

为什么django在使用 sync_playwright().start()方法后，使用orm查询提示You cannot call this from an async context - use a thread or sync_to_async

这是因为 sync_playwright() 方法使用了 asyncio 库来实现异步操作，而 Django ORM 是同步的，不能在异步上下文中使用。在这种情况下，你可以使用 Django 提供的 `sync_to_async` 函数将 ORM 查询转换为异步操作。这个函数将 ORM 查询封装在一个异步函数中，然后可以在异步上下文中使用它。例如：

from django.core.management import BaseCommand
from asgiref.sync import sync_to_async
from myapp.models import MyModel

class Command(BaseCommand):
    def handle(self, *args, **options):
        # 异步上下文中使用 ORM 查询
        async def query():
            return await sync_to_async(MyModel.objects.all)()

        # 启动 playwright
        with sync_playwright().start() as browser:
            page = browser.new_page()
            # 异步执行查询
            results = await query()
            # 处理查询结果
            for result in results:
                # ...

这样，就可以在异步上下文中使用 ORM 查询了。需要注意的是，`sync_to_async` 函数只能在异步上下文中使用，所以它必须在异步函数或协程中调用。