如何对open62541.h/open62541.c的UA_Client进行状态（在线/掉线）监控_open62541 清除client config

1.背景

目前在利用open62541.h/open62541.c编写了一个与PLC进行OPCUA通讯的上位机程序。
上位机这边会定时对PLC的某个opcua变量进行写操作。但是假如PLC离线或者说拔掉网线，上位机就会直接崩溃死机，并且报如下的错误：

[2024-08-12 10:07:25.528 (UTC+0800)] warn/channel	Connection 2516 | SecureChannel 28 | Receiving the response failed with StatusCode BadConnectionClosed
[2024-08-12 10:07:25.528 (UTC+0800)] warn/client	Received Publish Response with code BadSecureChannelClosed
[2024-08-12 10:07:25.528 (UTC+0800)] warn/client	Received Publish Response with code BadSecureChannelClosed
[2024-08-12 10:07:25.528 (UTC+0800)] warn/client	Received Publish Response with code BadSecureChannelClosed
[2024-08-12 10:07:25.528 (UTC+0800)] warn/client	Received Publish Response with code BadSecureChannelClosed
[2024-08-12 10:07:25.528 (UTC+0800)] warn/client	Received Publish Response with code BadSecureChannelClosed
[2024-08-12 10:07:25.528 (UTC+0800)] warn/client	Received Publish Response with code BadSecureChannelClosed
[2024-08-12 10:07:25.528 (UTC+0800)] warn/client	Received Publish Response with code BadSecureChannelClosed
[2024-08-12 10:07:25.528 (UTC+0800)] warn/client	Received Publish Response with code BadSecureChannelClosed
[2024-08-12 10:07:25.528 (UTC+0800)] warn/client	Received Publish Response with code BadSecureChannelClosed
[2024-08-12 10:07:25.528 (UTC+0800)] warn/client	Received Publish Response with code BadSecureChannelClosed
[2024-08-12 10:07:25.528 (UTC+0800)] warn/channel	Connection 0 | SecureChannel 0 | Could not receive with StatusCode BadConnectionClosed
[2024-08-12 10:07:25.528 (UTC+0800)] info/client	Client Status: ChannelState: Closed, SessionState: Created, ConnectStatus: Good
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有没有什么办法可以监控client的状态，了解其是什么时候掉线，掉线了我就不写就行了。

2.解决方案

幸亏，是有这么一个回调函数的，参考官方的例子【open62541/examples/client_async.c】可以发现，我们可以对UA_ClientConfig中的stateCallback进行赋值，也就是注册一个状态回调函数，从而获取客户端的状态变化。

static void
onConnect(UA_Client *client, UA_SecureChannelState channelState,
          UA_SessionState sessionState, UA_StatusCode connectStatus) {
    printf("Async connect returned with status code %s\n",
           UA_StatusCode_name(connectStatus));
}

---

 UA_ClientConfig *cc = UA_Client_getConfig(client);
 cc->stateCallback = onConnect;
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一般检测channelState就行

// 状态变化回调函数，可以通过这个监测客户端是否断开连接
// 此回调函数的线程，貌似就是UA_Client_run_iterate所在的线程？起始应该不是，恐怕是在哪个线程调用了UA_Client相关的函数，就在那个线程；
static void
onStateChanged(UA_Client *client,
          UA_SecureChannelState channelState,
          UA_SessionState sessionState,
          UA_StatusCode connectStatus)
{
    if(channelState == UA_SECURECHANNELSTATE_CLOSED) // 连接已断开
    {
        qDebug() << "callback thread:" << QThread::currentThread();
        qDebug() << "连接已断开--------" << QDateTime::currentDateTime();

       ---
       
    }
}
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3.异步连接

在实际使用中，假如opcua服务端是离线的话，执行UA_Client_connect会导致线程卡住很长一段时间。假如不希望卡住的话，得使用异步连接：UA_Client_connectAsync；
使用UA_Client_connectAsync后，需要在stateCallback函数中判断是否连接上。完全连接上的话，
UA_SessionState sessionState会等于UA_SESSIONSTATE_ACTIVATED;

static void
onStateChanged(UA_Client *client,
          UA_SecureChannelState channelState,
          UA_SessionState sessionState,
          UA_StatusCode connectStatus)
{
}
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假如你在onStateChanged中将channelState、sessionState打印出来的话，会发现channelState先从上往下满足下面的顺序：

然后是sessionState从上往下满足下面的顺序：

此外，一定要记得在某个地方周期地调用UA_Client_run_iterate，否则，不会进入回调函数。

4.注意事项

4.1.线程问题

这个opcua库对多线程的处理比较差，很容易造成冲突。一定要自己加个互斥锁，否则程序很容易就崩掉。

4.2.UA_Client_run_iterate

一定要在某个地方周期调用。但是最好不是在子线程中调用，除非你有把握控制好线程冲突（比如加互斥锁之类）。

---

参考：
【open62541/examples/client_async.c】