C++ 等待但允许事件触发

Question

使用 Visual Studio 2013 构建 SignalR C++ 客户端，我从 NuGet 包 Microsoft.AspNet.SignalR.Client.Cpp.v120.WinDesktop 中的工作示例代码开始, 来源 here

查看库源代码，在我看来，事件处理过程是基于 Concurrency Runtime (pplx::task) which relies on C++11 features

void chat(const utility::string_t& name)
{
    signalr::hub_connection connection{ U("https://testsite") };
    auto proxy = connection.create_hub_proxy(U("ChatHub"));
    proxy.on(U("broadcastMessage"), [](const web::json::value& m)
    {
        ucout << std::endl << m.at(0).as_string() << U(" wrote:") << m.at(1).as_string() << std::endl << U("Enter your message: ");
    });

    connection.start()
        .then([proxy, name]()
    {
        for (;;)
        {
            utility::string_t message;
            std::getline(ucin, message);

            if (message == U(":q"))
            {
                break;
            }

            send_message(proxy, name, message);
        }
    })
        .then([&connection]() // fine to capture by reference - we are blocking so it is guaranteed to be valid
    {
        return connection.stop();
    })
        .then([](pplx::task<void> stop_task)
    {
        try
        {
            stop_task.get();
            ucout << U("connection stopped successfully") << std::endl;
        }
        catch (const std::exception &e)
        {
            ucout << U("exception when starting or stopping connection: ") << e.what() << std::endl;
        }
    }).get();
}

我想删除 "user input" 组件；而是在收到特定 "broadcastMessage" 时退出循环。

如果我用睡眠语句替换 for 循环，broadcastMessage 事件将停止触发。

如果我在没有 getline 的情况下使用 for 循环，完成后将 bComplete 设置为 true，它会按我想要的方式工作，但会导致高 CPU 使用率（很明显）

for (;;)
{
if (bComplete) break;
}

理想情况下，我希望连接开始，然后等待 broadcastMessage 事件发出信号关闭连接。此外，在连接关闭之前，"chat" 函数不应 return。

Answer 1

我使用 WinAPI WaitForSingleObject 让这个工作正常：

HANDLE hEvent;
    void chat(const utility::string_t& name)
    {
        signalr::hub_connection connection{ U("https://testsite") };
        auto proxy = connection.create_hub_proxy(U("ChatHub"));
        proxy.on(U("broadcastMessage"), [](const web::json::value& m)
        {
            ucout << std::endl << m.at(0).as_string() << U(" wrote:") << m.at(1).as_string() << std::endl;
        if (m.at(1).as_string() == L"quit")
            {
                SetEvent(hEvent);
            }
        });

    hEvent = CreateEvent(0, TRUE, FALSE, 0);

        connection.start()
            .then([proxy, name]()
        {
            WaitForSingleObject(hEvent, INFINITE);
        })
            .then([&connection]() // fine to capture by reference - we are blocking so it is guaranteed to be valid
        {
            return connection.stop();
        })
            .then([](pplx::task<void> stop_task)
        {
            try
            {
                stop_task.get();
                ucout << U("connection stopped successfully") << std::endl;
            }
            catch (const std::exception &e)
            {
                ucout << U("exception when starting or stopping connection: ") << e.what() << std::endl;
            }`enter code here`
        }).get();
    }

Answer 2

我可以在 that you've already discovered Windows event objects; however, if you were looking for a C++11 platform-independent solution, consider std::condition_variable 中看到！

unsigned int accountAmount;
std::mutex mx;
std::condition_variable cv;

void depositMoney()
{
    // go to the bank etc...
    // wait in line...
    {
        std::unique_lock<std::mutex> lock(mx);
        std::cout << "Depositing money" << std::endl;
        accountAmount += 5000;
    }
    // Notify others we're finished
    cv.notify_all();
}
void withdrawMoney()
{
    std::unique_lock<std::mutex> lock(mx);
    // Wait until we know the money is there
    cv.wait(lock);
    std::cout << "Withdrawing money" << std::endl;
    accountAmount -= 2000;
}
int main()
{
    accountAmount = 0;
    std::thread deposit(&depositMoney);
    std::thread withdraw(&withdrawMoney);
    deposit.join();
    withdraw.join();
    std::cout << "All transactions processed. Final amount: " << accountAmount << std::endl;
    return 0;
}

在这个例子中，我们创建了两个线程：一个用于将钱存入帐户，一个用于取款。因为线程有可能先把钱取到运行，尤其是depositMoney()涉及的处理比较多，我们需要等到知道钱到了。我们在访问钱之前锁定我们的线程，然后告诉 condition_variable 我们在等待什么。 condition_variable 将解锁线程，一旦钱存入并调用 notify_all() 我们将被重新唤醒以完成我们的逻辑处理。

请注意，可以使用 Windows event objects 执行完全相同的操作。您将使用 SetEvent() 和 WaitForSingleObject()，而不是 std::condition_variable::wait() 和 std::condition_variable::notify_all()。不过这与平台无关。

C++ 等待但允许事件触发

C++ Wait But Allow Events To Fire

c++

winapi

concurrency-runtime

c++11

signalr