接收来自不同线程的用户输入

Question

我正在制作一个计时器程序来计算程序启动后经过的时间。在后台我也在检查键盘输入（进入/返回退出，点击窗口）；这是在我以 detached 身份运行的单独线程中完成的。

似乎第二个线程无法接收来自主线程的输入。当我使用键盘或鼠标时，没有任何反应。此外，屏幕上什么也没有出现，只有白色。

std::mutex g_mutex;
std::condition_variable cv;

// check for input from the user using the window object
// sets stopProgram to true if the user wishes to exit
void poll(sf::RenderWindow& window, bool& stopProgram) {
    std::unique_lock<std::mutex> lk(g_mutex);
    // wait for main thread to open window
    cv.wait(lk, [&] { return !stopProgram && window.isOpen(); });
    sf::Event event;
    while (true) {
        if (window.pollEvent(event)) {
            // if user wants to exit program
            if (event.type == sf::Event::Closed || (event.type == sf::Event::KeyPressed &&
                (event.key.code == sf::Keyboard::Return || event.key.code == sf::Keyboard::Escape))) {
                window.close();
                // main thread will explicitly exit the main loop
                stopProgram = true;
                break;
            }
        }
    }
}

int main()
{
    int hour = 0, minute = 0, second = 0;
    auto text = textObject();
    bool stopProgram = false;
    // run a background thread that checks for input while the main program runs
    std::thread(poll, std::ref(window), std::ref(stopProgram)).detach();
    std::once_flag flag;

    std::lock_guard<std::mutex> lk(g_mutex);
    while (window.isOpen()) {
        // notify once window opens
        std::call_once(flag, [&] { cv.notify_one(); });
        // set timestamp
        text->setString(makeTimeStamp(hour, minute, second));
        // if the background thread set stopProgram, end the program
        if (stopProgram) break;
        window.clear(sf::Color::White);
        window.draw(*text);
        window.display();
        // update time
        second = (second + 1) % MAX_SEC;
        if (second == 0) minute = (minute + 1) % MAX_MIN;
        if (second == 0 && minute == 0) hour = (hour + 1) % MAX_HOUR;
        // sleep one second
        std::this_thread::sleep_for(std::chrono::seconds(1));
    }
}

我对多线程的使用正确吗？如果是这样，只能主线程接收输入，这就是它不起作用的原因吗？

更新：如果我去掉 while (true) 并使用 while (window.pollEvent(event)) 并将 lock_guard 移动到 if (stopProgram) 之前，那么文本（时间戳）会出现在屏幕上，但我仍然无法处理输入。

Answer 1

主线程启动轮询线程。

std::thread(poll, std::ref(window), std::ref(stopProgram)).detach();

主线程获取 g_mutex 并且永远不会释放它。

std::lock_guard<std::mutex> lk(g_mutex);

poll 线程等待 g_mutex 被释放：

std::unique_lock<std::mutex> lk(g_mutex);

但是主线程从不释放它，所以轮询线程从不做任何事情。

修复它。更改 main() 函数的开头：

int main()
{
    int hour = 0, minute = 0, second = 0;
    auto text = textObject();
    volatile bool stopProgram = false;
    // run a background thread that checks for input while the main program runs
    std::thread(poll, std::ref(window), std::ref(stopProgram)).detach();

    while (!window.isOpen()) { /* busy loop */ }
    {
        std::lock_guard<std::mutex> lk(g_mutex);
        cv.notify_all();
    }
    while (window.isOpen()) {
       ...

这个 SFML API 比我用过的其他窗口框架更难处理。如果有线程安全的 window.pushCustomEvent() 函数会非常有用。