Question

我正在制作计时器程序，计算自程序启动以来经过的时间。在后台我也检查键盘输入（进入/返回退出，点击窗口）;这是在一个单独的线程中完成的，我已经分离。

似乎第二个线程无法从主线程接收输入。当我使用键盘或鼠标时，没有任何反应。此外，屏幕上没有任何内容，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));
    }
}

我对多线程的使用是否正确？如果是这样，主线程只能接收输入，这就是它无法正常工作的原因吗？

更新：如果我摆脱while (true)并使用while (window.pollEvent(event))并将lock_guard移至if (stopProgram)之前，则屏幕上会显示文字（时间戳），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，而永远不会释放它。

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

poll thread等待g_mutex被释放：

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

但是主线程永远不会释放它，所以poll线程永远不会做任何事情。

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比我使用过的其他窗口框架更难。如果有一个线程安全的window.pushCustomEvent（）函数，那将非常有用。

从不同的线程接收用户输入

1 个答案: