qml中如果捕获窗口关闭和程序结束事件

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在qml如何捕获程序结束事件，然后清理内存。比如下面的代码，创建了一个线程，不知道在哪调用清理线程函数j.join()。测试了QQmlApplicationEngine::quit不起作用

int main(int argc, char* argv[])
    std::thread j([]() {
    QGuiApplication app(argc, argv);
    QQmlApplicationEngine engine;
    const QUrl url(u"qrc:/untitled1/Main.qml"_qs);
    QObject::connect(
        &engine,
        &QQmlApplicationEngine::objectCreationFailed,
        [&](const QUrl& url) {
            j.join();
            QCoreApplication::exit(-1);
        Qt::QueuedConnection
    //以下代码捕获quit事件，但是永远不会被调用。
    QObject::connect(
        &engine,
        &QQmlApplicationEngine::quit,
        [&]() {
            j.join();
        Qt::QueuedConnection
    engine.load(url);
    return app.exec();