如果目标对象死亡，Qt::BlockingQueuedConnection emission 会发生什么？

Question

当我使用 invokeMethod 发送方法调用时，如果发送代码等待调用，但目标对象随后死亡，会发生什么情况？这会在无限等待中结束吗？或者 Qt 会唤醒调用者和 return false（这将是一种未记录的行为，并且是我自己的最佳猜测）？

Answer 1

以下示例在 invokeMethod 等待 BlockingQueuedConnection 时删除工作对象：

#include <QtCore>

//a thread that can be destroyed at any time
//see 
class SafeThread : public QThread{
    using QThread::run;
public:
    explicit SafeThread(QObject* parent= nullptr):QThread(parent){}
    ~SafeThread(){ quit(); wait(); }
};

//The function queues a functor to get executed in a specified worker's thread
template <typename Func>
void PostToThread(QThread* thread, Func&& f) {
    //see 
    QObject temporaryObject;
    QObject::connect(&temporaryObject, &QObject::destroyed,
                     thread->eventDispatcher(), std::forward<Func>(f),
                     Qt::QueuedConnection);
}

//a typical QObject worker that can "printName"
class Worker  : public QObject {
    Q_OBJECT
public:
    using QObject::QObject;
    ~Worker() {
        qInfo() << "destroying " << objectName()
                << " in " << QThread::currentThread()->objectName();
    }
    Q_SLOT void printName() {
        qInfo() << "my name is " << objectName()
                << " in " << QThread::currentThread()->objectName();
    }
};

int main(int argc, char *argv[]) {
    QCoreApplication a(argc, argv);

    //create worker
    Worker *worker = new Worker;
    worker->setObjectName("worker");

    //start worker thread and move worker to it
    SafeThread t;
    worker->moveToThread(&t);
    t.start();

    //set thread names (for better output)
    QThread::currentThread()->setObjectName("main_thread");
    t.setObjectName("worker_thread");

    //normal QMetaObject::invokeMethod usage
    if(QMetaObject::invokeMethod(worker, "printName",
                                 Qt::BlockingQueuedConnection)) {
        qInfo() << "printName called successfully before deletion";
    }
    //the lambda function will be executed in the worker thread
    PostToThread(&t, [worker]{
        qInfo() << "blocking " << QThread::currentThread()->objectName();
        QThread::sleep(2); //block worker thread for 2 seconds
        delete worker; //delete worker
    });
    //at this point the worker thread is about to destroy the worker object (but
    //hasn't done so yet)
    if(QMetaObject::invokeMethod(worker, "printName",
                                 Qt::BlockingQueuedConnection)) {
        qInfo() << "printName called successfully after deletion!";
    }

    QTimer::singleShot(100, &a, &QCoreApplication::quit);
    return a.exec();
}

#include "main.moc"

输出（在 Qt 5.9.1、Qt 5.7 - windows、debian 上测试）：

my name is  "worker"  in  "worker_thread"
printName called successfully before deletion
blocking  "worker_thread"
destroying  "worker"  in  "worker_thread"
printName called successfully after deletion!

所以简短的回答是：invokeMethod returns true 但没有任何调用。但是，请注意，在开始时，您必须保证 worker 对象仍然有效 （有关详细信息，请参阅最后一点） invokeMethod 调用主线程（否则, 是 UB).

以下是我通过挖掘 Qt 代码得出的结论列表：

ivokeMethod returns false 仅当传递给它的参数有问题时（例如插槽签名与参数不匹配 count/type, return 类型不匹配，未知连接类型，...）。参见 here。

当使用 Qt::BlockingQueuedConnection 时，invokeMethod 通过 acquiring a QSemaphore 阻塞调用线程。 QSemaphore 存储到发布到接收方对象的 QMetaCallEvent 中。

这个QSemaphoreis released当QMetaCallEvent被销毁

QObject的析构函数负责调用QCoreApplication::removePostedEvents() for the object being destructed. This means that all the events in the event queue that are targeted to an object are destroyed upon this object's destruction. See here.

您需要确保 worker 对象在调用线程执行 invokeMethod 时保持活动状态，直到获取提到的信号量，因为 invokeMethod 可能会在任何时候尝试访问 worker 对象。我认为这一要求在实践中会使事情变得复杂，因为最终可能不得不在整个 invokeMethod 调用期间保证对象的生命周期（因此避免了这整个问题）。

如果目标对象死亡，Qt::BlockingQueuedConnection emission 会发生什么？

What happens with Qt::BlockingQueuedConnection emission if target object dies?

c++

qt

conditional-statements

semaphore

signals-slots