Question

为了在一个单独的线程中创建一个Qt事件循环，从一个用Java编写的主应用程序调用的DLL中，我根据我读过here的建议完成了以下操作，哪个效果很好：

// Define a global namespace. We need to do this because the parameters 
// passed to QCoreApplication must have a lifetime exceeding that of the 
// QCoreApplication object
namespace ToolThreadGlobal
{
    static int argc = 1;
    static char * argv[] = { "MyVirtualMainApplication.exe", NULL };
    static QCoreApplication *coreApp = nullptr;
    static ToolThread *toolThread = nullptr;
};

//! The ToolThread class differs from a standard QThread only 
//! in its run() method
class ToolThread : public QThread
{
    //! Override QThread's run() method so that it calls 
    //! the QCoreApplication's exec() method rather than 
    //! the QThread's own
    void run() { ToolThreadGlobal::coreApp -> exec(); }
};

class ThreadStarter : public QObject
{
    Q_OBJECT

public:
    //! Constructor
    ThreadStarter()
    {
        // Set up the tool thread:
        if (!ToolThreadGlobal::toolThread)
        {
            ToolThreadGlobal::toolThread = new ToolThread();
            connect(ToolThreadGlobal::toolThread, &ToolThread::started,
                    this, &ThreadStarter::createApplication, Qt::DirectConnection);
                    // The thread's 'started' event is emitted after the thread
                    // is started but before its run() method is invoked. By 
                    // arranging for the createApplication subroutine to be 
                    // called before run(), we ensure that the required 
                    // QCoreApplication object is instantiated *within the 
                    // thread* before ToolThread's customised run() method 
                    // calls the application's exec() command.
            ToolThreadGlobal::toolThread->start();
        }
    }

    //! Destructor
    ~ThreadStarter()
    {
        // Ensure that the thread and the QCoreApplication are cleanly 
        // shut down:
        ToolThreadGlobal::coreApp -> quit();
        delete ToolThreadGlobal::coreApp;
        ToolThreadGlobal::coreApp = nullptr;
        delete ToolThreadGlobal::toolThread;
        ToolThreadGlobal::toolThread = nullptr;
    }

    //! Function to return a pointer to the actual tool thread:
    ToolThread* getThread() const { return ToolThreadGlobal::toolThread;  }

private:
    //! Create the QCoreApplication that will provide the tool 
    //! thread's event loop
    /*! This subroutine is intended to be called from the tool 
        thread itself as soon as the thread starts up.
    */
    void createApplication()
    {
        // Start the QCoreApplication event loop, so long as no 
        // other Qt event loop is already running
        if (QCoreApplication::instance() == NULL)
        {
            ToolThreadGlobal::coreApp = new QCoreApplication(ToolThreadGlobal::argc, 
                                                             ToolThreadGlobal::argv);
        }
    }
};

要使用它，从主Java应用程序的线程调用的子例程只需要创建一个ThreadStarter对象这将自动创建一个在其中运行的QCoreApplication的ToolThread：

itsThreadStarter = new ThreadStarter();
itsToolThread = itsThreadStarter -> getThread();

然后我们可以用通常的方式实例化一个QObject类，将它移动到线程并使用QMetaObject :: invokeMethod异步调用它的方法：

itsWorker = new Worker();
itsWorker -> moveToThread(itsToolThread);

QMetaObject::invokeMethod(itsWorker, “doSomethingInteresting”);

当我们完成后，我们只需删除ThreadStarter对象，一切都得到了很好的清理。除了恼人的消息说

WARNING: QApplication was not created in the main() thread

在启动时，似乎满足了我的所有要求。

除了......（在这里，最后，是我的问题）。

偶尔 - 并且没有我到目前为止能够辨别的任何模式 - 我在关机过程中遇到错误。通常它出现在

行

        delete ToolThreadGlobal::coreApp;

但有时候在

行

    ToolThreadGlobal::coreApp -> exec();

（当然这是在线程的run（）方法中执行的，直到ToolThreadGlobal :: coreApp - ＆gt; quit（）;完全执行后才会返回。）

错误消息通常是简单的访问冲突;有时它更为复杂：

ASSERT failure in QObjectPrivate::deleteChildren(): "isDeletingChildren already set, did this function recurse?", file ..\qtbase\src\corelib\kernel\qobject.cpp, line 1927

我认为这是因为，一旦我向QCoreApplication发出quit（）命令，我应该等待一段时间才能在删除之前正确关闭事件循环 - 正如人们通常会调用quit（）然后在删除它之前在普通QThread上等待（）。但是，QCoreApplication似乎没有等效的wait（）命令，我无法实现QTimer来强制延迟，因为一旦我用quit（）关闭事件循环它就无法工作。因此，我不知道该怎么做。我有一个问题，因为QCoreApplication是一个QObject，我可以调用它的deleteLater（）方法，但我看不到我应该从哪里调用它。

是否有专家能够很好地了解QCoreApplication和QThread的来龙去脉，以便为此提出解决方案？或者我设计的方式存在根本缺陷吗？

Answer 1

这似乎对我有用......

首先，我添加一个静态＆＃39;清理＆＃39;函数到我的全局命名空间：

namespace ToolThreadGlobal
{
    static int argc = 1;
    static char * argv[] = { "MyVirtualMainApplication.exe", NULL };
    static QCoreApplication *coreApp = nullptr;
    static ToolThread *toolThread = nullptr;
    static void cleanup() { coreApp->deleteLater();  coreApp = nullptr; }
};

然后，从我的ThreadStarter :: createApplication插槽中，我将QCoreApplication的aboutToQuit信号连接到它：

    void createApplication()
    {
        // Start the QCoreApplication event loop, so long as no other Qt event loop
        // is already running
        if (QCoreApplication::instance() == NULL)
        {
            ToolThreadGlobal::coreApp = new QCoreApplication(ToolThreadGlobal::argc, 
                                                             ToolThreadGlobal::argv);
            connect(ToolThreadGlobal::coreApp, &QCoreApplication::aboutToQuit,
                    ToolThreadGlobal::cleanup);
        }
    }

然后是＆＃39; ThreadStarter＆＃39;析构函数减少到只有五行（包括添加对QThread :: quit（）和QThread :: wait（）的调用，这应该是第一次出现的那样：

    ~ThreadStarter()
    {
        // Ensure that the thread and the QCoreApplication are cleanly shut down:
        ToolThreadGlobal::coreApp -> quit();
        ToolThreadGlobal::toolThread -> quit();
        ToolThreadGlobal::toolThread -> wait();
        delete ToolThreadGlobal::toolThread;
        ToolThreadGlobal::toolThread = nullptr;
    }

当ThreadStarter析构函数调用QCoreApplication :: quit（）时，QCoreApplication在其事件循环仍在运行时调用清理函数。这会调度QCoreApplication在它准备就绪时自行删除，同时将全局指针重置为NULL，以便应用程序的其余部分知道可以在需要时实例化新的QCoreApplication。

我想如果主应用程序立即创建一个新的QCoreApplication并尝试在其上运行exec（）而旧的QCoreApplication仍处于清理自身的过程中，那么可能存在冲突的风险非常小。我不认为这可能发生在我使用它的环境中。

在QThread中的QCoreApplication上调用quit（）时出错

1 个答案: