在QThread中的QCoreApplication上調用quit（）時發生錯誤

Question

爲了在一個單獨的線程中創建一個Qt事件循環，在由Java編寫的主應用程序調用的DLL中創建一個Qt事件循環，我做了以下工作，根據建議，我讀here，它的工作原理相當不錯：

// 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); 
     } 
    } 
}; 

要使用這一點，從主要的Java應用程序的線程調用的子程序只需要創建一個ThreadStarter對象 將自動與創建ToolThread在其中運行的QCoreApplication：

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

然後我們就可以實例化一個QObject類以通常的方式，將其移動到線程異步和使用調用它的方法QMetaObject :: invokeMethod中：

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

QMetaObject::invokeMethod(itsWorker, 「doSomethingInteresting」); 

當我們做，我們只是刪除ThreadStarter對象一切都很好地清理。除了令人討厭的消息說

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

在啓動時，它似乎滿足我所有的要求。

除...（以及最後，這是我的問題）。

有時 - 並且沒有任何我目前能夠辨別的模式 - 在關機過程中出現錯誤。它通常發生在行

 delete ToolThreadGlobal::coreApp; 

，但有時在該行

ToolThreadGlobal::coreApp -> exec(); 

（當然，在線程的run執行（）方法並不會返回，直到ToolThreadGlobal :: coreApp後 - > quit（）;已經完全執行）。

錯誤消息通常是一個簡單的訪問衝突;有時它是一個相當更復雜：

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

我以爲這是因爲，一旦我發出退出（）命令將QCoreApplication，我要等待一些時間，它在刪除之前，正確關閉事件循環它 - 就像人們通常在刪除它之前通常調用quit（）然後在普通的QThread上等待（）。然而，QCoreApplication似乎沒有等效的wait（）命令，並且我無法實現QTimer來強制延遲，因爲一旦我使用quit（）關閉了事件循環，它就無法工作。因此，我無所適從。我有一個暗示，因爲QCoreApplication是一個QObject，我可以調用它的deleteLater（）方法，但我看不到應該從哪裏調用它。

是否有專家瞭解QCoreApplication和QThread的來龍去脈並提出解決方案？或者我設計的方式存在根本性缺陷？

Answer 1

這似乎適用於我...

首先，我添加了一個靜態的 '清理' 功能，我的全局命名空間：

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; } 
}; 

然後，從我的ThreadStarter :: createApplication插槽我的QCoreApplication的aboutToQuit信號連接到它：

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); 
     } 
    } 

然後，'ThreadStarter'析構函數減少到只有五行（包括Qthread :: quit（）和QThread :: wait（）的調用，這應該是第一次出現）：

~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; 
    } 

當ThreadStarter析構函數調用QCoreApplication :: quit（）時，QCoreApplication在其事件循環仍在運行時調用清除函數。這會安排QCoreApplication在其準備好之後自行刪除，同時將全局指針重置爲NULL，以便應用程序的其餘部分知道可以在需要時實例化新的QCoreApplication。

如果主應用程序立即創建一個新的QCoreApplication並嘗試在其上運行exec（），而舊的QCoreApplication仍在清理自己的過程中，則可能會有衝突的風險很小。我不認爲這可能發生在我使用它的環境中。