关于Qt在子线程中使用QMessageBox的折衷方法

Qt将所有GUI相关的处理都限制在主线程中，这么做有助于防止意想不到的访问冲突产生，但也限制了线程中某些简单的UI交互的实现，比如QMessageBox。

因为QMessageBox必须在主线程中打开，为了使用它，便不得不自己动手实现一些信号和槽，从而增加了自己代码的复杂度。为降低使用QMessageBox时的设计负担，本文从QThread类继承一个新类MsgBoxThread，将这些实现细节封装为MsgBoxThread的成员函数about、aboutQt、critical、information、question、warning，分别与QMessageBox的静态成员about、aboutQt、critical、information、question、warning对应，使用时可从该类继承子类，并在run函数中使用这些函数：

#include <QThread>#include <QMessageBox>class MsgBoxThread : public QThread{   Q_OBJECT   typedef enum {      mbt_about = 0,      mbt_aboutqt = 1,      mbt_critical = 2,      mbt_information = 3,      mbt_question = 4,      mbt_warning = 5   } MSGBOXTYPE;protected:   int m_btnres;   void about(QWidget * parent, const QString &title, const QString &text) {      emit msgbox_sig(mbt_about, parent, title, text, QMessageBox::NoButton, QMessageBox::NoButton);   }   void aboutQt(QWidget *parent, const QString &title = QString()) {      emit msgbox_sig(mbt_aboutqt, parent, title, tr(""), QMessageBox::NoButton, QMessageBox::NoButton);   }   int critical(QWidget * parent, const QString &title, const QString &text, QMessageBox::StandardButtons buttons = QMessageBox::Ok, QMessageBox::StandardButtons defaultButton = QMessageBox::NoButton) {      emit msgbox_sig(mbt_critical, parent, title, text, buttons, defaultButton);      return m_btnres;   }   int information(QWidget * parent, const QString &title, const QString &text, QMessageBox::StandardButtons buttons = QMessageBox::Ok, QMessageBox::StandardButtons defaultButton = QMessageBox::NoButton)   {      emit msgbox_sig(mbt_information, parent, title, text, buttons, defaultButton);      return m_btnres;   }   int question(QWidget * parent, const QString &title, const QString &text, QMessageBox::StandardButtons buttons = QMessageBox::StandardButtons(QMessageBox::Yes | QMessageBox::No), QMessageBox::StandardButtons defaultButton = QMessageBox::NoButton) {      emit msgbox_sig(mbt_question, parent, title, text, buttons, defaultButton);      return m_btnres;   }   int warning(QWidget * parent, const QString &title, const QString &text, QMessageBox::StandardButtons buttons = QMessageBox::Ok, QMessageBox::StandardButtons defaultButton = QMessageBox::NoButton) {      emit msgbox_sig(mbt_warning, parent, title, text, buttons, defaultButton);      return m_btnres;   }public:   MsgBoxThread(QObject * parent = 0)      :QThread(parent), m_btnres(QMessageBox::NoButton) {      qRegisterMetaType<QMessageBox::StandardButtons>("QMessageBox::StandardButtons");      connect(this, SIGNAL(msgbox_sig(MSGBOXTYPE, QWidget *, const QString, const QString, QMessageBox::StandardButtons, QMessageBox::StandardButtons)), SLOT(on_information(MSGBOXTYPE, QWidget *, const QString , const QString, QMessageBox::StandardButtons , QMessageBox::StandardButtons )), Qt::BlockingQueuedConnection);   }signals:    void msgbox_sig(MSGBOXTYPE type, QWidget * parent, const QString &title, const QString &text, QMessageBox::StandardButtons buttons, QMessageBox::StandardButtons defaultButton);private slots:    void on_msgbox(MSGBOXTYPE type, QWidget * parent, const QString &title, const QString &text, QMessageBox::StandardButtons buttons, QMessageBox::StandardButtons defaultButton) {       switch(type) {       case mbt_about:          QMessageBox::about(parent,title,text);          break;       case mbt_aboutqt:          QMessageBox::aboutQt(parent, title);          break;       case mbt_critical:          m_btnres = QMessageBox::critical(parent, title, text, buttons, defaultButton);          break;       case mbt_information:          m_btnres = QMessageBox::information(parent, title, text, buttons, defaultButton);          break;       case mbt_question:          m_btnres = QMessageBox::question(parent, title, text, buttons, defaultButton);          break;       case mbt_warning:          m_btnres = QMessageBox::warning(parent, title, text, buttons, defaultButton);          break;       default:          Q_ASSERT_X(false,"QMessageBox in thread","invalid box type specified.");       }    }};

原理：

自定义的MsgBoxThread类继承自QThread类，其中声明了可在子线程中发射的信号msgbox_sig，并在MsgBoxThread的构造函数中将该信号以阻塞模式(Qt::BlockingQueuedConnection)连接到私有槽函数on_msgbox。因为槽函数的父对象在主线程中创建，这保证了信号msgbox_sig在主线程的事件循环中执行；又由于槽函数on_msgbox采用组色模式连接，使得子线程在调用了这些封装函数（about、aboutQt、information、critical、warning）后会等待主线程的槽函数执行并返回后才会继续执行；综上，即可保证使用QMessageBox实现简单的UI交互，又不违反Qt的GUI相关处理必须在主线程中的限制。