我们在新建一个Qt GUI项目时，main函数里会生成类似下面的代码：

int main(int argc, char *argv[])
{
    QApplication application(argc, argv);
    CQDialog dialog(NULL);
    dialog.show();
    return application.exec();
}

对应的步骤解释如下

1.构建QApplication对象
2.构建CQDialog主界面
3.主界面显示
4.QApplication对象进入事件循环处理直至退出

上述步骤包含QApplication对象构建过程、主界面显示过程、事件循环处理过程三个主题。

这篇博文主要讲解第一个主题，即QApplication对象构建过程。

QApplication类继承关系如下图所示

查看Qt源码QApplication的构造函数

#ifdef Q_QDOC
QApplication::QApplication(int &argc, char **argv)
#else
QApplication::QApplication(int &argc, char **argv, int _internal)
#endif
    : QGuiApplication(*new QApplicationPrivate(argc, argv, _internal))
{
    Q_D(QApplication);
    d->init();
}

QApplication父类QGuiApplication的构造函数

QGuiApplication::QGuiApplication(QGuiApplicationPrivate &p)
    : QCoreApplication(p)
{
}

可以看到QGuiApplication的构造函数为空内容,进入到QGuiApplication父类QCoreApplication的构造函数

QCoreApplication::QCoreApplication(QCoreApplicationPrivate &p)
#ifdef QT_NO_QOBJECT
    : d_ptr(&p)
#else
    : QObject(p, 0)
#endif
{
    d_func()->q_ptr = this;
    // note: it is the subclasses' job to call
    // QCoreApplicationPrivate::eventDispatcher->startingUp();
}

其也没有实际性的内容。

主要集中在QApplicationPrivate、QGuiApplicationPrivate、QCoreApplicationPrivate类的内部处理，这也是Qt一贯的用法，即信息隐藏。

其类关系图如下

因此函数调用返回到QApplication构造函数中，QApplicationPrivate::init函数被调用用于初始化操作

void QApplicationPrivate::init()
{
#if defined(Q_OS_MACOS)
    QMacAutoReleasePool pool;
#endif

    QGuiApplicationPrivate::init();

    initResources();

    qt_is_gui_used = (application_type != QApplicationPrivate::Tty);
    process_cmdline();

    // Must be called before initialize()
    qt_init(this, application_type);
    initialize();
    eventDispatcher->startingUp();

#ifdef QT_EVAL
    extern void qt_gui_eval_init(QCoreApplicationPrivate::Type);
    qt_gui_eval_init(application_type);
#endif
#ifndef QT_NO_ACCESSIBILITY
    // factory for accessible interfaces for widgets shipped with Qt
    QAccessible::installFactory(&qAccessibleFactory);
#endif

}

QGuiApplicationPrivate::init会调用QCoreApplicationPrivate::init，QCoreApplicationPrivate::init会进行eventDispatcher的创建，如下代码所示

#ifndef QT_NO_QOBJECT
    // use the event dispatcher created by the app programmer (if any)
    if (!eventDispatcher)
        eventDispatcher = threadData->eventDispatcher.load();
    // otherwise we create one
    if (!eventDispatcher)
        createEventDispatcher();
    Q_ASSERT(eventDispatcher);

    if (!eventDispatcher->parent()) {
        eventDispatcher->moveToThread(threadData->thread);
        eventDispatcher->setParent(q);
    }

    threadData->eventDispatcher = eventDispatcher;
    eventDispatcherReady();
#endif

基于多态性，QGuiApplicationPrivate::createEventDispatcher被调用

void QGuiApplicationPrivate::createEventDispatcher()
{
    Q_ASSERT(!eventDispatcher);

    if (platform_integration == 0)
        createPlatformIntegration();

    // The platform integration should not mess with the event dispatcher
    Q_ASSERT(!eventDispatcher);

    eventDispatcher = platform_integration->createEventDispatcher();
}

createEventDispatcher函数里做两件事情

1.创建平台插件(Windows、Linux)
2.根据平台插件创建eventDispatcher

以我在Windows平台上开发为例

1.创建QWindowsIntegration以及QWindowsGuiEventDispatcher
2.在QWindowsIntegration创建过程中会生成QWindowsContext对象

QEventDispatcherWin32类继承关系如下图所示

因此，QApplication构造时创建了eventDispatcher

关于QApplication对象构建过程就讲述完毕了，后续博文会看到eventDispatcher、QWindowsContext的用途

有部分代码位于qtbase\src\plugins\platforms源码目录