Qt事件分发机制源码分析之QApplication对象构建过程

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