QWidget 键盘事件焦点（源代码级别研究）

在Qt中，键盘事件和QWidget的focus密不可分：一般来说，一个拥有焦点(focus)的QWidget或者grabKeyboard()的QWidget才可以接受键盘事件。

键盘事件派发给谁？

如何确定谁来接收键盘事件，不妨看一点点QApplication的源码：

X11下

    QETWidget *keywidget=0;

    bool grabbed=false;

    if (event->type==XKeyPress || event->type==XKeyRelease) {

        keywidget = (QETWidget*)QWidget::keyboardGrabber();

        if (keywidget) {

            grabbed = true;

        } else if (!keywidget) {

            if (d->inPopupMode()) // no focus widget, see if we have a popup

                keywidget = (QETWidget*) (activePopupWidget()->focusWidget() ? activePopupWidget()->focusWidget() : activePopupWidget());

            else if (QApplicationPrivate::focus_widget)

                keywidget = (QETWidget*)QApplicationPrivate::focus_widget;

            else if (widget)

                keywidget = (QETWidget*)widget->window();

        }

    }

Windows下

            QWidget *g = QWidget::keyboardGrabber();

            if (g && qt_get_tablet_widget() && hwnd == qt_get_tablet_widget()->winId()) {

                // if we get an event for the internal tablet widget,

                // then don't send it to the keyboard grabber, but

                // send it to the widget itself (we don't use it right

                // now, just in case).

                g = 0;

            }

            if (g)

                widget = (QETWidget*)g;

            else if (QApplication::activePopupWidget())

                widget = (QETWidget*)QApplication::activePopupWidget()->focusWidget()

                       ? (QETWidget*)QApplication::activePopupWidget()->focusWidget()

                       : (QETWidget*)QApplication::activePopupWidget();

            else if (QApplication::focusWidget())

                widget = (QETWidget*)QApplication::focusWidget();

            else if (!widget || widget->internalWinId() == GetFocus()) // We faked the message to go to exactly that widget.

                widget = (QETWidget*)widget->window();

大致顺序：

QWidget::keyboardGrabber()
QApplication::activePopupWidget()
QApplication::focusWidget()
QWidget::window() [注：对于native的接收到键盘事件的widget，此时Qt将派发给其所属窗口]

在QWidget间切换焦点

在Qt键盘事件一文中我们提到这个和focusPolicy相关。我们可以通过Tab键或者鼠标单击来使得某个QWidget获得焦点。

问题：当我们按下Tab键(或者上下左右箭头键)时，下一个或获取焦点的QWidget是如何被确定的？

我们重新贴出上文最后贴出过的QWidget::event()的源码：

    case QEvent::KeyPress: {

        QKeyEvent *k = (QKeyEvent *)event;

        bool res = false;

        if (!(k->modifiers() & (Qt::ControlModifier | Qt::AltModifier))) {  //### Add MetaModifier?

            if (k->key() == Qt::Key_Backtab

                || (k->key() == Qt::Key_Tab && (k->modifiers() & Qt::ShiftModifier)))

                res = focusNextPrevChild(false);

            else if (k->key() == Qt::Key_Tab)

                res = focusNextPrevChild(true);

            if (res)

                break;

        }

        keyPressEvent(k);

老是觉得 QWidget::focusNextPrevChild() 这个函数有点名不符实（或者有点别扭），因为：

bool QWidget::focusNextPrevChild(bool next)

{

    Q_D(QWidget);

    QWidget* p = parentWidget();

    bool isSubWindow = (windowType() == Qt::SubWindow);

    if (!isWindow() && !isSubWindow && p)

        return p->focusNextPrevChild(next);

...

}

当我们调用一个Widget该成员时，最终将递归调用到其所在窗口的focusNextPrevChild成员。（不过这是一个protected的虚函数，在派生类中可以覆盖它，从而控制派生类实例中的焦点移动。）

prev/next

QWidgetPrivate内有3个成员变量：

class Q_GUI_EXPORT QWidgetPrivate : public QObjectPrivate

{

...

    QWidget *focus_next;

    QWidget *focus_prev;

    QWidget *focus_child;

这3个变量可以分别用：

QWidget::nextInFocusChain()
QWidget::previousInFocusChain()
QWidget::focusWidget() [注意区分：QApplication::focusWidget()]

进行获取。

前两个可以用来构成一个focus链表。

void QWidgetPrivate::init(QWidget *parentWidget, Qt::WindowFlags f)

{

    Q_Q(QWidget);

    focus_next = focus_prev = q;

...

void QWidgetPrivate::reparentFocusWidgets(QWidget * oldtlw)

{

...

通过QWidget::setTabOrder()可以调整Widgets在focus链表中的顺序

Widget上放置大量按钮怎么样？

比如一个类似软键盘的东西，在一个QWidget上面放置了大量的QPushButton。此时，除了Tab/Shift+Tab外，上下左右箭头也都可以用来移动焦点。

这是因为：

void QAbstractButton::keyPressEvent(QKeyEvent *e)

{

    bool next = true;

    switch (e->key()) {

    case Qt::Key_Up:

    case Qt::Key_Left:

        next = false;

        // fall through

    case Qt::Key_Right:

    case Qt::Key_Down:

...

            focusNextPrevChild(next);

}

focus proxy

QWidget::setFocusProxy()
QWidget::focusProxy()

Manual中说的比较清楚：

If there is a focus proxy, setFocus() and hasFocus() operate on the focus proxy.

简单列出源码：

void QWidget::setFocus(Qt::FocusReason reason)

{

    QWidget *f = this;

    while (f->d_func()->extra && f->d_func()->extra->focus_proxy)

        f = f->d_func()->extra->focus_proxy;

bool QWidget::hasFocus() const

{

    const QWidget* w = this;

    while (w->d_func()->extra && w->d_func()->extra->focus_proxy)

        w = w->d_func()->extra->focus_proxy;

其他

对于 Qt for Embedded Linux、Symbian 和 Windows CE，还有一个

QApplication::setNavigationMode()

设置可通过方向键来控制焦点进行上下左右的移动。

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setfocus() 是让某个窗体获得焦点

setfocusPolicy 是设置窗体怎么获得焦点

he focus policy is Qt::TabFocus if the widget accepts keyboard focus by tabbing, Qt::ClickFocus if the widget accepts focus by clicking, Qt::StrongFocus if it accepts both, and Qt::NoFocus (the default) if it does not accept focus at all.

void QWidget::setFocusProxy ( QWidget * w ) `[virtual] 就是把窗体获得焦点时的处理，委托给窗体w处理`

Sets this widget's focus proxy to w. If w is 0, this function resets this widget to not have any focus proxy.

Some widgets, such as QComboBox, can "have focus," but create a child widget to actually handle the focus. QComboBox, for example, creates a QLineEdit.

setFocusProxy() sets the widget which will actually get focus when "this widget" gets it. If there is a focus proxy, focusPolicy(), setFocusPolicy(), setFocus() and hasFocus() all operate on the focus proxy.