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
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()
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;