Detailed Description
The QEvent class is the base class of all event classes. Event objects contain event parameters.
Qt's main event loop (QCoreApplication::exec()) fetches native window system events from the event queue, translates them into QEvents, and sends the translated events to QObjects.
In general, events come from the underlying window system (spontaneous() returns true), but it is also possible to manually send events usingQCoreApplication::sendEvent() and QCoreApplication::postEvent() (spontaneous() returns false).
QObjects receive events by having their QObject::event() function called. The function can be reimplemented in subclasses to customize event handling and add additional event types; QWidget::event() is a notable example. By default, events are dispatched to event handlers like QObject::timerEvent() andQWidget::mouseMoveEvent(). QObject::installEventFilter() allows an object to intercept events destined for another object.
The basic QEvent contains only an event type parameter and an "accept" flag. The accept flag set with accept(), and cleared with ignore(). It is set by default, but don't rely on this as subclasses may choose to clear it in their constructor.
Subclasses of QEvent contain additional parameters that describe the particular event.
See also QObject::event(), QObject::installEventFilter(), QWidget::event(), QCoreApplication::sendEvent(), QCoreApplication::postEvent(), andQCoreApplication::processEvents().
如果是外部的事件(a system event)，就是spontaneous （自发的）。
accept默认是true的。但子类可能改写。