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<td align="right" valign="center"><img src="logo32.png" align="right" width="64" height="32" border="0"></td></tr></table><h1 align=center>Events and Event Filters</h1>
<p> In TQt, an event is an object that inherits <a href="qevent.html">TQEvent</a>. Events are
delivered to objects that inherit <a href="qobject.html">TQObject</a> through calling <a href="qobject.html#event">TQObject::event</a>(). Event delivery means that an event has occurred, the
<a href="qevent.html">TQEvent</a> indicates precisely what, and the <a href="qobject.html">TQObject</a> needs to respond. Most
events are specific to <a href="qwidget.html">TQWidget</a> and its subclasses, but there are
important events that aren't related to graphics, for example, socket
activation, which is the event used by <a href="qsocketnotifier.html">TQSocketNotifier</a> for its
work.
<p> Some events come from the window system, e.g. <a href="qmouseevent.html">TQMouseEvent</a>, some
from other sources, e.g. <a href="qtimerevent.html">TQTimerEvent</a>, and some come from the
application program. TQt is symmetric, as usual, so you can send
events in exactly the same ways as TQt's own event loop does.
<p> Most events types have special classes, most commonly <a href="qresizeevent.html">TQResizeEvent</a>,
<a href="qpaintevent.html">TQPaintEvent</a>, <a href="qmouseevent.html">TQMouseEvent</a>, <a href="qkeyevent.html">TQKeyEvent</a> and <a href="qcloseevent.html">TQCloseEvent</a>.
There are many others, perhaps forty or so, but most are rather odd.
<p> Each class subclasses <a href="qevent.html">TQEvent</a> and adds event-specific functions; see,
for example, <a href="qresizeevent.html">TQResizeEvent</a>. In the case of <a href="qresizeevent.html">TQResizeEvent</a>, <a href="qresizeevent.html#size">TQResizeEvent::size</a>() and <a href="qresizeevent.html#oldSize">TQResizeEvent::oldSize</a>() are added.
<p> Some classes support more than one event type. <a href="qmouseevent.html">TQMouseEvent</a>
supports mouse moves, presses, shift-presses, drags, clicks,
right-presses, etc.
<p> Since programs need to react in varied and complex ways, TQt's
event delivery mechanisms are flexible. The documentation for
<a href="qapplication.html#notify">TQApplication::notify</a>() concisely tells the whole story, here we
will explain enough for 99% of applications.
<p> The normal way for an event to be delivered is by calling a virtual
function. For example, <a href="qpaintevent.html">TQPaintEvent</a> is delivered by calling <a href="qwidget.html#paintEvent">TQWidget::paintEvent</a>(). This virtual function is responsible for
reacting appropriately, normally by repainting the widget. If you
do not perform all the necessary work in your implementation of the
virtual function, you may need to call the base class's
implementation; for example:
<pre>
MyTable::contentsMouseMoveEvent( <a href="qmouseevent.html">TQMouseEvent</a> *me )
{
// my implementation
TQTable::<a href="qscrollview.html#contentsMouseMoveEvent">contentsMouseMoveEvent</a>( me ); // hand it on
}
</pre>
If you want to replace the base class's function then you must
implement everything yourself; but if you only want to extend the base
class's functionality, then you implement what you want and then call
the base class.
<p> Occasionally there isn't such an event-specific function, or the
event-specific function isn't sufficient. The most common example is
tab key presses. Normally, those are interpreted by <a href="qwidget.html">TQWidget</a> to move
the <a href="focus.html#keyboard-focus">keyboard focus</a>, but a few widgets need the tab key for themselves.
<p> These objects can reimplement <a href="qobject.html#event">TQObject::event</a>(), the general event
handler, and either do their event handling before or after the usual
handling, or replace it completely. A very unusual widget that both
interprets tab and has an application-specific custom event might
contain:
<p> <pre>
bool MyClass:event( <a href="qevent.html">TQEvent</a> *evt ) {
if ( evt-><a href="qevent.html#type">type</a>() == TQEvent::KeyPress ) {
<a href="qkeyevent.html">TQKeyEvent</a> *ke = (TQKeyEvent *)evt;
if ( ke-><a href="qkeyevent.html#key">key</a>() == Key_Tab ) {
// special tab handling here
ke-><a href="qkeyevent.html#accept">accept</a>();
return TRUE;
}
} else if ( evt-><a href="qevent.html#type">type</a>() >= TQEvent::User ) {
<a href="qcustomevent.html">TQCustomEvent</a> *ce = (TQCustomEvent*) evt;
// custom event handling here
return TRUE;
}
return TQWidget::event( evt );
}
</pre>
<p> More commonly, an object needs to look at another's events. TQt
supports this using <a href="qobject.html#installEventFilter">TQObject::installEventFilter</a>() (and the
corresponding remove). For example, dialogs commonly want to filter
key presses for some widgets, e.g. to modify Return-key handling.
<p> An event filter gets to process events before the target object does.
The filter's <a href="qobject.html#eventFilter">TQObject::eventFilter</a>() implementation is called, and
can accept or reject the filter, and allow or deny further processing
of the event. If all the event filters allow further processing of an
event, the event is sent to the target object itself. If one of them
stops processing, the target and any later event filters don't get to
see the event at all.
<p> It's also possible to filter <em>all</em> events for the entire application,
by installing an event filter on <a href="qapplication.html">TQApplication</a>. This is what <a href="qtooltip.html">TQToolTip</a> does in order to see <em>all</em> the mouse and keyboard activity.
This is very powerful, but it also slows down event delivery of every
single event in the entire application, so it's best avoided.
<p> The global event filters are called before the object-specific
filters.
<p> Finally, many applications want to create and send their own events.
<p> Creating an event of a built-in type is very simple: create an object
of the relevant type, and then call <a href="qapplication.html#sendEvent">TQApplication::sendEvent</a>() or <a href="qapplication.html#postEvent">TQApplication::postEvent</a>().
<p> sendEvent() processes the event immediately - when sendEvent()
returns, (the event filters and) the object have already processed the
event. For many event classes there is a function called isAccepted()
that tells you whether the event was accepted or rejected by the last
handler that was called.
<p> postEvent() posts the event on a queue for later dispatch. The next
time TQt's main event loop runs, it dispatches all posted events, with
some optimization. For example, if there are several resize events,
they are are compacted into one. The same applies to paint events: <a href="qwidget.html#update">TQWidget::update</a>() calls postEvent(), which minimizes flickering and
increases speed by avoiding multiple repaints.
<p> postEvent() is also often used during object initialization, since the
posted event will typically be dispatched very soon after the
initialization of the object is complete.
<p> To create events of a custom type, you need to define an event number,
which must be greater than <a href="qevent.html#Type-enum">TQEvent::User</a>, and probably you also need
to subclass <a href="qcustomevent.html">TQCustomEvent</a> in order to pass characteristics about
your custom event. See the documentation to <a href="qcustomevent.html">TQCustomEvent</a> for
details.
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