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152 lines
8.8 KiB
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<title>Events and Event Filters</title>
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<font color="#004faf">Home</font></a>
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<font color="#004faf">All Classes</font></a>
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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>
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<p> In TQt, an event is an object that inherits <a href="ntqevent.html">TQEvent</a>. Events are
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delivered to objects that inherit <a href="tqobject.html">TQObject</a> through calling <a href="tqobject.html#event">TQObject::event</a>(). Event delivery means that an event has occurred, the
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<a href="ntqevent.html">TQEvent</a> indicates precisely what, and the <a href="tqobject.html">TQObject</a> needs to respond. Most
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events are specific to <a href="tqwidget.html">TQWidget</a> and its subclasses, but there are
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important events that aren't related to graphics, for example, socket
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activation, which is the event used by <a href="ntqsocketnotifier.html">TQSocketNotifier</a> for its
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work.
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<p> Some events come from the window system, e.g. <a href="qmouseevent.html">TQMouseEvent</a>, some
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from other sources, e.g. <a href="qtimerevent.html">TQTimerEvent</a>, and some come from the
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application program. TQt is symmetric, as usual, so you can send
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events in exactly the same ways as TQt's own event loop does.
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<p> Most events types have special classes, most commonly <a href="qresizeevent.html">TQResizeEvent</a>,
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<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>.
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There are many others, perhaps forty or so, but most are rather odd.
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<p> Each class subclasses <a href="ntqevent.html">TQEvent</a> and adds event-specific functions; see,
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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.
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<p> Some classes support more than one event type. <a href="qmouseevent.html">TQMouseEvent</a>
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supports mouse moves, presses, shift-presses, drags, clicks,
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right-presses, etc.
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<p> Since programs need to react in varied and complex ways, TQt's
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event delivery mechanisms are flexible. The documentation for
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<a href="ntqapplication.html#notify">TQApplication::notify</a>() concisely tells the whole story, here we
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will explain enough for 99% of applications.
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<p> The normal way for an event to be delivered is by calling a virtual
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function. For example, <a href="qpaintevent.html">TQPaintEvent</a> is delivered by calling <a href="tqwidget.html#paintEvent">TQWidget::paintEvent</a>(). This virtual function is responsible for
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reacting appropriately, normally by repainting the widget. If you
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do not perform all the necessary work in your implementation of the
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virtual function, you may need to call the base class's
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implementation; for example:
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<pre>
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MyTable::contentsMouseMoveEvent( <a href="qmouseevent.html">TQMouseEvent</a> *me )
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{
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// my implementation
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TQTable::<a href="ntqscrollview.html#contentsMouseMoveEvent">contentsMouseMoveEvent</a>( me ); // hand it on
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}
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</pre>
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If you want to replace the base class's function then you must
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implement everything yourself; but if you only want to extend the base
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class's functionality, then you implement what you want and then call
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the base class.
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<p> Occasionally there isn't such an event-specific function, or the
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event-specific function isn't sufficient. The most common example is
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tab key presses. Normally, those are interpreted by <a href="tqwidget.html">TQWidget</a> to move
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the <a href="focus.html#keyboard-focus">keyboard focus</a>, but a few widgets need the tab key for themselves.
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<p> These objects can reimplement <a href="tqobject.html#event">TQObject::event</a>(), the general event
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handler, and either do their event handling before or after the usual
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handling, or replace it completely. A very unusual widget that both
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interprets tab and has an application-specific custom event might
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contain:
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<p> <pre>
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bool MyClass:event( <a href="ntqevent.html">TQEvent</a> *evt ) {
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if ( evt-><a href="ntqevent.html#type">type</a>() == TQEvent::KeyPress ) {
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<a href="qkeyevent.html">TQKeyEvent</a> *ke = (TQKeyEvent *)evt;
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if ( ke-><a href="qkeyevent.html#key">key</a>() == Key_Tab ) {
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// special tab handling here
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ke-><a href="qkeyevent.html#accept">accept</a>();
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return TRUE;
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}
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} else if ( evt-><a href="ntqevent.html#type">type</a>() >= TQEvent::User ) {
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<a href="qcustomevent.html">TQCustomEvent</a> *ce = (TQCustomEvent*) evt;
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// custom event handling here
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return TRUE;
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}
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return TQWidget::event( evt );
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}
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</pre>
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<p> More commonly, an object needs to look at another's events. TQt
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supports this using <a href="tqobject.html#installEventFilter">TQObject::installEventFilter</a>() (and the
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corresponding remove). For example, dialogs commonly want to filter
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key presses for some widgets, e.g. to modify Return-key handling.
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<p> An event filter gets to process events before the target object does.
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The filter's <a href="tqobject.html#eventFilter">TQObject::eventFilter</a>() implementation is called, and
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can accept or reject the filter, and allow or deny further processing
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of the event. If all the event filters allow further processing of an
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event, the event is sent to the target object itself. If one of them
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stops processing, the target and any later event filters don't get to
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see the event at all.
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<p> It's also possible to filter <em>all</em> events for the entire application,
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by installing an event filter on <a href="ntqapplication.html">TQApplication</a>. This is what <a href="ntqtooltip.html">TQToolTip</a> does in order to see <em>all</em> the mouse and keyboard activity.
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This is very powerful, but it also slows down event delivery of every
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single event in the entire application, so it's best avoided.
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<p> The global event filters are called before the object-specific
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filters.
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<p> Finally, many applications want to create and send their own events.
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<p> Creating an event of a built-in type is very simple: create an object
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of the relevant type, and then call <a href="ntqapplication.html#sendEvent">TQApplication::sendEvent</a>() or <a href="ntqapplication.html#postEvent">TQApplication::postEvent</a>().
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<p> sendEvent() processes the event immediately - when sendEvent()
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returns, (the event filters and) the object have already processed the
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event. For many event classes there is a function called isAccepted()
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that tells you whether the event was accepted or rejected by the last
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handler that was called.
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<p> postEvent() posts the event on a queue for later dispatch. The next
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time TQt's main event loop runs, it dispatches all posted events, with
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some optimization. For example, if there are several resize events,
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they are are compacted into one. The same applies to paint events: <a href="tqwidget.html#update">TQWidget::update</a>() calls postEvent(), which minimizes flickering and
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increases speed by avoiding multiple repaints.
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<p> postEvent() is also often used during object initialization, since the
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posted event will typically be dispatched very soon after the
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initialization of the object is complete.
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<p> To create events of a custom type, you need to define an event number,
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which must be greater than <a href="ntqevent.html#Type-enum">TQEvent::User</a>, and probably you also need
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to subclass <a href="qcustomevent.html">TQCustomEvent</a> in order to pass characteristics about
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your custom event. See the documentation to <a href="qcustomevent.html">TQCustomEvent</a> for
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details.
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<p>
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<a href="troll.html">Trolltech</a><td align=center><a href="trademarks.html">Trademarks</a>
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<td align=right><div align=right>TQt 3.3.8</div>
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