Applying data warehousing principles to event processing
by Philip Howard,* Bloor ResearchIt is now pretty much agreed that in data warehousing environments you need a massively parallel processing (MPP) architecture in order to handle very large data volumes. The important factor is that you have local processing close to each disk in order to improve performance rather than moving all of the [...]
XtEventHandler() XtEventHandler()
Name
XtEventHandler - interface definition for event handler procedure.
Synopsis
typedef void (*XtEventHandler)(Widget, XtPointer, XEvent *, Boolean *);
Widget w;
XtPointer client_data;
XEvent *event;
Boolean *continue_to_dispatch_return
Inputs
w Specifies the widget for which the handler was registered.
client_data
Specifies data registered with this event handler.
event Specifies the event that triggered this call.
Outputs
continue_to_dispatch_return
Returns a Boolean indicating whether to call the remaining event handlers that are registered for the current event.
Description
An XtEventHandler is registered with XtAddEventHandler(), XtAddRawEventHandler(), XtInsertEventHandler(), or XtInsertRawEventHandler(). It
is called when one of the events that it was registered to handle occurs on the widget it was registered for.
An XtEventHandler should do whatever processing is necessary for the widget or application to handle the event event that occurred on wid-
get w. The client_data argument can be arbitrary data registered with the event handler procedure. It is cast to an XtPointer when regis-
tered and should be cast back to the appropriate type within the event handler.
The continue_to_dispatch_return is the address of a Boolean variable which is intialized to True by the Intrinsics before the event handler
is called. If a handler sets this variable to False, then no more handlers will be dispatched for the event. Doing this may lead to
portability problems because implementations of the Intrinsics are allowed to add event handlers for any widget at any time. If you pre-
vent these potential "invisible" event handlers from receiving events, the Intrinsics are not guaranteed to behave as expected.
Usage
Most widgets and applications do not need to use event handlers explicitly. Instead they can use translation tables and action procedures.
Example
The procedure below is an event handler from the xmag client. It is a special handler that follows the location of the mouse while it is
dragged with the button down and a pointer grab is in effect. When the button is released, it magnifies a new area of the screen,
releases the pointer grab, and calls XtRemoveEventHandler() on itself.
/*
* ResizeEH() -- Event Handler for resize of selection box.
*/
static void
ResizeEH(w, closure, event, continue_to_dispatch) /* ARGSUSED */
Widget w; XtPointer closure; XEvent *event; Boolean *continue_to_dispatch;
{
hlPtr data = (hlPtr)closure;
switch (event->type) {
case MotionNotify:
data->x = event->xmotion.x_root;
data->y = event->xmotion.y_root;
break;
case ButtonRelease:
GetImageAndAttributes(FindWindow(event->xmotion.x_root,
event->xmotion.y_root),
min(data->homeX,event->xbutton.x_root),
min(data->homeY,event->xbutton.y_root),
abs(data->homeX - event->xbutton.x_root),
abs(data->homeY - event->xbutton.y_root),
data);
if (data->newScale)
PopupNewScale(data);
else
SWSetImage(data->scaleInstance, data->image);
XtUngrabPointer(w, CurrentTime);
XtRemoveEventHandler(w, PointerMotionMask ButtonReleaseMask,
True, ResizeEH, (XtPointer)data);
data->selectMode = done;
break;
}
}
This event handler is registered with the following code, invoked when the user presses mouse button 2.
XtAddEventHandler(w, PointerMotionMask ButtonReleaseMask,
True, ResizeEH, (XtPointer)data);
See AlsoXtAddEventHandler(1), XtAddRawEventHandler(1), XtAppAddActions(1), XtRemoveEventHandler(1), XtRemoveRawEventHandler(1).
Xt - Event Handling XtEventHandler()