XtAppPeekEvent() XtAppPeekEvent()
Name
XtAppPeekEvent - return, but do not remove the event at the head of an application's input queue; block if no events are available.
Synopsis
Boolean XtAppPeekEvent(app_context, event_return)
XtAppContext app_context;
XEvent *event_return;
Inputs
app_context
Specifies the application context.
Outputs
event_return
Returns the event from the head of the queue, if that event is an X event.
Returns
True if the event at the head of the queue is an X event; False if it is a timer event or an alternate input source event.
Description
If there are X events pending on any of the displays in app_context, XtAppPeekEvent() copies the event from the head of the application
event queue into event_return (without removing the event from the queue) and returns True. If there are no events, it flushes the output
buffers of each display and checks again. If there are still no pending X events on any of the displays, but there are timer or alternate
input events ready, XtAppPeekEvent() returns False.
If there are no events of any kind, XtAppPeekEvent() blocks until one occurs, and then if it is an X event, copies the event (without
removing it from the queue) and returns True or returns False otherwise. Note that XtAppPeekEvent() never calls background work procedures
registered with XtAppAddWorkProc() .
Usage
Programs rarely need this much control over the event dispatching mechanism. Most programs use XtAppMainLoop().
If you want to get X events and remove them from the input queue, consider XtAppNextEvent(). This function also dispatches timer and
alternate input events.
If you want to check for input events without blocking, use XtAppPending(). This function returns a value that indicates which types of
events are pending for an application context, or 0 if no events are pending.
See AlsoXtAppMainLoop(1), XtAppNextEvent(1), XtAppPending(1).
Xt - Event Handling XtAppPeekEvent()
Check Out this Related Man Page
XtAppNextEvent(3Xt)XtAppNextEvent(3Xt)NAME
XtAppNextEvent, XtAppPending, XtAppPeekEvent, XtAppProcessEvent, XtDispatchEvent, XtAppMainLoop - query and process events and input
SYNOPSIS
void XtAppNextEvent(app_context, event_return)
XtAppContext app_context;
XEvent *event_return;
Boolean XtAppPeekEvent(app_context, event_return)
XtAppContext app_context;
XEvent *event_return;
XtInputMask XtAppPending(app_context)
XtAppContext app_context;
void XtAppProcessEvent(app_context, mask)
XtAppContext app_context;
XtInputMask mask;
Boolean XtDispatchEvent(event)
XEvent *event;
void XtAppMainLoop(app_context)
XtAppContext app_context;
ARGUMENTS
Specifies the application context that identifies the application. Specifies a pointer to the event structure that is to be dispatched to
the appropriate event handler. Returns the event information to the specified event structure. Specifies what types of events to process.
The mask is the bitwise inclusive OR of any combination of XtIMXEvent, XtIMTimer, XtIMAlternateInput, and XtIMSignal. As a convenience, the
X Toolkit defines the symbolic name XtIMAll to be the bitwise inclusive OR of all event types.
DESCRIPTION
If the X event queue is empty, XtAppNextEvent flushes the X output buffers of each Display in the application context and waits for an
event while looking at the other input sources, timeout values, and signal handlers and calling any callback procedures triggered by them.
This wait time can be used for background processing (see Section 7.8).
If there is an event in the queue, XtAppPeekEvent fills in the event and returns a nonzero value. If no X input is on the queue,
XtAppPeekEvent flushes the output buffer and blocks until input is available (possibly calling some timeout callbacks in the process). If
the input is an event, XtAppPeekEvent fills in the event and returns a nonzero value. Otherwise, the input is for an alternate input
source, and XtAppPeekEvent returns zero.
The XtAppPending function returns a nonzero value if there are events pending from the X server, timer pending, or other input sources
pending. The value returned is a bit mask that is the OR of XtIMXEvent, XtIMTimer, XtIMAlternateInput, and XtIMSignal (see XtAppProcessEv-
ent). If there are no events pending, XtAppPending flushes the output buffer and returns zero.
The XtAppProcessEvent function processes one timer, alternate input, signal source, or X event. If there is nothing of the appropriate
type to process, XtAppProcessEvent blocks until there is. If there is more than one type of thing available to process, it is undefined
which will get processed. Usually, this procedure is not called by client applications (see XtAppMainLoop). XtAppProcessEvent processes
timer events by calling any appropriate timer callbacks, alternate input by calling any appropriate alternate input callbacks, signal
source by calling any appropriate signal callbacks, and X events by calling XtDispatchEvent.
When an X event is received, it is passed to XtDispatchEvent, which calls the appropriate event handlers and passes them the widget, the
event, and client-specific data registered with each procedure. If there are no handlers for that event registered, the event is ignored
and the dispatcher simply returns. The order in which the handlers are called is undefined.
The XtDispatchEvent function sends those events to the event handler functions that have been previously registered with the dispatch rou-
tine. XtDispatchEvent returns True if it dispatched the event to some handler and False if it found no handler to dispatch the event to.
The most common use of XtDispatchEvent is to dispatch events acquired with the XtAppNextEvent procedure. However, it also can be used to
dispatch user-constructed events. XtDispatchEvent also is responsible for implementing the grab semantics for XtAddGrab.
The XtAppMainLoop function first reads the next incoming X event by calling XtAppNextEvent and then it dispatches the event to the appro-
priate registered procedure by calling XtDispatchEvent. This constitutes the main loop of X Toolkit applications, and, as such, it does not
return. Applications are expected to exit in response to some user action. There is nothing special about XtAppMainLoop; it is simply an
infinite loop that calls XtAppNextEvent and then XtDispatchEvent.
Applications can provide their own version of this loop, which tests some global termination flag or tests that the number of top-level
widgets is larger than zero before circling back to the call to XtAppNextEvent.
SEE ALSO
X Toolkit Intrinsics -- C Language Interface
Xlib -- C Language X Interface
XtAppNextEvent(3Xt)