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Tk_RestrictEvents(3)  Tk Library Procedures



Tk_RestrictEvents - filter and selectively delay X events


#include <tk.h> Tk_RestrictProc * Tk_RestrictEvents(proc, clientData, prevClientDataPtr)


Tk_RestrictProc *proc (in) Predicate procedure to call to filter incoming X events. NULL means do not restrict events at all. ClientData clientData (in) Arbitrary argument to pass to proc. ClientData *prevClientDataPtr (out) Pointer to place to save argu- ment to previous restrict procedure. _________________________________________________________________


This procedure is useful in certain situations where appli- cations are only prepared to receive certain X events. After Tk_RestrictEvents is called, Tk_DoOneEvent (and hence Tk_MainLoop) will filter X input events through proc. Proc indicates whether a given event is to be processed immedi- ately, deferred until some later time (e.g. when the event restriction is lifted), or discarded. Proc is a procedure with arguments and result that match the type Tk_RestrictProc: typedef Tk_RestrictAction Tk_RestrictProc( ClientData clientData, XEvent *eventPtr); Tk 1 Tk_RestrictEvents(3) Tk Library Procedures The clientData argument is a copy of the clientData passed to Tk_RestrictEvents; it may be used to provide proc with information it needs to filter events. The eventPtr points to an event under consideration. Proc returns a restrict action (enumerated type Tk_RestrictAction) that indicates what Tk_DoOneEvent should do with the event. If the return value is TK_PROCESS_EVENT, then the event will be handled immediately. If the return value is TK_DEFER_EVENT, then the event will be left on the event queue for later process- ing. If the return value is TK_DISCARD_EVENT, then the event will be removed from the event queue and discarded without being processed. Tk_RestrictEvents uses its return value and prevClientDa- taPtr to return information about the current event restric- tion procedure (a NULL return value means there are currently no restrictions). These values may be used to restore the previous restriction state when there is no longer any need for the current restriction. There are very few places where Tk_RestrictEvents is needed. In most cases, the best way to restrict events is by chang- ing the bindings with the bind Tcl command or by calling Tk_CreateEventHandler and Tk_DeleteEventHandler from C. The main place where Tk_RestrictEvents must be used is when per- forming synchronous actions (for example, if you need to wait for a particular event to occur on a particular window but you don't want to invoke any handlers for any other events). The ``obvious'' solution in these situations is to call XNextEvent or XWindowEvent, but these procedures cannot be used because Tk keeps its own event queue that is separate from the X event queue. Instead, call Tk_RestrictEvents to set up a filter, then call Tk_DoOneEvent to retrieve the desired event(s).


delay, event, filter, restriction Tk 2 Prepared by Ready-to-Run Software, Inc.


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