Tcl8.6.14/Tk8.6.14 Documentation > Tk C API, version 8.6.14 > RestrictEv

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Tk_RestrictEvents — filter and selectively delay X events


#include <tk.h>
Tk_RestrictProc *
Tk_RestrictEvents(proc, arg, prevArgPtr)


Tk_RestrictProc *proc (in)
Predicate procedure to call to filter incoming X events. NULL means do not restrict events at all.

ClientData arg (in)
Arbitrary argument to pass to proc.

ClientData *prevArgPtr (out)
Pointer to place to save argument to previous restrict procedure.


This procedure is useful in certain situations where applications are only prepared to receive certain X events. After Tk_RestrictEvents is called, Tcl_DoOneEvent (and hence Tk_MainLoop) will filter X input events through proc. Proc indicates whether a given event is to be processed immediately, 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 arg,
        XEvent *eventPtr);
The arg argument is a copy of the arg 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 Tcl_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 processing. 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 prevArgPtr to return information about the current event restriction 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 changing 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 performing synchronous actions (for example, if you need to wait for a particular event to occur on a particular window but you do not 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 Tcl_DoOneEvent to retrieve the desired event(s).


delay, event, filter, restriction
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