Notifier(3) Tcl (8.0) Notifier(3)
_________________________________________________________________
NAME |
Tcl_CreateEventSource, Tcl_DeleteEventSource, |
Tcl_SetMaxBlockTime, Tcl_QueueEvent, Tcl_DeleteEvents, |
Tcl_WaitForEvent, Tcl_SetTimer, Tcl_ServiceAll, |
Tcl_ServiceEvent, Tcl_GetServiceMode, Tcl_SetServiceMode - |
the event queue and notifier interfaces |
SYNOPSIS |
#include <tcl.h> |
Tcl_CreateEventSource(setupProc, checkProc, clientData) |
Tcl_DeleteEventSource(setupProc, checkProc, clientData) |
Tcl_SetMaxBlockTime(timePtr) |
Tcl_QueueEvent(evPtr, position) |
Tcl_DeleteEvents(deleteProc, clientData) |
int |
Tcl_WaitForEvent(timePtr) |
Tcl_SetTimer(timePtr) |
int |
Tcl_ServiceAll() |
int |
Tcl_ServiceEvent(flags) |
int |
Tcl_GetServiceMode() |
int |
Tcl_SetServiceMode(mode) |
ARGUMENTS
Tcl_EventSetupProc *setupProc (in) Procedure to
invoke to
prepare for
event wait in
Tcl_DoOneEvent.
Tcl_EventCheckProc *checkProc (in) Procedure for
Tcl_DoOneEvent
to invoke after
waiting for
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events. Checks
to see if any
events have
occurred and, if
so, queues them.
ClientData clientData (in) Arbitrary one-
word value to
pass to
setupProc,
checkProc, or
deleteProc.
Tcl_Time *timePtr (in) Indicates the
maximum amount
of time to wait
for an event.
This is
specified as an
interval (how
long to wait),
not an absolute
time (when to
wakeup). If the
pointer passed
to
Tcl_WaitForEvent
is NULL, it
means there is
no maximum wait
time: wait
forever if
necessary.
Tcl_Event *evPtr (in) An event to add
to the event
queue. The
storage for the
event must have
been allocated
by the caller
using Tcl_Alloc
or ckalloc.
Tcl_QueuePosition position (in) Where to add the
new event in the
queue:
TCL_QUEUE_TAIL,
TCL_QUEUE_HEAD,
or
TCL_QUEUE_MARK.
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int flags (in) What types of
events to
service. These
flags are the
same as those
passed to
Tcl_DoOneEvent.
Tcl_EventDeleteProc *deleteProc (in) Procedure to
invoke for each
queued event in
Tcl_DeleteEvents.
int mode (in) ||
Inidicates |
whether events |
should be |
serviced by |
Tcl_ServiceAll. |
Must be one of |
TCL_SERVICE_NONE |
or |
TCL_SERVICE_ALL.
_________________________________________________________________
INTRODUCTION
The interfaces described here are used to customize the Tcl |
event loop. The two most common customizations are to add |
new sources of events and to merge Tcl's event loop with |
some other event loop, such as one provided by an |
application in which Tcl is embedded. Each of these tasks |
is described in a separate section below.
The procedures in this manual entry are the building blocks
out of which the Tcl event notifier is constructed. The
event notifier is the lowest layer in the Tcl event
mechanism. It consists of three things:
[1] Event sources: these represent the ways in which events
can be generated. For example, there is a timer event
source that implements the Tcl_CreateTimerHandler
procedure and the after command, and there is a file
event source that implements the Tcl_CreateFileHandler
procedure on Unix systems. An event source must work
with the notifier to detect events at the right times,
record them on the event queue, and eventually notify
higher-level software that they have occurred. The
procedures Tcl_CreateEventSource,
Tcl_DeleteEventSource, and Tcl_SetMaxBlockTime,
Tcl_QueueEvent, and Tcl_DeleteEvents are used primarily
by event sources.
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[2] The event queue: there is a single queue for the whole
application, containing events that have been detected
but not yet serviced. Event sources place events onto
the queue so that they may be processed in order at
appropriate times during the event loop. The event
queue guarantees a fair discipline of event handling,
so that no event source can starve the others. It also
allows events to be saved for servicing at a future
time. Tcl_QueueEvent is used (primarily by event |
sources) to add events to the event queue and |
Tcl_DeleteEvents is used to remove events from the |
queue without processing them. |
[3] ||
The event loop: in order to detect and process events, |
the application enters a loop that waits for events to |
occur, places them on the event queue, and then |
processes them. Most applications will do this by |
calling the procedure Tcl_DoOneEvent, which is |
described in a separate manual entry. |
Most Tcl applications need not worry about any of the |
internals of the Tcl notifier. However, the notifier now |
has enough flexibility to be retargeted either for a new |
platform or to use an external event loop (such as the Motif |
event loop, when Tcl is embedded in a Motif application). |
The procedures Tcl_WaitForEvent and Tcl_SetTimer are |
normally implemented by Tcl, but may be replaced with new |
versions to retarget the notifier (the Tcl_Sleep, |
Tcl_CreateFileHandler, and Tcl_DeleteFileHandler must also |
be replaced; see CREATING A NEW NOTIFIER below for details). |
The procedures Tcl_ServiceAll, Tcl_ServiceEvent, |
Tcl_GetServiceMode, and Tcl_SetServiceMode are provided to |
help connect Tcl's event loop to an external event loop such |
as Motif's. |
NOTIFIER BASICS |
The easiest way to understand how the notifier works is to
consider what happens when Tcl_DoOneEvent is called.
Tcl_DoOneEvent is passed a flags argument that indicates
what sort of events it is OK to process and also whether or
not to block if no events are ready. Tcl_DoOneEvent does
the following things:
[1] Check the event queue to see if it contains any events
that can be serviced. If so, service the first
possible event, remove it from the queue, and return. |
It does this by calling Tcl_ServiceEvent and passing in |
the flags argument.
[2] Prepare to block for an event. To do this,
Tcl_DoOneEvent invokes a setup procedure in each event
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source. The event source will perform event-source
specific initialization and possibly call |
Tcl_SetMaxBlockTime to limit how long Tcl_WaitForEvent
will block if no new events occur.
[3] Call Tcl_WaitForEvent. This procedure is implemented
differently on different platforms; it waits for an
event to occur, based on the information provided by
the event sources. It may cause the application to
block if timePtr specifies an interval other than 0.
Tcl_WaitForEvent returns when something has happened,
such as a file becoming readable or the interval given
by timePtr expiring. If there are no events for
Tcl_WaitForEvent to wait for, so that it would block
forever, then it returns immediately and Tcl_DoOneEvent
returns 0.
[4] Call a check procedure in each event source. The check
procedure determines whether any events of interest to
this source occurred. If so, the events are added to
the event queue.
[5] Check the event queue to see if it contains any events
that can be serviced. If so, service the first
possible event, remove it from the queue, and return.
[6] See if there are idle callbacks pending. If so, invoke
all of them and return.
[7] Either return 0 to indicate that no events were ready,
or go back to step [2] if blocking was requested by the
caller.
CREATING A NEW EVENT SOURCE
An event source consists of three procedures invoked by the
notifier, plus additional C procedures that are invoked by
higher-level code to arrange for event-driven callbacks.
The three procedures called by the notifier consist of the
setup and check procedures described above, plus an
additional procedure that is invoked when an event is
removed from the event queue for servicing.
The procedure Tcl_CreateEventSource creates a new event
source. Its arguments specify the setup procedure and check
procedure for the event source. SetupProc should match the
following prototype:
typedef void Tcl_EventSetupProc(
ClientData clientData,
int flags);
The clientData argument will be the same as the clientData
argument to Tcl_CreateEventSource; it is typically used to
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point to private information managed by the event source.
The flags argument will be the same as the flags argument
passed to Tcl_DoOneEvent except that it will never be 0
(Tcl_DoOneEvent replaces 0 with TCL_ALL_EVENTS). Flags
indicates what kinds of events should be considered; if the
bit corresponding to this event source isn't set, the event
source should return immediately without doing anything.
For example, the file event source checks for the
TCL_FILE_EVENTS bit.
SetupProc's job is to make sure that the application wakes
up when events of the desired type occur. This is typically
done in a platform-dependent fashion. For example, under
Unix an event source might call Tcl_CreateFileHandler; under
Windows it might request notification with a Windows event.
For timer-driven event sources such as timer events or any
polled event, the event source can call Tcl_SetMaxBlockTime
to force the application to wake up after a specified time
even if no events have occurred. If no event source calls |
Tcl_SetMaxBlockTime then Tcl_WaitForEvent will wait as long |
as necessary for an event to occur; otherwise, it will only |
wait as long as the shortest interval passed to |
Tcl_SetMaxBlockTime by one of the event sources. If an |
event source knows that it already has events ready to |
report, it can request a zero maximum block time. For |
example, the setup procedure for the X event source looks to |
see if there are events already queued. If there are, it |
calls Tcl_SetMaxBlockTime with a 0 block time so that |
Tcl_WaitForEvent does not block if there is no new data on |
the X connection. The timePtr argument to Tcl_WaitForEvent
points to a structure that describes a time interval in
seconds and microseconds:
typedef struct Tcl_Time {
long sec;
long usec;
} Tcl_Time;
The usec field should be less than 1000000.
Information provided to Tcl_SetMaxBlockTime is only used for |
the next call to Tcl_WaitForEvent; it is discarded after |
Tcl_WaitForEvent returns. The next time an event wait is
done each of the event sources' setup procedures will be
called again, and they can specify new information for that
event wait.
If the application uses an external event loop rather than |
Tcl_DoOneEvent, the event sources may need to call |
Tcl_SetMaxBlockTime at other times. For example, if a new |
event handler is registered that needs to poll for events, |
the event source may call Tcl_SetMaxBlockTime to set the |
block time to zero to force the external event loop to call |
Tcl. In this case, Tcl_SetMaxBlockTime invokes Tcl_SetTimer |
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with the shortest interval seen since the last call to |
Tcl_DoOneEvent or Tcl_ServiceAll. |
In addition to the generic procedure Tcl_SetMaxBlockTime, |
other platform-specific procedures may also be available for |
setupProc, if there is additional information needed by |
Tcl_WaitForEvent on that platform. For example, on Unix |
systems the Tcl_CreateFileHandler interface can be used to |
wait for file events.
The second procedure provided by each event source is its
check procedure, indicated by the checkProc argument to
Tcl_CreateEventSource. CheckProc must match the following
prototype:
typedef void Tcl_EventCheckProc(
ClientData clientData,
int flags);
The arguments to this procedure are the same as those for
setupProc. CheckProc is invoked by Tcl_DoOneEvent after it
has waited for events. Presumably at least one event source
is now prepared to queue an event. Tcl_DoOneEvent calls
each of the event sources in turn, so they all have a chance
to queue any events that are ready. The check procedure
does two things. First, it must see if any events have
triggered. Different event sources do this in different
ways.
If an event source's check procedure detects an interesting
event, it must add the event to Tcl's event queue. To do
this, the event source calls Tcl_QueueEvent. The evPtr
argument is a pointer to a dynamically allocated structure
containing the event (see below for more information on
memory management issues). Each event source can define its
own event structure with whatever information is relevant to
that event source. However, the first element of the
structure must be a structure of type Tcl_Event, and the
address of this structure is used when communicating between
the event source and the rest of the notifier. A Tcl_Event
has the following definition:
typedef struct Tcl_Event {
Tcl_EventProc *proc;
struct Tcl_Event *nextPtr;
};
The event source must fill in the proc field of the event
before calling Tcl_QueueEvent. The nextPtr is used to link
together the events in the queue and should not be modified
by the event source.
An event may be added to the queue at any of three
positions, depending on the position argument to
Tcl_QueueEvent:
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TCL_QUEUE_TAIL Add the event at the back of the
queue, so that all other pending
events will be serviced first. This
is almost always the right place for
new events.
TCL_QUEUE_HEAD Add the event at the front of the
queue, so that it will be serviced
before all other queued events.
TCL_QUEUE_MARK Add the event at the front of the
queue, unless there are other events
at the front whose position is
TCL_QUEUE_MARK; if so, add the new
event just after all other
TCL_QUEUE_MARK events. This value
of position is used to insert an
ordered sequence of events at the
front of the queue, such as a series
of Enter and Leave events
synthesized during a grab or ungrab
operation in Tk.
When it is time to handle an event from the queue (steps 1 |
and 4 above) Tcl_ServiceEvent will invoke the proc specified
in the first queued Tcl_Event structure. Proc must match
the following prototype:
typedef int Tcl_EventProc(
Tcl_Event *evPtr,
int flags);
The first argument to proc is a pointer to the event, which
will be the same as the first argument to the Tcl_QueueEvent
call that added the event to the queue. The second argument
to proc is the flags argument for the current call to |
Tcl_ServiceEvent; this is used by the event source to
return immediately if its events are not relevant.
It is up to proc to handle the event, typically by invoking
one or more Tcl commands or C-level callbacks. Once the
event source has finished handling the event it returns 1 to
indicate that the event can be removed from the queue. If
for some reason the event source decides that the event
cannot be handled at this time, it may return 0 to indicate
that the event should be deferred for processing later; in |
this case Tcl_ServiceEvent will go on to the next event in
the queue and attempt to service it. There are several
reasons why an event source might defer an event. One
possibility is that events of this type are excluded by the
flags argument. For example, the file event source will
always return 0 if the TCL_FILE_EVENTS bit isn't set in
flags. Another example of deferring events happens in Tk if
Tk_RestrictEvents has been invoked to defer certain kinds of
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window events.
When proc returns 1, Tcl_ServiceEvent will remove the event |
from the event queue and free its storage. Note that the |
storage for an event must be allocated by the event source |
(using Tcl_Alloc or the Tcl macro ckalloc) before calling |
Tcl_QueueEvent, but it will be freed by Tcl_ServiceEvent, |
not by the event source. |
Tcl_DeleteEvents can be used to explicitly remove one or |
more events from the event queue. Tcl_DeleteEvents calls |
proc for each event in the queue, deleting those for with |
the procedure returns 1. Events for which the procedure |
returns 0 are left in the queue. Proc should match the |
following prototype: |
typedef int Tcl_EventDeleteProc( |
Tcl_Event *evPtr, |
ClientData clientData); |
The clientData argument will be the same as the clientData |
argument to Tcl_DeleteEvents; it is typically used to point |
to private information managed by the event source. The |
evPtr will point to the next event in the queue.
CREATING A NEW NOTIFIER
The notifier consists of all the procedures described in
this manual entry, plus Tcl_DoOneEvent and Tcl_Sleep, which
are available on all platforms, and Tcl_CreateFileHandler |
and Tcl_DeleteFileHandler, which are Unix-specific. Most of |
these procedures are generic, in that they are the same for |
all notifiers. However, five of the procedures are |
notifier-dependent: Tcl_SetTimer, Tcl_Sleep, |
Tcl_WaitForEvent, Tcl_CreateFileHandler and |
Tcl_DeleteFileHandler. To support a new platform or to |
integrate Tcl with an application-specific event loop, you |
must write new versions of these procedures. |
Tcl_WaitForEvent is the lowest-level procedure in the |
notifier; it is responsible for waiting for an |
``interesting'' event to occur or for a given time to |
elapse. Before Tcl_WaitForEvent is invoked, each of the |
event sources' setup procedure will have been invoked. The |
timePtr argument to Tcl_WaitForEvent gives the maximum time |
to block for an event, based on calls to Tcl_SetMaxBlockTime |
made by setup procedures and on other information (such as |
the TCL_DONT_WAIT bit in flags). |
Ideally, Tcl_WaitForEvent should only wait for an event to |
occur; it should not actually process the event in any way. |
Later on, the event sources will process the raw events and |
create Tcl_Events on the event queue in their checkProc |
procedures. However, on some platforms (such as Windows) |
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this isn't possible; events may be processed in |
Tcl_WaitForEvent, including queuing Tcl_Events and more (for |
example, callbacks for native widgets may be invoked). The |
return value from Tcl_WaitForEvent must be either 0, 1, or |
-1. On platforms such as Windows where events get processed |
in Tcl_WaitForEvent, a return value of 1 means that there |
may be more events still pending that haven't been |
processed. This is a sign to the caller that it must call |
Tcl_WaitForEvent again if it wants all pending events to be |
processed. A 0 return value means that calling |
Tcl_WaitForEvent again will not have any effect: either this |
is a platform where Tcl_WaitForEvent only waits without |
doing any event processing, or Tcl_WaitForEvent knows for |
sure that there are no additional events to process (e.g. it |
returned because the time elapsed). Finally, a return value |
of -1 means that the event loop is no longer operational and |
the application should probably unwind and terminate. Under |
Windows this happens when a WM_QUIT message is received; |
under Unix it happens when Tcl_WaitForEvent would have |
waited forever because there were no active event sources |
and the timeout was infinite. |
If the notifier will be used with an external event loop, |
then it must also support the Tcl_SetTimer interface. |
Tcl_SetTimer is invoked by Tcl_SetMaxBlockTime whenever the |
maximum blocking time has been reduced. Tcl_SetTimer should |
arrange for the external event loop to invoke Tcl_ServiceAll |
after the specified interval even if no events have |
occurred. This interface is needed because Tcl_WaitForEvent |
isn't invoked when there is an external event loop. If the |
notifier will only be used from Tcl_DoOneEvent, then |
Tcl_SetTimer need not do anything. |
On Unix systems, the file event source also needs support |
from the notifier. The file event source consists of the |
Tcl_CreateFileHandler and Tcl_DeleteFileHandler procedures, |
which are described elsewhere. |
The Tcl_Sleep and Tcl_DoOneEvent interfaces are described |
elsewhere. |
The easiest way to create a new notifier is to look at the |
code for an existing notifier, such as the files |
unix/tclUnixNotfy.c or win/tclWinNotify.c in the Tcl source |
distribution. |
EXTERNAL EVENT LOOPS |
The notifier interfaces are designed so that Tcl can be |
embedded into applications that have their own private event |
loops. In this case, the application does not call |
Tcl_DoOneEvent except in the case of recursive event loops |
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such as calls to the Tcl commands update or vwait. Most of |
the time is spent in the external event loop of the |
application. In this case the notifier must arrange for the |
external event loop to call back into Tcl when something |
happens on the various Tcl event sources. These callbacks |
should arrange for appropriate Tcl events to be placed on |
the Tcl event queue. |
Because the external event loop is not calling |
Tcl_DoOneEvent on a regular basis, it is up to the notifier |
to arrange for Tcl_ServiceEvent to be called whenever events |
are pending on the Tcl event queue. The easiest way to do |
this is to invoke Tcl_ServiceAll at the end of each callback |
from the external event loop. This will ensure that all of |
the event sources are polled, any queued events are |
serviced, and any pending idle handlers are processed before |
returning control to the application. In addition, event |
sources that need to poll for events can call |
Tcl_SetMaxBlockTime to force the external event loop to call |
Tcl even if no events are available on the system event |
queue. |
As a side effect of processing events detected in the main |
external event loop, Tcl may invoke Tcl_DoOneEvent to start |
a recursive event loop in commands like vwait. |
Tcl_DoOneEvent will invoke the external event loop, which |
will result in callbacks as described in the preceding |
paragraph, which will result in calls to Tcl_ServiceAll. |
However, in these cases it is undesirable to service events |
in Tcl_ServiceAll. Servicing events there is unnecessary |
because control will immediately return to the external |
event loop and hence to Tcl_DoOneEvent, which can service |
the events itself. Furthermore, Tcl_DoOneEvent is supposed |
to service only a single event, whereas Tcl_ServiceAll |
normally services all pending events. To handle this |
situation, Tcl_DoOneEvent sets a flag for Tcl_ServiceAll |
that causes it to return without servicing any events. This |
flag is called the service mode; Tcl_DoOneEvent restores it |
to its previous value before it returns. |
In some cases, however, it may be necessary for |
Tcl_ServiceAll to service events even when it has been |
invoked from Tcl_DoOneEvent. This happens when there is yet |
another recursive event loop invoked via an event handler |
called by Tcl_DoOneEvent (such as one that is part of a |
native widget). In this case, Tcl_DoOneEvent may not have a |
chance to service events so Tcl_ServiceAll must service them |
all. Any recursive event loop that calls an external event |
loop rather than Tcl_DoOneEvent must reset the service mode |
so that all events get processed in Tcl_ServiceAll. This is |
done by invoking the Tcl_SetServiceMode procedure. If |
Tcl_SetServiceMode is passed TCL_SERVICE_NONE, then calls to |
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Tcl_ServiceAll will return immediately without processing |
any events. If Tcl_SetServiceMode is passed |
TCL_SERVICE_ALL, then calls to Tcl_ServiceAll will behave |
normally. Tcl_SetServiceMode returns the previous value of |
the service mode, which should be restored when the |
recursive loop exits. Tcl_GetServiceMode returns the |
current value of the service mode.
KEYWORDS
event, notifier, event queue, event sources, file events,
timer, idle, service mode
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