                    #tocTable of Contents #toc0Name 
watch - call Tcl procedures before and after each
command 
#toc1Synopsis 
watch create watchName ?options? watch activate watchNamewatch deactivate watchName watch delete watchName watch configure watchName?
options watch info watchName watch names  
#toc2Description 
The 
watch command
arranges for Tcl procedures to be invoked before and after the execution
of each Tcl command. 
#toc3Introduction 
When an error occurs in Tcl, the global
variable 
errorInfo will contain a stack-trace of the active procedures when
the error occured. Sometimes, however, the stack trace is insufficient. 
You may need to know exactly where in the program's execution the error
occured.  In cases like this, a more general tracing facility would be useful.
The 
watch command lets you designate Tcl procedures to be invoked before
and after the execution of each Tcl command.  This means you can display
the command line and its results for each command as it executes.  Another
use is to profile your Tcl commands. You can profile any Tcl command (like
if and set), not just Tcl procedures. 
#toc4Example 
The following example use 
watchto trace Tcl commands  (printing to standard error) both before and after
they are executed.  
proc preCmd { level command argv } {    set name [lindex $argv 0]
    puts stderr "$level $name => $command"
}
proc postCmd { level command argv retcode results } {
    set name [lindex $argv 0]
    puts stderr "$level $name => $argv0= ($retcode) $results"
}
watch create trace \
    -postcmd postCmd -precmd preCmd #toc5Operations 
The following operations are available for the 
watch command:
watch activate watchName  Activates the watch, causing Tcl commands the
be traced to the maximum depth selected. 
watch create watchName ?options?...
Creates a new watch watchName. It's an error if another watch  watchNamealready exists and an error message will be returned. 
Options may have any
of the values accepted by the  
watch configure command. This command returns
the empty string.   
watch configure watchName ?options...? Queries or modifies
the configuration options of the watch 
watchName. WatchName is the name
of a watch. 
Options may have any of the following values: -active booleanSpecifies if the watch is active. By default, watches are active when created.
-postcmd string Specifies a Tcl procedure to be called immediately after
each Tcl command.  
String is name of a Tcl procedure and any extra arguments
to be passed to it.  Before 
string is invoked, five more arguments are appended:
1) the current level 2) the current command line 3) a list containing the
command after substitutions and split into words 4) the return code of
the command, and 5) the results of the command.  The return status of the
postcmd procedure is always ignored. 
-precmd string  Specifies a Tcl procedure
to be called immediately before each Tcl command.  
String is name of a Tcl
procedure and any extra arguments to be passed to it.  Before 
string is
invoked, three arguments are appended: 1) the current level 2) the current
command line, and 3) a list containing the command after substitutions
and split into words.  The return status of the 
-precmd procedure is always
ignored. 
-maxlevel number Specifies the maximum evaluation depth to watch
Tcl commands. The default maximum level is 10000. 
watch deactivate watchName 
Deactivates the watch.  The -precmd and -postcmd procedures will no longer
be invoked. 
watch info watchName  Returns the configuration information
associated with the  watch 
watchName.  WatchName is the name of a watch.
watch names ?state? Lists the names of the watches for a given state.  Statemay be one of the following: 
active, idle, or ignore.  If a state argument
isn't specified,  all watches are
 listed. 
#toc6Keywords 
debug, profile 
Table of Contents #sect0Name #sect1Synopsis #sect2Description #sect3Introduction #sect4Example #sect5Operations #sect6Keywords 