#toc
Table of Contents
#toc0
Name
watch - call Tcl procedures before and after each
command
#toc1
Synopsis
watch create
watchName
?
options
?
watch activate
watchName
watch deactivate
watchName
watch delete
watchName
watch configure
watchName
?
options
watch info
watchName
watch names
#toc2
Description
The
watch
command
arranges for Tcl procedures to be invoked before and after the execution
of each Tcl command.
#toc3
Introduction
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.
#toc4
Example
The following example use
watch
to 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
#toc5
Operations
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
watchName
already 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
boolean
Specifies 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.
State
may be one of the following:
active
,
idle
, or
ignore
.  If a
state
argument
isn't specified,  all watches are
listed.
#toc6
Keywords
debug, profile
Table of Contents
#sect0
Name
#sect1
Synopsis
#sect2
Description
#sect3
Introduction
#sect4
Example
#sect5
Operations
#sect6
Keywords
