watch - call Tcl procedures before and after each command
watch create watchName ?options?
watch activate watchName
watch deactivate watchName
watch delete watchName
watch configure watchName ?options
watch info watchName
watch names
The watch command arranges for Tcl procedures to be invoked
before and after the execution of each Tcl command.
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.
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
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.