New to tcl and trying to understand the "trace add variable" command.
I understand the need to invoke a callback function when a variable is "written" to.
But what is the use of the "read" option? For debugging?
2 Answers
One example use might be a global counter:
proc init { } {
set ::globalcounter 0
trace add variable ::globalcounter read ::gcountUpdate
}
proc gcountUpdate { } {
incr ::globalcounter
}
proc main { } {
init
puts $::globalcounter
puts $::globalcounter
}
main
I'm sure there are other uses. As you pointed out, debugging.
It could be used for enforcement of variable access by specific procedures.
Comments
One of the uses for read callbacks (which are indeed quite a bit less common than write callbacks) is in linking a Tcl variable to a C variable; the read callback is used to enforce the reading of the C variable and synchronizing the Tcl variable to it. (The write callback would ensure that the update of the Tcl variable gets reflected to the C variable.) Tcl's got a built-in C API that uses this mechanism, though it's using the underlying C API for variable traces rather than the Tcl API that is built on top of it.
You could also use a read callback to make an apparently read-only variable:
trace add variable foo read {apply {args {
upvar "#0" foo v
set v "definitely this"
}}}
puts $foo
set foo "that"
puts $foo
I don't recommend using variable traces on local variables. They have a lot more overhead (and internal complexity) than for global/namespace variables.
