Please explain BLT code

[fabien]

Hi!

I managed to make the following out of an example of the blt demos:

#!bin/sh
#exec wish ${1+"$@"}

package require BWidget
package require BLT

if { $tcl_version >= 8.0 } {
namespace import blt::*
namespace import -force blt::tile::*
}


set command [list tar -tvf tutu.tar]

proc InsertText { string tag } {
.text insert end "$tag: " "" $string $tag
update idletasks
}

proc DisplayOutput { name1 name2 how } {
upvar #0 $name1 arr

InsertText "$arr($name2)\n" stdout
set arr($name2) {}
}

proc DisplayErrors { name1 name2 how } {
upvar #0 $name1 arr

InsertText "$arr($name2)\n" stderr
set arr($name2) {}
}

trace variable results(stdout) w DisplayOutput
trace variable results(stderr) w DisplayErrors

# Create widgets
text .text
.text tag configure stdout -font -*-Helvetica-Bold-R-*-*-18-*-*-*-*-*-*-* -foreground green2
.text tag configure stderr -font -*-Helvetica-Medium-O-*-*-18-*-*-*-*-*-*-* -foreground red2

pack .text -fill both

.text delete 1.0 end
set results(status) {}
eval "bgexec results(status) -lasterror results(stderr) -lastoutput results(stdout) $command &"


I understand most of it except for

"trace variable results(stdout) w DisplayOutput"

DisplayOutput has three parameters:

"proc DisplayOutput { name1 name2 how }"

what does this do?
"upvar #0 $name1 arr"

and in
"InsertText "$arr($name2)\n" stdout
set arr($name2) {}"

how are name1, name2 and how been updated?

Thanks!