hook is a Tcllib package tht implements the subject/observer (publish/subscribe) pattern.
hook allows subjects, which may be modules, objects, widgets, and so forth, to synchronously call hooks which may be bound to an arbitrary number of subscribers, called observers. A subject may call any number of distinct hooks, and any number of observers can bind callbacks to a particular hook called by a particular subject. Hook bindings can be queried and deleted.
An incomplete but illustratie example from Emiliano Gavilán:
set connbutton [ttk::button $f.startstop \
-text Connect \
-command [fqcmd connect] \
-image ::img::connect0]
hook bind Reader <<Connecting>> $connbutton [lambda {w} {
$w state disabled
status set-clear "Connecting to reader"
} $connbutton]
hook bind Reader <<Connected>> $connbutton [lambda {w conn} {
$w state !disabled
if {$conn} {
set spec [list Disconnect [fqcmd disconnect] ::img::connect4]
status set-clear "Reader connected"
} else {
set spec [list Connect [fqcmd connect] ::img::connect0]
status set-clear "Reader disconnected"
}
foreach {txt cmd img} $spec {
$w configure -text $txt -command $cmd -image $img
}
} $connbutton]