'''`[Tcllib%|%hook]`''' is a [Tcllib] package tht implements the subject/observer ([Tcl implementations of publish-subscribe mechanisms%|%publish/subscribe]) pattern.



** What the Critics Say **

   ''[hook] has become an essential part of my toolbox. I can't imagine developing a medium size app without having such functionality'':   [eg%|%Emiliano Gavilán], [Tcl Chatroom], 2022-04-08



** Description **

`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 [eg%|%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]
======


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