Provided by: tcllib_1.17-dfsg-1_all 
NAME
hook - Hooks
SYNOPSIS
package require Tcl 8.5
package require hook ?0.1?
hook bind ?subject? ?hook? ?observer? ?cmdPrefix?
hook call subject hook ?args...?
hook forget object
hook cget option
hook configure option value ...
_________________________________________________________________________________________________
DESCRIPTION
This package provides the hook ensemble command, which implements the Subject/Observer
pattern. It 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.
This man page is intended to be a reference only.
CONCEPTS
INTRODUCTION
Tcl modules usually send notifications to other modules in two ways: via Tk events, and
via callback options like the text widget's -yscrollcommand option. Tk events are
available only in Tk, and callback options require tight coupling between the modules
sending and receiving the notification.
Loose coupling between sender and receiver is often desirable, however. In
Model/View/Controller terms, a View can send a command (stemming from user input) to the
Controller, which updates the Model. The Model can then call a hook to which all relevant
Views subscribe. The Model is decoupled from the Views, and indeed need not know whether
any Views actually exist. At present, Tcl/Tk has no standard mechanism for implementing
loose coupling of this kind. This package defines a new command, hook, which implements
just such a mechanism.
BINDINGS
The hook command manages a collection of hook bindings. A hook binding has four elements:
[1] A subject: the name of the entity that will be calling the hook.
[2] The hook itself. A hook usually reflects some occurrence in the life of the subject
that other entities might care to know about. A hook has a name, and may also have
arguments. Hook names are arbitrary strings. Each subject must document the names
and arguments of the hooks it can call.
[3] The name of the observer that wishes to receive the hook from the subject.
[4] A command prefix to which the hook arguments will be appended when the binding is
executed.
SUBJECTS AND OBSERVERS
For convenience, this document collectively refers to subjects and observers as objects,
while placing no requirements on how these objects are actually implemented. An object can
be a TclOO or Snit or XOTcl object, a Tcl command, a namespace, a module, a pseudo-object
managed by some other object (as tags are managed by the Tk text widget) or simply a well-
known name.
Subject and observer names are arbitrary strings; however, as hook might be used at the
package level, it's necessary to have conventions that avoid name collisions between
packages written by different people.
Therefore, any subject or observer name used in core or package level code should look
like a Tcl command name, and should be defined in a namespace owned by the package.
Consider, for example, an ensemble command ::foo that creates a set of pseudo-objects and
uses hook to send notifications. The pseudo-objects have names that are not commands and
exist in their own namespace, rather like file handles do. To avoid name collisions with
subjects defined by other packages, users of hook, these ::foo handles should have names
like ::foo::1, ::foo::2, and so on.
Because object names are arbitrary strings, application code can use whatever additional
conventions are dictated by the needs of the application.
REFERENCE
Hook provides the following commands:
hook bind ?subject? ?hook? ?observer? ?cmdPrefix?
This subcommand is used to create, update, delete, and query hook bindings.
Called with no arguments it returns a list of the subjects with hooks to which
observers are currently bound.
Called with one argument, a subject, it returns a list of the subject's hooks to
which observers are currently bound.
Called with two arguments, a subject and a hook, it returns a list of the observers
which are currently bound to this subject and hook.
Called with three arguments, a subject, a hook, and an observer, it returns the
binding proper, the command prefix to be called when the hook is called, or the
empty string if there is no such binding.
Called with four arguments, it creates, updates, or deletes a binding. If cmdPrefix
is the empty string, it deletes any existing binding for the subject, hook, and
observer; nothing is returned. Otherwise, cmdPrefix must be a command prefix taking
as many additional arguments as are documented for the subject and hook. The
binding is added or updated, and the observer is returned.
If the observer is the empty string, "", it will create a new binding using an
automatically generated observer name of the form ::hook::ob<number>. The
automatically generated name will be returned, and can be used to query, update,
and delete the binding as usual. If automated observer names are always used, the
observer name effectively becomes a unique binding ID.
It is possible to call hook bind to create or delete a binding to a subject and
hook while in an observer binding for that same subject and hook. The following
rules determine what happens when
hook bind $s $h $o $binding
is called during the execution of
hook call $s $h
[1] No binding is ever called after it is deleted.
[2] When a binding is called, the most recently given command prefix is always
used.
[3] The set of observers whose bindings are to be called is determined when this
method begins to execute, and does not change thereafter, except that
deleted bindings are not called.
In particular:
[1] If $os binding to $s and $h is deleted, and $os binding has not yet been
called during this execution of
hook call $s $h
it will not be called. (Note that it might already have been called; and in
all likelihood, it is probably deleting itself.)
[2] If $o changes the command prefix that's bound to $s and $h, and if $os
binding has not yet been called during this execution of
hook call $s $h
the new binding will be called when the time comes. (But again, it is
probably $os binding that is is making the change.)
[3] If a new observer is bound to $s and $h, its binding will not be called
until the next invocation of
hook call $s $h
hook call subject hook ?args...?
This command is called when the named subject wishes to call the named hook. All
relevant bindings are called with the specified arguments in the global namespace.
Note that the bindings are called synchronously, before the command returns; this
allows the args to include references to entities that will be cleaned up as soon
as the hook has been called.
The order in which the bindings are called is not guaranteed. If sequence among
observers must be preserved, define one observer and have its bindings call the
other callbacks directly in the proper sequence.
Because the hook mechanism is intended to support loose coupling, it is presumed
that the subject has no knowledge of the observers, nor any expectation regarding
return values. This has a number of implications:
[1] hook call returns the empty string.
[2] Normal return values from observer bindings are ignored.
[3] Errors and other exceptional returns propagate normally by default. This
will rarely be what is wanted, because the subjects usually have no
knowledge of the observers and will therefore have no particular competence
at handling their errors. That makes it an application issue, and so
applications will usually want to define an -errorcommand.
If the -errorcommand configuration option has a non-empty value, its value will be
invoked for all errors and other exceptional returns in observer bindings. See hook
configure, below, for more information on configuration options.
hook forget object
This command deletes any existing bindings in which the named object appears as
either the subject or the observer. Bindings deleted by this method will never be
called again. In particular,
[1] If an observer is forgotten during a call to hook call, any uncalled binding
it might have had to the relevant subject and hook will not be called
subsequently.
[2] If a subject $s is forgotten during a call to
hook call $s $h
then hook call will return as soon as the current binding returns. No
further bindings will be called.
hook cget option
This command returns the value of one of the hook command's configuration options.
hook configure option value ...
This command sets the value of one or more of the hook command's configuration
options:
-errorcommand cmdPrefix
If the value of this option is the empty string, "", then errors and other
exception returns in binding scripts are propagated normally. Otherwise, it
must be a command prefix taking three additional arguments:
[1] a 4-element list {subject hook arglist observer},
[2] the result string, and
[3] the return options dictionary.
Given this information, the -errorcommand can choose to log the error, call
interp bgerror, delete the errant binding (thus preventing the error from
arising a second time) and so forth.
-tracecommand cmdPrefix
The option's value should be a command prefix taking four arguments:
[1] a subject,
[2] a hook,
[3] a list of the hook's argument values, and
[4] a list of objects the hook was called for.
The command will be called for each hook that is called. This allows the
application to trace hook execution for debugging purposes.
EXAMPLE
The ::model module calls the <Update> hook in response to commands that change the model's
data:
hook call ::model <Update>
The .view megawidget displays the model state, and needs to know about model updates.
Consequently, it subscribes to the ::model's <Update> hook.
hook bind ::model <Update> .view [list .view ModelUpdate]
When the ::model calls the hook, the .views ModelUpdate subcommand will be called.
Later the .view megawidget is destroyed. In its destructor, it tells the hook that it no
longer exists:
hook forget .view
All bindings involving .view are deleted.
CREDITS
Hook has been designed and implemented by William H. Duquette.
BUGS, IDEAS, FEEDBACK
This document, and the package it describes, will undoubtedly contain bugs and other
problems. Please report such in the category hook of the Tcllib Trackers
[http://core.tcl.tk/tcllib/reportlist]. Please also report any ideas for enhancements you
may have for either package and/or documentation.
SEE ALSO
uevent(3tcl)
KEYWORDS
callback, event, hook, observer, producer, publisher, subject, subscriber, uevent
CATEGORY
Programming tools
COPYRIGHT
Copyright (c) 2010, by William H. Duquette