hook(3tcl) Hooks hook(3tcl)
______________________________________________________________________________
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, ob-
jects, widgets, and so forth, to synchronously call hooks which may be
bound to an arbitrary number of subscribers, called observers. A sub-
ject 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 call-
back options require tight coupling between the modules sending and re-
ceiving 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 im-
plementing loose coupling of this kind. This package defines a new com-
mand, 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 ob-
servers as objects, while placing no requirements on how these objects
are actually implemented. An object can be a TclOO or Snit or XOTcl ob-
ject, 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 peo-
ple.
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 com-
mand ::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 appli-
cation.
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 exist-
ing binding for the subject, hook, and observer; nothing is re-
turned. 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 re-
turned.
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 delet-
ing 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 cou-
pling, 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 han-
dling their errors. That makes it an application issue,
and so applications will usually want to define an -er-
rorcommand.
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 particu-
lar,
[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 con-
figuration 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 pre-
fix 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.
When proposing code changes, please provide unified diffs, i.e the out-
put of diff -u.
Note further that attachments are strongly preferred over inlined
patches. Attachments can be made by going to the Edit form of the
ticket immediately after its creation, and then using the left-most
button in the secondary navigation bar.
SEE ALSO
uevent(3tcl)
KEYWORDS
callback, event, hook, observer, producer, publisher, subject, sub-
scriber, uevent
CATEGORY
Programming tools
COPYRIGHT
Copyright (c) 2010, by William H. Duquette
tcllib 0.1 hook(3tcl)