'\"
'\" Generated from file 'hook\&.man' by tcllib/doctools with format 'nroff'
'\" Copyright (c) 2010, by William H\&. Duquette
'\"
.TH "hook" n 0\&.1 tcllib "Hooks"
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.SH "STANDARD OPTIONS"
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.BS
.SH NAME
hook \- Hooks
.SH SYNOPSIS
package require \fBTcl  8\&.5\fR
.sp
package require \fBhook  ?0\&.1?\fR
.sp
\fBhook\fR \fBbind\fR ?\fIsubject\fR? ?\fIhook\fR? ?\fIobserver\fR? ?\fIcmdPrefix\fR?
.sp
\fBhook\fR \fBcall\fR \fIsubject\fR \fIhook\fR ?\fIargs\fR\&.\&.\&.?
.sp
\fBhook\fR \fBforget\fR \fIobject\fR
.sp
\fBhook\fR \fBcget\fR \fIoption\fR
.sp
\fBhook\fR \fBconfigure\fR \fBoption\fR \fIvalue\fR \&.\&.\&.
.sp
.BE
.SH DESCRIPTION
.PP
This package provides the \fBhook\fR ensemble command, which
implements the Subject/Observer pattern\&. It allows \fIsubjects\fR,
which may be \fImodules\fR, \fIobjects\fR, \fIwidgets\fR, and so
forth, to synchronously call \fIhooks\fR which may be bound to an
arbitrary number of subscribers, called \fIobservers\fR\&. 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\&.
.PP
This man page is intended to be a reference only\&.
.SH CONCEPTS
.SS INTRODUCTION
Tcl modules usually send notifications to other modules in two ways:
via Tk events, and via callback options like the text widget's
\fB-yscrollcommand\fR option\&. Tk events are available only in Tk,
and callback options require tight coupling between the modules
sending and receiving the notification\&.
.PP
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 \fIto which all relevant
Views subscribe\&.\fR 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, \fBhook\fR,
which implements just such a mechanism\&.
.SS BINDINGS
The \fBhook\fR command manages a collection of hook bindings\&. A hook
binding has four elements:
.IP [1]
A \fIsubject\fR: the name of the entity that will be calling the
hook\&.
.IP [2]
The \fIhook\fR itself\&. A hook usually reflects some occurrence in the
life of the \fIsubject\fR that other entities might care to know
about\&. A \fIhook\fR has a name, and may also have arguments\&. Hook
names are arbitrary strings\&. Each \fIsubject\fR must document the
names and arguments of the hooks it can call\&.
.IP [3]
The name of the \fIobserver\fR that wishes to receive the \fIhook\fR
from the \fIsubject\fR\&.
.IP [4]
A command prefix to which the \fIhook\fR arguments will be appended
when the binding is executed\&.
.PP
.SS "SUBJECTS AND OBSERVERS"
For convenience, this document collectively refers to subjects and
observers as \fIobjects\fR, while placing no requirements on how
these \fIobjects\fR are actually implemented\&. An object can be a
\fBTclOO\fR or \fBSnit\fR or \fBXOTcl\fR 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\&.
.PP
Subject and observer names are arbitrary strings; however, as
\fBhook\fR might be used at the package level, it's necessary to have
conventions that avoid name collisions between packages written by
different people\&.
.PP
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 \fB::foo\fR that creates a set of pseudo-objects and uses
\fBhook\fR 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 \fBhook\fR, these \fB::foo\fR handles
should have names like \fB::foo::1\fR, \fB::foo::2\fR, and so on\&.
.PP
Because object names are arbitrary strings, application code can use
whatever additional conventions are dictated by the needs of the
application\&.
.SH REFERENCE
Hook provides the following commands:
.TP
\fBhook\fR \fBbind\fR ?\fIsubject\fR? ?\fIhook\fR? ?\fIobserver\fR? ?\fIcmdPrefix\fR?
This subcommand is used to create, update, delete, and query hook
bindings\&.
.sp
Called with no arguments it returns a list of the subjects with
hooks to which observers are currently bound\&.
.sp
Called with one argument, a \fIsubject\fR, it returns a list of
the subject's hooks to which observers are currently bound\&.
.sp
Called with two arguments, a \fIsubject\fR and a \fIhook\fR, it
returns a list of the observers which are currently bound to this
\fIsubject\fR and \fIhook\fR\&.
.sp
Called with three arguments, a \fIsubject\fR, a \fIhook\fR, and
an \fIobserver\fR, 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\&.
.sp
Called with four arguments, it creates, updates, or deletes a
binding\&. If \fIcmdPrefix\fR is the empty string, it deletes any
existing binding for the \fIsubject\fR, \fIhook\fR, and
\fIobserver\fR; nothing is returned\&. Otherwise, \fIcmdPrefix\fR must
be a command prefix taking as many additional arguments as are
documented for the \fIsubject\fR and \fIhook\fR\&. The binding is added
or updated, and the observer is returned\&.
.sp
If the \fIobserver\fR is the empty string, "", it will create a
new binding using an automatically generated observer name of the form
\fB::hook::ob\fR<\fBnumber\fR>\&. 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\&.
.sp
It is possible to call \fBhook bind\fR to create or delete a
binding to a \fIsubject\fR and \fIhook\fR while in an observer binding
for that same \fIsubject\fR and \fIhook\fR\&. The following rules
determine what happens when
.CS


    hook bind $s $h $o $binding

.CE
.IP
is called during the execution of
.CS


    hook call $s $h

.CE
.RS
.IP [1]
No binding is ever called after it is deleted\&.
.IP [2]
When a binding is called, the most recently given command prefix is
always used\&.
.IP [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\&.
.RE
.IP
In particular:
.RS
.IP [1]
If \fB$o\fRs binding to \fB$s\fR and \fB$h\fR is deleted, and
\fB$o\fRs binding has not yet been called during this execution of
.CS


    hook call $s $h

.CE
.IP
it will not be called\&. (Note that it might already have been called;
and in all likelihood, it is probably deleting itself\&.)
.IP [2]
If \fB$o\fR changes the command prefix that's bound to \fB$s\fR and
\fB$h\fR, and if \fB$o\fRs binding has not yet been called during
this execution of
.CS


    hook call $s $h

.CE
.IP
the new binding will be called when the time comes\&. (But again, it is
probably \fB$o\fRs binding that is is making the change\&.)
.IP [3]
If a new observer is bound to \fB$s\fR and \fB$h\fR, its binding will
not be called until the next invocation of
.CS


    hook call $s $h

.CE
.RE
.TP
\fBhook\fR \fBcall\fR \fIsubject\fR \fIhook\fR ?\fIargs\fR\&.\&.\&.?
This command is called when the named \fIsubject\fR wishes to call the
named \fIhook\fR\&. 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 \fIargs\fR
to include references to entities that will be cleaned up as soon as
the hook has been called\&.
.sp
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\&.
.sp
Because the \fBhook\fR mechanism is intended to support loose
coupling, it is presumed that the \fIsubject\fR has no knowledge of
the observers, nor any expectation regarding return values\&. This has a
number of implications:
.RS
.IP [1]
\fBhook call\fR returns the empty string\&.
.IP [2]
Normal return values from observer bindings are ignored\&.
.IP [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
\fB-errorcommand\fR\&.
.RE
.IP
If the \fB-errorcommand\fR configuration option has a non-empty
value, its value will be invoked for all errors and other exceptional
returns in observer bindings\&. See \fBhook configure\fR, below, for
more information on configuration options\&.
.TP
\fBhook\fR \fBforget\fR \fIobject\fR
This command deletes any existing bindings in which the named
\fIobject\fR appears as either the \fIsubject\fR or the
\fIobserver\fR\&.
Bindings deleted by this method will never be called again\&. In
particular,
.RS
.IP [1]
If an observer is forgotten during a call to \fBhook call\fR, any
uncalled binding it might have had to the relevant subject and hook
will \fInot\fR be called subsequently\&.
.IP [2]
If a subject \fB$s\fR is forgotten during a call to
.CS

hook call $s $h
.CE
.IP
then \fBhook call\fR will return as soon as the current binding
returns\&.  No further bindings will be called\&.
.RE
.TP
\fBhook\fR \fBcget\fR \fIoption\fR
This command returns the value of one of the \fBhook\fR command's
configuration options\&.
.TP
\fBhook\fR \fBconfigure\fR \fBoption\fR \fIvalue\fR \&.\&.\&.
This command sets the value of one or more of the \fBhook\fR command's
configuration options:
.RS
.TP
\fB-errorcommand\fR \fIcmdPrefix\fR
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:
.RS
.IP [1]
a 4-element list {subject hook arglist observer},
.IP [2]
the result string, and
.IP [3]
the return options dictionary\&.
.RE
.IP
Given this information, the \fB-errorcommand\fR can choose to log
the error, call \fBinterp bgerror\fR, delete the errant binding
(thus preventing the error from arising a second time) and so forth\&.
.TP
\fB-tracecommand\fR \fIcmdPrefix\fR
The option's value should be a command prefix taking four
arguments:
.RS
.IP [1]
a \fIsubject\fR,
.IP [2]
a \fIhook\fR,
.IP [3]
a list of the hook's argument values, and
.IP [4]
a list of \fIobjects\fR the hook was called for\&.
.RE
.IP
The command will be called for each hook that is called\&. This allows
the application to trace hook execution for debugging purposes\&.
.RE
.PP
.SH EXAMPLE
The \fB::model\fR module calls the <Update> hook in response to
commands that change the model's data:
.CS


     hook call ::model <Update>

.CE
The \fB\&.view\fR megawidget displays the model state, and needs to
know about model updates\&. Consequently, it subscribes to the ::model's
<Update> hook\&.
.CS


     hook bind ::model <Update> \&.view [list \&.view ModelUpdate]

.CE
When the \fB::model\fR calls the hook, the \fB\&.view\fRs
ModelUpdate subcommand will be called\&.
.PP
Later the \fB\&.view\fR megawidget is destroyed\&. In its destructor,
it tells the \fIhook\fR that it no longer exists:
.CS


     hook forget \&.view

.CE
All bindings involving \fB\&.view\fR are deleted\&.
.SH CREDITS
Hook has been designed and implemented by William H\&. Duquette\&.
.SH "BUGS, IDEAS, FEEDBACK"
This document, and the package it describes, will undoubtedly contain
bugs and other problems\&.
Please report such in the category \fIhook\fR of the
\fITcllib Trackers\fR [http://core\&.tcl\&.tk/tcllib/reportlist]\&.
Please also report any ideas for enhancements you may have for either
package and/or documentation\&.
.PP
When proposing code changes, please provide \fIunified diffs\fR,
i\&.e the output of \fBdiff -u\fR\&.
.PP
Note further that \fIattachments\fR are strongly preferred over
inlined patches\&. Attachments can be made by going to the \fBEdit\fR
form of the ticket immediately after its creation, and then using the
left-most button in the secondary navigation bar\&.
.SH "SEE ALSO"
uevent(n)
.SH KEYWORDS
callback, event, hook, observer, producer, publisher, subject, subscriber, uevent
.SH CATEGORY
Programming tools
.SH COPYRIGHT
.nf
Copyright (c) 2010, by William H\&. Duquette

.fi
