TIP #379: Add a Command for Delivering Events Without Tk


TIP:379
Title:Add a Command for Delivering Events Without Tk
Version:$Revision: 1.9 $
Author:Will Duquette <will at wjduquette dot com>
State:Draft
Type:Project
Tcl-Version:8.7
Vote:Pending
Created:Sunday, 17 October 2010
Keywords:event

Abstract

This proposal defines 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.

Rationale

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 proposal defines a new command, hook, which implements just such a mechanism.

Hook Bindings

The hook command manages a collection of hook bindings. A hook binding has four elements:

Subjects and Observers

For convenience, this TIP collectively refers to subjects and observers as objects, while placing no requirements on how these objects are actually implemented. An object can be a 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.

Specification

The hook command is an ensemble command with the following subcommands:

Bind Subcommand

This subcommand is used to create, update, delete, and query hook bindings.

hook bind ?subject? ?hook? ?observer? ?cmdPrefix?

Called with no arguments, hook bind returns a list of the subjects with hooks to which observers are currently bound.

Called with one argument, a subject, hook bind returns a list of the subject's hooks to which observers are currently bound.

Called with two arguments, a subject and a hook, hook bind 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, hook bind 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, hook bind creates, updates, or deletes a binding. If cmdPrefix is the empty string, hook bind 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, "", hook 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.

Binds During Calls

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:

In particular:

Discussion

Optional Observers: It has been suggested that the observer argument should follow the cmdprefix argument; if it is omitted, an observer name would be automatically generated. However, the observer name is frequently used in practice, and is likely to be much shorter than the cmdprefix, which might be quite long. As a general rule, short arguments following long ones tend to get lost visually; keeping the observer before the cmdprefix leads to more easily readable code.

Call Subcommand

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 hook call 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:

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.

Also, see below for possible extensions to hook call.

Forget Subcommand

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 hook forget will never be called again. In particular,

Configuration Subcommands

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

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: a list {subject hook arglist observer}, the result string, and 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

The option's value should be a command prefix taking four arguments: a subject, a hook, a list of the hook's argument values, and a list of objects the hook. 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 .view's 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.

Possible Additions

During discussions on the tcl-core mailing list, members suggested a number of possible additions to the functionality described in the first draft of this TIP. Some small capabilities have been added in this draft; however, there are two significant ones that I have elected to defer, for two reasons:

Consequently, I'd rather not delay this TIP until these suggested additions are mature. If this TIP is accepted, then these additions can be considered as TIPs in their own right.

On the other hand, they are genuinely interesting, so I want to mention them here.

Asynchronous Dispatch

The hook call command calls bindings synchronously, returning after all bindings have been called. An asychronous mode has been proposed, where bindings would be called in the context of the event loop for even looser coupling between the subject and the observers. This is certainly doable; however, the same effect can be achieved by calling hook call in an after handler.

I've often considered adding a mode like this to our existing implementation, but have always thought better of it in the end.

Accumulating Binding Return Values

Two members of tcl-core have suggested that hook call would be useful to support plug-in architectures if the return values of all bindings called were properly captured. This is an interesting notion; but I'm not sure how to get it right. In some use cases, it would be enough just to get a list of the return values. In other uses cases, the caller might want to know the observer, the return value, and the complete dictionary of return options.

I don't want to build up all of this return information in the usual case; if it isn't needed, there's no reason to spend the time accumulating it. Consequently, what makes sense to me is an option or options that determine what kind of information should be returned, e.g., hook call ?options...? $s $h ...

Given that hook call currently returns the empty string, this functionality can easily be added at a later time.

Prototype Implementation

A prototype implementation is available at [1]. It is written in Tcl. The prototype implementation should work in both Tcl 8.5 and Tcl 8.6.

Copyright

This document has been placed in the public domain.


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