TIP #306 Version 1.6: Auto-Naming Widgets

This is not necessarily the current version of this TIP.

Abstract

A Tk programmer must give every widget a unique name. This is often quite annoying, especially for widgets whose name is stored in a variable (which also must be given a name). This TIP proposes a small extension to generate automatic names for widgets.

Rationale

Every Tk widget must be given a unique name. This is needed internally for Tk and also for the programmer to be able to refer to it. Quite often, and as recommended in many Tk coding guidelines (e.g. http://www.beedub.com/book/2nd/TKINTRO.doc.html), the widget names are stored in variables and only referred to indirectly. These variables must also be given a name, which may lead to confusion, and requires more inventivity of the programmer than needed.

An example from the BWidget source code:

 set status   [frame $path.status -background $bg]
 set label    [label $status.label]
 set indframe [frame $status.indf -background $bg]
 set prgframe [frame $status.prgf -background $bg]

Other cases where widget names are unimportant from a programmer's viewpoint, are widgets which are never referred to after creation, and those which are only referred to by other means (e.g. by a textvariable). Example:

 pack [label .name_label -text "Enter name:"] -side left
 pack [entry .name_entry -textvariable name] -side left

In all those cases, it would be helpful if Tk could generate the widget names itself.

Currently the widget creation commands already return the widget name, which is always the exact name the programmer has supplied. This makes it easy to make an (almost) backwards compatible extension, as presented below.

Specification

When creating a new widget with a name that ends on "%", the "%" will be replaced by a counter. The actual widget name is then returned. Note that each parent widget has its own counter.

 % button .%
 .1
 % set f [frame .%]
 .2
 % label $f.%
 .2.1
 % set p [frame .prefix%]
 .prefix3
 % label $p.%a%b%c%%
 .prefix3.%a%b%c%1

The examples above can now be written as:

 set status   [frame $path.% -background $bg]
 set label    [label $status.%]
 set indframe [frame $status.% -background $bg]
 set prgframe [frame $status.% -background $bg]

 pack [label .% -text "Enter name:"] -side left
 pack [entry .% -textvariable name] -side left

Care is taken that the automatic name does not refer to an already existing (manually specified) widget. Example:

 % frame .2
 .2
 % frame .%
 .1
 % frame .%
 .3

Reference Implementation

See SourceForge patch #1735008[1].

Basically, the TkWindow structure gets a new member "autocount", which is initalized to 0 when the widget is created, and which is incremented when the auto-name feature is used for creating child widgets (this is done in the NameWindow() function).

Compatibility

Backward Compatibility

The presented extension is backward compatible, except for existing code which uses widget names ending on "%". Hope this is acceptible...

Forward Compatibility

This is another issue. Megawidgets which want to support the new naming scheme, will probably have to be adapted. In particular, when they are written as:

 proc mymegawidget {win args} {
     # do some stuff with $win
     # ...

     # Create the hull frame
     frame $win -class MyMegaWidget

     # Create internals
     label $win.title -text "Title"
     # ...
 }

they will have to be rewritten in the following way:

 proc mymegawidget {w args} {
     # Create the hull frame (returns the actual widget name)
     set win [frame $w -class MyMegaWidget]

     # do some stuff with $win
     # ...

     # Create internals
     label $win.title -text "Title"
     # ...
 }

In Tk itself, there is one such example which has to be rewritten: tk_optionMenu.

Alternatives

1. Implementing this TIP in Tcl. This would require all widget commands to be wrapped. That's a lot of work compared to the simple patch to the C code used here.

2. Make a separate autoname command, and use this when creating widgets. For example:

    set frame [frame [autoname .%]]
    label [autoname $frame.%]
   

   This makes the code a longer and more difficult to read, which is the opposite of what this TIP tries to achieve.

A very simple autonaming can be done by just setting aside one global variable and define an alias that increments that variable:

 set ::# 0 ;# from 8.5 not even needed
 interp alias {} % {} incr ::#
 set frame [frame .[%]]
 label $frame.[%]

Copyright

This document has been placed in the public domain.

This is not necessarily the current version of this TIP.