Tk - An overview of an Object Oriented Tk8.0 extension for perl5
$main = MainWindow->new();
$widget = $main->Widget(...);
In writing the perl Tk extension, the goals were to provide a complete interface to the latest production version of John Ousterhout's Tk, while providing an Object Oriented interface to perl code.
The package is composed of three loosely connected parts:
- pTk - Converted Tk source
The pTk sub-directory is a copy of the C code of Tk4.0, modified to allow use by languages other than the original Tcl. (The pTk can be read as 'perl' Tk or 'portable' Tk, depending on your sensibilities.)
- Tk to Perl 'Glue'
The top level directory provides Tk.xs and tkGlue.c which provide the perl-callable interfaces to pTk
- Perl code for 'Widget' Classes
The Tk/Tk sub-directory contains the various perl modules that comprise the "Classes" that are visible to Tk applications.
The "major" widgets such as Tk::Text are actually in separate directories at the top level (e.g. Text/* for Tk::Text) and are dynamically loaded as needed on platforms which support perl5's DynaLoader.
- package Tk; - the 'base class'
All the "command names" documented in Tcl/Tk are made to look like perl sub's and reside in the Tk package. Their names are all lower case. Typically there are very few commands at this level which are called directly by applications.
- package Tk::Widget; - the 'Widget class'
There are no actual objects of the Tk::Widget class; however all the various Tk window "widgets" inherit from it, and it in turn inherits all the core Tk functions from Tk.
Tk::Widget provides various functions and interfaces which are common to all Widgets.
A widget is represented to perl as a blessed reference to a hash. There are some members of the hash which are private to Tk and its tkGlue code. Keys starting with '.' and of the form /_[A-Z][A-Za-z_]+_/ (i.e. starting and ending in _ and with first char after _ being upper case) should be considered reserved to Tk.
- Tk::Button, Tk::Entry, Tk::Text ...
There is one class for each of the "Tk" widget item types. Some of them like Tk::Frame do very little indeed, and really only exist so that they can be derived from or so that focus or menu traversal can discover the "kind" of window being processed.
Other classes, Tk::Text for example, provide a lot of methods used with Tk's "bind" to provide a rich keyboard/mouse interface to the widgets' data.
These widget classes also include conversions of the Tcl code for event bindings, keyboard focus traversal, menu bars, and menu keyboard traversal. All the Tcl functions have been converted, but the names have changed (systematically) and they have been split up between the various classes in what I hope is an appropriate manner. Name changes are normally: dropping initial tk_ as the Tk-ness is implicit in the Tk:: prefix, and similarly dropping say Menu from the name if it has been moved the Tk::Menu class. Thus 'proc tkMenuNextEntry' becomes 'sub NextEntry' in the Tk::Menu package.
This does for Tk4.0's "images" what Tk::Widget does for widgets. Images are new to Tk4.0 and the class structure is not mature either.
There are three sub-classes Tk::Bitmap, Tk::Pixmap and Tk::Photo.
It is expected that Tk::Image hierarchy will evolve during the "beta" phase of Tk to allow dynamic or auto-loaded image types or photo formats (e.g. support for JPEG format for photos).
- Composite Widgets
A composite is some kind of 'frame' with subwidgets which give it useful behaviour. Tk::Dialog is an example of a composite widget classes built from the basic Tk ones. It is intended that user code should not need to be aware that a particular class is a composite, and create and configure such widgets in the same manner as any other kind. The configure mechanism and the methods of the class manipulate the subwidgets as required.
Composite widgets are implemented via Tk::Frame and multiple inheritance. The two 'frame' base classes Tk::Frame and Tk::Toplevel include the additional class Tk::Derived in their inheritance. Tk::Derived provides methods to allow additional configure options to be defined for a widget.
A Composite widget is typically defined as derived from Tk::Frame or Tk::Toplevel (e.g. Tk::Dialog).