The class hierarchy looks complicated, but in actual practice, application programmers
almost always refer to the classes at the very bottom of the hierarchy.
- These classes are provided for the purposes of organizing certain functions under one
namespace. They aren't meant to be instantiated independently.
- The Tk class is meant to be instantiated only once in an
application. Application programmers need not instantiate one explicitly, the system
creates one whenever any of the other classes are instantiated.
- The Widget class is not meant to be instantiated, it is meant
only for subclassing to make ``real'' widgets (in C++, this is called an `abstract
To make use of this reference material, there will be times when you will need to know how
to read short passages of Tk and how to identify the various parts of a Tk command. (See
section 16.1.4 for the Tkinter equivalents of what's below.)
Tk scripts are Tcl programs. Like all Tcl programs, Tk scripts are just lists of tokens
separated by spaces. A Tk widget is just its class, the options that help
configure it, and the actions that make it do useful things.
To make a widget in Tk, the command is always of the form:
classCommand newPathname options
- denotes which kind of widget to make (a button, a label, a menu...)
- is the new name for this widget. All names in Tk must be unique. To help enforce this,
widgets in Tk are named with pathnames, just like files in a file system. The top
level widget, the root, is called
. (period) and children are
delimited by more periods. For example,
.myApp.controlPanel.okButton might be
the name of a widget.
- configure the widget's appearance and in some cases, its behavior. The options come in
the form of a list of flags and values. Flags are proceeded by a `-', like unix shell
command flags, and values are put in quotes if they are more than one word.
button .fred -fg red -text "hi there"
^ ^ \_____________________/
| | |
class new options
command widget (-opt val -opt val ...)
Once created, the pathname to the widget becomes a new command. This new widget
command is the programmer's handle for getting the new widget to perform some action.
In C, you'd express this as someAction(fred, someOptions), in C++, you would express this as
fred.someAction(someOptions), and in Tk, you say:
.fred someAction someOptions
Note that the object name,
.fred, starts with a dot.
As you'd expect, the legal values for someAction will depend on the widget's
.fred disable works if fred is a button (fred gets greyed out), but does
not work if fred is a label (disabling of labels is not supported in Tk).
The legal values of someOptions is action dependent. Some actions, like
require no arguments, others, like a text-entry box's
delete command, would need
arguments to specify what range of text to delete.