TIP:            110
Title:          Add a Tristate Mode to the Checkbutton and Radiobutton
Version:        $Revision: 1.12 $
Author:         Brian Griffin <bgriffin@model.com>
State:          Final
Type:           Project
Vote:           Done
Created:        01-Oct-2002
Post-History:   
Tcl-Version:    8.5

~ Abstract

This TIP proposes adding a third value (tristate) to the checkbutton
and radiobutton widgets and corresponding display.

~ Rationale

In order to meet more demanding requirements of todays complex
graphical user interfaces, it would help add to the functionality of
the basic checkbutton and radiobutton widgets. To support using
checkbuttons and radiobuttons along with multiple selection, it is
necessary for these buttons to be able to display a third state,
i.e., both On and Off.  This indicates the situation where a property
has a particular value for some members of a selection set, but
not others.

~ Proposed Change

The change would add a third "tristate" value to current On Off values
of the checkbutton and radiobutton widgets.  The widget would then
display the check/dot mark (as appropriate) along with a "grayed"
background, for example.  When the checkbutton or radiobutton is 
invoked (i.e. clicked on) it would behave exactly as it does currently, 
setting the variable to the On value.

There is a concern that the Unix version of these widgets do not have
a sufficiently different appearance when in the tristate state
compared with the On and Off states.  This issue is addressed in
[109].

~ Propsal Specifics

   1. Identify a third, platform specific, presentation (e.g. check
      with grayed background) to represent the tristate value,

   1. Add the option "-tristatevalue" to specify the match value
      only. The default value of this option will be "{}",

   1. Change the behavior to display the indeterminate or tristate
      presentation when the associated variable's value matches the
      -tristatevalue, and

   1. Add the option "-tristateimage" to specify an image to display
      (in place of the image option) when the checkbutton or
      radiobutton has the tristate value (as defined above.)

~ Copyright

This document has been placed in the public domain.
