TIP:		19
Title:		Add a Text Changed Flag to Tk's Text Widget
Version:	$Revision: 1.6 $
Author:		Neil McKay <mckay@eecs.umich.edu>
State:		Final
Type:		Project
Vote:		Done
Created:	03-Jan-2001
Tcl-Version:	8.4a2
Obsoleted-By:	26
Post-History:

~Abstract

This TIP adds a ''text changed'' flag to the Tk text widget.  The flag
would initially be reset, but would be set whenever the contents of
the text widget changes.

~Rationale

When creating a text editor, it is often useful to know when the
contents of the edit buffer have changed, e.g. in order to ask the
user whether or not to save changes on exit. It is possible to create
key bindings in Tk's text widget that will set a flag whenever the
user changes the widget's contents; however, this is awkward, and it
still requires that the programmer set the flag whenever text is
changed programmatically.  A better solution is to include a ''text
changed'' flag in the code for the text widget itself; this can be
accomplished with a relatively small amount of code.

~Flag Behavior

The ''text changed'' flag should behave as follows:

 * It should be reset when the text widget is created

 * It should be set whenever characters are inserted into or deleted
   from the widget

 * It must be resettable programmatically via a Tcl command

~Reference Implementation

At the Tcl level, one possible implementation is to add a ''changed''
widget command to the text widget. One possible syntax for this
command is:

|    .txt changed ?boolean?

where .txt is a text widget.  With no ''boolean'' argument, the
command returns the state of the text-changed flag; with an argument,
it sets the state of the text-changed flag to the value of the
argument.

~Example

A typical sequence of commands in a text editor would be

 1. Create a text widget

 2. Read a file and put its contents into the text widget

 3. Mark the text as unchanged

 4. Edit the text

 5. Write the text out, if it has changed.

This could be accomplished by the following Tcl code fragment:

| grid [button .b -text Quit -command EndEdit]
| grid [text .t]
|
| proc EndEdit {} {
|     if {[.t changed]} {
|         set result [tk_messageBox -type yesno -message "Save changes?"]
|         if {[string compare $result "yes"] == 0} {
|             set fh [open $fileName "w"]
|             puts -nonewline $fh [.t get 1.0 end-1c]
|             close $fh
|         }
|     }
|     exit
| }
|
| set fh [open $fileName "r"]
| .t insert end [read $fh]
| close $fh
| .t changed false

~Copyright

This document is in the public domain.

~Patch

The ''changed'' text widget command, as described above, may be added
to Tk8.4a2 by applying the patch at
http://www.cs.man.ac.uk/fellowsd-bin/TIP/19.patch