TIP 19: Add a Text Changed Flag to Tk's Text Widget

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by Aug 21.
Author:		Neil McKay <mckay@eecs.umich.edu>
State:		Final
Type:		Project
Vote:		Done
Created:	03-Jan-2001
Tcl-Version:	8.4a2
Obsoleted-By:	26


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.


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:

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.


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

 set fh [open $fileName "r"]
 .t insert end [read $fh]
 close $fh
 .t changed false


This document is in the public domain.


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