TIP #19: Add a Text Changed Flag to Tk's Text Widget
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TIP:	19
Title:	Add a Text Changed Flag to Tk's Text Widget
Version:	$Revision: 1.6 $
Authors:	Neil McKay <mckay@eecs.umich.edu>
State:	Final
Type:	Project
Tcl-Version:	8.4a2
Vote:	Done
Created:	Wed Jan 03 00:00:00 UTC 2001
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
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