The Tk Text Widget

by Derek Fountain

All script writers need to deal with textual data at one time or another. One of the most powerful tools for manipulating text in the free software world is the text widget found in the Tk graphical user interface (GUI) toolkit. This widget is available to script writers working with Tcl, Perl/Tk and Tkinter in Python, and it boasts features and functionality that can solve almost any text-related requirement a script writer is likely to encounter.

Key features include multi-line text display and editing, comprehensive text formatting, embedded images, embedded widgets and a unique, almost boundless mechanism for endowing dynamic behaviour on areas of text. This article discusses the features of the Tk text widget that make it a rich and dynamic text manipulation utility. Examples are presented in Tcl/Tk code, but users of other languages shouldn't have much trouble translating the concepts and examples to their favoured environments. Before we get started, however, we need to take a quick look at a couple of concepts employed by the widget.

Indices in the Text

Each character stored in a text widget can be addressed by an index. An index most often is defined by two numbers, the line number and the character position on that line. As an example, the index 10.45 refers to the character on line 10, position 45.

The text widget has built-in functionality for index arithmetic; it supports expressions that allow you to find a location, say, 10 lines and 15 characters from a given point. It also handles special indices containing a pixel location (specified as @x,y) or containing certain words. For example, 1.end means the character at the end of the first line, insert means the character at the input cursor and current means the character under the mouse pointer.

Tagging Parts of the Text

The feature that provides the Tk text widget with much of its power is known as tagging. Any area of the text, defined as starting at one index and ending at another, can be tagged with a logical tag name. Each tag can be assigned certain attributes and properties, and all the areas of text given that tag immediately take on the assigned properties. A good part of this article discusses the properties that can be controlled and the uses to which this tagging feature can be put.

The Tk text widget provides all the facilities required for rich text editing. This includes flexible font handling, adjustable line spacing and margins, word wrap, colour support, cut, copy and paste, an undo stack, plus the usual array of bold, underline, italic and other formatting options. Figure 1 is a screenshot of one of the demonstration scripts supplied with Tk. It shows some of the formatting options available. Figure 1 here

All formatting is implemented using the tags mechanism. The script writer defines a tag with the formatting required, then either inserts the required text with that tag or applies that tag to an area of text already in the widget. For example, for an individual text widget named .t, a title tag might be defined with a large, underlined font and centre justification:


.t tag configure title -font "helvetica 14" \
                       -justify "center" \
                       -underline on


To insert some title text into the widget, this code can be used:


.t insert 1.0 "The Legend of Black Cave\n" title


This inserts the text at line 1, position 0, with the tag title applied. Alternatively, the tag can be applied to text already in the widget by using code like:


.t tag add title 1.0 1.end


This adds the title tag to the text between the given indices, in this case between the start and end of line 1. In a more realistic situation those indices wouldn't be hard coded. They are more likely to be calculated from the location of the selection, for example, or the results of a search. Notice that the command used to assign a tag to an area of text is add. This is because an area of text can have several tags assigned to it. For example, our title text also might need to be highlighted as the result of a search or to indicate that its status has changed to urgent.

Figure 2 is a screenshot of a small script (Listing 1) that demonstrates formatting by way of tag manipulation. It also demonstrates the text widget's useful ability to save marks, or named locations, anywhere in the text. Figure 2 here

Embedding Images and Widgets

In addition to text formatting, the Tk text widget also supports embedding images and other GUI control widgets. When images are inserted into the text, they float, so editing of the text around them causes them to move in accordance with the formatting rules configured for that area of text. The same image can be inserted into the widget multiple times, and if an image is modified dynamically elsewhere in the script, its representation in the text widget is updated immediately.

Other Tk GUI widgets can be inserted into the text widget. Simple buttons and dropdown lists, as might be found amongst the text of a web page, are only the beginning. It is possible to embed anything--a drawing canvas, a table or even another text widget. If you have a complicated set of widgets that need to be presented inside some text, the text widget has a mechanism whereby it creates only those widgets when necessary--when the appropriate area is scrolled into view, for example.

Advanced Features

Text formatting and image and widget presentation is the top layer of the text widget's box of tricks. Tags provide an extensive array of facilities that can add intriguing dynamic abilities to pieces of text.

In order to make some of the following concepts a little clearer, another small demonstration script is provided in Listing 2. This script creates a text widget and loads it with the contents of an XML file. Areas of the text are tagged according to their position in the XML structure. Figure 3 shows this script running with my Konqueror bookmarks file as its input. Figure 3 here

Searching

The text widget has its own built in search mechanism. A simple search command looks like this:


.t search $searchText 1.0 end


This command returns the index of the first occurrence of the string in $searchText between the start of the text (index 1.0) and the end (index end). Implementing a find next feature is as simple as changing the starting position for the search to the location the last find returned. Searches can be configured to go forwards or backwards, be case insensitive or be made to look for regular expressions as opposed to fixed strings.

Tag and Bind

All of the above demonstrates that the Tk text widget is a flexible solution to most text formatting and editing requirements. More power is waiting to be explored, though, and this is where the programmer needs to start employing some imagination. This extra power is opened up with the text widget's bind subcommand.

The bind subcommand associates a Tcl script with the occurrence of an X event within a text widget often, but not necessarily, over a tagged area of text. A simple example might be a mouse over; when the X system reports that the mouse pointer has been placed over a piece of tagged text, we might want to change the mouse pointer shape. Code to bind such an event looks like this:


.t tag bind mouseover <Enter> {.t configure -cursor center_ptr}
.t tag bind mouseover <Leave> {.t configure -cursor xterm}


The X <Enter> event is received when the mouse pointer enters a certain screen location, and the <Leave> event is received when it leaves it. In this case the screen location is any area of text in the .t widget is tagged with the mouseover tag. In this simple example, the script run in each case is a one-liner that reconfigures the text widget with a new mouse pointer shape. In a more advanced example, perhaps some bubble help would appear or an acronym would be expanded or maybe more details could be presented from a database lookup. The only limitation is what can be achieved in the fairly limited timeframe. Given modern hardware and fast scripting languages, such as Tcl, Perl or Python, a surprisingly large amount of code can be executed within a mouse movement.

Hyperlinks

All X events can be utilised by the bind command, including mouse clicks. An obvious use for this is a hyperlink. Setting up the binding is as simple as before:


.t tag bind hyperlink <Button-1> {clickLink %x %y}


Now, whenever mouse button 1 is clicked over any text tagged with the hyperlink tag, the Tcl procedure clickLink is called. The %x and %y are replaced by the bind mechanism with the pixel coordinates of where the mouse click happened. This allows us to find the clicked piece of text. The clickLink procedure, which assumes the clicked text contains a URL, looks like this:


proc clickLink { xpos ypos } {

    set i [.t index @$xpos,$ypos]
    set range [.t tag prevrange hyperlink $i]
    set url [eval .t get $range]
    catch { exec mozilla $url & }
}


This code finds the index of the character clicked on using the @x,y syntax of the text widget's index subcommand. It uses that index to find the range of text covered by that occurrence of the hyperlink tag. It then gets that piece of text--the URL--and hands it over to, in this case, Mozilla. The catch command prevents the script from stopping if there's a problem starting Mozilla. I've omitted error handling for simplicity; in a real example a problem with Mozilla would need to be handled smoothly. Witness the power of this tag-and-bind approach--hyperlinks implemented in less than 10 lines of code.

Hyperlinks are a fairly obvious use for this functionality, so let's consider some alternatives. Imagine an e-mail client in which the user clicks on a subject line in one text widget and sees the text of the e-mail in another. Imagine a programmer's editor where you can click on a keyword or function and see a summary of the syntax it requires. How about a database front end, where the user hovers the mouse pointer over a table name and sees the structure of that table? These sorts of features are simple to implement with the tag and bind approach.

Caveats

A few problems can be found in using the Tk text widget. When a tool has been around for as long as this one, the number of problems become fairly small. There is one significant problem with the widget as it stands, however. In order to get the full benefits of margin handling and the like, text needs to be inserted without newline characters. That is, each paragraph needs to be one long string with a newline at the end. This allows the word wrapping to work properly. However, the widget's default keyboard navigation behaviour is that pressing the up or down arrow key moves up or down a real line, rather than a displayed line. The effect is to skip up or down a paragraph at a time instead of a line, and so the only way to move to the middle of a paragraph is to hold the right arrow key and allow the cursor to work its way across and downwards.

Given the quality of thought and implementation that clearly has gone into the rest of the widget, it's hard to see how this behaviour has been tolerated for so long. The problem finally was addressed for Tk 8.5, which, as of this writing, is in alpha release. In the meantime, the Tcler's Wiki has a workaround that installs more sensible behaviour, see Resources.

Conclusion

The Tk text widget is a remarkably powerful tool available to script writers who work with Tcl, Python and Perl. In its basic form it allows multi-line text entry, with rich text formatting and undo facilities where appropriate. Its more powerful features provide a framework for implementing all sorts of interesting facilities, including image handling, embedded GUI controls, mouse overs and hyperlinks. If your application involves the manipulation or presentation of textual information, the abilities of Tk's text widget should not be overlooked.

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