4 - Designing a Simple Text Editor

This chapter takes you through the design of a simple pyFLTK-based text editor.

Determining the Goals of the Text Editor

Since this will be the first big project you'll be doing with pyFLTK, lets define what we want our text editor to do:

  1. Provide a menubar/menus for all functions.
  2. Edit a single text file, possibly with multiple views.
  3. Load from a file.
  4. Save to a file.
  5. Cut/copy/delete/paste functions.
  6. Search and replace functions.
  7. Keep track of when the file has been changed.

Designing the Main Window

Now that we've outlined the goals for our editor, we can begin with the design of our GUI. Obviously the first thing that we need is a window, which we'll place inside a class called EditorWindow:


Our text editor will need some global variables to keep track of things:

The textbuf variable is the text editor buffer for our window class described previously. We'll cover the other variables as we build the application.

Menubars and Menus

The first goal requires us to use a menubar and menus that define each function the editor needs to perform. The Fl_Menu_Item structure is used to define the menus and items in a menubar:

Once we have the menus defined we can create the Fl_Menu_Bar widget and assign the menus to it with:

We'll define the callback functions later.

Editing the Text

To keep things simple our text editor will use the Fl_Text_Editor widget to edit the text:

So that we can keep track of changes to the file, we also want to add a "modify" callback:

Finally, we want to use a mono-spaced font like FL_COURIER:

The Replace Dialog

We can use the pyFLTK convenience functions for many of the editor's dialogs, however the replace dialog needs its own custom window. To keep things simple we will have a "find" string, a "replace" string, and "replace all", "replace next", and "cancel" buttons. The strings are just Fl_Input widgets, the "replace all" and "cancel" buttons are Fl_Button widgets, and the "replace next " button is a Fl_Return_Button widget:

The search and replace dialog.
Figure 4-1: The search and replace dialog.


Now that we've defined the GUI components of our editor, we need to define our callback functions.


This function will be called whenever the user changes any text in the editor widget:

The set_title() function is one that we will write to set the changed status on the current file. We're doing it this way because we want to show the changed status in the window's title bar.


This callback function will call kf_copy() to copy the currently selected text to the clipboard:


This callback function will call kf_cut() to cut the currently selected text to the clipboard:


This callback function will call remove_selection() to delete the currently selected text to the clipboard:


This callback function asks for a search string using the fl_input() convenience function and then calls the find2_cb() function to find the string:


This function will find the next occurrence of the search string. If the search string is blank then we want to pop up the search dialog:

If the search string cannot be found we use the fl_alert() convenience function to display a message to that effect.


This callback function will clear the editor widget and current filename. It also calls the check_save() function to give the user the opportunity to save the current file first as needed:


This callback function will ask the user for a filename and then load the specified file into the input widget and current filename. It also calls the check_save() function to give the user the opportunity to save the current file first as needed:

We call the load_file() function to actually load the file.


This callback function will call kf_paste() to paste the clipboard at the current position:


The quit callback will first see if the current file has been modified, and if so give the user a chance to save it. It then exits from the program:


The replace callback just shows the replace dialog:


This callback will replace the next occurence of the replacement string. If nothing has been entered for the replacement string, then the replace dialog is displayed instead:


This callback will replace all occurences of the search string in the file:


This callback just hides the replace dialog:


This callback saves the current file. If the current filename is blank it calls the "save as" callback:

The save_file() function saves the current file to the specified filename.


This callback asks the user for a filename and saves the current file:

The save_file() function saves the current file to the specified filename.

Other Functions

Now that we've defined the callback functions, we need our support functions to make it all work:


This function checks to see if the current file needs to be saved. If so, it asks the user if they want to save it:


This function loads the specified file into the textbuf class:

When loading the file we use the loadfile() method to "replace" the text in the buffer, or the insertfile() method to insert text in the buffer from the named file.


This function saves the current buffer to the specified file:


This function checks the changed variable and updates the window label accordingly:

The main code

Once we've created all of the support functions, the only thing left is to tie them all together. What remains is to create a new text buffer, create a new view (window) for the text, show the window, load the file on the command-line (if any), and then enter the FLTK event loop:

Running the Editor

The complete source for our text editor can be found in the test/editor.py source file. You can run the program as:

Congratulations, you've just built your own text editor!

The Final Product

The final editor window should look like the image in Figure 4-2.

The completed editor window.
Figure 4-2: The completed editor window

Advanced Features

Now that we've implemented the basic functionality, it is time to show off some of the advanced features of the Fl_Text_Editor widget.

Syntax Highlighting

The Fl_Text_Editor widget supports highlighting of text with different fonts, colors, and sizes. The implementation is based on the excellent NEdit text editor core, which uses a parallel "style" buffer which tracks the font, color, and size of the text that is drawn.

Styles are defined using Python arrays of the form: Fl_Text_Display::Style_Table_Entry structure defined in <FL/Fl_Text_Display.H>:

The color member sets the color for the text, the font member sets the FLTK font index to use, and the size member sets the pixel size of the text. The attr member is currently not used.

For our text editor we'll define 7 styles for plain code, comments, keywords, and preprocessor directives:

You'll notice that the comments show a letter next to each style - each style in the style buffer is referenced using a character starting with the letter 'A'.

You call the highlight_data() method to associate the style data and buffer with the text editor widget:

Finally, you need to add a callback to the main text buffer so that changes to the text buffer are mirrored in the style buffer:

The style_update() function, like the change_cb() function described earlier, is called whenever text is added or removed from the text buffer. It mirrors the changes in the style buffer and then updates the style data as necessary:

The style_parse() function scans a copy of the text in the buffer and generates the necessary style characters for display. It assumes that parsing begins at the start of a line: