4 - Designing a Simple Text Editor

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

Determining the Goals of the Text Editor

Since this will be the first big project you'll be doing with FLTK, lets define what we want our text editor to do:
  1. Menubar/menus for all functions.
  2. Edit a single text file.
  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:

Variables

Our text editor will need some global variables to keep track of things: The window variable is our top-level window 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_Multiline_Input widget to edit the text: So that we can keep track of changes to the file, we also want to add a "changed" callback: Finally, we want to use a mono-spaced font like FL_COURIER:

The Replace Dialog

We can use the FLTK 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.

Callbacks

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

changed_cb()

This function will be called whenever the user changes any text in the input widget: The set_changed() function is one that we will write to set the changed status on the current file. We're doing it this way because some of the other callbacks will set the changed status to 0, and also because we want to show the changed status in the window's title bar.

copy_cb()

This callback function will call input->copy() to copy the currently selected text to the clipboard:

cut_cb()

This callback function will call input->copy() to copy the currently selected text to the clipboard and then input->cut() to delete it:

delete_cb()

This callback function will call input->cut() to delete the selected text:

find_cb()

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:

find2_cb()

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.

new_cb()

This callback function will clear 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:

open_cb()

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.

paste_cb()

This callback function will send a FL_PASTE message to the input widget using the Fl::paste() method:

quit_cb()

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 hides the main window:

replace_cb()

The replace callback just shows the replace dialog:

replace2_cb()

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:

replall_cb()

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

replcan_cb()

This callback just hides the replace dialog:

save_cb()

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.

saveas_cb()

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.

undo_cb()

The undo callback just calls the undo() method:

Other Functions

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

check_save()

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:

load_file()

This function loads the specified file into the input widget: When loading the file we use the input->replace() method to "replace" the text at the end of the buffer. The pos variable keeps track of the end of the buffer.

save_file()

This function saves the current buffer to the specified file:

set_changed()

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

Compiling the Editor

The complete source for our text editor can be found in the test/editor.cxx source file. Both the Makefile and Visual C++ workspace include the necessary rules to build the editor. You can also compile it using a standard compiler with: As noted in Chapter 1, you may need to include compiler and linker options to tell them where to find the FLTK library. Also, the CC command may also be called gcc or c++ on your system.

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

The Final Product

The final editor window should look like the image on the next page.

The completed editor window.