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:

Menubar/menus for all functions.
Edit a single text file.
Load from a file.
Save to a file.
Cut/copy/delete/paste functions.
Search and replace functions.
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:

Fl_Window *window;

window = new Fl_Window(640, 480, "Text Editor");

Variables

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

Fl_Window          *window;
Fl_Menu_Bar        *menubar;
Fl_Multiline_Input *input;
Fl_Window          *replace_dlg;
Fl_Input           *replace_find;
Fl_Input           *replace_with;
Fl_Button          *replace_all;
Fl_Return_Button   *replace_next;
Fl_Button          *replace_cancel;

int                changed = 0;
char               filename[1024] = "";
char               search[256] = "";

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:

Fl_Menu_Item menuitems[] = {
  { "&File", 0, 0, 0, FL_SUBMENU },
    { "&New",        FL_ALT + 'n', (Fl_Callback *)new_cb },
    { "&Open...",    FL_ALT + 'o', (Fl_Callback *)open_cb, 0, FL_MENU_DIVIDER },
    { "&Save",       FL_ALT + 's', (Fl_Callback *)save_cb },
    { "Save &As...", FL_ALT + FL_SHIFT + 's', (Fl_Callback *)saveas_cb, 0, FL_MENU_DIVIDER },
    { "&Quit", FL_ALT + 'q', (Fl_Callback *)quit_cb },
    { 0 },

  { "&Edit", 0, 0, 0, FL_SUBMENU },
    { "&Undo",       FL_ALT + 'z', (Fl_Callback *)undo_cb, 0, FL_MENU_DIVIDER },
    { "Cu&t",        FL_ALT + 'x', (Fl_Callback *)cut_cb },
    { "&Copy",       FL_ALT + 'c', (Fl_Callback *)copy_cb },
    { "&Paste",      FL_ALT + 'v', (Fl_Callback *)paste_cb },
    { "&Delete",     0, (Fl_Callback *)delete_cb },
    { 0 },

  { "&Search", 0, 0, 0, FL_SUBMENU },
    { "&Find...",       FL_ALT + 'f', (Fl_Callback *)find_cb },
    { "F&ind Again",    FL_ALT + 'g', (Fl_Callback *)find2_cb },
    { "&Replace...",    FL_ALT + 'r', (Fl_Callback *)replace_cb },
    { "Re&place Again", FL_ALT + 't', (Fl_Callback *)replace2_cb },
    { 0 },

  { 0 }
};

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

Fl_Menu_Bar *menubar = new Fl_Menu_Bar(0, 0, 640, 30);
menubar->menu(menuitems);

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:

Fl_Multiline_Input *input = new Fl_Multiline_Input(0, 30, 640, 450);

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

input->callback(changed_cb);
input->when(FL_WHEN_CHANGED);

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

input->textfont(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:

Fl_Window *replace_dlg = new Fl_Window(300, 105, "Replace");
Fl_Input *replace_find = new Fl_Input(70, 10, 200, 25, "Find:");
Fl_Input *replace_with = new Fl_Input(70, 40, 200, 25, "Replace:");
Fl_Button *replace_all = new Fl_Button(10, 70, 90, 25, "Replace All");
Fl_Button *replace_next = new Fl_Button(105, 70, 120, 25, "Replace Next");
Fl_Button *replace_cancel = new Fl_Button(230, 70, 60, 25, "Cancel");

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:

void changed_cb(void) {
  set_changed(1);
}

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:

void copy_cb(void) {
  input->copy();
}

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:

void cut_cb(void) {
  input->copy();
  input->cut();
}

delete_cb()

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

void delete_cb(void) {
  input->cut();
}

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:

void find_cb(void) {
  const char *val;

  val = fl_input("Search String:", search);
  if (val != NULL) {
    // User entered a string - go find it!
    strcpy(search, val);
    find2_cb();
  }
}

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:

void find2_cb(void) {
  const char *val, *found;
  int pos;

  if (search[0] == '\0') {
    // Search string is blank; get a new one...
    find_cb();
    return;
  }

  val   = input->value() + input->mark();
  found = strstr(val, search);

  if (found != NULL) {
    // Found a match; update the position and mark...
    pos = input->mark() + found - val;
    input->position(pos, pos + strlen(search));
  }
  else fl_alert("No occurrences of \'%s\' found!", search);
}

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:

void new_cb(void) {
  if (changed)
    if (!check_save()) return;

  filename[0] = '\0';
  input->value("");
  set_changed(0);
}

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:

void open_cb(void) {
  char *newfile;

  if (changed)
    if (!check_save()) return;

  newfile = fl_file_chooser("Open File?", "*", filename);
  if (newfile != NULL) load_file(newfile);
}

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:

void paste_cb(void) {
  Fl::paste(*input);
}

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:

void quit_cb(void) {
  if (changed)
    if (!check_save())
      return;

  window->hide();
}

replace_cb()

The replace callback just shows the replace dialog:

void replace_cb(void) {
  replace_dlg->show();
}

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:

void replace2_cb() {
  const char *find, *val, *found;
  int pos;

  find = replace_find->value();
  if (find[0] == '\0') {
    // Search string is blank; get a new one...
    replace_dlg->show();
    return;
  }

  val   = input->value() + input->position();
  found = strstr(val, find);

  if (found != NULL) {
    // Found a match; update the position and replace text...
    pos = input->position() + found - val;
    input->replace(pos, pos + strlen(find), replace_with->value());
    input->position(pos + strlen(replace_with->value()));
  }
  else fl_alert("No occurrences of \'%s\' found!", find);
}

replall_cb()

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

void replall_cb() {
  const char *find, *val, *found;
  int pos;
  int times;

  find = replace_find->value();
  if (find[0] == '\0') {
    // Search string is blank; get a new one...
    replace_dlg->show();
    return;
  }

  input->position(0);
  times = 0;

  // Loop through the whole string
  do {
    val   = input->value() + input->position();
    found = strstr(val, find);

    if (found != NULL) {
      // Found a match; update the position and replace text...
      times ++;
      pos = input->position() + found - val;
      input->replace(pos, pos + strlen(find), replace_with->value());
      input->position(pos + strlen(replace_with->value()));
    }
  } while (found != NULL);

  if (times > 0) fl_message("Replaced %d occurrences.", times);
  else fl_alert("No occurrences of \'%s\' found!", find);
}

replcan_cb()

This callback just hides the replace dialog:

void replcan_cb() {
  replace_dlg->hide();
}

save_cb()

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

void save_cb(void) {
  if (filename[0] == '\0') {
    // No filename - get one!
    saveas_cb();
    return;
  }
  else save_file(filename);
}

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:

void saveas_cb(void) {
  char *newfile;

  newfile = fl_file_chooser("Save File As?", "*", filename);
  if (newfile != NULL) save_file(newfile);
}

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

undo_cb()

The undo callback just calls the undo() method:

void undo_cb(void) {
  input->undo();
}

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:

int check_save(void) {
  if (!changed) return 1;

  if (fl_ask("The current file has not been saved.\n"
             "Would you like to save it now?")) {
    // Save the file...
    save_cb();

    return !changed;
  }
  else return (1);
}

load_file()

This function loads the specified file into the input widget:

void load_file(char *newfile) {
  FILE *fp;
  char buffer[8192];
  int  nbytes;
  int  pos;

  input->value("");

  fp = fopen(newfile, "r");
  if (fp != NULL) {
    // Was able to open file; let's read from it...
    strcpy(filename, newfile);
    pos = 0;

    while ((nbytes = fread(buffer, 1, sizeof(buffer), fp)) > 0) {
      input->replace(pos, pos, buffer, nbytes);
      pos += nbytes;
    }

    fclose(fp);
    input->position(0);
    set_changed(0);
  } else {
    // Couldn't open file - say so...
    fl_alert("Unable to open \'%s\' for reading!");
  }
}

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:

void save_file(char *newfile) {
  FILE *fp;

  fp = fopen(newfile, "w");
  if (fp != NULL) {
    // Was able to create file; let's write to it...
    strcpy(filename, newfile);

    if (fwrite(input->value(), 1, input->size(), fp) < 1) {
      fl_alert("Unable to write file!");
      fclose(fp);
      return;
    }

    fclose(fp);
    set_changed(0);
  } else {
    // Couldn't open file - say so...
    fl_alert("Unable to create \'%s\' for writing!");
  }
}

set_changed()

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

void set_changed(int c) {
  if (c != changed) {
    char title[1024];
    char *slash;

    changed = c;

    if (filename[0] == '\0') strcpy(title, "Untitled");
    else {
      slash = strrchr(filename, '/');
      if (slash == NULL) slash = strrchr(filename, '\\');

      if (slash != NULL) strcpy(title, slash + 1);
      else strcpy(title, filename);
    }

    if (changed) strcat(title, " (modified)");

    window->label(title);
  }
}

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:

CC -o editor editor.cxx -lfltk -lXext -lX11 -lm

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 below: