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9. Windows

Windows form the most important concept in curses. You have seen the standard window stdscr above where all the functions implicitly operated on this window. Now to make design even a simplest GUI, you need to resort to windows. The main reason you may want to use windows is to manipulate parts of the screen separately, for better efficiency, by updating only the windows that need to be changed and for a better design. I would say the last reason is the most important in going for windows. You should always strive for a better and easy-to-manage design in your programs. If you are writing big, complex GUIs this is of pivotal importance before you start doing anything.

9.1. The basics

A Window can be created by calling the function newwin(). It doesn't create any thing on the screen actually. It allocates memory for a structure to manipulate the window and updates the structure with data regarding the window like it's size, beginy, beginx etc.. Hence in curses, a window is just an abstraction of an imaginary window, which can be manipulated independent of other parts of screen. The function newwin() returns a pointer to structure WINDOW, which can be passed to window related functions like wprintw() etc.. Finally the window can be destroyed with delwin(). It will deallocate the memory associated with the window structure.

9.2. Let there be a Window !!!

What fun is it, if a window is created and we can't see it. So the fun part begins by displaying the window. The function box() can be used to draw a border around the window. Let's explore these functions in more detail in this example.

Example 7. Window Border example

#include <ncurses.h>


WINDOW *create_newwin(int height, int width, int starty, int startx);
void destroy_win(WINDOW *local_win);

int main(int argc, char *argv[])
{       WINDOW *my_win;
        int startx, starty, width, height;
        int ch;

        initscr();                      /* Start curses mode            */
        cbreak();                       /* Line buffering disabled, Pass on
                                         * everty thing to me           */
        keypad(stdscr, TRUE);           /* I need that nifty F1         */

        height = 3;
        width = 10;
        starty = (LINES - height) / 2;  /* Calculating for a center placement */
        startx = (COLS - width) / 2;    /* of the window                */
        printw("Press F1 to exit");
        refresh();
        my_win = create_newwin(height, width, starty, startx);

        while((ch = getch()) != KEY_F(1))
        {       switch(ch)
                {       case KEY_LEFT:
                                destroy_win(my_win);
                                my_win = create_newwin(height, width, starty,--startx);
                                break;
                        case KEY_RIGHT:
                                destroy_win(my_win);
                                my_win = create_newwin(height, width, starty,++startx);
                                break;
                        case KEY_UP:
                                destroy_win(my_win);
                                my_win = create_newwin(height, width, --starty,startx);
                                break;
                        case KEY_DOWN:
                                destroy_win(my_win);
                                my_win = create_newwin(height, width, ++starty,startx);
                                break;  
                }
        }
                
        endwin();                       /* End curses mode                */
        return 0;
}

WINDOW *create_newwin(int height, int width, int starty, int startx)
{       WINDOW *local_win;

        local_win = newwin(height, width, starty, startx);
        box(local_win, 0 , 0);          /* 0, 0 gives default characters 
                                         * for the vertical and horizontal
                                         * lines                        */
        wrefresh(local_win);            /* Show that box                */

        return local_win;
}

void destroy_win(WINDOW *local_win)
{       
        /* box(local_win, ' ', ' '); : This won't produce the desired
         * result of erasing the window. It will leave it's four corners 
         * and so an ugly remnant of window. 
         */
        wborder(local_win, ' ', ' ', ' ',' ',' ',' ',' ',' ');
        /* The parameters taken are 
         * 1. win: the window on which to operate
         * 2. ls: character to be used for the left side of the window 
         * 3. rs: character to be used for the right side of the window 
         * 4. ts: character to be used for the top side of the window 
         * 5. bs: character to be used for the bottom side of the window 
         * 6. tl: character to be used for the top left corner of the window 
         * 7. tr: character to be used for the top right corner of the window 
         * 8. bl: character to be used for the bottom left corner of the window 
         * 9. br: character to be used for the bottom right corner of the window
         */
        wrefresh(local_win);
        delwin(local_win);
}

9.3. Explanation

Don't scream. I know it's a big example. But I have to explain some important things here :-). This program creates a rectangular window that can be moved with left, right, up, down arrow keys. It repeatedly creates and destroys windows as user press a key. Don't go beyond the screen limits. Checking for those limits is left as an exercise for the reader. Let's dissect it by line by line.

The create_newwin() function creates a window with newwin() and displays a border around it with box. The function destroy_win() first erases the window from screen by painting a border with ' ' character and then calling delwin() to deallocate memory related to it. Depending on the key the user presses, starty or startx is changed and a new window is created.

In the destroy_win, as you can see, I used wborder instead of box. The reason is written in the comments (You missed it. I know. Read the code :-)). wborder draws a border around the window with the characters given to it as the 4 corner points and the 4 lines. To put it clearly, if you have called wborder as below:

    wborder(win, '|', '|', '-', '-', '+', '+', '+', '+');

it produces some thing like

    +------------+
    |            |
    |            |
    |            |
    |            |
    |            |
    |            |
    +------------+

9.4. The other stuff in the example

You can also see in the above examples, that I have used the variables COLS, LINES which are initialized to the screen sizes after initscr(). They can be useful in finding screen dimensions and finding the center co-ordinate of the screen as above. The function getch() as usual gets the key from keyboard and according to the key it does the corresponding work. This type of switch- case is very common in any GUI based programs.

9.5. Other Border functions

Above program is grossly inefficient in that with each press of a key, a window is destroyed and another is created. So let's write a more efficient program which uses other border related functions.

The following program uses mvhline() and mvvline() to achieve similar effect. These two functions are simple. They create a horizontal or vertical line of the specified length at the specified position.

Example 8. More border functions

#include <ncurses.h>

typedef struct _win_border_struct {
        chtype  ls, rs, ts, bs, 
                tl, tr, bl, br;
}WIN_BORDER;

typedef struct _WIN_struct {

        int startx, starty;
        int height, width;
        WIN_BORDER border;
}WIN;

void init_win_params(WIN *p_win);
void print_win_params(WIN *p_win);
void create_box(WIN *win, bool flag);

int main(int argc, char *argv[])
{       WIN win;
        int ch;

        initscr();                      /* Start curses mode            */
        start_color();                  /* Start the color functionality */
        cbreak();                       /* Line buffering disabled, Pass on
                                         * everty thing to me           */
        keypad(stdscr, TRUE);           /* I need that nifty F1         */
        noecho();
        init_pair(1, COLOR_CYAN, COLOR_BLACK);

        /* Initialize the window parameters */
        init_win_params(&win);
        print_win_params(&win);

        attron(COLOR_PAIR(1));
        printw("Press F1 to exit");
        refresh();
        attroff(COLOR_PAIR(1));
        
        create_box(&win, TRUE);
        while((ch = getch()) != KEY_F(1))
        {       switch(ch)
                {       case KEY_LEFT:
                                create_box(&win, FALSE);
                                --win.startx;
                                create_box(&win, TRUE);
                                break;
                        case KEY_RIGHT:
                                create_box(&win, FALSE);
                                ++win.startx;
                                create_box(&win, TRUE);
                                break;
                        case KEY_UP:
                                create_box(&win, FALSE);
                                --win.starty;
                                create_box(&win, TRUE);
                                break;
                        case KEY_DOWN:
                                create_box(&win, FALSE);
                                ++win.starty;
                                create_box(&win, TRUE);
                                break;  
                }
        }
        endwin();                       /* End curses mode                */
        return 0;
}
void init_win_params(WIN *p_win)
{
        p_win->height = 3;
        p_win->width = 10;
        p_win->starty = (LINES - p_win->height)/2;      
        p_win->startx = (COLS - p_win->width)/2;

        p_win->border.ls = '|';
        p_win->border.rs = '|';
        p_win->border.ts = '-';
        p_win->border.bs = '-';
        p_win->border.tl = '+';
        p_win->border.tr = '+';
        p_win->border.bl = '+';
        p_win->border.br = '+';

}
void print_win_params(WIN *p_win)
{
#ifdef _DEBUG
        mvprintw(25, 0, "%d %d %d %d", p_win->startx, p_win->starty, 
                                p_win->width, p_win->height);
        refresh();
#endif
}
void create_box(WIN *p_win, bool flag)
{       int i, j;
        int x, y, w, h;

        x = p_win->startx;
        y = p_win->starty;
        w = p_win->width;
        h = p_win->height;

        if(flag == TRUE)
        {       mvaddch(y, x, p_win->border.tl);
                mvaddch(y, x + w, p_win->border.tr);
                mvaddch(y + h, x, p_win->border.bl);
                mvaddch(y + h, x + w, p_win->border.br);
                mvhline(y, x + 1, p_win->border.ts, w - 1);
                mvhline(y + h, x + 1, p_win->border.bs, w - 1);
                mvvline(y + 1, x, p_win->border.ls, h - 1);
                mvvline(y + 1, x + w, p_win->border.rs, h - 1);

        }
        else
                for(j = y; j <= y + h; ++j)
                        for(i = x; i <= x + w; ++i)
                                mvaddch(j, i, ' ');
                                
        refresh();

}