How to prevent gtk redrawing the entire GtkDrawingArea? - linux

Question

This is my code. I hope it can draw a circle when users press button. The circel is centered around the point where the mouse key pressed. But the program doesn't run as expected. It clears the pre-circles and draws a new circle. Are there some functions to solve this problem?
My codes:
#include <gtk/gtk.h>
static void do_drawing(cairo_t *);
struct {
int count;
double coordx[100];
double coordy[100];
} glob;
static gboolean on_draw_event(GtkWidget *widget, cairo_t *cr,
gpointer user_data)
{
do_drawing(cr);
return FALSE;
}
static void do_drawing(cairo_t *cr)
{
cairo_set_source_rgb(cr, 0, 0, 0);
cairo_set_line_width(cr, 0.5);
if(glob.count)
{
cairo_set_font_size(cr, 20);
cairo_new_sub_path(cr);
cairo_arc(cr, glob.coordx[glob.count-1], glob.coordy[glob.count-1], 20, 0, 2*G_PI);
cairo_move_to(cr, glob.coordx[glob.count-1]-3, glob.coordy[glob.count-1] + 7);
cairo_show_text(cr, "i");
cairo_stroke(cr);
}
}
static gboolean clicked(GtkWidget *widget, GdkEventButton *event,
gpointer user_data)
{
if (event->button == 1) {
glob.coordx[glob.count] = event->x;
glob.coordy[glob.count++] = event->y;
gtk_widget_queue_draw(widget);
}
return TRUE;
}
int main(int argc, char *argv[])
{
GtkWidget *window;
GtkWidget *darea;
glob.count = 0;
gtk_init(&argc, &argv);
window = gtk_window_new(GTK_WINDOW_TOPLEVEL);
darea = gtk_drawing_area_new();
gtk_container_add(GTK_CONTAINER(window), darea);
gtk_widget_add_events(darea, GDK_BUTTON_PRESS_MASK);
g_signal_connect(G_OBJECT(darea), "draw",
G_CALLBACK(on_draw_event), NULL);
g_signal_connect(window, "destroy",
G_CALLBACK(gtk_main_quit), NULL);
g_signal_connect(darea, "button-press-event",
G_CALLBACK(clicked), NULL);
gtk_window_set_position(GTK_WINDOW(window), GTK_WIN_POS_CENTER);
gtk_window_set_default_size(GTK_WINDOW(window), 400, 300);
gtk_window_set_title(GTK_WINDOW(window), "Lines");
gtk_widget_show_all(window);
gtk_main();
return 0;
}
I know the gtk_widget_queue_draw(widget) will redraw the GtkDrawingArea entirely, So which function can replace it? I have another question: If I wan to draw on GtkDrawingArea, should I create cairo_t before drawing? How to create a cairo_t for GtkDrawingArea?