GTK3+ - How to update graphics based on mouse click? - c

Question

I am trying to modify the cairo graphics rendering I found in http://zetcode.com/gfx/cairo/basicdrawing/ so that it updates the graphics when I click the drawing area with the left mouse button. Currently the code in the website uses the right mouse button click to update the graphics.
I have tried adding gtk_widget_queue_draw(widget); to gboolean clicked(), when event->button == 1:
#include <cairo.h>
#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);
int i, j;
for (i = 0; i <= glob.count - 1; i++ ) {
for (j = 0; j <= glob.count - 1; j++ ) {
cairo_move_to(cr, glob.coordx[i], glob.coordy[i]);
cairo_line_to(cr, glob.coordx[j], glob.coordy[j]);
}
}
glob.count = 0;
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);
}
if (event->button == 3) {
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(window, 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(window, "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;
}
However, this does not work since now the graphics aren't updated at all.
Is there a way of updating the graphics by pressing the left mouse button (button 1) and simultaneously carry out inserting the coordinates to struct glob as before?
Thanks!

Answer 1

You are setting glob.count = 0 after drawing the lines, however glob.count is only updated by one in the clicked() callback, so it will never draw more than a single line. Also the first line is from point (glob.coordx[0], glob.coordy[0]) to (glob.coordx[0], glob.coordy[0]) (that is: it starts and ends at the same point) so it has no length, and will not be visible.
Solution: remove the line glob.count = 0; from do_drawing().

Answer 2

g_signal_connect(window, "button-press-event", G_CALLBACK(clicked), NULL);
When your callback is called, the window (which emits "button-press-event") is passed as first parameter. The queue_draw call queues redraw of window. However, redraw of DrawingArea is not requested and will not happen.
You can:
Queue redraw of DrawingArea
Connect to "button-press-event" of DrawingArea
Without any changes of code: press a mouse button and resize the window. Gtk will see, that DrawingArea has changed (it's resized too) and queue redraw of this widget.