GTK3 cairo g_signal_connect: Dont get the point [closed] - c

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i´m trying to fix it out that when i click on "button1" the "on_draw_event"-funcion is called (that works) and a line is shown in my "darea"-drawing_area (that does not work).
Here is my code:
#include <gtk/gtk.h>
static gboolean on_draw_event(GtkWidget *button, cairo_t *darea)
{
printf ("function: on_draw_event\n");
cairo_set_source_rgb(darea, 0, 255, 0);
cairo_set_line_width(darea, 0.5);
cairo_move_to(darea, 0, 100);
cairo_line_to(darea, 400, 100);
cairo_stroke(darea);
return FALSE;
}
int main (int argc, char *argv[])
{
GtkWidget *window;
GtkWidget *darea;
GtkWidget *myGrid;
GtkWidget *button1;
GtkWidget *button2;
gtk_init(&argc, &argv);
window = gtk_window_new(GTK_WINDOW_POPUP);
myGrid = gtk_grid_new ();
darea = gtk_drawing_area_new();
gtk_widget_set_size_request(darea, 400, 400);
button1 = gtk_button_new_with_label ("BUTTON 1");
button2 = gtk_button_new_with_label ("QUIT");
gtk_grid_attach(GTK_GRID(myGrid), button1, 0, 0, 1, 1);
gtk_grid_attach(GTK_GRID(myGrid), darea, 0, 1, 1, 1);
gtk_grid_attach(GTK_GRID(myGrid), button2, 0, 2, 1, 1);
gtk_container_add(GTK_CONTAINER(window), myGrid);
g_signal_connect(G_OBJECT(button2), "clicked", G_CALLBACK(gtk_main_quit), NULL);
g_signal_connect(button1, "clicked", G_CALLBACK(on_draw_event), darea);
gtk_window_set_position(GTK_WINDOW(window), GTK_WIN_POS_CENTER);
gtk_widget_show_all(window);
gtk_main();
return 0;
}
If i cange
"g_signal_connect(button1, "clicked", G_CALLBACK(on_draw_event), darea);"
to
"g_signal_connect(G_OBJECT(darea), "draw", G_CALLBACK(on_draw_event), NULL);"
the line will be drawn, but i want that it waits for my button click. Hope someone can halp me, cant be a big problem.
Thank you.

Answer 1

As posted, your example crashes because you are connecting a drawing callback to the clicked signal. When called, the callback gets nonsensical arguments and dies.
To "wait for the button click", introduce a flag that tells you whether the button has been clicked. Consult that flag in on_draw_event and set it in the button's clicked callback.
Your example modified to implement the above would look like this:
#include <gtk/gtk.h>
static gboolean on_draw_event(GtkWidget *darea, cairo_t *cr,
gboolean *draw_line)
{
if (!*draw_line)
return FALSE;
printf ("function: on_draw_event\n");
cairo_set_source_rgb(cr, 0, 255, 0);
cairo_set_line_width(cr, 0.5);
cairo_move_to(cr, 0, 100);
cairo_line_to(cr, 400, 100);
cairo_stroke(cr);
return FALSE;
}
static void on_button1_clicked(GtkWidget *button, gboolean *draw_line)
{
*draw_line = TRUE;
/* Make sure the widget is repainted after the click.
gtk_widget_queue_draw() should better be invoked with just the
drawing area, but this code accesses the common parent for
simplicity. Passing darea in a struct whose address is provided as
user_data (along with gboolean *draw_line) is left as exercise. */
gtk_widget_queue_draw(gtk_widget_get_parent(button));
}
int main (int argc, char *argv[])
{
GtkWidget *window;
GtkWidget *darea;
GtkWidget *myGrid;
GtkWidget *button1;
GtkWidget *button2;
gboolean draw_line = FALSE;
gtk_init(&argc, &argv);
window = gtk_window_new(GTK_WINDOW_POPUP);
myGrid = gtk_grid_new ();
darea = gtk_drawing_area_new();
gtk_widget_set_size_request(darea, 400, 400);
button1 = gtk_button_new_with_label ("BUTTON 1");
button2 = gtk_button_new_with_label ("QUIT");
gtk_grid_attach(GTK_GRID(myGrid), button1, 0, 0, 1, 1);
gtk_grid_attach(GTK_GRID(myGrid), darea, 0, 1, 1, 1);
gtk_grid_attach(GTK_GRID(myGrid), button2, 0, 2, 1, 1);
gtk_container_add(GTK_CONTAINER(window), myGrid);
g_signal_connect(G_OBJECT(button2), "clicked", G_CALLBACK(gtk_main_quit), NULL);
g_signal_connect(button1, "clicked", G_CALLBACK(on_button1_clicked), &draw_line);
g_signal_connect(G_OBJECT(darea), "draw", G_CALLBACK(on_draw_event), &draw_line);
gtk_window_set_position(GTK_WINDOW(window), GTK_WIN_POS_CENTER);
gtk_widget_show_all(window);
gtk_main();
return 0;
}