Align label over GtkButton

button_label.c

#include <stdio.h>
#include <gtk/gtk.h>


typedef struct {
  GtkWidget           *button;
  GtkWidget           *button_label;
  GtkStyleProvider    *button_style_provider;
} app_widgets;


void destroy_handler (GtkApplication* app, gpointer data)
{
    g_application_quit(G_APPLICATION (data));
}


static void
button_size_allocate (GtkWidget       *label,
                            GdkRectangle    *allocation,
                            gpointer         user_data)
{
  app_widgets *widgets = (app_widgets *) user_data;
  char            *css_text;

  css_text = g_strdup_printf ("button,\n"
                              "button\n"
                              "{\n"
                              "  padding: 0em %dpx 0em %dpx;\n"
                              "}\n",
                              20+allocation->height, 10+allocation->height);

  gtk_css_provider_load_from_data (GTK_CSS_PROVIDER (widgets->button_style_provider),
                                   css_text, -1, NULL);
  g_free (css_text);
}


void
button_clicked_callback (GtkButton *button,
                          app_widgets *widgets)
{
   GtkAlign align = gtk_widget_get_halign(widgets->button_label);

  switch (align) {
      case GTK_ALIGN_CENTER:
         gtk_widget_set_halign (widgets->button_label,
                         GTK_ALIGN_END);
         gtk_label_set_text (GTK_LABEL(widgets->button_label), "Right");
         return;
      case GTK_ALIGN_END:
         gtk_widget_set_halign (widgets->button_label,
                        GTK_ALIGN_START);
         gtk_label_set_text (GTK_LABEL(widgets->button_label), "Left");
         return;
      default:
        gtk_widget_set_halign (widgets->button_label,
                         GTK_ALIGN_CENTER);
         gtk_label_set_text (GTK_LABEL(widgets->button_label), "Center");
         return;
      }
}


static void
activate (GtkApplication *app,
          gpointer        user_data)
{
  GtkSizeGroup    *size_group;
  GtkWidget       *window;
  GtkWidget       *grid;
  GtkWidget       *button_overlay;
  GtkWidget       *button_cancel;

  app_widgets *widgets = g_slice_new(app_widgets);

  window = gtk_application_window_new (app);
  gtk_window_set_title (GTK_WINDOW (window), "Button");
  gtk_window_set_default_size (GTK_WINDOW (window), 400, 60);


  grid = gtk_grid_new ();
  gtk_grid_set_column_homogeneous (GTK_GRID (grid),
                                TRUE);
  gtk_grid_set_row_homogeneous (GTK_GRID (grid),
                                FALSE);
  gtk_grid_set_row_spacing (GTK_GRID (grid), 10);

  widgets->button = gtk_button_new_with_label("");
  button_cancel = gtk_button_new_with_label("Cancel");
  button_overlay = gtk_overlay_new ();

  gtk_widget_set_hexpand (button_overlay, TRUE);

  gtk_container_add (GTK_CONTAINER (button_overlay),
                     widgets->button);
  widgets->button_label = gtk_label_new ("Center");
  gtk_widget_set_valign (widgets->button_label, GTK_ALIGN_CENTER);
  gtk_overlay_add_overlay (GTK_OVERLAY (button_overlay),
                            widgets->button_label);
  gtk_overlay_set_overlay_pass_through (GTK_OVERLAY (button_overlay),
                            widgets->button_label, TRUE);
  size_group = gtk_size_group_new (GTK_SIZE_GROUP_HORIZONTAL);
  gtk_size_group_add_widget (size_group, widgets->button);
  gtk_size_group_add_widget (size_group, widgets->button_label);

  widgets->button_style_provider = GTK_STYLE_PROVIDER (gtk_css_provider_new ());
  gtk_style_context_add_provider (gtk_widget_get_style_context (widgets->button),
                                  widgets->button_style_provider,
                                  GTK_STYLE_PROVIDER_PRIORITY_APPLICATION);
  gtk_widget_set_halign (widgets->button_label,
                        GTK_ALIGN_CENTER);

  gtk_grid_attach (GTK_GRID (grid), button_overlay, 0, 0, 4, 1);
  gtk_grid_attach (GTK_GRID (grid), button_cancel, 3, 1, 1, 1);
  gtk_container_add (GTK_CONTAINER (window), grid);
  gtk_container_set_border_width(GTK_CONTAINER(window),5);
  g_signal_connect (widgets->button_label,
                    "size-allocate",
                    G_CALLBACK (button_size_allocate), widgets);
  g_signal_connect (G_OBJECT(window), "destroy",
                    G_CALLBACK (destroy_handler), app);
  g_signal_connect (widgets->button, "clicked",
                    G_CALLBACK (button_clicked_callback), widgets);
  g_signal_connect (G_OBJECT(button_cancel), "clicked",
                    G_CALLBACK (destroy_handler), app);
  gtk_widget_show_all (window);
}


int
main (int argc,
      char **argv)
{
  GtkApplication *app;
  int status;

  app = gtk_application_new ("org.gtk.button_overlay", G_APPLICATION_FLAGS_NONE);
  g_signal_connect (app, "activate", G_CALLBACK (activate), NULL);
  status = g_application_run (G_APPLICATION (app), argc, argv);
  g_object_unref (app);

  return status;
}