Controlling All Widgets in GTK Callbacks With GtkBuilder

TL;DR: gtk_builder_connect_signals(builder, builder)

GtkBuilder allows one to conveniently create graphical user interface (GUI) using an interface designer (i.e. Glade), which is convenient because we can draw GUI by dragging an dropping, without writing a loong piece of code in a GUI initialization function.

In a GtkBuilder UI definition file, widget signals can also be assigned callbacks in the designer so that one can conveniently connect them with a single function call (this is what previously I wrote):

gtk_builder_connect_signals(builder, NULL);

Callbacks are declared by the caller, and thus forms of callbacks are fixed for implementors. In GTK (and almost all G* stuff), one gpointer user_data parameter is reserved for extra argument passing. It is natural if we only pass at most one object to the callbacks. If we want to pass more to a callback, we may want to do the following:

1. Pack all things into one data structure. In C, struct is certainly used.
2. Dynamically allocate a piece of memory somewhere in code.
3. Put (pointers of) all things to pass in that memory.
4. Tell GObject to pass the pointer to that piece of memory to the callback.

struct _PrivateFoo {
  GtkWidget *btn_left;
  GtkWidget *btn_right;
  GtkWidget *entry;
};

void btn_xxx_clicked_cb(GtkWidget *self, gpointer data);

/* ... */

struct _PrivateFoo *data = malloc(sizeof(struct _PrivateFoo));

data->btn_left  = btn_foo;
data->btn_right = btn_bar;
data->entry     = entry_baz;

/* Connect "clicked" signal of a button to the callback declared before */
g_signal_connect(G_OBJECT(btn_xxx), "clicked", G_CALLBACK(btn_xxx_clicked_cb), data);

This must be written in code, and preferably during the initialization process of a program. If we have to do these for all callbacks that controls on multiple widgets¹, we need to declare many private structs and write a long piece of code in order to initialize the signals. This eliminates benefits that GtkBuilder brings us.

At some point, I noticed the prototype of gtk_builder_connect_signals²:

void
gtk_builder_connect_signals (GtkBuilder *builder,
                             gpointer user_data);

In which:

• builder: a GtkBuilder
• user_data: user data to pass back with all signals

After some search and experiments, I found out that this user_data is what being passed to all callbacks by default. If we pass NULL here, all callbacks without user_data declared in Glade will receive NULL. So, if we do:

/* GtkBuilder used to connect signals is also passed to all callbacks */
gtk_builder_connect_signals(builder, builder);

Then all callbacks written in the UI definition file without a designated widget as user_data will receive a GtkBuilder *. We can, therefore, extract multiple widgets from the GtkBuilder in callbacks:

void some_cb(GtkWidget *self, GtkBuilder *builder) {
  /* Widget IDs are declared in Glade */
  GtkWidget *btn_left  = GTK_WIDGET(gtk_builder_get_object(builder, "btn_foo"));
  GtkWidget *btn_right = GTK_WIDGET(gtk_builder_get_object(builder, "btn_bar"));
  GtkWidget *entry     = GTK_WIDGET(gtk_builder_get_object(builder, "entry_baz"));

  /* Operate on these widgets! */
}

This is a graceful way to pass multiple widgets to all callbacks, with the help of GtkBuilder.

Notes