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Gtk+/Gnome Application Development
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Emitting A Signal

It's your object's responsibility to emit its signals at appropriate times. This is very simple; if you've saved the return value from gtk_signal_new(), that identifier can be used to emit the signal. Otherwise, you can emit the signal by name (with some cost in execution speed, since GTK+ will have to look up the identifier in a hash table).

Here is code from gtk/gtkbutton.c which is used to emit the "button_pressed" signal:


void
gtk_button_pressed (GtkButton *button)
{
  g_return_if_fail (button != NULL);
  g_return_if_fail (GTK_IS_BUTTON (button));

  gtk_signal_emit (GTK_OBJECT (button), button_signals[PRESSED]);
}


If a signal has arguments (other than the standard two), you must specify those as a variable argument list:


  gtk_signal_emit (GTK_OBJECT (widget), widget_signals[SIZE_REQUEST],
                   &widget->requisition);

If a signal returns a value, you must pass a location for the returned value as the final argument:


  gint return_val;

  return_val = FALSE;

  gtk_signal_emit (GTK_OBJECT (widget), widget_signals[EVENT], event,
                   &return_val);

Notice that return_val is initialized to something sane; if there are no signal handlers, none of them will assign a value to return_val. So you must initialize the variable. Each signal handler's return value will be assigned to the same location, so the final value of return_val is determined by the last signal handler to run. Note that certain return values (such as strings) must be freed by the signal emitter.

gtk_signal_emit_by_name() is the same as gtk_signal_emit(), except that the second argument is a signal name rather than a signal ID number. There are also variants of both emission functions that take a vector of GtkArg instead of a variable argument list. These variants expect arrays of n+1GtkArg structs, where n is the number of signal arguments and there is an additional GtkArg for the return value. The GtkArg structs should be initialized with sane values. If the function returns no value, the return value GtkArg will have GTK_TYPE_NONE.

All four signal emission functions are summarized in Figure 5.

#include <gtk/gtksignal.h>

void gtk_signal_emit(GtkObject* object, guint signal_id, ...);

void gtk_signal_emit_by_name(GtkObject* object, const gchar* name, ...);

void gtk_signal_emitv(GtkObject* object, guint signal_id, GtkArg* params);

void gtk_signal_emitv_by_name(GtkObject* object, const gchar* name, GtkArg* params);

Figure 5. Signal Emission

Keep in mind that it is usually inappropriate to simply emit a signal outside of an object's implementation. Only GTK_RUN_ACTION signals are guaranteed to work properly without special setup or shutdown. Objects often export functions you can use to emit signals properly; for example, to emit the "size_request" signal, GtkWidget provides this function:


void
gtk_widget_size_request (GtkWidget      *widget,
                         GtkRequisition *requisition)
{
  g_return_if_fail (widget != NULL);
  g_return_if_fail (GTK_IS_WIDGET (widget));

  gtk_widget_ref (widget);
  gtk_widget_ensure_style (widget);
  gtk_signal_emit (GTK_OBJECT (widget), widget_signals[SIZE_REQUEST],
                   &widget->requisition);

  if (requisition)
    gtk_widget_get_child_requisition (widget, requisition);

  gtk_widget_unref (widget);
}

As you can see, particular actions are required before and after emitting the signal; thus it should only be emitted via the gtk_widget_size_request() function.

Gtk+/Gnome Application Development
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