What are signals?

Signals are a way to get notification when something happens and to customize object behavior according to the user's needs. Every signal is uniquely identified by a name, "class_name::signal_name", where signal_name might be something like "clicked" and class_name might be "GtkButton". Note that some other class may also define a "clicked" callback, so long as it doesn't derive from GtkButton.

When they are created, they are also assigned a unique positive integer, the signal id (1 is the first signal id- 0 is used to flag an error). Each is also tied to an array of types that describes the prototype of the function pointer(s) (handlers) you may connect to the signal. Finally, every signal has a default handler that is given by a function pointer in its class structure: it is run by default whenever the signal is emitted. (It is possible that a signal will be emitted and a user-defined handler will prevent the default handler from being run.)

Signals are used by everyone, but they are only created on a per class basis -- so you should not call call gtk_signal_new() unless you are writing a new GtkObject type. However, if you want to make a new signal for an existing type, you may use gtk_object_class_user_signal_new() to create a signal that doesn't correspond to a class's builtin methods.

How are signals used?

There are two basic actions in the signal handling game. If you want notification of an event, you must connect a function pointer and a data pointer to that signal; the data pointer will be passed as the last argument to the function (so long as you are using the default marshalling functions). You will receive a connection id, a unique positive integer corresponding to that attachment.

Functions that want to notify the user of certain actions, emit signals.

Basic Terminology

A brief note on how they work.

The functions responsible for translating an array of GtkArgs to your C compiler's normal semantics are called Marshallers. They are identified by gtk_marshal_return_value__parameter_list() for example a C function returning a gboolean and taking a gint can be invoked by using gtk_marshal_BOOL__INT(). Not all possibly combinations of return/params are available, of course, so if you are writing a GtkObject with parameters you might have to write a marshaller.

gtk_signal_new ()

guint
gtk_signal_new (const gchar *name,
                GtkSignalRunType signal_flags,
                GType object_type,
                guint function_offset,
                GSignalCMarshaller marshaller,
                GType return_val,
                guint n_args,
                ...);

Creates a new signal type. (This is usually done in the class initializer.)

gtk_signal_newv ()

guint
gtk_signal_newv (const gchar *name,
                 GtkSignalRunType signal_flags,
                 GType object_type,
                 guint function_offset,
                 GSignalCMarshaller marshaller,
                 GType return_val,
                 guint n_args,
                 GType *args);

Creates a new signal type. (This is usually done in a class initializer.)

This function take the types as an array, instead of a list following the arguments. Otherwise the same as gtk_signal_new().

gtk_signal_lookup()

#define             gtk_signal_lookup(name,object_type)

Given the name of the signal and the type of object it connects to, get the signal's identifying integer. Emitting the signal by number is somewhat faster than using the name each time.

It also tries the ancestors of the given type.

gtk_signal_name()

#define             gtk_signal_name(signal_id)

Given the signal's identifier, finds its name.

Two different signals may have the same name, if they have differing types.

gtk_signal_emit ()

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

Emits a signal. This causes the default handler and user-defined handlers to be run.

Here is what gtk_signal_emit() does:

1. Calls the default handler and the user-connected handlers. The default handler will be called first if GTK_RUN_FIRST is set, and last if GTK_RUN_LAST is set.

2. Calls all handlers connected with the "after" flag set.

gtk_signal_emit_by_name ()

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

Emits a signal. This causes the default handler and user-connected handlers to be run.

gtk_signal_emitv ()

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

Emits a signal. This causes the default handler and user-connected handlers to be run. This differs from gtk_signal_emit() by taking an array of GtkArgs instead of using C's varargs mechanism.

gtk_signal_emitv_by_name ()

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

Emits a signal by name. This causes the default handler and user-connected handlers to be run. This differs from gtk_signal_emit() by taking an array of GtkArgs instead of using C's varargs mechanism.

gtk_signal_emit_stop()

#define             gtk_signal_emit_stop(object,signal_id)

This function aborts a signal's current emission.

It will prevent the default method from running, if the signal was GTK_RUN_LAST and you connected normally (i.e. without the "after" flag).

It will print a warning if used on a signal which isn't being emitted.

gtk_signal_emit_stop_by_name ()

void
gtk_signal_emit_stop_by_name (GtkObject *object,
                              const gchar *name);

This function aborts a signal's current emission.

It is just like gtk_signal_emit_stop() except it will lookup the signal id for you.

gtk_signal_connect()

#define             gtk_signal_connect(object,name,func,func_data)

Attaches a function pointer and user data to a signal for a particular object.

The GtkSignalFunction takes a GtkObject as its first parameter. It will be the same object as the one you're connecting the hook to. The func_data will be passed as the last parameter to the hook.

All else being equal, signal handlers are invoked in the order connected (see gtk_signal_emit() for the other details of which order things are called in).

Here is how one passes an integer as user data, for when you just want to specify a constant int as parameter to your function:

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static void button_clicked_int (GtkButton* button, gpointer func_data)
{
    g_print ("button pressed: %d\n", GPOINTER_TO_INT (func_data));
}
/* By calling this function, you will make the g_print above
 * execute, printing the number passed as `to_print'. */
static void attach_print_signal (GtkButton* button, gint to_print)
{
    gtk_signal_connect (GTK_OBJECT (button), "clicked",
        GTK_SIGNAL_FUNC (button_clicked_int),
        GINT_TO_POINTER (to_print));
}

gtk_signal_connect_after()

#define             gtk_signal_connect_after(object,name,func,func_data)

Attaches a function pointer and user data to a signal so that this handler will be called after the other handlers.

gtk_signal_connect_object()

#define             gtk_signal_connect_object(object,name,func,slot_object)

This function is for registering a callback that will call another object's callback. That is, instead of passing the object which is responsible for the event as the first parameter of the callback, it is switched with the user data (so the object which emits the signal will be the last parameter, which is where the user data usually is).

This is useful for passing a standard function in as a callback. For example, if you wanted a button's press to gtk_widget_show() some widget, you could write:

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gtk_signal_connect_object (button, "clicked", gtk_widget_show, window);

gtk_signal_connect_object_after()

#define             gtk_signal_connect_object_after(object,name,func,slot_object)

Attaches a signal hook to a signal, passing in an alternate object as the first parameter, and guaranteeing that the default handler and all normal handlers are called first.

gtk_signal_connect_full ()

gulong
gtk_signal_connect_full (GtkObject *object,
                         const gchar *name,
                         GCallback func,
                         GtkCallbackMarshal unsupported,
                         gpointer data,
                         GDestroyNotify destroy_func,
                         gint object_signal,
                         gint after);

Attaches a function pointer and user data to a signal with more control.

gtk_signal_connect_while_alive ()

void
gtk_signal_connect_while_alive (GtkObject *object,
                                const gchar *name,
                                GCallback func,
                                gpointer func_data,
                                GtkObject *alive_object);

Attaches a function pointer and another GtkObject to a signal.

This function takes an object whose "destroy" signal should be trapped. That way, you don't have to clean up the signal handler when you destroy the object. It is a little less efficient though.

(Instead you may call gtk_signal_disconnect_by_data(), if you want to explicitly delete all attachments to this object. This is perhaps not recommended since it could be confused with an integer masquerading as a pointer (through GINT_TO_POINTER()).)

gtk_signal_connect_object_while_alive ()

void
gtk_signal_connect_object_while_alive (GtkObject *object,
                                       const gchar *name,
                                       GCallback func,
                                       GtkObject *alive_object);

These signal connectors are for signals which refer to objects, so they must not be called after the object is deleted.

Unlike gtk_signal_connect_while_alive(), this swaps the object and user data, making it suitable for use with functions which primarily operate on the user data.

This function acts just like gtk_signal_connect_object() except it traps the "destroy" signal to prevent you from having to clean up the handler.

gtk_signal_disconnect()

#define             gtk_signal_disconnect(object,handler_id)

Destroys a user-defined handler connection.

gtk_signal_disconnect_by_func()

#define             gtk_signal_disconnect_by_func(object,func,data)

Destroys all connections for a particular object, with the given function-pointer and user-data.

gtk_signal_disconnect_by_data()

#define             gtk_signal_disconnect_by_data(object,data)

Destroys all connections for a particular object, with the given user-data.

gtk_signal_handler_block()

#define             gtk_signal_handler_block(object,handler_id)

Prevents a user-defined handler from being invoked. All other signal processing will go on as normal, but this particular handler will ignore it.

gtk_signal_handler_block_by_func()

#define             gtk_signal_handler_block_by_func(object,func,data)

Prevents a user-defined handler from being invoked, by reference to the user-defined handler's function pointer and user data. (It may result in multiple hooks being blocked, if you've called connect multiple times.)

gtk_signal_handler_block_by_data()

#define             gtk_signal_handler_block_by_data(object,data)

Prevents all user-defined handlers with a certain user data from being invoked.

gtk_signal_handler_unblock()

#define             gtk_signal_handler_unblock(object,handler_id)

Undoes a block, by connection id. Note that undoing a block doesn't necessarily make the hook callable, because if you block a hook twice, you must unblock it twice.

gtk_signal_handler_unblock_by_func()

#define             gtk_signal_handler_unblock_by_func(object,func,data)

Undoes a block, by function pointer and data. Note that undoing a block doesn't necessarily make the hook callable, because if you block a hook twice, you must unblock it twice.

gtk_signal_handler_unblock_by_data()

#define             gtk_signal_handler_unblock_by_data(object,data)

Undoes block(s), to all signals for a particular object with a particular user-data pointer

gtk_signal_handler_pending()

#define             gtk_signal_handler_pending(object,signal_id,may_be_blocked)

Returns a connection id corresponding to a given signal id and object.

One example of when you might use this is when the arguments to the signal are difficult to compute. A class implementor may opt to not emit the signal if no one is attached anyway, thus saving the cost of building the arguments.

gtk_signal_handler_pending_by_func()

#define             gtk_signal_handler_pending_by_func(object,signal_id,may_be_blocked,func,data)

Returns a connection id corresponding to a given signal id, object, function pointer and user data.

GTK_SIGNAL_OFFSET

#define GTK_SIGNAL_OFFSET	                      G_STRUCT_OFFSET

Use in place of offsetof(), which is used if it exists.

enum GtkSignalRunType

These configure the signal's emission. They control whether the signal can be emitted recursively on an object and whether to run the default method before or after the user-defined handlers.

gtk_signal_default_marshaller

#define gtk_signal_default_marshaller g_cclosure_marshal_VOID__VOID

A marshaller that returns void and takes no extra parameters.