Signals

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 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_OFFSET()

#define GTK_SIGNAL_OFFSET(struct, field)	(GTK_STRUCT_OFFSET (struct, field))

GtkSignalMarshal ()

void        (*GtkSignalMarshal)             (GtkObject *object,
                                             gpointer data,
                                             guint nparams,
                                             GtkArg *args,
                                             GtkType *arg_types,
                                             GtkType return_type);

This is currently a hack left in for a scheme wrapper library. It may be removed.

Don't use it.

GtkSignalDestroy ()

void        (*GtkSignalDestroy)             (gpointer data);

A function which you can use to clean up when the signal handler is destroyed.

For example, if your handler requires a few variables that you made into a struct and allocated (using g_new() or something), then you will probably want to free it as soon as the hook is destroyed. This will allow you to do that. (For this in particular it is convenient to pass g_free() as a GtkSignalDestroy function).

GtkEmissionHook ()

gboolean    (*GtkEmissionHook)              (GtkObject *object,
                                             guint signal_id,
                                             guint n_params,
                                             GtkArg *params,
                                             gpointer data);

A simple function pointer to get invoked when the signal is emitted. This allows you tie a hook to the signal type, so that it will trap all emissions of that signal, from any object.

You may not attach these to signals created with the GTK_RUN_NO_HOOKS flag.

struct GtkSignalQuery

struct GtkSignalQuery
{
  GtkType	   object_type;
  guint		   signal_id;
  const gchar	  *signal_name;
  guint		   is_user_signal : 1;
  GtkSignalRunType signal_flags;
  GtkType	   return_val;
  guint		   nparams;
  const GtkType	  *params;
};

This structure contains all the information about a particular signal: its name, the type it affects, the signature of the handlers, and its unique identifying integer.

enum GtkSignalRunType

typedef enum			/*< flags >*/
{
  GTK_RUN_FIRST      = 1 << 0,
  GTK_RUN_LAST       = 1 << 1,
  GTK_RUN_BOTH       = (GTK_RUN_FIRST | GTK_RUN_LAST),
  GTK_RUN_NO_RECURSE = 1 << 2,
  GTK_RUN_ACTION     = 1 << 3,
  GTK_RUN_NO_HOOKS   = 1 << 4
} 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_init ()

void        gtk_signal_init                 (void);

gtk_signal_new ()

guint       gtk_signal_new                  (const gchar *name,
                                             GtkSignalRunType signal_flags,
                                             GtkType object_type,
                                             guint function_offset,
                                             GtkSignalMarshaller marshaller,
                                             GtkType return_val,
                                             guint nparams,
                                             ...);

Create 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,
                                             GtkType object_type,
                                             guint function_offset,
                                             GtkSignalMarshaller marshaller,
                                             GtkType return_val,
                                             guint nparams,
                                             GtkType *params);

Create 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 ()

guint       gtk_signal_lookup               (const gchar *name,
                                             GtkType 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 ()

gchar*      gtk_signal_name                 (guint signal_id);

Given the signal's identifier, find 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,
                                             ...);

Emit 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,
                                             ...);

Emit 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 *params);

Emit 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 *params);

Emit 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_n_emissions ()

guint       gtk_signal_n_emissions          (GtkObject *object,
                                             guint signal_id);

Find out the recursion depth of emissions for a particular type of signal and object. (So it will always return 0 or 1 if GTK_RUN_NO_RECURSE is specified) This is a way to avoid recursion: you can see if you are currently running in that signal handler and emit it only if you are.

Another way to look at it is that this number increases by one when #gtk_signal_emit(), et al, are called, and decreases by one when #gtk_signal_emit() returns.

gtk_signal_n_emissions_by_name ()

guint       gtk_signal_n_emissions_by_name  (GtkObject *object,
                                             const gchar *name);

Find out the recursion depth of emissions for a particular type of signal and object. Just like gtk_signal_n_emissions() except it will lookup the signal id for you.

gtk_signal_emit_stop ()

void        gtk_signal_emit_stop            (GtkObject *object,
                                             guint 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 ()

guint       gtk_signal_connect              (GtkObject *object,
                                             const gchar *name,
                                             GtkSignalFunc func,
                                             gpointer func_data);

Attach 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:

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 ()

guint       gtk_signal_connect_after        (GtkObject *object,
                                             const gchar *name,
                                             GtkSignalFunc func,
                                             gpointer func_data);

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

gtk_signal_connect_object ()

guint       gtk_signal_connect_object       (GtkObject *object,
                                             const gchar *name,
                                             GtkSignalFunc func,
                                             GtkObject *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:

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

gtk_signal_connect_object_after ()

guint       gtk_signal_connect_object_after (GtkObject *object,
                                             const gchar *name,
                                             GtkSignalFunc func,
                                             GtkObject *slot_object);

Attach 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 ()

guint       gtk_signal_connect_full         (GtkObject *object,
                                             const gchar *name,
                                             GtkSignalFunc func,
                                             GtkCallbackMarshal marshal,
                                             gpointer data,
                                             GtkDestroyNotify destroy_func,
                                             gint object_signal,
                                             gint after);

Attach 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 *signal,
                                             GtkSignalFunc func,
                                             gpointer func_data,
                                             GtkObject *alive_object);

Attach 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_AS_POINTER).)

gtk_signal_connect_object_while_alive ()

void        gtk_signal_connect_object_while_alive
                                            (GtkObject *object,
                                             const gchar *signal,
                                             GtkSignalFunc 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 ()

void        gtk_signal_disconnect           (GtkObject *object,
                                             guint handler_id);

Destroy a user-defined handler connection.

gtk_signal_disconnect_by_func ()

void        gtk_signal_disconnect_by_func   (GtkObject *object,
                                             GtkSignalFunc func,
                                             gpointer data);

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

gtk_signal_disconnect_by_data ()

void        gtk_signal_disconnect_by_data   (GtkObject *object,
                                             gpointer data);

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

gtk_signal_handler_block ()

void        gtk_signal_handler_block        (GtkObject *object,
                                             guint handler_id);

Prevent an 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 ()

void        gtk_signal_handler_block_by_func
                                            (GtkObject *object,
                                             GtkSignalFunc func,
                                             gpointer data);

Prevent 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 ()

void        gtk_signal_handler_block_by_data
                                            (GtkObject *object,
                                             gpointer data);

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

gtk_signal_handler_unblock ()

void        gtk_signal_handler_unblock      (GtkObject *object,
                                             guint handler_id);

Undo 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 ()

void        gtk_signal_handler_unblock_by_func
                                            (GtkObject *object,
                                             GtkSignalFunc func,
                                             gpointer data);

Undo 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 ()

void        gtk_signal_handler_unblock_by_data
                                            (GtkObject *object,
                                             gpointer data);

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

gtk_signal_handler_pending ()

guint       gtk_signal_handler_pending      (GtkObject *object,
                                             guint signal_id,
                                             gboolean 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 ()

guint       gtk_signal_handler_pending_by_func
                                            (GtkObject *object,
                                             guint signal_id,
                                             gboolean may_be_blocked,
                                             GtkSignalFunc func,
                                             gpointer data);

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

gtk_signal_handler_pending_by_id ()

gint        gtk_signal_handler_pending_by_id
                                            (GtkObject *object,
                                             guint handler_id,
                                             gboolean may_be_blocked);

Returns whether a connection id is valid (and optionally not blocked).

gtk_signal_handlers_destroy ()

void        gtk_signal_handlers_destroy     (GtkObject *object);

Destroy all the signal handlers connected to an object. This is done automatically when the object is destroyed.

This function is labeled private.

gtk_signal_set_funcs ()

void        gtk_signal_set_funcs            (GtkSignalMarshal marshal_func,
                                             GtkSignalDestroy destroy_func);

These set default functions to call when the user didn't supply a function when connecting. (These are rarely used, and probably only for language bindings)

By default, there are no such functions.

gtk_signal_query ()

GtkSignalQuery* gtk_signal_query            (guint signal_id);

Obtain information about a signal.

gtk_signal_add_emission_hook ()

guint       gtk_signal_add_emission_hook    (guint signal_id,
                                             GtkEmissionHook hook_func,
                                             gpointer data);

Add an emission hook for a type of signal, for any object.

gtk_signal_add_emission_hook_full ()

guint       gtk_signal_add_emission_hook_full
                                            (guint signal_id,
                                             GtkEmissionHook hook_func,
                                             gpointer data,
                                             GDestroyNotify destroy);

Add an emission hook for a type of signal, for any object. (with control of what happens when the hook is destroyed).

gtk_signal_remove_emission_hook ()

void        gtk_signal_remove_emission_hook (guint signal_id,
                                             guint hook_id);

Delete an emission hook. (see gtk_signal_add_emission_hook())