doc/SIGNAL-HOWTO.dox

/** @page signal-howto Signals HOWTO

 @section Introduction
  The libpurple signals interface is used for general event notification, such
  as plugins being loaded or unloaded, allowing the GUI frontend to respond
  appropriately to changing internal data. Unfortunately, its use is not at all
  obvious from the information in the header files. This document uses code
  snippets from the Pidgin/libpurple plugin systems to illustrate the proper
  use of signals.

 @section overview Overview of Signals
  Signals in libpurple are very similar to those in GTK+. When certain events
  happen, a named signal is "emitted" from a certain object. Emitting the
  signal triggers a series of callbacks that have been "connected" to that
  signal for that object. These callbacks take appropriate action in response
  to the signal.

 @section registering_signal Registering a Signal
  The first step of using a signal is registering it with libpurple so that
  callbacks may be connected to it. This is done using purple_signal_register()
  Here is a slightly modified example from @c purple_plugins_init in 
  @c libpurple/plugin.c :

  @code
	purple_signal_register( purple_plugins_get_handle(), /* Instance */
				"plugin-load",               /* Signal name */
				purple_marshal_VOID__POINTER,/* Marshal function */
				NULL,                        /* Callback return value type */
				1,                           /* Number of callback arguments (not including void *data) */
				purple_value_new(PURPLE_TYPE_SUBTYPE,PURPLE_SUBTYPE_PLUGIN) /* Type of first callback argument */
				);
  @endcode

  @subsection Instance
  A reference to the object from which this signal is emitted, and to which
  potential callbacks should be connected. In this case, it will be the entire
  plugin module emitting the signal.
  
  @subsection signalname Signal Name
  Unique identifier for the signal itself.

  @subsection therest Callback function definition
  The rest of the arguments specify the form of the callback function.

  @subsubsection marshalfunc Marshal Function
  @c purple_marshal_VOID__POINTER represents the callback function prototype,
  not including a "data" argument, explained later. The form is 
  @c purple_marshal_RETURNVALUETYPE__ARG1TYPE_ARG2TYPE_ETC. See signals.h for
  more possible types.

  In this case, the callback will have the form
  @code
	void cb(void *arg1, void *data)
  @endcode

  If @c purple_marshal_BOOLEAN__POINTER_POINTER_POINTER were specified, it
  would be:
  @code
	gboolean cb(void *arg1, void *arg2, void *arg3, void *data)
  @endcode

  The @c void @c *data argument at the end of each callback function
  provides the data argument given to purple_signal_connect() .

  @subsubsection cb_ret_type Callback return value type
  In our case, this is NULL, meaning "returns void".
  @todo This could be described better.

  @subsubsection num_args Number of arguments
  The number of arguments (not including @c data ) that the callback function
  will take.

  @subsubsection type_arg Type of argument
  @c purple_value_new(PURPLE_TYPE_SUBTYPE,PURPLE_SUBTYPE_PLUGIN) specifies that
  the first argument given to the callback will be a @c PurplePlugin* . You
  will need as many "type of argument" arguments to purple_signal_register() as
  you specified in "Number of arguments" above.

  @todo Describe this more.

  @see value.h

  @section connect Connecting to the signal
  Once the signal is registered, you can connect callbacks to it. First, you
  must define a callback function, such as this one from gtkplugin.c :
  @code
static void plugin_load_cb(PurplePlugin *plugin, gpointer data)
{
	GtkTreeView *view = (GtkTreeView *)data;
	plugin_loading_common(plugin, view, TRUE);
}
  @endcode
  Note that the callback function prototype matches that specified in the call
  to purple_signal_register() above.

  Once the callback function is defined, you can connect it to the signal.
  Again from gtkplugin.c , in @c pidgin_plugin_dialog_show() :
  @code
	purple_signal_connect(purple_plugins_get_handle(), "plugin-load", /* What to connect to */
		plugin_dialog, /* Object receiving the signal */
		PURPLE_CALLBACK(plugin_load_cb), /* Callback function */
		event_view, /* Data to pass to the callback function
		);
  @endcode

  The first two arguments ("What to connect to") specify the object emitting
  the signal (the plugin module) and what signal to listen for ("plugin-load").

  The object receiving the signal is @c plugin_dialog , the Pidgin plugins
  dialog. When @c plugin_dialog is deleted, then 
  @c purple_signals_disconnect_by_handle(plugin_dialog) should be called to
  remove all signal connections it is associated with.

  The callback function is given using a helper macro, and finally the
  @c data argument to be passed to @c plugin_load_cb is given as @c event_view,
  a pointer to the GTK widget that @c plugin_load_cb needs to update.

  @section emit-signal Emitting a signal
  Connecting callbacks to signals is all well and good, but how do you "fire"
  the signal and trigger the callback? At some point, you must "emit" the
  signal, which immediately calls all connected callbacks.

  As seen in @c purple_plugin_load() in plugin.c :
  @code
	purple_signal_emit(purple_plugins_get_handle(), "plugin-load", plugin);
  @endcode
  This causes the signal "plugin-load" to be emitted from the plugin module
  (given by @c purple_plugins_get_handle() ), with the newly loaded plugin as
  the argument to pass to any registered callback functions.

  In our example, @c plugin_load_cb is called immediately as
  @code
	plugin_load_cb(plugin, event_view);
  @endcode
  and does whatever it does.
  
 */