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/** @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.

pidgin/pidgin