<title>Signals tutorial</title>

<title>Introduction</title>

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.

<title>Overview of Purple-signals</title>

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.

<title>Registering a Signal</title>

The first step of using a signal is registering it with libpurple so that

callbacks may be connected to it. This is done using

<link linkend="purple-signal-register"><function>purple_signal_register()</function></link>.

Here is a slightly modified example from

<link linkend="purple-plugins-init"><function>purple_plugins_init()</function></link>

in <literal>plugins.c</literal>:

purple_signal_register(purple_plugins_get_handle(), /* Instance */

"plugin-load", /* Signal name */

purple_marshal_VOID__POINTER,/* Marshal function */

G_TYPE_NONE, /* Callback return type */

1, /* Number of callback arguments (not including void *data) */

PURPLE_TYPE_PLUGIN /* Type of first callback argument */

);

<title>Instance</title>

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.

<title>Signal Name</title>

Unique identifier for the signal itself.

<title>Callback function definition</title>

The rest of the arguments specify the form of the callback function.

<listitem><para><emphasis>Marshal function</emphasis></para><para>

<literal>purple_marshal_VOID__POINTER</literal> represents the callback

function prototype, not including a "data" argument, explained later. The form

is <literal>purple_marshal_RETURNVALUETYPE__ARG1TYPE_ARG2TYPE_ETC</literal>.

See <link linkend="libpurple-signals">signals.h</link> for more possible types.

In this case, the callback will have the form

void cb(void *arg1, void *data)

If <literal>purple_marshal_BOOLEAN__POINTER_POINTER_POINTER</literal> were

specified, it would be:

gboolean cb(void *arg1, void *arg2, void *arg3, void *data)

The <literal>void *data</literal> argument at the end of each callback function

provides the data argument given to

<link linkend="purple-signal-connect"><function>purple_signal_connect()</function></link>.

<listitem><para><emphasis>Callback return type</emphasis></para><para>

In our case, this is G_TYPE_NONE, meaning "returns void".

<listitem><para><emphasis>Number of callback arguments</emphasis></para><para>

The number of arguments (not including <literal>data</literal>) that the callback function

will take.

<listitem><para><emphasis>Type of argument</emphasis></para><para>

<literal>PURPLE_TYPE_PLUGIN</literal> specifies that the first argument given to the callback

will be a <literal>PurplePlugin*</literal>. You will need as many "type of argument"

arguments to

<link linkend="purple-signal-register"><function>purple_signal_register()</function></link>

as you specified in

"Number of arguments" above.

<title>Connecting to the signal</title>

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 :

static void plugin_load_cb(PurplePlugin *plugin, gpointer data)

{

GtkTreeView *view = (GtkTreeView *)data;

plugin_loading_common(plugin, view, TRUE);

}

Note that the callback function prototype matches that specified in the call

to <link linkend="purple-signal-register"><function>purple_signal_register()</function></link>

above.

Once the callback function is defined, you can connect it to the signal.

Again from gtkplugin.c , in <function>pidgin_plugin_dialog_show()</function>:

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

);

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 <literal>plugin_dialog</literal> , the Pidgin plugins

dialog. When <literal>plugin_dialog</literal> is deleted, then

<literal>purple_signals_disconnect_by_handle(plugin_dialog)</literal> should be called to

remove all signal connections it is associated with.

The callback function is given using a helper macro, and finally the

<literal>data</literal> argument to be passed to

<literal>plugin_load_cb</literal> is given as <literal>event_view</literal>,

a pointer to the GTK widget that <literal>plugin_load_cb</literal> needs to update.

<title>Emitting a signal</title>

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 <link linkend="purple-plugin-load"><function>purple_plugin_load()</function></link>

in plugin.c:

purple_signal_emit(purple_plugins_get_handle(), "plugin-load", plugin);

This causes the signal "plugin-load" to be emitted from the plugin module

(given by

<link linkend="purple-plugins-get-handle"><function>purple_plugins_get_handle()</function></link>),

with the newly loaded plugin as

the argument to pass to any registered callback functions.

In our example, <literal>plugin_load_cb</literal> is called immediately as

plugin_load_cb(plugin, event_view);

and does whatever it does.