[~andy/gtk] / docs / reference / gtk / tmpl / gtksignal.sgml

<!-- ##### SECTION Title ##### -->
Signals

<!-- ##### SECTION Short_Description ##### -->
Object methods and callbacks.

<!-- ##### SECTION Long_Description ##### -->
<para>
The GTK+ signal system merely proxies the GLib signal system now. For future 
usage, direct use of the <link linkend="gobject-Signals">GSignal</link> API is recommended, this avoids significant 
performance hits where #GtkArg structures have to be converted into #GValue<!>s.
</para>

<refsect2>
<title>What are signals?</title>
<para>
Signals are a way to get notification when something happens
and to customize object behavior according to the
user's needs.
Every <WordAsWord>signal</WordAsWord> 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.
</para>
<para>
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.)
</para>
<para>
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.
</para>
</refsect2>
<refsect2>
<title>How are signals used?</title>
<para>
There are two basic actions in the signal handling game.
If you want notification of an event, you must <emphasis>connect</emphasis>
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.
</para>
<para>
Functions that want to notify the user of certain actions,
<Emphasis>emit</Emphasis> signals.
</para>
</refsect2>
<refsect2>
<title>Basic Terminology</title>
<variablelist>

<varlistentry>
<term>signal</term>
<listitem><para>A class method, e.g. GtkButton::clicked.
More precisely it is a unique class-branch/signal-name pair.
This means you may not define a signal handler for a class which
derives from #GtkButton that is called clicked,
but it is okay to share signals names if they are separate in
the class tree.
</para></listitem>
</varlistentry>

<varlistentry>
<term>default handler</term>
<listitem><para>The object's internal method which is invoked
when the signal is emitted.</para>
</listitem>
</varlistentry>

<varlistentry>
<term>user-defined handler</term>
<listitem><para>A function pointer and data connected
to a signal (for a particular object).</para>
<para>There are really two types: those which are connected
normally, and those which are connected by one 
of the connect_after functions.  The connect_after handlers
are always run after the default handler.</para>
<para>Many toolkits refer to these as <wordasword>callbacks</wordasword>.</para>
</listitem>
</varlistentry>

<varlistentry>
<term>emission</term>
<listitem><para>the whole process of emitting a signal,
including the invocation of all
the different handler types mentioned above.</para>
</listitem>
</varlistentry>

<varlistentry>
<term>signal id</term>
<listitem><para>The unique positive (nonzero) integer
used to identify a signal.  It can be used instead of 
a name to many functions for a slight performance
improvement.</para>
</listitem>
</varlistentry>

<varlistentry>
<term>connection id</term>
<listitem><para>The unique positive (nonzero) integer
used to identify the connection of a user-defined handler
to a signal.  Notice that it is allowed to connect the
same function-pointer/user-data pair twice, so
there is no guarantee that a function-pointer/user-data
maps to a unique connection id.
</para>
</listitem>
</varlistentry>

</variablelist>
</refsect2>

<refsect2><title>A brief note on how they work.</title>
<para>
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_<replaceable>return_value</replaceable>__<replaceable>parameter_list</replaceable>()
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.
</para>
</refsect2>

<!-- ##### SECTION See_Also ##### -->
<para>
<variablelist>

<varlistentry>
<term>#GtkObject</term>
<listitem><para>The base class for things which emit signals.</para></listitem>
</varlistentry>

<varlistentry>
<term><link linkend="gobject-Signals">GSignal</link></term>
<listitem><para>The GLib signal system.</para></listitem>
</varlistentry>
</variablelist>
</para>

<!-- ##### MACRO GTK_SIGNAL_OFFSET ##### -->
<para>
Use in place of <function>offsetof()</function>, which is used if it exists.
</para>

<!-- # Unused Parameters # -->
@struct: 
@field: 


<!-- ##### ENUM GtkSignalRunType ##### -->
<para>
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.
</para>

<variablelist>

<varlistentry>
<term>GTK_RUN_FIRST</term>
<listitem><para>Run the default handler before the connected user-defined
handlers.
</para></listitem>
</varlistentry>

<varlistentry>
<term>GTK_RUN_LAST</term>
<listitem><para>Run the default handler after the connected
user-defined handlers.
(Handlers registered as "after" always run after the default handler though)
</para></listitem>
</varlistentry>

<varlistentry>
<term>GTK_RUN_BOTH</term>
<listitem><para>Run the default handler twice,
once before the user-defined handlers,
and
once after.
</para></listitem>
</varlistentry>

<varlistentry>
<term>GTK_RUN_NO_RECURSE</term>
<listitem><para>Whether to prevent a handler or hook
from reemitting the signal from within itself.
Attempts to
emit the signal while it is running will result in the signal
emission being restarted once it is done with the current processing.
</para><para>
You must be
careful to avoid having two handlers endlessly reemitting signals,
gtk_signal_n_emissions() can be helpful.
</para></listitem>
</varlistentry>

<varlistentry>
<term>GTK_RUN_ACTION</term>
<listitem><para>The signal is an action you can 
invoke without any particular setup or cleanup.
The signal is treated no differently, but some
other code can determine if the signal is appropriate to
delegate to user control.  For example, key binding sets
only allow bindings of ACTION signals to keystrokes.
</para></listitem>
</varlistentry>

<varlistentry>
<term>GTK_RUN_NO_HOOKS</term>
<listitem><para>This prevents the connection of emission hooks
to the signal.
</para></listitem>
</varlistentry>

</variablelist>

@GTK_RUN_FIRST: 
@GTK_RUN_LAST: 
@GTK_RUN_BOTH: 
@GTK_RUN_NO_RECURSE: 
@GTK_RUN_ACTION: 
@GTK_RUN_NO_HOOKS: 

<!-- ##### FUNCTION gtk_signal_new ##### -->
<para>
Creates a new signal type.  (This is usually done in the
class initializer.)
</para>

@name: the event name for the signal, e.g. "clicked".
@signal_flags: a combination of #GTK_RUN flags
specifying detail of when the default handler is to be invoked.
You should at least specify #GTK_RUN_FIRST
or #GTK_RUN_LAST.
@object_type: the type of object this signal pertains to.
It will also pertain to derivers of this type automatically.
@function_offset: How many bytes the function pointer is in
the class structure for this type.  Used to invoke a class
method generically.
@marshaller: the function to translate between an array
of GtkArgs and the native calling convention.  Usually they
are identified just by the type of arguments they take:
for example, gtk_marshal_BOOL__STRING() describes a marshaller
which takes a string and returns a boolean value.
@return_val: the type of return value, or #GTK_TYPE_NONE for a signal
without a return value.
@n_args: 
@Varargs: a list of #GTK_TYPE_*, one for each parameter.
@Returns: the signal id.
<!-- # Unused Parameters # -->
@nparams: the number of parameter the handlers may take.


<!-- ##### FUNCTION gtk_signal_newv ##### -->
<para>
Creates a new signal type.  (This is usually done in a
class initializer.)
</para>
<para>
This function take the types as an array, instead of a list
following the arguments.  Otherwise the same as gtk_signal_new().
</para>

@name: the name of the signal to create.
@signal_flags: see gtk_signal_new().
@object_type: the type of #GtkObject to associate the signal with.
@function_offset: how many bytes the function pointer is in
the class structure for this type.
@marshaller: 
@return_val: the type of the return value, or #GTK_TYPE_NONE if
you don't want a return value.
@n_args: 
@args: 
@Returns: the signal id.
<!-- # Unused Parameters # -->
@nparams: the number of parameters to the user-defined handlers.
@params: an array of GtkTypes, describing the prototype to
the callbacks.


<!-- ##### MACRO gtk_signal_lookup ##### -->
<para>
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.
</para>
<para>
It also tries the ancestors of the given type.
</para>

@name: the signal's name, e.g. clicked.
@object_type: the type that the signal operates on, e.g. #GTK_TYPE_BUTTON.
@Returns: the signal's identifying number, or 0 if no signal was found.


<!-- ##### MACRO gtk_signal_name ##### -->
<para>
Given the signal's identifier, finds its name.
</para>
<para>
Two different signals may have the same name, if they have differing types.
</para>

@signal_id: the signal's identifying number.
@Returns: the signal name, or %NULL if the signal number was invalid.


<!-- ##### FUNCTION gtk_signal_emit ##### -->
<para>
Emits a signal.  This causes the default handler and user-defined
handlers to be run.
</para>
<para>
Here is what gtk_signal_emit() does:
</para>
<para>
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.
</para>
<para>
2.  Calls all handlers connected with the "after" flag set.
</para>

@object: the object that emits the signal.
@signal_id: the signal identifier.
@Varargs: the parameters to the function, followed
by a pointer to the return type, if any.


<!-- ##### FUNCTION gtk_signal_emit_by_name ##### -->
<para>
Emits a signal.  This causes the default handler and user-connected
handlers to be run.
</para>

@object: the object that emits the signal.
@name: the name of the signal.
@Varargs: the parameters to the function, followed
by a pointer to the return type, if any.


<!-- ##### FUNCTION gtk_signal_emitv ##### -->
<para>
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.
</para>

@object: the object to emit the signal to.
@signal_id: the signal identifier.
@args: 
<!-- # Unused Parameters # -->
@params: an array of GtkArgs, one for each parameter,
followed by one which is a pointer to the return type.


<!-- ##### FUNCTION gtk_signal_emitv_by_name ##### -->
<para>
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.
</para>

@object: the object to emit the signal to.
@name: the name of the signal.
@args: 
<!-- # Unused Parameters # -->
@params: an array of GtkArgs, one for each parameter,
followed by one which is a pointer to the return type.


<!-- ##### MACRO gtk_signal_emit_stop ##### -->
<para>
This function aborts a signal's current emission.
</para>
<para>
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).
</para>
<para>
It will print a warning if used on a signal which
isn't being emitted.
</para>

@object: the object whose signal handlers you wish to stop.
@signal_id: the signal identifier, as returned by g_signal_lookup().
<!-- # Unused Parameters # -->
@i: 
@s: 


<!-- ##### FUNCTION gtk_signal_emit_stop_by_name ##### -->
<para>
This function aborts a signal's current emission.
</para>
<para>It is just like
gtk_signal_emit_stop()
except it will lookup the signal id for you.
</para>

@object: the object whose signal handlers you wish to stop.
@name: the name of the signal you wish to stop.


<!-- ##### MACRO gtk_signal_connect ##### -->
<para>
Attaches a function pointer and user data to a signal for
a particular object.
</para>
<para>
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.
</para>
<para>
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).
</para>
<para>
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:
</para>
<informalexample>
<programlisting>
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));
}
</programlisting>
</informalexample>

@object: the object associated with the signal, e.g. if a button
is getting pressed, this is that button.
@name: name of the signal.
@func: function pointer to attach to the signal.
@func_data: value to pass as to your function (through the marshaller).
@Returns: the connection id.
<!-- # Unused Parameters # -->
@o: 
@s: 
@f: 
@d: 


<!-- ##### MACRO gtk_signal_connect_after ##### -->
<para>
Attaches a function pointer and user data to a signal
so that this handler will be called after the other handlers.
</para>

@object: the object associated with the signal.
@name: name of the signal.
@func: function pointer to attach to the signal.
@func_data: value to pass as to your function (through the marshaller).
@Returns: the unique identifier for this attachment:  the connection id.
<!-- # Unused Parameters # -->
@o: 
@s: 
@f: 
@d: 


<!-- ##### MACRO gtk_signal_connect_object ##### -->
<para>
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).
</para>
<para>
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:
</para>
<informalexample>
<programlisting>
gtk_signal_connect_object (button, "clicked", gtk_widget_show, window);
</programlisting>
</informalexample>

@object: the object which emits the signal.
@name: the name of the signal.
@func: the function to callback.
@slot_object: the object to pass as the first parameter to func.
(Though it pretends to take an object, you can
really pass any gpointer as the #slot_object .)
@Returns: the connection id.
<!-- # Unused Parameters # -->
@o: 
@s: 
@f: 
@d: 


<!-- ##### MACRO gtk_signal_connect_object_after ##### -->
<para>
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.
</para>

@object: the object associated with the signal.
@name: name of the signal.
@func: function pointer to attach to the signal.
@slot_object: the object to pass as the first parameter to #func.
@Returns: the connection id.
<!-- # Unused Parameters # -->
@o: 
@s: 
@f: 
@d: 


<!-- ##### FUNCTION gtk_signal_connect_full ##### -->
<para>
Attaches a function pointer and user data to a signal with
more control.
</para>

@object: the object which emits the signal.  For example, a button
in the button press signal.
@name: the name of the signal.
@func: function pointer to attach to the signal.
@unsupported: 
@data: the user data associated with the function.
@destroy_func: function to call when this particular hook is 
disconnected.
@object_signal: whether this is an object signal-- basically an "object
signal" is one that wants its user_data and object fields switched,
which is useful for calling functions which operate on another
object primarily.
@after: whether to invoke the user-defined handler after the signal, or to let 
the signal's default behavior preside (i.e. depending on #GTK_RUN_FIRST
and #GTK_RUN_LAST).
@Returns: the connection id.
<!-- # Unused Parameters # -->
@marshal: the function marshal, see the gtkmarshall documentation for
more details, but briefly: the marshaller is a function which takes
the #GtkObject which emits the signal, the user data, the number of the
arguments, and the array of arguments.  It is responsible for
calling the function in the appropriate calling convention.
gtk_signal_default_marshaller is usually fine.
(This shows up, for example, when worrying about matching
c++ or other languages' calling conventions.)


<!-- ##### FUNCTION gtk_signal_connect_while_alive ##### -->
<para>
Attaches a function pointer and another #GtkObject to a signal.
</para>
<para>
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.
</para>
<para>
(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()).)
</para>

@object: the object that emits the signal.
@signal: 
@func: function pointer to attach to the signal.
@func_data: pointer to pass to func.
@alive_object: object whose death should cause the handler connection
to be destroyed.
<!-- # Unused Parameters # -->
@name: name of the signal.


<!-- ##### FUNCTION gtk_signal_connect_object_while_alive ##### -->
<para>
These signal connectors are for signals which refer to objects,
so they must not be called after the object is deleted.
</para>
<para>
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.
</para>
<para>
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.
</para>

@object: the object associated with the signal.
@signal: 
@func: function pointer to attach to the signal.
@alive_object: the user data, which must be an object, whose destruction
should signal the removal of this signal.
<!-- # Unused Parameters # -->
@name: name of the signal.


<!-- ##### MACRO gtk_signal_disconnect ##### -->
<para>
Destroys a user-defined handler connection.
</para>

@object: the object which the handler pertains to.
@handler_id: the connection id.


<!-- ##### MACRO gtk_signal_disconnect_by_func ##### -->
<para>
Destroys all connections for a particular object, with
the given function-pointer and user-data.
</para>

@object: the object which emits the signal.
@func: the function pointer to search for.
@data: the user data to search for.
<!-- # Unused Parameters # -->
@o: 
@f: 
@d: 


<!-- ##### MACRO gtk_signal_disconnect_by_data ##### -->
<para>
Destroys all connections for a particular object, with
the given user-data.
</para>

@object: the object which emits the signal.
@data: the user data to search for.
<!-- # Unused Parameters # -->
@o: 
@d: 


<!-- ##### MACRO gtk_signal_handler_block ##### -->
<para>
Prevents a user-defined handler from being invoked.  All other
signal processing will go on as normal, but this particular
handler will ignore it.
</para>

@object: the object which emits the signal to block.
@handler_id: the connection id.


<!-- ##### MACRO gtk_signal_handler_block_by_func ##### -->
<para>
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.)
</para>

@object: the object which emits the signal to block.
@func: the function pointer of the handler to block.
@data: the user data of the handler to block.
<!-- # Unused Parameters # -->
@o: 
@f: 
@d: 


<!-- ##### MACRO gtk_signal_handler_block_by_data ##### -->
<para>
Prevents all user-defined handlers with a certain user data from being invoked.
</para>

@object: the object which emits the signal we want to block.
@data: the user data of the handlers to block.
<!-- # Unused Parameters # -->
@o: 
@d: 


<!-- ##### MACRO gtk_signal_handler_unblock ##### -->
<para>
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.
</para>

@object: the object which emits the signal we want to unblock.
@handler_id: the emission handler identifier, as returned by
gtk_signal_connect(), etc.


<!-- ##### MACRO gtk_signal_handler_unblock_by_func ##### -->
<para>
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.
</para>

@object: the object which emits the signal we want to unblock.
@func: the function pointer to search for.
@data: the user data to search for.
<!-- # Unused Parameters # -->
@o: 
@f: 
@d: 


<!-- ##### MACRO gtk_signal_handler_unblock_by_data ##### -->
<para>
Undoes block(s), to all signals for a particular object
with a particular user-data pointer
</para>

@object: the object which emits the signal we want to unblock.
@data: the user data to search for.
<!-- # Unused Parameters # -->
@o: 
@d: 


<!-- ##### MACRO gtk_signal_handler_pending ##### -->
<para>
Returns a connection id corresponding to a given signal id and object.
</para>
<para>
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.
</para>

@object: the object to search for the desired user-defined handler.
@signal_id: the number of the signal to search for.
@may_be_blocked: whether it is acceptable to return a blocked
handler.
@Returns: the connection id, if a connection was found.  0 otherwise.
<!-- # Unused Parameters # -->
@i: 
@s: 
@b: 


<!-- ##### MACRO gtk_signal_handler_pending_by_func ##### -->
<para>
Returns a connection id corresponding to a given signal id, object, function
pointer and user data.
</para>

@object: the object to search for the desired handler.
@signal_id: the number of the signal to search for.
@may_be_blocked: whether it is acceptable to return a blocked
handler.
@func: the function pointer to search for.
@data: the user data to search for.
@Returns: the connection id, if a handler was found.  0 otherwise.
<!-- # Unused Parameters # -->
@o: 
@s: 
@b: 
@f: 
@d: 


<!-- ##### MACRO gtk_signal_default_marshaller ##### -->
<para>
A marshaller that returns void and takes no extra parameters.
</para>