gclosure.sgml

This GLib version 2.16.1. GLib is the low-level core library that forms the basis for projects such

<!-- ##### SECTION Title ##### -->
Closures

<!-- ##### SECTION Short_Description ##### -->
Functions as first-class objects

<!-- ##### SECTION Long_Description ##### -->
<para>
A #GClosure represents a callback supplied by the programmer. It will generally
comprise a function of some kind and a marshaller used to call it. It is the 
reponsibility of the marshaller to convert the arguments for the invocation 
from #GValue<!-- -->s into a suitable form, perform the callback on the 
converted arguments, and transform the return value back into a #GValue.
</para>
<para>
In the case of C programs, a closure usually just holds a pointer to a function
and maybe a data argument, and the marshaller converts between #GValue<!-- -->
and native C types. The GObject library provides the #GCClosure type for this
purpose. Bindings for other languages need marshallers which 
convert between #GValue<!-- -->s and suitable representations in the runtime
of the language in order to use functions written in that languages as 
callbacks.
</para>
<para>
Within GObject, closures play an important role in the implementation of 
signals. When a signal is registered, the @c_marshaller argument to 
g_signal_new() specifies the default C marshaller for any closure which is 
connected to this signal. GObject provides a number of C marshallers  
for this purpose, see the g_cclosure_marshal_*() functions. Additional
C marshallers can be generated with the <link linkend="glib-genmarshal"
>glib-genmarshal</link> utility.
Closures can be explicitly connected to signals with 
g_signal_connect_closure(), but it usually more convenient to let GObject 
create a closure automatically by using one of the g_signal_connect_*() 
functions which take a callback function/user data pair.
</para>
<para>
Using closures has a number of important advantages over a simple
callback function/data pointer combination:
<itemizedlist>
<listitem><para>
Closures allow the callee to get the types of the callback parameters, 
which means that language bindings don't have to write individual glue 
for each callback type.
</para></listitem>
<listitem><para>
The reference counting of #GClosure makes it easy to handle reentrancy 
right; if a callback is removed while it is being invoked, the closure 
and it's parameters won't be freed until the invocation finishes. 
</para></listitem>
<listitem><para>
g_closure_invalidate() and invalidation notifiers allow callbacks to be
automatically removed when the objects they point to go away.
</para></listitem>
</itemizedlist>
</para>

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

</para>

<!-- ##### SECTION Stability_Level ##### -->


<!-- ##### MACRO G_CLOSURE_NEEDS_MARSHAL ##### -->
<para>
Returns %TRUE if a #GClosureMarshal marshaller has not yet been set on 
@closure. See g_closure_set_marshal().
</para>

@closure: a #GClosure


<!-- ##### MACRO G_CLOSURE_N_NOTIFIERS ##### -->
<para>
Returns the total number of notifiers connected with the closure @cl. 
The count includes the meta marshaller, the finalize and invalidate notifiers 
and the marshal guards. Note that each guard counts as two notifiers. 
See g_closure_set_meta_marshal(), g_closure_add_finalize_notifier(),
g_closure_add_invalidate_notifier() and g_closure_add_marshal_guards().
</para>

@cl: a #GClosure


<!-- ##### MACRO G_CCLOSURE_SWAP_DATA ##### -->
<para>
Returns whether the user data of the #GCClosure should be passed as the
first parameter to the callback. See g_cclosure_new_swap().
</para>

@cclosure: a #GCClosure


<!-- ##### MACRO G_CALLBACK ##### -->
<para>
Cast a function pointer to a #GCallback.
</para>

@f: a function pointer.


<!-- ##### USER_FUNCTION GCallback ##### -->
<para>
The type used for callback functions in structure definitions and function 
signatures. This doesn't mean that all callback functions must take no 
parameters and return void. The required signature of a callback function 
is determined by the context in which is used (e.g. the signal to which it 
is connected). Use G_CALLBACK() to cast the callback function to a #GCallback. 
</para>



<!-- ##### STRUCT GClosure ##### -->
<para>
A #GClosure represents a callback supplied by the programmer.
</para>

@in_marshal: Indicates whether the closure is currently being invoked with 
  g_closure_invoke()
@is_invalid: Indicates whether the closure has been invalidated by 
  g_closure_invalidate()

<!-- ##### MACRO G_TYPE_CLOSURE ##### -->
<para>
The #GType for #GClosure.
</para>



<!-- ##### STRUCT GCClosure ##### -->
<para>
A #GCClosure is a specialization of #GClosure for C function callbacks.
</para>

@closure: the #GClosure
@callback: the callback function

<!-- ##### USER_FUNCTION GClosureMarshal ##### -->
<para>
The type used for marshaller functions.
</para>

@closure: the #GClosure to which the marshaller belongs
@return_value: a #GValue to store the return value. May be %NULL if the
  callback of @closure doesn't return a value.
@n_param_values: the length of the @param_values array
@param_values: an array of #GValue<!-- -->s holding the arguments on
  which to invoke the callback of @closure
@invocation_hint: the invocation hint given as the the last argument
  to g_closure_invoke()
@marshal_data: additional data specified when registering the marshaller,
  see g_closure_set_marshal() and g_closure_set_meta_marshal()


<!-- ##### USER_FUNCTION GClosureNotify ##### -->
<para>
The type used for the various notification callbacks which can be registered
on closures.
</para>

@data: data specified when registering the notification callback
@closure: the #GClosure on which the notification is emitted


<!-- ##### FUNCTION g_cclosure_new ##### -->
<para>
Creates a new closure which invokes @callback_func with @user_data as 
the last parameter. 
</para>

@callback_func: the function to invoke
@user_data: user data to pass to @callback_func
@destroy_data: destroy notify to be called when @user_data is no longer used
@Returns: a new #GCClosure


<!-- ##### FUNCTION g_cclosure_new_swap ##### -->
<para>
Creates a new closure which invokes @callback_func with @user_data as 
the first parameter. 
</para>

@callback_func: the function to invoke
@user_data: user data to pass to @callback_func
@destroy_data: destroy notify to be called when @user_data is no longer used
@Returns: a new #GCClosure


<!-- ##### FUNCTION g_cclosure_new_object ##### -->
<para>
A variant of g_cclosure_new() which uses @object as @user_data and calls 
g_object_watch_closure() on @object and the created closure. This function 
is useful when you have a callback closely associated with a #GObject,
and want the callback to no longer run after the object is is freed.
</para>

@callback_func: the function to invoke
@object: a #GObject pointer to pass to @callback_func
@Returns: a new #GCClosure


<!-- ##### FUNCTION g_cclosure_new_object_swap ##### -->
<para>
A variant of g_cclosure_new_swap() which uses @object as @user_data and calls 
g_object_watch_closure() on @object and the created closure. This function 
is useful when you have a callback closely associated with a #GObject,
and want the callback to no longer run after the object is is freed.
</para>

@callback_func: the function to invoke
@object: a #GObject pointer to pass to @callback_func
@Returns: a new #GCClosure


<!-- ##### FUNCTION g_closure_new_object ##### -->
<para>
A variant of g_closure_new_simple() which stores @object in the @data
field of the closure and calls g_object_watch_closure() on @object and the 
created closure. This function is mainly useful when implementing new types 
of closures.
</para>

@sizeof_closure: the size of the structure to allocate, must be at least 
<literal>sizeof (GClosure)</literal>
@object: a #GObject pointer to store in the @data field of the newly 
  allocated #GClosure
@Returns: a newly allocated #GClosure


<!-- ##### FUNCTION g_closure_ref ##### -->
<para>
Increments the reference count on a closure to force it staying
alive while the caller holds a pointer to it.
</para>

@closure: #GClosure to increment the reference count on
@Returns: The @closure passed in, for convenience


<!-- ##### FUNCTION g_closure_sink ##### -->
<para>
Takes over the initial ownership of a closure.
Each closure is initially created in a <firstterm>floating</firstterm> state, 
which means that the initial reference count is not owned by any caller. 
g_closure_sink() checks to see if the object is still floating, and if so, 
unsets the floating state and decreases the reference count. If the closure 
is not floating, g_closure_sink() does nothing. The reason for the existance 
of the floating state is to prevent cumbersome code sequences like: 
<programlisting>
closure = g_cclosure_new (cb_func, cb_data); 
g_source_set_closure (source, closure); 
g_closure_unref (closure); /* XXX GObject doesn't really need this */
</programlisting>
Because g_source_set_closure() (and similar functions) take ownership of the 
initial reference count, if it is unowned, we instead can write: 
<programlisting>
g_source_set_closure (source, g_cclosure_new (cb_func, cb_data));
</programlisting>
</para>
<para>
Generally, this function is used together with g_closure_ref(). Ane example 
of storing a closure for later notification looks like:
<informalexample><programlisting>
static GClosure *notify_closure = NULL;
void
foo_notify_set_closure (GClosure *closure)
{
  if (notify_closure)
    g_closure_unref (notify_closure);
  notify_closure = closure;
  if (notify_closure)
    {
      g_closure_ref (notify_closure);
      g_closure_sink (notify_closure);
    }
}
</programlisting></informalexample>
</para>
<para>
Because g_closure_sink() may decrement the reference count of a closure
(if it hasn't been called on @closure yet) just like g_closure_unref(),
g_closure_ref() should be called prior to this function.
</para>

@closure: #GClosure to decrement the initial reference count on, if it's
          still being held


<!-- ##### FUNCTION g_closure_unref ##### -->
<para>
Decrements the reference count of a closure after it was previously 
incremented by the same caller. If no other callers are using the closure,
then the closure will be destroyed and freed.
</para>

@closure: #GClosure to decrement the reference count on


<!-- ##### FUNCTION g_closure_invoke ##### -->
<para>
Invokes the closure, i.e. executes the callback represented by the @closure.
</para>

@closure: a #GClosure
@return_value: a #GValue to store the return value. May be %NULL if the
  callback of @closure doesn't return a value.
@n_param_values: the length of the @param_values array
@param_values: an array of #GValue<!-- -->s holding the arguments on
  which to invoke the callback of @closure
@invocation_hint: a context-dependent invocation hint


<!-- ##### FUNCTION g_closure_invalidate ##### -->
<para>
Sets a flag on the closure to indicate that it's calling environment has 
become invalid, and thus causes any future invocations of g_closure_invoke() 
on this @closure to be ignored. Also, invalidation notifiers installed on 
the closure will be called at this point. Note that unless you are holding 
a reference to the closure yourself, the invalidation notifiers may unref 
the closure and cause it to be destroyed, so if you need to access the 
closure after calling g_closure_invalidate(), make sure that you've 
previously called g_closure_ref().
</para>
<para>
Note that g_closure_invalidate() will also be called when the reference count
of a closure drops to zero (unless it has already been invalidated before).
</para>

@closure: GClosure to invalidate


<!-- ##### FUNCTION g_closure_add_finalize_notifier ##### -->
<para>
Registers a finalization notifier which will be called when the reference
count of @closure goes down to 0. Multiple finalization notifiers on a 
single closure are invoked in unspecified order. If a single call to 
g_closure_unref() results in the closure being both invalidated and 
finalized, then the invalidate notifiers will be run before the finalize 
notifiers.
</para>

@closure: a #GClosure
@notify_data: data to pass to @notify_func
@notify_func: the callback function to register


<!-- ##### FUNCTION g_closure_add_invalidate_notifier ##### -->
<para>
Registers an invalidation notifier which will be called when the @closure
is invalidated with g_closure_invalidate(). Invalidation notifiers are 
invoked before finalization notifiers, in an unspecified order.
</para>

@closure: a #GClosure
@notify_data: data to pass to @notify_func
@notify_func: the callback function to register


<!-- ##### FUNCTION g_closure_remove_finalize_notifier ##### -->
<para>
Removes a finalization notifier.
</para>
<para>
Notice that notifiers are automatically removed after they are run.
</para>

@closure: a #GClosure
@notify_data: data which was passed to g_closure_add_finalize_notifier()
 when registering @notify_func
@notify_func: the callback function to remove


<!-- ##### FUNCTION g_closure_remove_invalidate_notifier ##### -->
<para>
Removes an invalidation notifier.
</para>

<para>
Notice that notifiers are automatically removed after they are run.
</para>

@closure: a #GClosure
@notify_data: data which was passed to g_closure_add_invalidate_notifier()
 when registering @notify_func
@notify_func: the callback function to remove


<!-- ##### FUNCTION g_closure_new_simple ##### -->
<para>
Allocates a struct of the given size and initializes the initial part
as a #GClosure. This function is mainly useful when implementing new types 
of closures.
</para>
<informalexample>
<programlisting>
typedef struct _MyClosure MyClosure;
struct _MyClosure 
{
  GClosure closure;
  /* extra data goes here */
};
    

static void
my_closure_finalize (gpointer  notify_data, 
                     GClosure *closure)
{
  MyClosure *my_closure = (MyClosure *)closure;

  /* free extra data here */
}