Closure

Closure

Object Hierarchy:

Description:

public class Closure

A Closure 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 Values into a suitable form, perform the callback on the converted arguments, and transform the return value back into a Value.

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 Value and native C types. The GObject library provides the GCClosure type for this purpose. Bindings for other languages need marshallers which convert between Value s and suitable representations in the runtime of the language in order to use functions written in that languages as callbacks.

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 glib-genmarshal utility. Closures can be explicitly connected to signals with 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.

Using closures has a number of important advantages over a simple callback function/data pointer combination:

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.
The reference counting of Closure makes it easy to handle reentrancy right; if a callback is removed while it is being invoked, the closure and its parameters won't be freed until the invocation finishes.
invalidate and invalidation notifiers allow callbacks to be automatically removed when the objects they point to go away.

Namespace: GLib

Package: gobject-2.0

Content:

Creation methods:

public Closure (ulong sizeof_closure, Object object)
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.

Methods:

public void sink ()
Takes over the initial ownership of a closure. Each closure is initially created in a floating state, which means that the initial reference count is not owned by any caller. 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, sink does nothing. The reason for the existence of the floating state is to prevent cumbersome code sequences like:
public void invoke (out Value? return_value, Value[] param_values, void* invocation_hint)
Invokes the closure, i.e. executes the callback represented by the closure.
public void invalidate ()
Sets a flag on the closure to indicate that its calling environment has become invalid, and thus causes any future invocations of 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 invalidate, make sure that you've previously called g_closure_ref.
public void add_finalize_notifier (void* notify_data, ClosureNotify notify_func)
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.
public void add_invalidate_notifier (void* notify_data, ClosureNotify notify_func)
Registers an invalidation notifier which will be called when the closure is invalidated with invalidate. Invalidation notifiers are invoked before finalization notifiers, in an unspecified order.
public void remove_finalize_notifier (void* notify_data, ClosureNotify notify_func)
Removes a finalization notifier.
public void remove_invalidate_notifier (void* notify_data, ClosureNotify notify_func)
Removes an invalidation notifier.
public void set_marshal (ClosureMarshal marshal)
Sets the marshaller of closure. The marshal_data of marshal provides a way for a meta marshaller to provide additional information to the marshaller. (See set_meta_marshal.) For GObject's C predefined marshallers (the g_cclosure_marshal_*() functions), what it provides is a callback function to use instead of closure->callback.
public void add_marshal_guards (void* pre_marshal_data, ClosureNotify pre_marshal_notify, void* post_marshal_data, ClosureNotify post_marshal_notify)
Adds a pair of notifiers which get invoked before and after the closure callback, respectively. This is typically used to protect the extra arguments for the duration of the callback. See g_object_watch_closure for an example of marshal guards.
public void set_meta_marshal (void* marshal_data, ClosureMarshal meta_marshal)
Sets the meta marshaller of closure. A meta marshaller wraps closure-> marshal and modifies the way it is called in some fashion. The most common use of this facility is for C callbacks. The same marshallers (generated by glib-genmarshal) are used everywhere, but the way that we get the callback function differs. In most cases we want to use closure->callback, but in other cases we want to use some different technique to retrieve the callback function.