Closure


Object Hierarchy:

Object hierarchy for Closure

Description:

[ Compact ]
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 responsibility 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 Values and suitable representations in the runtime of the language in order to use functions written in that language as callbacks. Use set_marshal to set the marshaller on such a custom closure implementation.

Within GObject, closures play an important role in the implementation of signals. When a signal is registered, the c_marshaller argument to @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

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