glib/gobject/gtypeplugin.c

/* GObject - GLib Type, Object, Parameter and Signal Library
 * Copyright (C) 2000 Red Hat, Inc.
 *
 * SPDX-License-Identifier: LGPL-2.1-or-later
 *
 * This library is free software; you can redistribute it and/or
 * modify it under the terms of the GNU Lesser General Public
 * License as published by the Free Software Foundation; either
 * version 2.1 of the License, or (at your option) any later version.
 *
 * This library is distributed in the hope that it will be useful,
 * but WITHOUT ANY WARRANTY; without even the implied warranty of
 * MERCHANTABILITY or FITNESS FOR A PARTICULAR PURPOSE.  See the GNU
 * Lesser General Public License for more details.
 *
 * You should have received a copy of the GNU Lesser General
 * Public License along with this library; if not, see <http://www.gnu.org/licenses/>.
 */

#include "config.h"

#include "gtypeplugin.h"


/**
 * GTypePlugin:
 *
 * An interface that handles the lifecycle of dynamically loaded types.
 *
 * The GObject type system supports dynamic loading of types.
 * It goes as follows:
 *
 * 1. The type is initially introduced (usually upon loading the module
 *    the first time, or by your main application that knows what modules
 *    introduces what types), like this:
 *    ```c
 *    new_type_id = g_type_register_dynamic (parent_type_id,
 *                                           "TypeName",
 *                                           new_type_plugin,
 *                                           type_flags);
 *    ```
 *    where `new_type_plugin` is an implementation of the
 *    `GTypePlugin` interface.
 *
 * 2. The type's implementation is referenced, e.g. through
 *    [func@GObject.TypeClass.ref] or through [func@GObject.type_create_instance]
 *    (this is being called by [ctor@GObject.Object.new]) or through one of the above
 *    done on a type derived from `new_type_id`.
 *
 * 3. This causes the type system to load the type's implementation by calling
 *    [method@GObject.TypePlugin.use] and [method@GObject.TypePlugin.complete_type_info]
 *    on `new_type_plugin`.
 *
 * 4. At some point the type's implementation isn't required anymore, e.g. after
 *    [method@GObject.TypeClass.unref] or [func@GObject.type_free_instance]
 *    (called when the reference count of an instance drops to zero).
 *
 * 5. This causes the type system to throw away the information retrieved
 *    from [method@GObject.TypePlugin.complete_type_info] and then it calls
 *    [method@GObject.TypePlugin.unuse] on `new_type_plugin`.
 *
 * 6. Things may repeat from the second step.
 *
 * So basically, you need to implement a `GTypePlugin` type that
 * carries a use_count, once use_count goes from zero to one, you need
 * to load the implementation to successfully handle the upcoming
 * [method@GObject.TypePlugin.complete_type_info] call. Later, maybe after
 * succeeding use/unuse calls, once use_count drops to zero, you can
 * unload the implementation again. The type system makes sure to call
 * [method@GObject.TypePlugin.use] and [method@GObject.TypePlugin.complete_type_info]
 * again when the type is needed again.
 *
 * [class@GObject.TypeModule] is an implementation of `GTypePlugin` that
 * already implements most of this except for the actual module loading and
 * unloading. It even handles multiple registered types per module.
 */


/* --- functions --- */
GType
g_type_plugin_get_type (void)
{
  static GType type_plugin_type = 0;
  
  if (!type_plugin_type)
    {
      const GTypeInfo type_plugin_info = {
        sizeof (GTypePluginClass),
        NULL,           /* base_init */
        NULL,           /* base_finalize */
        0,              /* class_init */
        NULL,           /* class_destroy */
        NULL,           /* class_data */
        0,              /* instance_size */
        0,              /* n_preallocs */
        NULL,           /* instance_init */
        NULL,           /* value_table */
      };
      
      type_plugin_type = g_type_register_static (G_TYPE_INTERFACE, g_intern_static_string ("GTypePlugin"), &type_plugin_info, 0);
    }
  
  return type_plugin_type;
}

/**
 * g_type_plugin_use:
 * @plugin: a #GTypePlugin
 *
 * Calls the @use_plugin function from the #GTypePluginClass of
 * @plugin.  There should be no need to use this function outside of
 * the GObject type system itself.
 */
void
g_type_plugin_use (GTypePlugin *plugin)
{
  GTypePluginClass *iface;
  
  g_return_if_fail (G_IS_TYPE_PLUGIN (plugin));
  
  iface = G_TYPE_PLUGIN_GET_CLASS (plugin);
  iface->use_plugin (plugin);
}

/**
 * g_type_plugin_unuse:
 * @plugin: a #GTypePlugin
 *
 * Calls the @unuse_plugin function from the #GTypePluginClass of
 * @plugin.  There should be no need to use this function outside of
 * the GObject type system itself.
 */
void
g_type_plugin_unuse (GTypePlugin *plugin)
{
  GTypePluginClass *iface;
  
  g_return_if_fail (G_IS_TYPE_PLUGIN (plugin));
  
  iface = G_TYPE_PLUGIN_GET_CLASS (plugin);
  iface->unuse_plugin (plugin);
}

/**
 * g_type_plugin_complete_type_info:
 * @plugin: a #GTypePlugin
 * @g_type: the #GType whose info is completed
 * @info: the #GTypeInfo struct to fill in
 * @value_table: the #GTypeValueTable to fill in
 * 
 * Calls the @complete_type_info function from the #GTypePluginClass of @plugin.
 * There should be no need to use this function outside of the GObject 
 * type system itself.
 */
void
g_type_plugin_complete_type_info (GTypePlugin     *plugin,
				  GType            g_type,
				  GTypeInfo       *info,
				  GTypeValueTable *value_table)
{
  GTypePluginClass *iface;
  
  g_return_if_fail (G_IS_TYPE_PLUGIN (plugin));
  g_return_if_fail (info != NULL);
  g_return_if_fail (value_table != NULL);
  
  iface = G_TYPE_PLUGIN_GET_CLASS (plugin);
  iface->complete_type_info (plugin,
			     g_type,
			     info,
			     value_table);
}

/**
 * g_type_plugin_complete_interface_info:
 * @plugin: the #GTypePlugin
 * @instance_type: the #GType of an instantiatable type to which the interface
 *  is added
 * @interface_type: the #GType of the interface whose info is completed
 * @info: the #GInterfaceInfo to fill in
 *
 * Calls the @complete_interface_info function from the
 * #GTypePluginClass of @plugin. There should be no need to use this
 * function outside of the GObject type system itself.
 */
void
g_type_plugin_complete_interface_info (GTypePlugin    *plugin,
				       GType           instance_type,
				       GType           interface_type,
				       GInterfaceInfo *info)
{
  GTypePluginClass *iface;
  
  g_return_if_fail (G_IS_TYPE_PLUGIN (plugin));
  g_return_if_fail (info != NULL);
  
  iface = G_TYPE_PLUGIN_GET_CLASS (plugin);
  iface->complete_interface_info (plugin,
				  instance_type,
				  interface_type,
				  info);
}