path: root/calendar/pcs/cal-backend.c

/* -*- Mode: C; tab-width: 8; indent-tabs-mode: t; c-basic-offset: 8 -*- */
/* Evolution calendar - generic backend class
 *
 * Copyright (C) 2000 Ximian, Inc.
 * Copyright (C) 2000 Ximian, Inc.
 *
 * Authors: Federico Mena-Quintero <federico@ximian.com>
 *          JP Rosevear <jpr@ximian.com>
 *          Rodrigo Moya <rodrigo@ximian.com>    
 *
 * This program is free software; you can redistribute it and/or modify
 * it under the terms of the GNU General Public License as published by
 * the Free Software Foundation; either version 2 of the License, or
 * (at your option) any later version.
 *
 * This program 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 General Public License for more details.
 *
 * You should have received a copy of the GNU General Public License
 * along with this program; if not, write to the Free Software
 * Foundation, Inc., 59 Temple Place, Suite 330, Boston, MA 02111-1307, USA.
 */

#include <config.h>
#include <gtk/gtkobject.h>
#include <gtk/gtksignal.h>
#include <gnome-xml/parser.h>
#include <gnome-xml/parserInternals.h>
#include <gnome-xml/xmlmemory.h>

#include "cal-backend.h"
#include "libversit/vcc.h"



/* Signal IDs */
enum {
    LAST_CLIENT_GONE,
    CAL_ADDED,
    OPENED,
    OBJ_UPDATED,
    OBJ_REMOVED,
    LAST_SIGNAL
};

static void cal_backend_class_init (CalBackendClass *class);

static guint cal_backend_signals[LAST_SIGNAL];

#define CLASS(backend) (CAL_BACKEND_CLASS (GTK_OBJECT (backend)->klass))



/**
 * cal_backend_get_type:
 * @void:
 *
 * Registers the #CalBackend class if necessary, and returns the type ID
 * associated to it.
 *
 * Return value: The type ID of the #CalBackend class.
 **/
GtkType
cal_backend_get_type (void)
{
    static GtkType cal_backend_type = 0;

    if (!cal_backend_type) {
        static const GtkTypeInfo cal_backend_info = {
            "CalBackend",
            sizeof (CalBackend),
            sizeof (CalBackendClass),
            (GtkClassInitFunc) cal_backend_class_init,
            (GtkObjectInitFunc) NULL,
            NULL, /* reserved_1 */
            NULL, /* reserved_2 */
            (GtkClassInitFunc) NULL
        };

        cal_backend_type =
            gtk_type_unique (GTK_TYPE_OBJECT, &cal_backend_info);
    }

    return cal_backend_type;
}

/* Class initialization function for the calendar backend */
static void
cal_backend_class_init (CalBackendClass *class)
{
    GtkObjectClass *object_class;

    object_class = (GtkObjectClass *) class;

    cal_backend_signals[LAST_CLIENT_GONE] =
        gtk_signal_new ("last_client_gone",
                GTK_RUN_FIRST,
                object_class->type,
                GTK_SIGNAL_OFFSET (CalBackendClass, last_client_gone),
                gtk_marshal_NONE__NONE,
                GTK_TYPE_NONE, 0);
    cal_backend_signals[CAL_ADDED] =
        gtk_signal_new ("cal_added",
                GTK_RUN_FIRST,
                object_class->type,
                GTK_SIGNAL_OFFSET (CalBackendClass, cal_added),
                gtk_marshal_NONE__POINTER,
                GTK_TYPE_NONE, 1,
                GTK_TYPE_POINTER);
    cal_backend_signals[OPENED] =
        gtk_signal_new ("opened",
                GTK_RUN_FIRST,
                object_class->type,
                GTK_SIGNAL_OFFSET (CalBackendClass, opened),
                gtk_marshal_NONE__ENUM,
                GTK_TYPE_NONE, 1,
                GTK_TYPE_ENUM);
    cal_backend_signals[OBJ_UPDATED] =
        gtk_signal_new ("obj_updated",
                GTK_RUN_FIRST,
                object_class->type,
                GTK_SIGNAL_OFFSET (CalBackendClass, obj_updated),
                gtk_marshal_NONE__STRING,
                GTK_TYPE_NONE, 1,
                GTK_TYPE_STRING);
    cal_backend_signals[OBJ_REMOVED] =
        gtk_signal_new ("obj_removed",
                GTK_RUN_FIRST,
                object_class->type,
                GTK_SIGNAL_OFFSET (CalBackendClass, obj_removed),
                gtk_marshal_NONE__STRING,
                GTK_TYPE_NONE, 1,
                GTK_TYPE_STRING);

    gtk_object_class_add_signals (object_class, cal_backend_signals, LAST_SIGNAL);

    class->last_client_gone = NULL;
    class->opened = NULL;
    class->obj_updated = NULL;
    class->obj_removed = NULL;

    class->get_uri = NULL;
    class->open = NULL;
    class->is_loaded = NULL;
    class->get_mode = NULL;
    class->set_mode = NULL; 
    class->get_n_objects = NULL;
    class->get_object = NULL;
    class->get_object_component = NULL;
    class->get_timezone_object = NULL;
    class->get_uids = NULL;
    class->get_objects_in_range = NULL;
    class->get_free_busy = NULL;
    class->get_changes = NULL;
    class->get_alarms_in_range = NULL;
    class->get_alarms_for_object = NULL;
    class->update_objects = NULL;
    class->remove_object = NULL;
}



/**
 * cal_backend_get_uri:
 * @backend: A calendar backend.
 *
 * Queries the URI of a calendar backend, which must already have an open
 * calendar.
 *
 * Return value: The URI where the calendar is stored.
 **/
const char *
cal_backend_get_uri (CalBackend *backend)
{
    g_return_val_if_fail (backend != NULL, NULL);
    g_return_val_if_fail (IS_CAL_BACKEND (backend), NULL);

    g_assert (CLASS (backend)->get_uri != NULL);
    return (* CLASS (backend)->get_uri) (backend);
}

/* Callback used when a Cal is destroyed */
static void
cal_destroy_cb (GtkObject *object, gpointer data)
{
    Cal *cal;
    Cal *lcal;
    CalBackend *backend;
    GList *l;

    cal = CAL (object);

    backend = CAL_BACKEND (data);

    /* Find the cal in the list of clients */

    for (l = backend->clients; l; l = l->next) {
        lcal = CAL (l->data);

        if (lcal == cal)
            break;
    }

    g_assert (l != NULL);

    /* Disconnect */

    backend->clients = g_list_remove_link (backend->clients, l);
    g_list_free_1 (l);

    /* When all clients go away, notify the parent factory about it so that
     * it may decide whether to kill the backend or not.
     */
    if (!backend->clients)
        cal_backend_last_client_gone (backend);
}

/**
 * cal_backend_add_cal:
 * @backend: A calendar backend.
 * @cal: A calendar client interface object.
 *
 * Adds a calendar client interface object to a calendar @backend.
 * The calendar backend must already have an open calendar.
 **/
void
cal_backend_add_cal (CalBackend *backend, Cal *cal)
{
    g_return_if_fail (backend != NULL);
    g_return_if_fail (IS_CAL_BACKEND (backend));
    g_return_if_fail (IS_CAL (cal));

    /* we do not keep a reference to the Cal since the Calendar
     * user agent owns it */
    gtk_signal_connect (GTK_OBJECT (cal), "destroy",
                GTK_SIGNAL_FUNC (cal_destroy_cb),
                backend);

    backend->clients = g_list_prepend (backend->clients, cal);

    /* notify backend that a new Cal has been added */
    gtk_signal_emit (GTK_OBJECT (backend),
             cal_backend_signals[CAL_ADDED],
             cal);
}

/**
 * cal_backend_open:
 * @backend: A calendar backend.
 * @uristr: URI that contains the calendar data.
 * @only_if_exists: Whether the calendar should be opened only if it already
 * exists.  If FALSE, a new calendar will be created when the specified @uri
 * does not exist.
 *
 * Opens a calendar backend with data from a calendar stored at the specified
 * URI.
 *
 * Return value: An operation status code.
 **/
CalBackendOpenStatus
cal_backend_open (CalBackend *backend, const char *uristr, gboolean only_if_exists)
{
    CalBackendOpenStatus result;

    g_return_val_if_fail (backend != NULL, CAL_BACKEND_OPEN_ERROR);
    g_return_val_if_fail (IS_CAL_BACKEND (backend), CAL_BACKEND_OPEN_ERROR);
    g_return_val_if_fail (uristr != NULL, CAL_BACKEND_OPEN_ERROR);

    g_assert (CLASS (backend)->open != NULL);
    result = (* CLASS (backend)->open) (backend, uristr, only_if_exists);

    return result;
}

/**
 * cal_backend_is_loaded:
 * @backend: A calendar backend.
 * 
 * Queries whether a calendar backend has been loaded yet.
 * 
 * Return value: TRUE if the backend has been loaded with data, FALSE
 * otherwise.
 **/
gboolean
cal_backend_is_loaded (CalBackend *backend)
{
    gboolean result;

    g_return_val_if_fail (backend != NULL, FALSE);
    g_return_val_if_fail (IS_CAL_BACKEND (backend), FALSE);

    g_assert (CLASS (backend)->is_loaded != NULL);
    result = (* CLASS (backend)->is_loaded) (backend);

    return result;
}

/**
 * cal_backend_get_mode:
 * @backend: A calendar backend. 
 * 
 * Queries whether a calendar backend is connected remotely.
 * 
 * Return value: The current mode the calendar is in
 **/
CalMode
cal_backend_get_mode (CalBackend *backend)
{
    CalMode result;

    g_return_val_if_fail (backend != NULL, FALSE);
    g_return_val_if_fail (IS_CAL_BACKEND (backend), FALSE);

    g_assert (CLASS (backend)->get_mode != NULL);
    result = (* CLASS (backend)->get_mode) (backend);

    return result;
}


/**
 * cal_backend_set_mode:
 * @backend: A calendar backend
 * @mode: Mode to change to
 * 
 * Sets the mode of the calendar
 * 
 **/
void
cal_backend_set_mode (CalBackend *backend, CalMode mode)
{
    g_return_if_fail (backend != NULL);
    g_return_if_fail (IS_CAL_BACKEND (backend));

    g_assert (CLASS (backend)->set_mode != NULL);
    (* CLASS (backend)->set_mode) (backend, mode);
}

/**
 * cal_backend_get_n_objects:
 * @backend: A calendar backend.
 * @type: Types of objects that will be included in the count.
 * 
 * Queries the number of calendar objects of a particular type.
 * 
 * Return value: Number of objects of the specified @type.
 **/
int
cal_backend_get_n_objects (CalBackend *backend, CalObjType type)
{
    g_return_val_if_fail (backend != NULL, -1);
    g_return_val_if_fail (IS_CAL_BACKEND (backend), -1);

    g_assert (CLASS (backend)->get_n_objects != NULL);
    return (* CLASS (backend)->get_n_objects) (backend, type);
}

/**
 * cal_backend_get_object:
 * @backend: A calendar backend.
 * @uid: Unique identifier for a calendar object.
 *
 * Queries a calendar backend for a calendar object based on its unique
 * identifier.
 *
 * Return value: The string representation of a complete calendar wrapping the
 * the sought object, or NULL if no object had the specified UID.
 **/
char *
cal_backend_get_object (CalBackend *backend, const char *uid)
{
    g_return_val_if_fail (backend != NULL, NULL);
    g_return_val_if_fail (IS_CAL_BACKEND (backend), NULL);
    g_return_val_if_fail (uid != NULL, NULL);

    g_assert (CLASS (backend)->get_object != NULL);
    return (* CLASS (backend)->get_object) (backend, uid);
}

/**
 * cal_backend_get_object_component:
 * @backend: A calendar backend.
 * @uid: Unique identifier for a calendar object.
 *
 * Queries a calendar backend for a calendar object based on its unique
 * identifier. It returns the CalComponent rather than the string
 * representation.
 *
 * Return value: The CalComponent of the sought object, or NULL if no object
 * had the specified UID.
 **/
CalComponent *
cal_backend_get_object_component (CalBackend *backend, const char *uid)
{
    g_return_val_if_fail (backend != NULL, NULL);
    g_return_val_if_fail (IS_CAL_BACKEND (backend), NULL);
    g_return_val_if_fail (uid != NULL, NULL);

    g_assert (CLASS (backend)->get_object_component != NULL);
    return (* CLASS (backend)->get_object_component) (backend, uid);
}

/**
 * cal_backend_get_timezone_object:
 * @backend: A calendar backend.
 * @tzid: Unique identifier for a calendar VTIMEZONE object.
 *
 * Queries a calendar backend for a VTIMEZONE calendar object based on its
 * unique TZID identifier.
 *
 * Return value: The string representation of a VTIMEZONE component, or NULL
 * if no VTIMEZONE object had the specified TZID.
 **/
char *
cal_backend_get_timezone_object (CalBackend *backend, const char *tzid)
{
    g_return_val_if_fail (backend != NULL, NULL);
    g_return_val_if_fail (IS_CAL_BACKEND (backend), NULL);
    g_return_val_if_fail (tzid != NULL, NULL);

    g_assert (CLASS (backend)->get_timezone_object != NULL);
    return (* CLASS (backend)->get_timezone_object) (backend, tzid);
}

/**
 * cal_backend_get_type_by_uid
 * @backend: A calendar backend.
 * @uid: Unique identifier for a Calendar object.
 *
 * Returns the type of the object identified by the @uid argument
 */
CalObjType
cal_backend_get_type_by_uid (CalBackend *backend, const char *uid)
{
    icalcomponent *icalcomp;
    char *comp_str;
    CalObjType type = CAL_COMPONENT_NO_TYPE;

    g_return_val_if_fail (IS_CAL_BACKEND (backend), CAL_COMPONENT_NO_TYPE);
    g_return_val_if_fail (uid != NULL, CAL_COMPONENT_NO_TYPE);

    comp_str = cal_backend_get_object (backend, uid);
    if (!comp_str)
        return CAL_COMPONENT_NO_TYPE;

    icalcomp = icalparser_parse_string (comp_str);
    if (icalcomp) {
        switch (icalcomponent_isa (icalcomp)) {
        case ICAL_VEVENT_COMPONENT :
            type = CALOBJ_TYPE_EVENT;
            break;
        case ICAL_VTODO_COMPONENT :
            type = CALOBJ_TYPE_TODO;
            break;
        case ICAL_VJOURNAL_COMPONENT :
            type = CALOBJ_TYPE_JOURNAL;
            break;
        default :
            type = CAL_COMPONENT_NO_TYPE;
        }

        icalcomponent_free (icalcomp);
    }

    g_free (comp_str);

    return type;
}

/**
 * cal_backend_get_uids:
 * @backend: A calendar backend.
 * @type: Bitmask with types of objects to return.
 *
 * Builds a list of unique identifiers corresponding to calendar objects whose
 * type matches one of the types specified in the @type flags.
 *
 * Return value: A list of strings that are the sought UIDs.  The list should be
 * freed using the cal_obj_uid_list_free() function.
 **/
GList *
cal_backend_get_uids (CalBackend *backend, CalObjType type)
{
    g_return_val_if_fail (backend != NULL, NULL);
    g_return_val_if_fail (IS_CAL_BACKEND (backend), NULL);

    g_assert (CLASS (backend)->get_uids != NULL);
    return (* CLASS (backend)->get_uids) (backend, type);
}


/**
 * cal_backend_get_objects_in_range:
 * @backend: A calendar backend.
 * @type: Bitmask with types of objects to return.
 * @start: Start time for query.
 * @end: End time for query.
 * 
 * Builds a list of unique identifiers corresponding to calendar objects of the
 * specified type that occur or recur within the specified time range.
 * 
 * Return value: A list of UID strings.  The list should be freed using the
 * cal_obj_uid_list_free() function.
 **/
GList *
cal_backend_get_objects_in_range (CalBackend *backend, CalObjType type,
                  time_t start, time_t end)
{
    g_return_val_if_fail (backend != NULL, NULL);
    g_return_val_if_fail (IS_CAL_BACKEND (backend), NULL);
    g_return_val_if_fail (start != -1 && end != -1, NULL);
    g_return_val_if_fail (start <= end, NULL);

    g_assert (CLASS (backend)->get_objects_in_range != NULL);
    return (* CLASS (backend)->get_objects_in_range) (backend, type, start, end);
}

/**
 * cal_backend_get_free_busy:
 * @backend: A calendar backend.
 * @users: List of users to get free/busy information for.
 * @start: Start time for query.
 * @end: End time for query.
 * 
 * Gets a free/busy object for the given time interval
 * 
 * Return value: a list of CalObj's
 **/
GList *
cal_backend_get_free_busy (CalBackend *backend, GList *users, time_t start, time_t end)
{
    g_return_val_if_fail (backend != NULL, NULL);
    g_return_val_if_fail (IS_CAL_BACKEND (backend), NULL);
    g_return_val_if_fail (start != -1 && end != -1, NULL);
    g_return_val_if_fail (start <= end, NULL);

    g_assert (CLASS (backend)->get_free_busy != NULL);
    return (* CLASS (backend)->get_free_busy) (backend, users, start, end);
}

/**
 * cal_backend_get_changes:
 * @backend: A calendar backend
 * @type: Bitmask with types of objects to return.
 * @change_id: A unique uid for the callers change list
 * 
 * Builds a sequence of objects and the type of change that occurred on them since
 * the last time the give change_id was seen
 * 
 * Return value: A list of the objects that changed and the type of change
 **/
GNOME_Evolution_Calendar_CalObjChangeSeq *
cal_backend_get_changes (CalBackend *backend, CalObjType type, const char *change_id) 
{
    g_return_val_if_fail (backend != NULL, NULL);
    g_return_val_if_fail (IS_CAL_BACKEND (backend), NULL);
    g_return_val_if_fail (change_id != NULL, NULL);

    g_assert (CLASS (backend)->get_changes != NULL);
    return (* CLASS (backend)->get_changes) (backend, type, change_id);
}

/**
 * cal_backend_get_alarms_in_range:
 * @backend: A calendar backend.
 * @start: Start time for query.
 * @end: End time for query.
 * @valid_range: Return value that says whether the range is valid or not.
 * 
 * Builds a sorted list of the alarms that trigger in the specified time range.
 * 
 * Return value: A sequence of component alarm instances structures, or NULL
 * if @valid_range returns FALSE.
 **/
GNOME_Evolution_Calendar_CalComponentAlarmsSeq *
cal_backend_get_alarms_in_range (CalBackend *backend, time_t start, time_t end, gboolean *valid_range)
{
    g_return_val_if_fail (backend != NULL, NULL);
    g_return_val_if_fail (IS_CAL_BACKEND (backend), NULL);
    g_return_val_if_fail (valid_range != NULL, NULL);

    g_assert (CLASS (backend)->get_alarms_in_range != NULL);

    if (!(start != -1 && end != -1 && start <= end)) {
        *valid_range = FALSE;
        return NULL;
    } else {
        *valid_range = TRUE;
        return (* CLASS (backend)->get_alarms_in_range) (backend, start, end);
    }
}

/**
 * cal_backend_get_alarms_for_object:
 * @backend: A calendar backend.
 * @uid: Unique identifier for a calendar object.
 * @start: Start time for query.
 * @end: End time for query.
 * @result: Return value for the result code for the operation.
 * 
 * Builds a sorted list of the alarms of the specified event that trigger in a
 * particular time range.
 * 
 * Return value: A structure of the component's alarm instances, or NULL if @result
 * returns something other than #CAL_BACKEND_GET_ALARMS_SUCCESS.
 **/
GNOME_Evolution_Calendar_CalComponentAlarms *
cal_backend_get_alarms_for_object (CalBackend *backend, const char *uid,
                   time_t start, time_t end,
                   CalBackendGetAlarmsForObjectResult *result)
{
    g_return_val_if_fail (backend != NULL, NULL);
    g_return_val_if_fail (IS_CAL_BACKEND (backend), NULL);
    g_return_val_if_fail (uid != NULL, NULL);
    g_return_val_if_fail (result != NULL, NULL);

    g_assert (CLASS (backend)->get_alarms_for_object != NULL);

    if (!(start != -1 && end != -1 && start <= end)) {
        *result = CAL_BACKEND_GET_ALARMS_INVALID_RANGE;
        return NULL;
    } else {
        gboolean object_found;
        GNOME_Evolution_Calendar_CalComponentAlarms *alarms;

        alarms = (* CLASS (backend)->get_alarms_for_object) (backend, uid, start, end,
                                     &object_found);

        if (object_found)
            *result = CAL_BACKEND_GET_ALARMS_SUCCESS;
        else
            *result = CAL_BACKEND_GET_ALARMS_NOT_FOUND;

        return alarms;
    }
}

/**
 * cal_backend_update_objects:
 * @backend: A calendar backend.
 * @calobj: String representation of the new calendar object(s).
 * 
 * Updates an object in a calendar backend.  It will replace any existing
 * object that has the same UID as the specified one.  The backend will in
 * turn notify all of its clients about the change.
 * 
 * Return value: TRUE on success, FALSE on being passed an invalid object or one
 * with an unsupported type.
 **/
gboolean
cal_backend_update_objects (CalBackend *backend, const char *calobj)
{
    g_return_val_if_fail (backend != NULL, FALSE);
    g_return_val_if_fail (IS_CAL_BACKEND (backend), FALSE);
    g_return_val_if_fail (calobj != NULL, FALSE);

    g_assert (CLASS (backend)->update_objects != NULL);
    return (* CLASS (backend)->update_objects) (backend, calobj);
}

/**
 * cal_backend_remove_object:
 * @backend: A calendar backend.
 * @uid: Unique identifier of the object to remove.
 * 
 * Removes an object in a calendar backend.  The backend will notify all of its
 * clients about the change.
 * 
 * Return value: TRUE on success, FALSE on being passed an UID for an object
 * that does not exist in the backend.
 **/
gboolean
cal_backend_remove_object (CalBackend *backend, const char *uid)
{
    g_return_val_if_fail (backend != NULL, FALSE);
    g_return_val_if_fail (IS_CAL_BACKEND (backend), FALSE);
    g_return_val_if_fail (uid != NULL, FALSE);

    g_assert (CLASS (backend)->remove_object != NULL);
    return (* CLASS (backend)->remove_object) (backend, uid);
}

/**
 * cal_backend_last_client_gone:
 * @backend: A calendar backend.
 * 
 * Emits the "last_client_gone" signal of a calendar backend.  This function is
 * to be used only by backend implementations.
 **/
void
cal_backend_last_client_gone (CalBackend *backend)
{
    g_return_if_fail (backend != NULL);
    g_return_if_fail (IS_CAL_BACKEND (backend));

    gtk_signal_emit (GTK_OBJECT (backend), cal_backend_signals[LAST_CLIENT_GONE]);
}

/**
 * cal_backend_opened:
 * @backend: A calendar backend.
 * @status: Open status code.
 * 
 * Emits the "opened" signal of a calendar backend.  This function is to be used
 * only by backend implementations.
 **/
void
cal_backend_opened (CalBackend *backend, CalBackendOpenStatus status)
{
    g_return_if_fail (backend != NULL);
    g_return_if_fail (IS_CAL_BACKEND (backend));

    gtk_signal_emit (GTK_OBJECT (backend), cal_backend_signals[OPENED],
             status);
}

/**
 * cal_backend_obj_updated:
 * @backend: A calendar backend.
 * @uid: Unique identifier of the component that was updated.
 * 
 * Emits the "obj_updated" signal of a calendar backend.  This function is to be
 * used only by backend implementations.
 **/
void
cal_backend_obj_updated (CalBackend *backend, const char *uid)
{
    g_return_if_fail (backend != NULL);
    g_return_if_fail (IS_CAL_BACKEND (backend));
    g_return_if_fail (uid != NULL);

    gtk_signal_emit (GTK_OBJECT (backend), cal_backend_signals[OBJ_UPDATED],
             uid);
}

/**
 * cal_backend_obj_removed:
 * @backend: A calendar backend.
 * @uid: Unique identifier of the component that was removed.
 * 
 * Emits the "obj_removed" signal of a calendar backend.  This function is to be
 * used only by backend implementations.
 **/
void
cal_backend_obj_removed (CalBackend *backend, const char *uid)
{
    g_return_if_fail (backend != NULL);
    g_return_if_fail (IS_CAL_BACKEND (backend));
    g_return_if_fail (uid != NULL);

    gtk_signal_emit (GTK_OBJECT (backend), cal_backend_signals[OBJ_REMOVED],
             uid);
}


/**
 * cal_backend_get_timezone:
 * @backend: A calendar backend.
 * @tzid: Unique identifier of a VTIMEZONE object. Note that this must not be
 * NULL.
 * 
 * Returns the icaltimezone* corresponding to the TZID, or NULL if the TZID
 * can't be found.
 * 
 * Return value: TRUE on success, FALSE on being passed an UID for an object
 * that does not exist in the backend.
 **/
icaltimezone*
cal_backend_get_timezone (CalBackend *backend, const char *tzid)
{
    g_return_val_if_fail (backend != NULL, NULL);
    g_return_val_if_fail (IS_CAL_BACKEND (backend), NULL);
    g_return_val_if_fail (tzid != NULL, NULL);

    g_assert (CLASS (backend)->get_timezone != NULL);
    return (* CLASS (backend)->get_timezone) (backend, tzid);
}