##############################################################################
#
# Copyright (c) 2002 Zope Corporation and Contributors. All Rights Reserved.
#
# This software is subject to the provisions of the Zope Public License,
# Version 2.1 (ZPL).  A copy of the ZPL should accompany this distribution.
# THIS SOFTWARE IS PROVIDED "AS IS" AND ANY AND ALL EXPRESS OR IMPLIED
# WARRANTIES ARE DISCLAIMED, INCLUDING, BUT NOT LIMITED TO, THE IMPLIED
# WARRANTIES OF TITLE, MERCHANTABILITY, AGAINST INFRINGEMENT, AND FITNESS
# FOR A PARTICULAR PURPOSE.
#
##############################################################################
"""Property sheets

$Id: PropertySheets.py 40300 2005-11-21 16:54:03Z efge $
"""
import time,  App.Management, Globals, sys
from webdav.interfaces import IWriteLock
from webdav.WriteLockInterface import WriteLockInterface
from ZPublisher.Converters import type_converters
from Globals import InitializeClass
from Globals import DTMLFile, MessageDialog
from Acquisition import Implicit, Explicit
from App.Common import rfc1123_date, iso8601_date
from webdav.common import urlbase
from ExtensionClass import Base
from Globals import Persistent
from Traversable import Traversable
from Acquisition import aq_base
from AccessControl import ClassSecurityInfo
from AccessControl.Permissions import access_contents_information
from AccessControl.Permissions import manage_properties
from AccessControl.Permissions import view_management_screens
from AccessControl import getSecurityManager
from webdav.common import isDavCollection
from zExceptions import BadRequest, Redirect
from cgi import escape
from types import ListType

# DM: we would like to import this from somewhere
BadRequestException= 'Bad Request'


class View(App.Management.Tabs, Base):
    """A view of an object, typically used for management purposes

    This class provides bits of management framework needed by propertysheets
    to be used as a view on an object.
    """

    def manage_workspace(self, URL1, RESPONSE):
        '''Implement a "management" interface
        '''
        RESPONSE.redirect(URL1+'/manage')

    def tpURL(self): return self.getId()

    def manage_options(self):
        """Return a manage option data structure for me instance
        """
        try: r=self.REQUEST
        except: r=None
        if r is None:
            pre='../../'
        else:
            pre=r['URL']
            for i in (1,2,3):
                l=pre.rfind('/')
                if l >= 0:
                    pre=pre[:l]
            pre=pre+'/'

        r=[]
        for d in self.aq_parent.aq_parent.manage_options:
            path=d['action']
            option={'label': d['label'],
                      'action': pre+path,
                      'path': '../../'+path}
            help=d.get('help')
            if help is not None:
                option['help']=help
            r.append(option)
        return r

    def tabs_path_info(self, script, path):
        l=path.rfind('/')
        if l >= 0:
            path=path[:l]
            l=path.rfind('/')
            if l >= 0: path=path[:l]
        return View.inheritedAttribute('tabs_path_info')(
            self, script, path)

    def meta_type(self):
        try: return self.aq_parent.aq_parent.meta_type
        except: return ''


class PropertySheet(Traversable, Persistent, Implicit):
    """A PropertySheet is a container for a set of related properties and
       metadata describing those properties. PropertySheets may or may not
       provide a web interface for managing its properties."""

    _properties=()
    _extensible=1
    icon='p_/Properties_icon'

    security = ClassSecurityInfo()
    security.declareObjectProtected(access_contents_information)
    security.setPermissionDefault(access_contents_information,
                                  ('Anonymous', 'Manager'))

    __reserved_ids= ('values','items')

    def property_extensible_schema__(self):
        """Return a flag indicating whether new properties may be
        added or removed."""
        return self._extensible

    def __init__(self, id, md=None):
        # Create a new property set, using the given id and namespace
        # string. The namespace string should be usable as an xml name-
        # space identifier.

        if id in self.__reserved_ids:
            raise ValueError(
                  "'%s' is a reserved Id (forbidden Ids are: %s) " % (
                  id, self.__reserved_ids
                  ))

        self.id=id
        self._md=md or {}

    def getId(self):
        return self.id

    security.declareProtected(access_contents_information, 'xml_namespace')
    def xml_namespace(self):
        # Return a namespace string usable as an xml namespace
        # for this property set.
        return self._md.get('xmlns', '')

    def v_self(self):
        return self

    def p_self(self):
        return self.v_self()

    def valid_property_id(self, id):
        if not id or id[:1]=='_' or (id[:3]=='aq_') \
           or (' ' in id) or escape(id) != id:
            return 0
        return 1

    security.declareProtected(access_contents_information, 'hasProperty')
    def hasProperty(self, id):
        # Return a true value if a property exists with the given id.
        for prop in self._propertyMap():
            if id==prop['id']:
                return 1
        return 0

    security.declareProtected(access_contents_information, 'getProperty')
    def getProperty(self, id, default=None):
        # Return the property with the given id, returning the optional
        # second argument or None if no such property is found.
        if self.hasProperty(id):
            return getattr(self.v_self(), id)
        return default

    security.declareProtected(access_contents_information, 'getPropertyType')
    def getPropertyType(self, id):
        """Get the type of property 'id', returning None if no
           such property exists"""
        pself=self.p_self()
        for md in pself._properties:
            if md['id']==id:
                return md.get('type', 'string')
        return None

    def _wrapperCheck(self, object):
        # Raise an error if an object is wrapped.
        if hasattr(object, 'aq_base'):
            raise ValueError, 'Invalid property value: wrapped object'
        return

    def _setProperty(self, id, value, type='string', meta=None):
        # Set a new property with the given id, value and optional type.
        # Note that different property sets may support different typing
        # systems.
        self._wrapperCheck(value)
        if not self.valid_property_id(id):
            raise BadRequest, 'Invalid property id, %s.' % escape(id)

        if not self.property_extensible_schema__():
            raise BadRequest, (
                'Properties cannot be added to this property sheet')
        pself=self.p_self()
        self=self.v_self()
        if hasattr(aq_base(self),id):
            if not (id=='title' and not self.__dict__.has_key(id)):
                raise BadRequest, (
                    'Invalid property id, <em>%s</em>. It is in use.' %
                        escape(id))
        if meta is None: meta={}
        prop={'id':id, 'type':type, 'meta':meta}
        pself._properties=pself._properties+(prop,)
        if type in ('selection', 'multiple selection'):
            if not value:
                raise BadRequest, (
                    'The value given for a new selection property '
                    'must be a variable name<p>')
            prop['select_variable']=value
            if type=='selection': value=None
            else: value=[]

        # bleah - can't change kw name in api, so use ugly workaround.
        if sys.modules['__builtin__'].type(value) == ListType:
            value = tuple(value)
        setattr(self, id, value)

    def _updateProperty(self, id, value, meta=None):
        # Update the value of an existing property. If value is a string,
        # an attempt will be made to convert the value to the type of the
        # existing property. If a mapping containing meta-data is passed,
        # it will used to _replace_ the properties meta data.
        self._wrapperCheck(value)
        if not self.hasProperty(id):
            raise BadRequest, 'The property %s does not exist.' % escape(id)
        propinfo=self.propertyInfo(id)
        if not 'w' in propinfo.get('mode', 'wd'):
            raise BadRequest, '%s cannot be changed.' % escape(id)
        if type(value)==type(''):
            proptype=propinfo.get('type', 'string')
            if type_converters.has_key(proptype):
                value=type_converters[proptype](value)
        if meta is not None:
            props=[]
            pself=self.p_self()
            for prop in pself._properties:
                if prop['id']==id: prop['meta']=meta
                props.append(prop)
            pself._properties=tuple(props)

        if type(value) == ListType:
            value = tuple(value)
        setattr(self.v_self(), id, value)

    def _delProperty(self, id):
        # Delete the property with the given id. If a property with the
        # given id does not exist, a ValueError is raised.
        if not self.hasProperty(id):
            raise BadRequest, 'The property %s does not exist.' % escape(id)
        vself=self.v_self()
        if hasattr(vself, '_reserved_names'):
            nd=vself._reserved_names
        else: nd=()
        if (not 'd' in self.propertyInfo(id).get('mode', 'wd')) or (id in nd):
            raise BadRequest, '%s cannot be deleted.' % escape(id)
        delattr(vself, id)
        pself=self.p_self()
        pself._properties=tuple(filter(lambda i, n=id: i['id'] != n,
                                       pself._properties))

    security.declareProtected(access_contents_information, 'propertyIds')
    def propertyIds(self):
        # Return a list of property ids.
        return map(lambda i: i['id'], self._propertyMap())

    security.declareProtected(access_contents_information, 'propertyValues')
    def propertyValues(self):
        # Return a list of property values.
        return map(lambda i, s=self: s.getProperty(i['id']),
                   self._propertyMap())

    security.declareProtected(access_contents_information, 'propertyItems')
    def propertyItems(self):
        # Return a list of (id, property) tuples.
        return map(lambda i, s=self: (i['id'], s.getProperty(i['id'])),
                   self._propertyMap())

    security.declareProtected(access_contents_information, 'propertyInfo')
    def propertyInfo(self, id):
        # Return a mapping containing property meta-data
        for p in self._propertyMap():
            if p['id']==id: return p
        raise ValueError, 'The property %s does not exist.' % escape(id)

    def _propertyMap(self):
        # Return a tuple of mappings, giving meta-data for properties.
        # Some ZClass instances dont seem to have an _properties, so
        # we have to fake it...
        return self.p_self()._properties

    security.declareProtected(access_contents_information, 'propertyMap')
    def propertyMap(self):
        # Returns a secure copy of the property definitions.
        return tuple(map(lambda dict: dict.copy(), self._propertyMap()))

    def _propdict(self):
        dict={}
        for p in self._propertyMap():
            dict[p['id']]=p
        return dict

    propstat='<d:propstat xmlns:n="%s">\n' \
             '  <d:prop>\n' \
             '%s\n' \
             '  </d:prop>\n' \
             '  <d:status>HTTP/1.1 %s</d:status>\n%s' \
             '</d:propstat>\n'

    propdesc='  <d:responsedescription>\n' \
             '  %s\n' \
             '  </d:responsedescription>\n'

    def dav__allprop(self, propstat=propstat ):
        # DAV helper method - return one or more propstat elements
        # indicating property names and values for all properties.
        result=[]
        for item in self._propertyMap():
            name, type=item['id'], item.get('type','string')
            value=self.getProperty(name)

            if type=='tokens':
                value=' '.join(map(str, value))
            elif type=='lines':
                value='\n'.join(map(str, value))
            # check for xml property
            attrs=item.get('meta', {}).get('__xml_attrs__', None)
            if attrs is not None:
                # It's a xml property. Don't escape value.
                attrs=map(lambda n: ' %s="%s"' % n, attrs.items())
                attrs=''.join(attrs)
            else:
                # It's a non-xml property. Escape value.
                attrs=''
                if not hasattr(self,"dav__"+name):
                    value = xml_escape(value)
            prop='  <n:%s%s>%s</n:%s>' % (name, attrs, value, name)

            result.append(prop)
        if not result: return ''
        result='\n'.join(result)

        return propstat % (self.xml_namespace(), result, '200 OK', '')

    def dav__propnames(self, propstat=propstat):
        # DAV helper method - return a propstat element indicating
        # property names for all properties in this PropertySheet.
        result=[]
        for name in self.propertyIds():
            result.append('  <n:%s/>' % name)
        if not result: return ''
        result='\n'.join(result)
        return propstat % (self.xml_namespace(), result, '200 OK', '')


    def dav__propstat(self, name, result,
                      propstat=propstat, propdesc=propdesc):
        # DAV helper method - return a propstat element indicating
        # property name and value for the requested property.
        xml_id=self.xml_namespace()
        propdict=self._propdict()
        if not propdict.has_key(name):
            prop='<n:%s xmlns:n="%s"/>\n' % (name, xml_id)
            code='404 Not Found'
            if not result.has_key(code):
                result[code]=[prop]
            else: result[code].append(prop)
            return
        else:
            item=propdict[name]
            name, type=item['id'], item.get('type','string')
            value=self.getProperty(name)
            if type=='tokens':
                value=' '.join(map(str, value))
            elif type=='lines':
                value='\n'.join(map(str, value))
            # allow for xml properties
            attrs=item.get('meta', {}).get('__xml_attrs__', None)
            if attrs is not None:
                # It's a xml property. Don't escape value.
                attrs=map(lambda n: ' %s="%s"' % n, attrs.items())
                attrs=''.join(attrs)
            else:
                # It's a non-xml property. Escape value.
                attrs=''
                if not hasattr(self, 'dav__%s' % name):
                    value = xml_escape(value)
            prop='<n:%s%s xmlns:n="%s">%s</n:%s>\n' % (
                name, attrs, xml_id, value, name)
            code='200 OK'
            if not result.has_key(code):
                result[code]=[prop]
            else: result[code].append(prop)
            return

    del propstat
    del propdesc


    # Web interface

    manage=DTMLFile('dtml/properties', globals())

    security.declareProtected(manage_properties, 'manage_propertiesForm')
    def manage_propertiesForm(self, URL1):
        " "
        raise Redirect, URL1+'/manage'

    security.declareProtected(manage_properties, 'manage_addProperty')
    def manage_addProperty(self, id, value, type, REQUEST=None):
        """Add a new property via the web. Sets a new property with
        the given id, type, and value."""
        if type_converters.has_key(type):
            value=type_converters[type](value)
        self._setProperty(id, value, type)
        if REQUEST is not None:
            return self.manage(self, REQUEST)

    security.declareProtected(manage_properties, 'manage_editProperties')
    def manage_editProperties(self, REQUEST):
        """Edit object properties via the web."""
        for prop in self._propertyMap():
            name=prop['id']
            if 'w' in prop.get('mode', 'wd'):
                value=REQUEST.get(name, '')
                self._updateProperty(name, value)
        return MessageDialog(
               title  ='Success!',
               message='Your changes have been saved',
               action ='manage')

    security.declareProtected(manage_properties, 'manage_changeProperties')
    def manage_changeProperties(self, REQUEST=None, **kw):
        """Change existing object properties by passing either a mapping
           object of name:value pairs {'foo':6} or passing name=value
           parameters."""
        if REQUEST is None:
            props={}
        else: props=REQUEST
        if kw:
            for name, value in kw.items():
                props[name]=value
        propdict=self._propdict()
        for name, value in props.items():
            if self.hasProperty(name):
                if not 'w' in propdict[name].get('mode', 'wd'):
                    raise BadRequest, '%s cannot be changed' % escape(name)
                self._updateProperty(name, value)
        if REQUEST is not None:
            return MessageDialog(
                title  ='Success!',
                message='Your changes have been saved.',
                action ='manage')

    security.declareProtected(manage_properties, 'manage_delProperties')
    def manage_delProperties(self, ids=None, REQUEST=None):
        """Delete one or more properties specified by 'ids'."""
        if REQUEST:
            # Bugfix for properties named "ids" (casey)
            if ids == self.getProperty('ids', None): ids = None
            ids = REQUEST.get('_ids', ids)
        if ids is None:
            return MessageDialog(
                   title='No property specified',
                   message='No properties were specified!',
                   action ='./manage',)
        for id in ids:
            self._delProperty(id)
        if REQUEST is not None:
            return self.manage(self, REQUEST)

InitializeClass(PropertySheet)


class Virtual:
    """A virtual propertysheet stores it's properties in it's instance."""

    def __init__(self):
        pass

    def v_self(self):
        return self.aq_parent.aq_parent


class DefaultProperties(Virtual, PropertySheet, View):
    """The default property set mimics the behavior of old-style Zope
       properties -- it stores its property values in the instance of
       its owner."""

    id='default'
    _md={'xmlns': 'http://www.zope.org/propsets/default'}

InitializeClass(DefaultProperties)


class DAVProperties(Virtual, PropertySheet, View):
    """WebDAV properties"""

    id='webdav'
    _md={'xmlns': 'DAV:'}
    pm=({'id':'creationdate',     'mode':'r'},
        {'id':'displayname',      'mode':'r'},
        {'id':'resourcetype',     'mode':'r'},
        {'id':'getcontenttype',   'mode':'r'},
        {'id':'getcontentlength', 'mode':'r'},
        {'id':'source',           'mode':'r'},
        {'id':'supportedlock',    'mode':'r'},
        {'id':'lockdiscovery',    'mode':'r'},
        )

    def getProperty(self, id, default=None):
        method='dav__%s' % id
        if not hasattr(self, method):
            return default
        return getattr(self, method)()

    def _setProperty(self, id, value, type='string', meta=None):
        raise ValueError, '%s cannot be set.' % escape(id)

    def _updateProperty(self, id, value):
        raise ValueError, '%s cannot be updated.' % escape(id)

    def _delProperty(self, id):
        raise ValueError, '%s cannot be deleted.' % escape(id)

    def _propertyMap(self):
        # Only use getlastmodified if returns a value
        if hasattr(self.v_self(), '_p_mtime'):
            return self.pm + ({'id':'getlastmodified',  'mode':'r'},)
        return self.pm

    def propertyMap(self):
        return map(lambda dict: dict.copy(), self._propertyMap())

    def dav__creationdate(self):
        return iso8601_date(43200.0)

    def dav__displayname(self):
        return absattr(xml_escape(self.v_self().title_or_id()))

    def dav__resourcetype(self):
        vself=self.v_self()
        if isDavCollection(vself):
            return '<n:collection/>'
        return ''

    def dav__getlastmodified(self):
        return rfc1123_date(self.v_self()._p_mtime)

    def dav__getcontenttype(self):
        vself=self.v_self()
        if hasattr(vself, 'content_type'):
            return absattr(vself.content_type)
        if hasattr(vself, 'default_content_type'):
            return absattr(vself.default_content_type)
        return ''

    def dav__getcontentlength(self):
        vself=self.v_self()
        if hasattr(vself, 'get_size'):
            return vself.get_size()
        return ''

    def dav__source(self):
        vself=self.v_self()
        if hasattr(vself, 'document_src'):
            url=urlbase(vself.absolute_url())
            return '\n  <n:link>\n' \
                   '  <n:src>%s</n:src>\n' \
                   '  <n:dst>%s/document_src</n:dst>\n' \
                   '  </n:link>\n  ' % (url, url)
        return ''

    def dav__supportedlock(self):
        vself = self.v_self()
        out = '\n'
        if IWriteLock.providedBy(vself) or \
                WriteLockInterface.isImplementedBy(vself):
            out += ('  <n:lockentry>\n'
                    '  <d:lockscope><d:exclusive/></d:lockscope>\n'
                    '  <d:locktype><d:write/></d:locktype>\n'
                    '  </n:lockentry>\n  ')
        return out

    def dav__lockdiscovery(self):
        security = getSecurityManager()
        user = security.getUser().getId()

        vself = self.v_self()
        out = '\n'
        if IWriteLock.providedBy(vself) or \
                WriteLockInterface.isImplementedBy(vself):
            locks = vself.wl_lockValues(killinvalids=1)
            for lock in locks:

                creator = lock.getCreator()[-1]
                if creator == user: fake=0
                else:               fake=1

                out = '%s\n%s' % (out, lock.asLockDiscoveryProperty('n',fake=fake))

            out = '%s\n' % out

        return out

InitializeClass(DAVProperties)


class PropertySheets(Traversable, Implicit, App.Management.Tabs):
    """A tricky container to keep property sets from polluting
       an object's direct attribute namespace."""

    id='propertysheets'

    security = ClassSecurityInfo()
    security.declareObjectProtected(access_contents_information)
    security.setPermissionDefault(access_contents_information,
                                  ('Anonymous', 'Manager'))

    # optionally to be overridden by derived classes
    PropertySheetClass= PropertySheet

    webdav =DAVProperties()
    def _get_defaults(self):
        return (self.webdav,)

    def __propsets__(self):
        propsets=self.aq_parent.__propsets__
        __traceback_info__= propsets, type(propsets)
        return self._get_defaults() + propsets

    def __bobo_traverse__(self, REQUEST, name=None):
        for propset in self.__propsets__():
            if propset.getId()==name:
                return propset.__of__(self)
        return getattr(self, name)

    def __getitem__(self, n):
        return self.__propsets__()[n].__of__(self)

    security.declareProtected(access_contents_information, 'values')
    def values(self):
        propsets=self.__propsets__()
        return map(lambda n, s=self: n.__of__(s), propsets)

    security.declareProtected(access_contents_information, 'items')
    def items(self):
        propsets=self.__propsets__()
        r=[]
        for n in propsets:
            if hasattr(n,'id'): id=n.id
            else: id=''
            r.append((id, n.__of__(self)))

        return r

    security.declareProtected(access_contents_information, 'get')
    def get(self, name, default=None):
        for propset in self.__propsets__():
            if propset.id==name or (hasattr(propset, 'xml_namespace') and \
                                    propset.xml_namespace()==name):
                return propset.__of__(self)
        return default

    security.declareProtected(manage_properties, 'manage_addPropertySheet')
    def manage_addPropertySheet(self, id, ns, REQUEST=None):
        """ """
        md={'xmlns':ns}
        ps= self.PropertySheetClass(id, md)
        self.addPropertySheet(ps)
        if REQUEST is None: return ps
        ps= self.get(id)
        REQUEST.RESPONSE.redirect('%s/manage' % ps.absolute_url())

    security.declareProtected(manage_properties, 'addPropertySheet')
    def addPropertySheet(self, propset):
        propsets=self.aq_parent.__propsets__
        propsets=propsets+(propset,)
        self.aq_parent.__propsets__=propsets

    security.declareProtected(manage_properties, 'delPropertySheet')
    def delPropertySheet(self, name):
        result=[]
        for propset in self.aq_parent.__propsets__:
            if propset.getId() != name and  propset.xml_namespace() != name:
                result.append(propset)
        self.aq_parent.__propsets__=tuple(result)

    ## DM: deletion support
    def isDeletable(self,name):
        '''currently, we say that *name* is deletable when it is not a
        default sheet. Later, we may further restrict deletability
        based on an instance attribute.'''
        ps= self.get(name)
        if ps is None: return 0
        if ps in self._get_defaults(): return 0
        return 1

    def manage_delPropertySheets(self, ids=(), REQUEST=None):
        '''delete all sheets identified by *ids*.'''
        for id in ids:
            if not self.isDeletable(id):
                raise BadRequestException, 'attempt to delete undeletable property sheet: ' + id
            self.delPropertySheet(id)
        if REQUEST is not None: 
            REQUEST.RESPONSE.redirect('%s/manage' % self.absolute_url())

    def __len__(self):
        return len(self.__propsets__())

    def getId(self):
        return self.id


    # Management interface:

    security.declareProtected(view_management_screens, 'manage')
    manage=Globals.DTMLFile('dtml/propertysheets', globals())

    def manage_options(self):
        """Return a manage option data structure for me instance
        """
        try: r=self.REQUEST
        except: r=None
        if r is None:
            pre='../'
        else:
            pre=r['URLPATH0']
            for i in (1,2):
                l=pre.rfind('/')
                if l >= 0:
                    pre=pre[:l]
            pre=pre+'/'

        r=[]
        for d in self.aq_parent.manage_options:
            r.append({'label': d['label'], 'action': pre+d['action']})
        return r

    def tabs_path_info(self, script, path):
        l=path.rfind('/')
        if l >= 0: path=path[:l]
        return PropertySheets.inheritedAttribute('tabs_path_info')(
            self, script, path)

InitializeClass(PropertySheets)


class DefaultPropertySheets(PropertySheets):
    """A PropertySheets container that contains a default property
       sheet for compatibility with the arbitrary property mgmt
       design of Zope PropertyManagers."""
    default=DefaultProperties()
    webdav =DAVProperties()
    def _get_defaults(self):
        return (self.default, self.webdav)

InitializeClass(DefaultPropertySheets)


class FixedSchema(PropertySheet):
    """A Fixed-schema property sheet has no control over it's schema

    It gets its schema from another proprtysheet but has control over
    its value storage.

    This mix-in is used for ZClass instance proprtysheets, which store
    their data in instances, but get their schema from the
    proprtysheet managed in the ZClass.
    """

    def __init__(self, id, base, md=None):
        FixedSchema.inheritedAttribute('__init__')(self, id, md)
        self._base=base

    def _propertyMap(self):
        # Return a tuple of mappings, giving meta-data for properties.
        r = []
        for d in self._base._propertyMap():
            d = d.copy()
            mode = d.get('mode', 'wd')
            if 'd' in mode:
                d['mode']=filter(lambda c: c != 'd', mode)
            r.append(d)

        return tuple(r)

    def propertyMap(self):
        return self._propertyMap()

    def property_extensible_schema__(self):
        return 0
        return self._base._extensible

InitializeClass(FixedSchema)


class vps(Base):
    """Virtual Propertysheets

    The vps object implements a "computed attribute" - it ensures
    that a PropertySheets instance is returned when the propertysheets
    attribute of a PropertyManager is accessed.
    """
    def __init__(self, c=PropertySheets):
        self.c=c

    def __of__(self, parent):
        return self.c().__of__(parent)

def absattr(attr):
    if callable(attr):
        return attr()
    return attr

def xml_escape(value):
    from webdav.xmltools import escape
    if not isinstance(value, basestring):
        value = unicode(value)
    if not isinstance(value, unicode):
        # XXX It really shouldn't be hardcoded to latin-1 here.
        value = value.decode('latin-1')
    value = escape(value)
    return value.encode('utf-8')