Parse/calc length, converting to pixels, calls callback(length, *args)
        when the length is first computed or changes

def _parse_length(self, value, font_relative, callback, *args):
        '''Parse/calc length, converting to pixels, calls callback(length, *args)
        when the length is first computed or changes'''
        if value.endswith('%'):
            frac = float(value[:-1])/100
            if font_relative:
                attrs = self._get_current_attributes()
                font_size = attrs.font.get_size() / pango.SCALE
                callback(frac*display_resolution*font_size, *args)
            else:
                ## CSS says "Percentage values: refer to width of the closest
                ##           block-level ancestor"
                ## This is difficult/impossible to implement, so we use
                ## textview width instead; a reasonable approximation..
                alloc = self.textview.get_allocation()
                self.__parse_length_frac_size_allocate(self.textview, alloc,
                                                       frac, callback, args)
                self.textview.connect("size-allocate",
                                      self.__parse_length_frac_size_allocate,
                                      frac, callback, args)

        elif value.endswith('pt'): # points
            callback(float(value[:-2])*display_resolution, *args)

        elif value.endswith('em'): # ems, the height of the element's font
            attrs = self._get_current_attributes()
            font_size = attrs.font.get_size() / pango.SCALE
            callback(float(value[:-2])*display_resolution*font_size, *args)

        elif value.endswith('ex'): # x-height, ~ the height of the letter 'x'
            ## FIXME: figure out how to calculate this correctly
            ##        for now 'em' size is used as approximation
            attrs = self._get_current_attributes()
            font_size = attrs.font.get_size() / pango.SCALE
            callback(float(value[:-2])*display_resolution*font_size, *args)

        elif value.endswith('px'): # pixels
            callback(int(value[:-2]), *args)

        else:
            warnings.warn("Unable to parse length value '%s'" % value)