Finds a view function by urlconf. If the function has attribute
    'navigation', it is used as breadcrumb title. Such title can be either a
    callable or an object with `__unicode__` attribute. If it is callable, it
    must follow the views API (i.e. the only required argument is request
    object). It is also expected to return a `unicode` value.

def _resolve_by_callback(request, url, urlconf=None):
    """
    Finds a view function by urlconf. If the function has attribute
    'navigation', it is used as breadcrumb title. Such title can be either a
    callable or an object with `__unicode__` attribute. If it is callable, it
    must follow the views API (i.e. the only required argument is request
    object). It is also expected to return a `unicode` value.
    """
    try:
        callback, args, kwargs = _resolve_url(url, request, urlconf=urlconf)
    except urlresolvers.Resolver404:
        return None

    bc = getattr(callback, 'breadcrumb', None)
    if bc is None:
        bc = getattr(callback, 'navigation', None)
        if bc is not None:  # pragma: nocover
            import warnings
            warnings.warn('The "navigation" attribute is deprecated, use '
                          '"breadcrumb" instead.')
    if bc is None:
        return None

    if hasattr(bc, '__call__'):
        # the breadcrumb is a function with an API identical to that of views.
        try:
            title = bc(request, *args, **kwargs)
        except http.Http404:
            return None
        assert isinstance(title, basestring), (
            'Breadcrumb function must return Unicode, not %s' % title)
    else:
        title = unicode(bc)    # handle i18n proxy objects

    return Crumb(url, title)