gistify333274.m

    @weakify(headerView)
    RAC(headerView.textField, font) = [headerView.textField.rac_textSignal map:^id(NSString * s)
                                       {
                                           @strongify(headerView)
                                           UIFont *thin = [UIFont fontWithName:FONT_THIN size:headerView.textField.font.pointSize];
                                           UIFont *normal = [UIFont fontWithName:FONT_LIGHT size:headerView.textField.font.pointSize];
                                           
                                           return [s length] == 0 ? thin : normal;
                                       }];