RAC 组合combine

RAC(_btn,enabled) = [RACSignal combineLatest:@[_accountTextFileld.rac_textSignal,_pwTextFileld.rac_textSignal] reduce:^id _Nullable(NSString *ac,NSString *pwd){

        NSLog(@"account:%@ pw:%@",ac,pwd);

        return @(ac.length && pwd.length);

        //注意reduceBlock参数：根据组合的信号关联的  必须 一一对应！！

    }];