flutter Row里面元素居中显示

直接上代码：

              new Expanded(
                  flex: ,
                  child: new Row(
                    children: <Widget>[
                      Expanded(
                          child: new Container(
                        alignment: Alignment.center,
                        color: Colors.black38,
                        child: new Text("這裡是logo"),
                      ))
                    ],
                  )),