代码创建FlexibleSpaceBarButton(弹性空白)

  //1.创建toolbar左边的按钮，调用initWithTitle方法进行初始化

  // UIBarButtonItem *extend = [[UIBarButtonItemalloc]initWithTitle:@"扩展"style:UIBarButtonItemStylePlain target:self action:nil];

   //2.创建toolbar中间的flexibleSpace(弹性空白)，调用initWithBarButtonSystemItem方法进行初始化

     UIBarButtonItem *flexibleSpace = [[UIBarButtonItemalloc]initWithBarButtonSystemItem:UIBarButtonSystemItemFlexibleSpace target:nil action:nil];