UIBarButtonItem改变系统rightbutton和leftbutton的位置

由于系统的UIBarButtonItem *rightButton和UIBarButtonItem *rightButton的位置已经固定，但有时我们又想改变他的位置，可以这么做
   // 设置右边的按钮

    UIButton *rightBtn = [[UIButton alloc] initWithFrame:CGRectMake(0, 0, 40, 30)];

    [rightBtn setImage:[UIImage imageNamed:@"white_Cart"] forState:UIControlStateNormal];

    [rightBtn addTarget:self action:@selector(clickRightBtn) forControlEvents:UIControlEventTouchUpInside];

    UIBarButtonItem *rightButton = [[UIBarButtonItem alloc] initWithCustomView:rightBtn];

    UIBarButtonItem *negativeSpacer = [[UIBarButtonItem alloc]initWithBarButtonSystemItem:UIBarButtonSystemItemFixedSpace target:nil action:nil];

    negativeSpacer.width = -15.0;  // 这样rightButton就会向右移动15，也就是更接近边框了

    self.navigationItem.rightBarButtonItems = @[negativeSpacer, rightButton];