// 初始化一个返回按钮
    UIButton *button = [[UIButton alloc] initWithFrame:CGRectMake(0, 0, 40, 40)];
//    button.backgroundColor = [UIColor greenColor];
    // 为返回按钮设置图片样式
    [button setImage:[UIImage imageNamed:@"goback"] forState:UIControlStateNormal];
    // 设置返回按钮触发的事件
    [button addTarget:self action:@selector(backBarButtonPressed:) forControlEvents:UIControlEventTouchUpInside];
    
    // 初始化一个BarButtonItem，并将其设置为返回的按钮的样式
    UIBarButtonItem *backButton = [[UIBarButtonItem alloc] initWithCustomView:button];
    
    if(@available(iOS 11.0, *)) {
        [[button.heightAnchor constraintEqualToConstant:40] setActive:YES];//关键地方
        [[button.widthAnchor constraintEqualToConstant:40] setActive:YES];//关键地方
    }
    // 将BarButtonItem添加到LeftBarButtonItem上
    self.navigationItem.leftBarButtonItem = backButton;