swift实现类似宏定义功能，举例实现safeAreaInsets

//注意：要获取safeAreaInsets最好的办法是重写viewSafeAreaInsetsDidChange函数，在里面获取。因为viewDidLoad之前，safeAreaInsets是0值。并且屏幕也会转动。
    override func viewSafeAreaInsetsDidChange () {
        print(view.safeAreaInsets.bottom)
    }
    
    //这里以safeAreaInsets举例建立类似宏定义的东西
    //全局的
    //建一个.swift文件
    //创建常量
    let SafeAreaInsets = UIEdgeInsets(top: 44, left: 0, bottom: 34, right: 0)
    
    //view.safeAreaInsets要在viewDidLoad之后调用才有正确的值，所以可以写一个全局常量，默认lazy属性，延时加载只赋值一次
    //注意，常量只会赋值一次，竖屏调用后，转横屏，再调用值不会变
    let SafeAreaInsets: UIEdgeInsets = {
        guard #available(iOS 11.0, *), let safeAreaInsets = UIApplication.shared.delegate?.window??.safeAreaInsets else {
            return UIEdgeInsets()
        }
        return safeAreaInsets
    } ()
    
    
    
    //本页的
    //如果常量很多且是类似的，可以在本页面建一个私有结构体，定义静态常量和方法
    private struct Constants {
        //静态常量
        static let SafeAreaInsets = UIEdgeInsets(top: 44, left: 0, bottom: 34, right: 0)
        
        static func safeAreaInsets () -> UIEdgeInsets {
            guard #available(iOS 11.0, *), let safeAreaInsets = UIApplication.shared.delegate?.window??.safeAreaInsets else {
                return UIEdgeInsets()
            }
            return safeAreaInsets
        }
    }
    //调用
    print(Constants.SafeAreaInsets, Constants.safeAreaInsets())