swift實現類似宏定義功能，舉例實現safeAreaInsets

//注意：要獲取safeAreaInsets最好的辦法是重寫viewSafeAreaInsetsDidChange函數，在里面獲取。因為viewDidLoad之前，safeAreaInsets是0值。並且屏幕也會轉動。
    override func viewSafeAreaInsetsDidChange () {
        print(view.safeAreaInsets.bottom)
    }
    
    //這里以safeAreaInsets舉例建立類似宏定義的東西
    //全局的
    //建一個.swift文件
    //創建常量
    let SafeAreaInsets = UIEdgeInsets(top: 44, left: 0, bottom: 34, right: 0)
    
    //view.safeAreaInsets要在viewDidLoad之后調用才有正確的值，所以可以寫一個全局常量，默認lazy屬性，延時加載只賦值一次
    //注意，常量只會賦值一次，豎屏調用后，轉橫屏，再調用值不會變
    let SafeAreaInsets: UIEdgeInsets = {
        guard #available(iOS 11.0, *), let safeAreaInsets = UIApplication.shared.delegate?.window??.safeAreaInsets else {
            return UIEdgeInsets()
        }
        return safeAreaInsets
    } ()
    
    
    
    //本頁的
    //如果常量很多且是類似的，可以在本頁面建一個私有結構體，定義靜態常量和方法
    private struct Constants {
        //靜態常量
        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
        }
    }
    //調用
    print(Constants.SafeAreaInsets, Constants.safeAreaInsets())