摘要：var optionalString: String? = "facial"; var greeting = "hi"; if let name = optionalString { //把optionalString复制给一个临时变量 name. 可以判断optionalString是否有值, g 阅读全文

摘要：There’s an even simpler way to include values in strings: Write the value in parentheses, and write a backslash (\) before the parentheses. This is kn 阅读全文