Swift 变量与常量
阐述
变量与常量在使用前必须首先声明，声明的时候可以赋值，也可以不赋值。如果不赋值，需要声明它的类型。var 用于声明变量，而 let 用于常量；常量在第一次赋值之后不能再次被赋值。
var environment = "development"
let maximumNumberOfLoginAttempts: Int
// maximumNumberOfLoginAttempts has no value yet.
if environment == "development" {
maximumNumberOfLoginAttempts = 100
} else {
maximumNumberOfLoginAttempts = 10
}
// Now maximumNumberOfLoginAttempts has a value, and can be read.