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- 可选型的声明
var errorCode: String? = "404" //表示变量可以为空值，通常用var声明
print(errorCode) // 输出：optional("404")
- 可选型的基本用法
"The errorCode is " + errorCode //报错，可选型不能直接使用
需要用解包(unwrapped)的方式使用"Hello" + errorCode! //强制解包，有风险性(errorCode可能为nil)if let errorCode = errorCode { // 推荐用法（“if-let式”解包，建立新的解包常量，可以在大括号内持续使用）
print("The errorCode is " + errorCode)
}// 输出: The errorCode is 404//一次性解包多个变量
iflet errorCode = errorCode ,
let errorMessage = errorMessage , errorCode == "404"{
print("Page Not Found!")//可选型链
errorCode?.uppercased() //安全解包?
var uppercasedErrorCode = errorCode?.uppercased() //仍然是可选型//可选型变量赋值给另一个变量
let errorCode1 = errorCode == nil ? "Error!" : errorCode!
let errorCode2 = errorCode ?? "nil-Error!"//更简洁的写法
}
- 可选型更多用法
// 区别下列3中可选型
var error1: (errorCode: Int, errorMessage: String?) = (404, "Not Found")
var error2: (errorCode: Int, errorMessage: String)? = (404, "Not Found")
var error3: (errorCode: Int, errorMessage: String?)? = (404, "Not Found")//可选型在实际项目中的应用
var ageInput: String = "16"//隐式可选型
var errorMessage: String! = nil
errorMessage = "Not Found"
print("The message is " + errorMessage)
- 隐式可选型的用例
//1. 可以存放nil，可以直接使用，不用强制解包
//2. 常用于类的定义中（一开始设置为nil，在初始化后赋值）var errorMessage: String! = nil
errorMessage = "Not Found"
"The message is " + errorMessage //但是如果是nil，则会直接报错（不安全）class City {
let cityName: String
var country: Country
init(cityName: String, country: Country) {
self.cityName = cityName
self.country = country
}
}class Country {
let countryName: String
var capitalCity: City! //初始化Country需要先初始化City
init(countryName: String, capitalCity: String) {
self.countryName = countryName
self.capitalCity = City(cityName: capitalCity, country: self)
}
func showInfo() {
print("This is \(countryName)")
print("The capital is \(capitalCity.cityName)")
}
}
let china = Country(countryName: "中国", capitalCity: "北京")
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