为什么我会使用if和let,而不仅仅是检查原始变量是否为零？(迅速)

Question

在"The Swift Programming Language."一书中,Apple提到在访问可选变量时使用if和let一起使用.

本书给出了以下代码:

var optionalString: String? = "Hello"
optionalString == nil

var optionalName: String? = "John Appleseed"
var greeting = "Hello!"
if let name = optionalName {
    greeting = "Hello, \(name)"
}

使用的优势是什么if let name = optionalName,而不是if optionalName != nil(并且总是称之为optionalName)？有什么区别,还是仅仅是惯例？

Answer 1

因为它还解包了可选值,所以这段代码:

if let name = optionalName {
    greeting = "Hello, \(name)"
}

相当于:

if optionalName != nil {
    let name:String = optionalName!
    greeting = "Hello, \(name)"
}

这种语言糖在Swift中称为Optional Binding.

可选类型

在夫特T和T?不同类型的,但可选的基础值T?可以很容易地通过使用能够实现类型!后缀操作符,例如:

let name:String = optionalName!

现在可以在String预期的地方使用,例如:

func greet(name:String) -> String {
    return "Hello, \(name)"
}

greet(name)

虽然这样做是安全的,但Swift确实允许你隐式转换为可选类型:

let name = "World"
let optionalName: String? = name

func greet(optionalName:String?) -> String? {
    if optionalName != nil {
        return "Hello, \(optionalName)"
    }
    return nil
}

//Can call with either String or String?
greet(optionalName)
greet(name)