Swift 中的本地参数名和外部参数名有什么区别？

Question

https://developer.apple.com/library/ios/documentation/Swift/Conceptual/Swift_Programming_Language/Initialization.html#//apple_ref/doc/uid/TP40014097-CH18-ID203

struct Celsius {
    var temperatureInCelsius: Double
    init(fromFahrenheit fahrenheit: Double) {
        temperatureInCelsius = (fahrenheit - 32.0) / 1.8
    }
    init(fromKelvin kelvin: Double) {
        temperatureInCelsius = kelvin - 273.15
    }
}


let boilingPointOfWater = Celsius(fromFahrenheit: 212.0)
// boilingPointOfWater.temperatureInCelsius is 100.0
let freezingPointOfWater = Celsius(fromKelvin: 273.15)
// freezingPointOfWater.temperatureInCelsius is 0.0

我对为什么init函数有(fromFahrenheit fahrenheit: Double)和(fromKelvin kelvin: Double)感到困惑。文档说：

"第一个初始化器有一个名为from Fahrenheit的外部名称和一个名为fahrenheit的本地名称。第二个初始化器有一个名为from Kelvin的外部名称和一个名为kelvin的本地名称。"

为什么我们需要这么多名称呢？

- user678392

---

我刚刚写了一个覆盖这个问题的 Swift 教程，可能会对你有帮助！内部参数名称：http://www.apeth.com/swiftBook/ch02.html#_function_parameters_and_return_value 外部参数名称：http://www.apeth.com/swiftBook/ch02.html#_external_parameter_names - matt

Answer 1

1. This is used for an easy interface with Objective C. Objective C methods have both named local parameters and external parameters.

   For example this is a typical Objective C method.

   -(void)setValue:(NSObject *)object forKey:(NSObject *)key
   

   This will be called as [dict setValue:object1 forKey:object2]

   The same function in swift will be called as

   dict.setValue(object1,forKey:object2)
   

2. It also clarifies the purpose of each parameter and helps in distinguishing functions with similar names and different parameters. Functions can also have same signature with different external names.

   init(fromFahrenheit fahrenheit: Double) {
       temperatureInCelsius = (fahrenheit - 32.0) / 1.8
   }
   init(fromKelvin kelvin: Double) {
       temperatureInCelsius = kelvin - 273.15
   }
   

   In your example, both functions take a Double. Without external parameters there will be no way to distinguish them when called.

- rakeshbs

Answer 2

在方法中使用本地参数，而在调用方法时使用外部参数。

- tounaobun