如何在Swift中使用键值编码获取全局常量？

Question

I am writing a method that will give localized names of CNPostalAddress elements. The localization keys I am attempting to retrieve are global constants.

/// Get the localised name of a CNPostalAddress element
///
/// - Parameter field: CNPostalAddress element name (Street, SubLocality, City, SubAdministrativeArea, State, PostalCode, Country, ISOCountryCode)
/// - Returns: localized name of the address field


func localizedAddressFieldName(for field: String) -> String? {
    let keyPathKey = "CNPostalAddress\(field)Key"

    if let localizationKey = value(forKey: keyPathKey) as? String {
        return CNPostalAddress.localizedString(forKey: localizationKey)
    } else {
        return nil
    }
}

However, the application crashes when getting the the localizationKey with the following log:

Terminating app due to uncaught exception 'NSUnknownKeyException', reason: '[<MyProject.MyViewController 0x7fd83d81c8b0> valueForUndefinedKey:]: this class is not key value coding-compliant for the key CNPostalAddressStreetKey.'

CNPostalAddressStreetKey is a valid key, as shown in Apple docs , but is a Global constant.

Solution

extension CNPostalAddress {
    class func localizedAddressFieldName(for field: String) -> String? {
        let localizationKey = field.decapitalizingFirstLetter()
        return CNPostalAddress.localizedString(forKey: localizationKey)
    }
}

extension String {
    func decapitalizingFirstLetter() -> String {
        let firstLetterLowercase = String(prefix(1)).lowercased()
        return firstLetterLowercase + String(dropFirst())
    }
}

Answer 1

it's because you may have to use those variables instead:

print("\(CNPostalAddress.localizedString(forKey: "subLocality"))")
print("\(CNPostalAddress.localizedString(forKey: "street"))")

so instead of

let keyPathKey = "CNPostalAddress\(field)Key"

you should use

let keyPathKey = "\(field)"

it's case sensitive, so you might have to make some adjustments.

see here

Answer 2

First value(forKey: keyPathKey) is a method defined in NSKeyValueCoding protocol, you are calling that method in your viewController that is why is crashing to avoid the crash you have to implement that method in your ViewController

override func value(forKey key: String) -> Any? {

}

but anyway I think you don't need call this method in first instance

func localizedAddressFieldName(for field: String) -> String? {
    let localizationKey = "CNPostalAddress\(field)Key"

    return CNPostalAddress.localizedString(forKey: localizationKey)
}

If your key is not defined in CNPostalAddress the result will be an empty string