How to override localizedDescription for custom Error in Swift 3? [duplicate]

The documentation about new Error bridging feature is not clear enough still now, so this answer may need some updates in the near future, but according to SE-0112 and the latest Swift source code, you may need to use LocalizedError rather than Error and implement errorDescription.

class MyError: NSObject, LocalizedError {    var desc = ""    init(str: String) {        desc = str    }    override var description: String {        get {            return "MyError: \(desc)"        }    }    //You need to implement `errorDescription`, not `localizedDescription`.    var errorDescription: String? {        get {            return self.description        }    }}func test_my_code() {    let error = MyError(str: "my test string")    let x = error as Error    print(x.localizedDescription)}test_my_code() //->MyError: my test string

Other than using LocalizedError, this default implementation works:

(NSError.swift, the link shown above)

public extension Error {    /// Retrieve the localized description for this error.    var localizedDescription: String {        return NSError(domain: _domain, code: _code, userInfo: nil).localizedDescription    }}

It is a little complicated how Swift defines _domain or _code from arbitrary types just conforming to Error, but it seems that NSError generates "The operation couldn’t be completed..." for unknown combinations of domain and code.

If custom type conforms to protocol CustomStringConvertible and provides localized description, then the following extension of LocalizedError might be useful:

extension LocalizedError where Self: CustomStringConvertible {   var errorDescription: String? {      return description   }}

Example code:

class MyError: LocalizedError, CustomStringConvertible {   let desc: String   init(str: String) {      desc = str   }   var description: String {      let format = NSLocalizedString("Operation error: %@", comment: "Error description")      return String.localizedStringWithFormat(format, desc)   }}let error = MyError(str: "my test string")let x = error as Errorprint(x.localizedDescription) // Prints "Operation error: my test string"print(String(describing: x))  // Prints "Operation error: my test string"