Simplest way to throw an error/exception with a custom message in Swift 2?

Question

I want to do something in Swift 2 that I\'m used to doing in multiple other languages: throw a runtime exception with a custom message. For example (in Java):

Answer 1

The simplest approach is probably to define one custom enum with just one case that has a String attached to it:

enum MyError: ErrorType {
    case runtimeError(String)
}

Or, as of Swift 4:

enum MyError: Error {
    case runtimeError(String)
}

Example usage would be something like:

func someFunction() throws {
    throw MyError.runtimeError("some message")
}
do {
    try someFunction()
} catch MyError.runtimeError(let errorMessage) {
    print(errorMessage)
}

If you wish to use existing Error types, the most general one would be an NSError, and you could make a factory method to create and throw one with a custom message.

Answer 2

I like @Alexander-Borisenko's answer, but the localized description was not returned when caught as an Error. It seems that you need to use LocalizedError instead:

struct RuntimeError: LocalizedError
{
    let message: String

    init(_ message: String)
    {
        self.message = message
    }

    public var errorDescription: String?
    {
        return message
    }
}

See this answer for more details.

Answer 3

Throwing code should make clear whether the error message is appropriate for display to end users or is only intended for developer debugging. To indicate a description is displayable to the user, I use a struct DisplayableError that implements the LocalizedError protocol.

struct DisplayableError: Error, LocalizedError {
    let errorDescription: String?

    init(_ description: String) {
        errorDescription = description
    }
}

Usage for throwing:

throw DisplayableError("Out of pixie dust.")

Usage for display:

let messageToDisplay = error.localizedDescription

Answer 4

Based on @Nick keets answer, here is a more complete example:

extension String: Error {} // Enables you to throw a string

extension String: LocalizedError { // Adds error.localizedDescription to Error instances
    public var errorDescription: String? { return self }
}

func test(color: NSColor) throws{
    if color == .red {
        throw "I don't like red"
    }else if color == .green {
        throw "I'm not into green"
    }else {
        throw "I like all other colors"
    }
}

do {
    try test(color: .green)
} catch let error where error.localizedDescription == "I don't like red"{
    Swift.print ("Error: \(error)") // "I don't like red"
}catch let error {
    Swift.print ("Other cases: Error: \(error.localizedDescription)") // I like all other colors
}

Originally published on my swift blog: http://eon.codes/blog/2017/09/01/throwing-simple-errors/

Answer 5

Swift 4:

As per:

https://developer.apple.com/documentation/foundation/nserror

if you don't want to define a custom exception, you could use a standard NSError object as follows:

import Foundation

do {
  throw NSError(domain: "my error domain", code: 42, userInfo: ["ui1":12, "ui2":"val2"] ) 
}
catch let error as NSError {
  print("Caught NSError: \(error.localizedDescription), \(error.domain), \(error.code)")
  let uis = error.userInfo 
  print("\tUser info:")
  for (key,value) in uis {
    print("\t\tkey=\(key), value=\(value)")
  }
}

Prints:

Caught NSError: The operation could not be completed, my error domain, 42
    User info:
        key=ui1, value=12
        key=ui2, value=val2

This allows you to provide a custom string (the error domain), plus a numeric code and a dictionary with all the additional data you need, of any type.

N.B.: this was tested on OS=Linux (Ubuntu 16.04 LTS).

Answer 6

In case you don't need to catch the error and you want to immediately stop the application you can use a fatalError: fatalError ("Custom message here")