根据@Nick keets的回答，这里有一个更完整的例子:

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
}

最初发表于我的swift博客:http://eon.codes/blog/2017/09/01/throwing-simple-errors/

看看这个很酷的版本。其思想是同时实现String和ErrorType协议，并使用错误的rawValue。

enum UserValidationError: String, Error {
  case noFirstNameProvided = "Please insert your first name."
  case noLastNameProvided = "Please insert your last name."
  case noAgeProvided = "Please insert your age."
  case noEmailProvided = "Please insert your email."
}

用法:

do {
  try User.define(firstName,
                  lastName: lastName,
                  age: age,
                  email: email,
                  gender: gender,
                  location: location,
                  phone: phone)
}
catch let error as User.UserValidationError {
  print(error.rawValue)
  return
}

斯威夫特4:

按:

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

如果你不想定义一个自定义异常，你可以使用一个标准NSError对象，如下所示:

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)")
  }
}

打印:

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

这允许您提供一个自定义字符串(错误域)，加上一个数字代码和一个字典，其中包含您需要的所有其他类型的数据。

注意:这是在OS=Linux (Ubuntu 16.04 LTS)上测试的。

最简单的方法可能是定义一个自定义enum，只有一个case，并附加一个String:

enum MyError: ErrorType {
    case runtimeError(String)
}

或者，在Swift 4中:

enum MyError: Error {
    case runtimeError(String)
}

示例用法如下:

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

如果您希望使用现有的错误类型，最常用的错误类型是NSError，您可以创建一个工厂方法来创建并抛出一个带有自定义消息的错误类型。

最简单的方法是使String符合Error:

extension String: Error {}

然后你可以抛出一个字符串:

throw "Some Error"

为了让字符串本身成为错误的localizedString，你可以扩展LocalizedError:

extension String: LocalizedError {
    public var errorDescription: String? { return self }
}

根据@Nick keets的回答，这里有一个更完整的例子:

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
}

最初发表于我的swift博客:http://eon.codes/blog/2017/09/01/throwing-simple-errors/