更新： Xcode 8.3.2•Swift 3.1

enum PasswordError: String, Error {
    case eightCharacters
    case oneUppercase
    case oneLowercase
    case oneDecimalDigit
}

extension String {
    func validatePassword() throws  {
        guard count > 7
            else { throw PasswordError.eightCharacters }
        guard rangeOfCharacter(from: .uppercaseLetters) != nil
            else { throw PasswordError.oneUppercase }
        guard rangeOfCharacter(from: .lowercaseLetters) != nil
            else { throw PasswordError.oneLowercase }
        guard rangeOfCharacter(from: .decimalDigits) != nil
            else { throw PasswordError.oneDecimalDigit }
    }
}

let myPass = "12345678"

do {
    try myPass.validatePassword()
    print("valid password action")
} catch let error as PasswordError {
    print("Password error:", error) 
    switch error {
    case .eightCharacters:
        print("Needs At Least Eight Characters action")
    case .oneUppercase:
        print("Needs At Least one Uppercase action")
    case .oneLowercase:
        print("Needs At Least one Lowercase action")
    case .oneDecimalDigit:
        print("Needs At Least One DecimalDigit action")
    }
} catch {
    print("error:", error)
}