URLString.swift

extension String {
  /// Remove all characters not allowed in a URL from the string
  /// - Precondition: String must not be empty and contain at least one character allowed in a URL
  /// - Postcondition: A valid String for creating a URL
  var urlString: String {
    precondition(self.isEmpty == false, "String cannot be empty")
    let returnValue = self.filter { $0.isAllowedInURL }
    precondition(URL(string: returnValue) != nil, "urlString \(returnValue) could not be used to make a URL")
    return returnValue
  }
}