尝试添加一个扩展来处理编码，然后在需要编码时调用该扩展。

extension String {
    func stringByAddingPercentEncodingForRFC3986() -> String? {
        let unreserved = "-._~/?:"
        let allowed = NSMutableCharacterSet.alphanumericCharacterSet()
        allowed.addCharactersInString(unreserved)
        return stringByAddingPercentEncodingWithAllowedCharacters(allowed)
    }
}

然后使用它：

let query = "http://test.com/param1& param2+ param3"
let encoded = query.stringByAddingPercentEncodingForRFC3986()!

extension String {
  func stringByAddingPercentEncodingForRFC3986() -> String? {
    let unreserved = "-._~/?:"
    let allowedCharacterSet = NSMutableCharacterSet.alphanumeric()
    allowedCharacterSet.addCharacters(in: unreserved)
    return self.addingPercentEncoding(withAllowedCharacters: allowedCharacterSet as CharacterSet)
  }
}

let encodedVal = usernameTextField.text.stringByAddingPercentEncodingForRFC3986()

• ------或----------

extension String {
   var urlEncoded: String? {
    let allowedCharacterSet = CharacterSet.alphanumerics.union(CharacterSet(charactersIn: "-._~/?:"))
    return self.addingPercentEncoding(withAllowedCharacters: allowedCharacterSet)
  }
}

let encodedVal = usernameTextField.text.urlEncoded