完成处理程序swift 3从函数返回一个变量

Question

我很困惑围绕swift 3中的完成处理程序的语法.

在下面的函数中,在xml从Web服务调用解析文件后,它应该返回一个变量(an array [String:String]).
我的尝试在下面,但显然是不正确的.

  enum HistoryKey {
  case success([String:String])
  case failure(String)
 }

 private func getHistoryKeys(searchterm: String, completion: @escaping () -> HistoryKey) {
    let url = PubmedAPI.createEsearchURL(searchString: searchterm)
    let request = URLRequest.init(url: url as URL)
    let task = session.dataTask(with: request) { (data, response, error) in

        if let theData = data{
            let myParser = XMLParser.init(data: theData)
            myParser.delegate = self
            myParser.parse()
        }
    }
    task.resume()

    if keys.isEmpty {
        return .failure("no historyKeyDictionary")
    }else{
        return .success(keys)
    }

}// End of func

我想使用此功能如下

 let result = self.getHistoryKeys(searchTerm)

Answer 1

两个问题:

• 完成处理程序传递一个HistoryKey实例并且没有返回值,因此签名必须相反.
• 完成处理程序的调用必须是内部数据任务的完成块.

为了能够在完成块之外解析接收的数据,返回data成功

enum ConnectionResult {
   case success(Data)
   case failure(Error)
}

private func getHistoryKeys(searchterm: String, completion: @escaping (ConnectionResult) -> ()) {
   let url = PubmedAPI.createEsearchURL(searchString: searchterm)
   let task = session.dataTask(with: url) { (data, response, error) in
       if let error = error {
          completion(.failure(error))
       } else {
          completion(.success(data!))
       }
  }
  task.resume()
}

并称之为

getHistoryKeys(searchterm: String) { connectionResult in 
    switch connectionResult {
       case .success(let data): 
           let myParser = XMLParser(data: data)
           myParser.delegate = self
           myParser.parse()
           // get the parsed data from the delegate methods

       case .failure(let error): print(error)
    }
}