两次调用 Swift 的 readline 到同一个变量会忽略第二次调用

Question

我正在从命令行读取用户输入，检查它是否是有效的文件路径，如果不是，请用户再试一次。

如果用户输入的是nil，第一次应该被当作任何其他错误输入，让用户输入一个新的值，但是第二次输入nil，程序应该强制退出.

（我假设nil 值是用户不会故意输入的，所以如果它发生两次以上，我假设出现问题并退出程序以避免无休止的循环要求新的输入。这可能是也可能不是一个好方法，但这不会影响问题。）

问题是readLine() 在收到行尾输入（产生nil 值）后第二次调用时不会要求用户输入。 （行尾可以用^D输入。）

这意味着readLine()所在的函数会自动返回nil，因为这是接收readLine()的变量的最新值。

问题

1. 不应该调用readLine()，不管接收变量已经有什么值？

2. 如果是这样，为什么在输入一次nil 时不要求用户输入？

这是代码：

import Foundation

/**
    Ask the user to provide a word list file, check if the file exists. If it doesn't exist, ask the user again.
 */
func askUserForWordList() -> String? {
    print("")
    print("Please drag a word list file here (or enter its path manually), to use as basis for the statistics:")

    var path = readLine(stripNewline: true) // THIS IS SKIPPED IF "PATH" IS ALREADY "NIL".

    return path
}

/**
    Check the user input // PROBABLY NOT RELEVANT FOR THIS QUESTION
 */
func fileExists(filePath: String) -> Bool {
    let fileManager = NSFileManager.defaultManager()

    if fileManager.fileExistsAtPath(filePath) {
        return true
    } else {
        return false
    }
}

/**
    Get the file from the user and make sure it’s valid.
 */
func getFilePathFromUser() throws -> String {

    enum inputError: ErrorType {
        case TwoConsecutiveEndOfFiles
    }
    var correctFile = false
    var path: String? = ""
    var numberOfConsecutiveNilFiles = 0

    repeat {
        // Check that the user did not enter a nil-value (end-of-file) – if they did so two times in a row, terminate the program as this might be some kind of error (so that we don't get an infinite loop).
        if numberOfConsecutiveNilFiles > 1 { // FIXME: entering ^D once is enough to end the program (it should require two ^D). Actually the problem seems to be in function "askUserForWordList()".
            throw inputError.TwoConsecutiveEndOfFiles
        }

        path = askUserForWordList()

        if path == nil {
            numberOfConsecutiveNilFiles += 1
        } else {
            numberOfConsecutiveNilFiles = 0

            correctFile = fileExists(path!)

            if !correctFile {
                print("")
                print("Oops, I couldn't recognize that file path. Please try again.")
            }
        }
    } while !correctFile

    return path!
}

// This is where the actual execution starts

print("")
print("=== Welcome to \"Word Statistics\", command line version ===")
print("")
print("This program will give you some statistics for the list of words you provide.")
do {
    let path = try getFilePathFromUser()
} catch {
    print("Error: \(error)")
    exit(-46) // Using closest error type from http://www.swiftview.com/tech/exitcodes.htm (which may not be standard at all. I could, however, not find any "standard" list of exit values).
}

备注

• 当输入任何其他无效路径（任何字符串，甚至是空的（只需按 Enter））时，循环将按预期工作。
• 最初askUserForWordList() 函数中的path 被声明为常量（let path = readLine(stripNewline: true)），但我将其更改为var，因为它应该在每次调用函数时更新。不过，这并不影响程序的运行方式。
• 我尝试在调用readLine()之前在行上单独声明path，没有任何区别。
• 我尝试完全跳过path 变量，让askUserForWordList() 函数直接返回readLine() 结果（return readLine(stripNewline: true)）。这没什么区别。

• 我一起跳过了askUserForWordList() 函数，并将要求用户输入的代码移到了函数“getFilePathFromUser()”的“主”代码中，但这并没有改变任何东西。

  修改代码：

  func getFilePathFromUser() throws -> String {
  
      enum inputError: ErrorType {
          case TwoConsecutiveEndOfFiles
      }
      var correctFile = false
      var path: String? = ""
      var numberOfConsecutiveNilFiles = 0
  
      repeat {
          // Check that the user did not enter a nil-value (end-of-file) – if they did so two times in a row, terminate the program as this might be some kind of error (so that we don't get an infinite loop).
          if numberOfConsecutiveNilFiles > 1 { // FIXME: entering ^D once is enough to end the program (it should require two ^D). Actually the problem seems to be in function "askUserForWordList()".
              throw inputError.TwoConsecutiveEndOfFiles
          }
  
          // MODIFIED – This code was previously located in "askUserForWordList()"
          print("")
          print("Please drag a word list file here (or enter its path manually), to use as basis for the statistics:")
  
          path = readLine(stripNewline: true)
  
          // END OF MODIFICATION
  
          if path == nil {
              numberOfConsecutiveNilFiles += 1
          } else {
              numberOfConsecutiveNilFiles = 0
  
              correctFile = fileExists(path!)
  
              if !correctFile {
                  print("")
                  print("Oops, I couldn't recognize that file path. Please try again.")
              }
          }
      } while !correctFile
  
      return path!
  }
  

Answer 1

readLine() 返回nil 如果（且仅当）标准输入文件描述符已到达文件末尾。这发生在（例如）读取输入时 来自 tty，并输入 Ctrl-D（"end-of-transmission character"）作为一行的第一个字符。

所有后续的readLine() 调用然后也返回nil，没有 方式检测到“Ctrl-D 输入了两次”。 换句话说，一旦标准输入处于文件结束状态 您无法再从中读取任何数据。如果您的程序需要 数据多了就只能报错，例如

guard let path = readLine(stripNewline: true) else {
    throw InputError.UnexpectedEndOfFile
}