Question

我从命令行读取用户输入，检查它是否是有效的文件路径，并且 - 如果它没有要求用户重试。

如果用户输入为nil，则应首次将其视为任何其他错误输入，让用户输入新值，但第二次输入nil时，程序应该强制退出。

（我假设nil值是用户不会故意进入的，所以如果它发生了两次以上，我会假设出错了并退出程序以避免永无止境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()如果收到了行结束输入（产生nil值），则第二次调用后不会要求用户输入。（可以用^ D输入行尾。）

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

无论接收变量已有多少值，都不应该readLine()被调用？
如果是这样，为什么用户在输入nil一次后才要求输入？

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，甚至是空字符串（只需按Enter键）时，循环按预期工作。
path函数中的原始askUserForWordList()被声明为常量（let path = readLine(stripNewline: true)），但我将其更改为var，因为它应该在每次函数更新时更新调用。但是，这并不影响该计划的运作方式。
我在调用path之前尝试单独声明readLine()，这没有任何区别。
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动到函数＆＃34; getFilePathFromUser（）＆＃34;的“主要”代码中，但是没有'改变任何事情。

修改后的代码：

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

如果（且仅当）标准输入文件描述符已到达文件结尾，

readLine()将返回nil。在读取输入时会发生这种情况（例如）从tty开始，Ctrl-D（"end-of-transmission character"）作为第一个字符输入。

所有后续readLine()次调用都会返回nil，但没有方式检测＆＃34; Ctrl-D输入两次＆＃34;。换句话说，一旦标准输入处于文件结束条件你不能再读取它的任何数据了。如果您的程序需要如果您有更多数据，则只能报告错误，例如

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

将Swift的readline调用到同一个变量两次会忽略第二次调用

1 个答案: