在Swift中逐行读取文件/URL

Question

我正在尝试读取NSURL中给定的文件，并将其加载到数组中，其中的项由换行符分隔\n。

以下是我到目前为止的工作方式：

var possList: NSString? = NSString.stringWithContentsOfURL(filePath.URL) as? NSString
if var list = possList {
    list = list.componentsSeparatedByString("\n") as NSString[]
    return list
}
else {
    //return empty list
}

由于几个原因，我对此不是很满意。首先，我正在处理大小从几千字节到数百MB的文件。您可以想象，处理这么大的琴弦既缓慢又笨拙。其次，这会在执行时冻结UI - 再次，不好。

我已经研究过在单独的线程中运行这段代码，但我一直遇到问题，而且，它仍然不能解决处理大型字符串的问题。

我想做的是类似于以下伪代码的事情：

var aStreamReader = new StreamReader(from_file_or_url)
while aStreamReader.hasNextLine == true {
    currentline = aStreamReader.nextLine()
    list.addItem(currentline)
}

我如何在Swift中实现这一点？

关于我正在读取的文件的一些注意事项:所有文件都由短(




Answer 1

Swift 4.2 安全语法

class LineReader {

    let path: String

    init?(path: String) {
        self.path = path
        guard let file = fopen(path, "r") else {
            return nil
        }
        self.file = file
    }
    deinit {
        fclose(file)
    }

    var nextLine: String? {
        var line: UnsafeMutablePointer<CChar>?
        var linecap = 0
        defer {
            free(line)
        }
        let status = getline(&line, &linecap, file)
        guard status > 0, let unwrappedLine = line else {
            return nil
        }
        return String(cString: unwrappedLine)
    }

    private let file: UnsafeMutablePointer<FILE>
}

extension LineReader: Sequence {
    func makeIterator() -> AnyIterator<String> {
        return AnyIterator<String> {
            return self.nextLine
        }
    }
}

用法：

guard let reader = LineReader(path: "/Path/to/file.txt") else {
    return
}
reader.forEach { line in
    print(line.trimmingCharacters(in: .whitespacesAndNewlines))      
}

Answer 2

用于逐行读取文本文件的高效便捷类（Swift 4、Swift 5）

注意：此代码独立于平台（macOS、iOS、ubuntu）

import Foundation

/// Read text file line by line in efficient way
public class LineReader {
   public let path: String

   fileprivate let file: UnsafeMutablePointer<FILE>!

   init?(path: String) {
      self.path = path
      file = fopen(path, "r")
      guard file != nil else { return nil }
   }

   public var nextLine: String? {
      var line:UnsafeMutablePointer<CChar>? = nil
      var linecap:Int = 0
      defer { free(line) }
      return getline(&line, &linecap, file) > 0 ? String(cString: line!) : nil
   }

   deinit {
      fclose(file)
   }
}

extension LineReader: Sequence {
   public func  makeIterator() -> AnyIterator<String> {
      return AnyIterator<String> {
         return self.nextLine
      }
   }
}

用法：

guard let reader = LineReader(path: "/Path/to/file.txt") else {
    return; // cannot open file
}

for line in reader {
    print(">" + line.trimmingCharacters(in: .whitespacesAndNewlines))      
}

github上的存储库

Answer 3

匿名用户

（代码现在适用于 Swift 2.2/Xcode 7.3。如果有人需要，可以在编辑历史记录中找到旧版本。最后提供了 Swift 3 的更新版本。

以下Swift代码深受“如何从NSFileHandle逐行读取数据？”的各种答案的启发？。它从文件中分块读取，并将整行转换为字符串。

可以使用可选参数设置默认的行分隔符(< code>\n)、字符串编码(UTF-8)和块大小(4096)。

class StreamReader  {

    let encoding : UInt
    let chunkSize : Int

    var fileHandle : NSFileHandle!
    let buffer : NSMutableData!
    let delimData : NSData!
    var atEof : Bool = false

    init?(path: String, delimiter: String = "\n", encoding : UInt = NSUTF8StringEncoding, chunkSize : Int = 4096) {
        self.chunkSize = chunkSize
        self.encoding = encoding

        if let fileHandle = NSFileHandle(forReadingAtPath: path),
            delimData = delimiter.dataUsingEncoding(encoding),
            buffer = NSMutableData(capacity: chunkSize)
        {
            self.fileHandle = fileHandle
            self.delimData = delimData
            self.buffer = buffer
        } else {
            self.fileHandle = nil
            self.delimData = nil
            self.buffer = nil
            return nil
        }
    }

    deinit {
        self.close()
    }

    /// Return next line, or nil on EOF.
    func nextLine() -> String? {
        precondition(fileHandle != nil, "Attempt to read from closed file")

        if atEof {
            return nil
        }

        // Read data chunks from file until a line delimiter is found:
        var range = buffer.rangeOfData(delimData, options: [], range: NSMakeRange(0, buffer.length))
        while range.location == NSNotFound {
            let tmpData = fileHandle.readDataOfLength(chunkSize)
            if tmpData.length == 0 {
                // EOF or read error.
                atEof = true
                if buffer.length > 0 {
                    // Buffer contains last line in file (not terminated by delimiter).
                    let line = NSString(data: buffer, encoding: encoding)

                    buffer.length = 0
                    return line as String?
                }
                // No more lines.
                return nil
            }
            buffer.appendData(tmpData)
            range = buffer.rangeOfData(delimData, options: [], range: NSMakeRange(0, buffer.length))
        }

        // Convert complete line (excluding the delimiter) to a string:
        let line = NSString(data: buffer.subdataWithRange(NSMakeRange(0, range.location)),
            encoding: encoding)
        // Remove line (and the delimiter) from the buffer:
        buffer.replaceBytesInRange(NSMakeRange(0, range.location + range.length), withBytes: nil, length: 0)

        return line as String?
    }

    /// Start reading from the beginning of file.
    func rewind() -> Void {
        fileHandle.seekToFileOffset(0)
        buffer.length = 0
        atEof = false
    }

    /// Close the underlying file. No reading must be done after calling this method.
    func close() -> Void {
        fileHandle?.closeFile()
        fileHandle = nil
    }
}

用法：

if let aStreamReader = StreamReader(path: "/path/to/file") {
    defer {
        aStreamReader.close()
    }
    while let line = aStreamReader.nextLine() {
        print(line)
    }
}

你甚至可以使用带有for-in循环的阅读器

for line in aStreamReader {
    print(line)
}

通过实现SequenceType协议（比较http://robots.thoughtbot.com/swift-sequences):

extension StreamReader : SequenceType {
    func generate() -> AnyGenerator<String> {
        return AnyGenerator {
            return self.nextLine()
        }
    }
}

Swift 3/Xcode 8 beta 6的更新:也进行了“现代化”以使用< code>guard和新的< code>Data值类型:

class StreamReader  {

    let encoding : String.Encoding
    let chunkSize : Int
    var fileHandle : FileHandle!
    let delimData : Data
    var buffer : Data
    var atEof : Bool

    init?(path: String, delimiter: String = "\n", encoding: String.Encoding = .utf8,
          chunkSize: Int = 4096) {

        guard let fileHandle = FileHandle(forReadingAtPath: path),
            let delimData = delimiter.data(using: encoding) else {
                return nil
        }
        self.encoding = encoding
        self.chunkSize = chunkSize
        self.fileHandle = fileHandle
        self.delimData = delimData
        self.buffer = Data(capacity: chunkSize)
        self.atEof = false
    }

    deinit {
        self.close()
    }

    /// Return next line, or nil on EOF.
    func nextLine() -> String? {
        precondition(fileHandle != nil, "Attempt to read from closed file")

        // Read data chunks from file until a line delimiter is found:
        while !atEof {
            if let range = buffer.range(of: delimData) {
                // Convert complete line (excluding the delimiter) to a string:
                let line = String(data: buffer.subdata(in: 0..<range.lowerBound), encoding: encoding)
                // Remove line (and the delimiter) from the buffer:
                buffer.removeSubrange(0..<range.upperBound)
                return line
            }
            let tmpData = fileHandle.readData(ofLength: chunkSize)
            if tmpData.count > 0 {
                buffer.append(tmpData)
            } else {
                // EOF or read error.
                atEof = true
                if buffer.count > 0 {
                    // Buffer contains last line in file (not terminated by delimiter).
                    let line = String(data: buffer as Data, encoding: encoding)
                    buffer.count = 0
                    return line
                }
            }
        }
        return nil
    }

    /// Start reading from the beginning of file.
    func rewind() -> Void {
        fileHandle.seek(toFileOffset: 0)
        buffer.count = 0
        atEof = false
    }

    /// Close the underlying file. No reading must be done after calling this method.
    func close() -> Void {
        fileHandle?.closeFile()
        fileHandle = nil
    }
}

extension StreamReader : Sequence {
    func makeIterator() -> AnyIterator<String> {
        return AnyIterator {
            return self.nextLine()
        }
    }
}