macos - 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，项目由换行符 \n 分隔。

这是我目前的做法:

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
}

出于几个原因，我对此不是很满意。第一，我正在处理大小从几千字节到数百 MB 不等的文件。可以想象，使用这么大的字符串既慢又笨重。其次，这会在执行时卡住 UI——同样不好。

我研究过在一个单独的线程中运行这段代码，但我一直遇到麻烦，此外，它仍然没有解决处理巨大字符串的问题。

我想做的是遵循以下伪代码的思路:

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

我如何在 Swift 中完成此操作？

关于我正在读取的文件的几点说明:所有文件均由短字符串(<255 个字符)组成，字符串由 \n 或 \分隔r\n。文件的长度从大约 100 行到超过 5000 万行不等。它们可能包含欧洲字符和/或带有重音符号的字符。

Answer 1

(该代码目前适用于 Swift 2.2/Xcode 7.3。如果有人需要，可以在编辑历史中找到旧版本。最后提供了 Swift 3 的更新版本。)

以下 Swift 代码很大程度上受到了各种答案的启发 How to read data from NSFileHandle line by line? .它以 block 的形式从文件中读取，并将完整的行转换为字符串。

默认行分隔符(\n)、字符串编码(UTF-8)和 block 大小(4096)可以使用可选参数进行设置。

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)
    }
}

您甚至可以将读取器与 for-in 循环一起使用

for line in aStreamReader {
    print(line)
}

通过实现 SequenceType 协议(protocol)(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“现代化”到使用 guard 和新的 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()
        }
    }
}