在 Swift 中逐行读取文本文件？

Question

我刚开始学习 Swift。我已经从文本文件中读取了我的代码，并且应用程序显示了整个文本文件的内容。如何逐行显示并多次调用该行？

TextFile.txt 包含以下内容：

1. Banana 
2. Apple
3. pear
4. strawberry
5. blueberry
6. blackcurrant

以下是目前拥有的..

  if let path = NSBundle.mainBundle().pathForResource("TextFile", ofType: "txt"){
        var data = String(contentsOfFile:path, encoding: NSUTF8StringEncoding, error: nil)
            if let content = (data){
                TextView.text = content
    }

如果有其他方法，请告诉我。将不胜感激。

Answer 1

Swift 3.0

if let path = Bundle.main.path(forResource: "TextFile", ofType: "txt") {
    do {
        let data = try String(contentsOfFile: path, encoding: .utf8)
        let myStrings = data.components(separatedBy: .newlines)
        TextView.text = myStrings.joined(separator: ", ")
    } catch {
        print(error)
    }
}

变量myStrings应该是数据的每一行。

使用的代码来自： Reading file line by line in iOS SDK 用 Obj-C 编写并使用 NSString

检查以前版本的 Swift 的编辑历史记录。

Answer 2

Swift 5.5

下面的解决方案展示了如何一次读取一行。这与将全部内容读入内存完全不同。如果您要读取大文件，则逐行读取可以很好地扩展。将整个文件放入内存并不能很好地适应大文件。

下面的示例使用了一个 while 循环，当没有更多行时退出，但您可以根据需要选择不同的行数来读取。

代码工作如下：

1. 创建一个告诉文件所在位置的 URL
2. 确保文件存在
3. 打开文件进行阅读
4. 设置一些初始变量以供读取
5. 使用getLine()读取每一行
6. 完成后关闭文件并释放缓冲区

如果你愿意，你可以让代码不那么冗长；我已经包含了 cmets 来解释变量的用途。

斯威夫特 5.5

import Cocoa

// get URL to the the documents directory in the sandbox
let home = FileManager.default.homeDirectoryForCurrentUser

// add a filename
let fileUrl = home
    .appendingPathComponent("Documents")
    .appendingPathComponent("my_file")
    .appendingPathExtension("txt")


// make sure the file exists
guard FileManager.default.fileExists(atPath: fileUrl.path) else {
    preconditionFailure("file expected at \(fileUrl.absoluteString) is missing")
}

// open the file for reading
// note: user should be prompted the first time to allow reading from this location
guard let filePointer:UnsafeMutablePointer<FILE> = fopen(fileUrl.path,"r") else {
    preconditionFailure("Could not open file at \(fileUrl.absoluteString)")
}

// a pointer to a null-terminated, UTF-8 encoded sequence of bytes
var lineByteArrayPointer: UnsafeMutablePointer<CChar>? = nil

// see the official Swift documentation for more information on the `defer` statement
// https://docs.swift.org/swift-book/ReferenceManual/Statements.html#grammar_defer-statement
defer {
    // remember to close the file when done
    fclose(filePointer)

    // The buffer should be freed by even if getline() failed.
    lineByteArrayPointer?.deallocate()
}

// the smallest multiple of 16 that will fit the byte array for this line
var lineCap: Int = 0

// initial iteration
var bytesRead = getline(&lineByteArrayPointer, &lineCap, filePointer)


while (bytesRead > 0) {
    
    // note: this translates the sequence of bytes to a string using UTF-8 interpretation
    let lineAsString = String.init(cString:lineByteArrayPointer!)
    
    // do whatever you need to do with this single line of text
    // for debugging, can print it
    print(lineAsString)
    
    // updates number of bytes read, for the next iteration
    bytesRead = getline(&lineByteArrayPointer, &lineCap, filePointer)
}

Answer 3

这并不漂亮，但我相信它有效（在 Swift 5 上）。这使用底层 POSIX getline 命令进行迭代和文件读取。

typealias LineState = (
  // pointer to a C string representing a line
  linePtr:UnsafeMutablePointer<CChar>?,
  linecap:Int,
  filePtr:UnsafeMutablePointer<FILE>?
)

/// Returns a sequence which iterates through all lines of the the file at the URL.
///
/// - Parameter url: file URL of a file to read
/// - Returns: a Sequence which lazily iterates through lines of the file
///
/// - warning: the caller of this function **must** iterate through all lines of the file, since aborting iteration midway will leak memory and a file pointer
/// - precondition: the file must be UTF8-encoded (which includes, ASCII-encoded)
func lines(ofFile url:URL) -> UnfoldSequence<String,LineState>
{
  let initialState:LineState = (linePtr:nil, linecap:0, filePtr:fopen(fileURL.path,"r"))
  return sequence(state: initialState, next: { (state) -> String? in
    if getline(&state.linePtr, &state.linecap, state.filePtr) > 0,
      let theLine = state.linePtr  {
      return String.init(cString:theLine)
    }
    else {
      if let actualLine = state.linePtr  { free(actualLine) }
      fclose(state.filePtr)
      return nil
    }
  })
}

以下是您可以如何使用它：

for line in lines(ofFile:myFileURL) {
  print(line)
}

Answer 4

Swift 2.0 / Xcode 7.2 更新

    do {
        if let path = NSBundle.mainBundle().pathForResource("TextFile", ofType: "txt"){
            let data = try String(contentsOfFile:path, encoding: NSUTF8StringEncoding)

            let myStrings = data.componentsSeparatedByCharactersInSet(NSCharacterSet.newlineCharacterSet())
            print(myStrings)
        }
    } catch let err as NSError {
        //do sth with Error
        print(err)
    }

另外值得一提的是，此代码读取项目文件夹中的文件（因为使用了 pathForResource），而不是例如设备的文件夹

Answer 5

在 Swift 5.0 中执行此操作的最简单、最简单的方法可能如下：

import Foundation

// Determine the file name
let filename = "main.swift"

// Read the contents of the specified file
let contents = try! String(contentsOfFile: filename)

// Split the file into separate lines
let lines = contents.split(separator:"\n")

// Iterate over each line and print the line
for line in lines {
    print("\(line)")
}

注意：这会将整个文件读入内存，然后只遍历内存中的文件以生成行......

归功于：https://wiki.codermerlin.com/mediawiki/index.php/Code_Snippet:_Print_a_File_Line-by-Line

Answer 6

您可能确实想一次读取整个文件。我敢打赌它很小。

但是您想将生成的字符串拆分为一个数组，然后将数组的内容分配给各种 UI 元素，例如表格单元格。

一个简单的例子：

    var x: String = "abc\ndef"
    var y = x.componentsSeparatedByString("\n")
    // y is now a [String]: ["abc", "def"]

Answer 7

在 >= Swift 5.0 中，您可以：

如果文件在主 Bundle 中：

if let bundleURL = Bundle.main.url(forResource: "YOUR_FILE_NAME", withExtension: "txt"),
   let contentsOfFile = try? String(contentsOfFile: bundleURL.path, encoding: .utf8) {
    let components = contentsOfFile.components(separatedBy: .newlines)
    print(components)
}

components 属性将返回一个字符串数组。

Answer 8

另一种 getline 解决方案：

• 易于使用。只是复制过去。
• 在实际项目中测试。

extension URL
{
    func foreachRow(_ mode:String, _ rowParcer:((String, Int)->Bool) )
    {
        //Here we should use path not the absoluteString (wich contains file://)
        let path = self.path
        
        guard let cfilePath = (path as NSString).utf8String,
              let m = (mode as NSString).utf8String
        else {return}
        
        //Open file with specific mode (just use "r")
        guard let file = fopen(cfilePath, m)
        else {
            print("fopen can't open file: \"\(path)\", mode: \"\(mode)\"")
            return
        }
        
        //Row capacity for getline() 
        var cap = 0
        
        var row_index = 0
        
        //Row container for getline()
        var cline:UnsafeMutablePointer<CChar>? = nil
        
        //Free memory and close file at the end
        defer{free(cline); fclose(file)}
                    
        while getline(&cline, &cap, file) > 0
        {
            if let crow = cline,
               // the output line may contain '\n' that's why we filtered it
               let s = String(utf8String: crow)?.filter({($0.asciiValue ?? 0) >= 32}) 
            {
                if rowParcer(s, row_index)
                {
                    break
                }
            }
            
            row_index += 1
        }
    }
}

用法：

    let token = "mtllib "

    var mtlRow = ""

    largeObjFileURL.foreachRow("r"){ (row, i) -> Bool in

        if row.hasPrefix(token)
        {
            mtlRow = row

            return true // end of file reading
        }

        return false // continue file reading
    }