在Swift？中逐行读取文本文件？

Question

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

文本文件.txt包含以下内容：

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

以下是目前的…

  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
    }

如果有别的方法，请告诉我。非常感谢。

Answer 1

这并不漂亮，但我相信它是可行的(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
    }
  })
}

以下是您可以使用它的方法：

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

Answer 2

雨燕5.5

下面的解决方案显示了如何一次读取一行。这与将整个内容读入内存完全不同。如果您有一个大文件要读取，则逐行读取可以很好地缩放。对于大文件，将整个文件放入内存并不能很好地扩展。

下面的例子使用了一个while循环，当没有更多的行时会退出，但是如果愿意，您可以选择不同的行数来读取。

代码的工作原理如下：

1. 创建一个告诉文件位置的URL
2. 确保文件存在
3. 打开文件读取
4. 设置一些读取的初始变量
5. 使用getLine（）
读取每一行
6. 完成后关闭文件并释放缓冲区

如果您愿意，您可以使代码不那么冗长；我已经包含了注释来解释变量的用途。

雨燕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

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：在用Obj-C编写的iOS SDK中逐行读取文件并使用NSString

检查Swift以前版本的编辑历史。