Append text or data to text file in Swift

问题:

I already have read Read and write data from text file

I need to append the data (a string) to the end of my text file.
One obvious way to do it is to read the file from disk and append the string to the end of it and write it back, but it is not efficient, especially if you are dealing with large files and doing in often.

So the question is "How to append string to the end of a text file, without reading the file and writing the whole thing back"?

so far I have:

    let dir:NSURL = NSFileManager.defaultManager().URLsForDirectory(NSSearchPathDirectory.CachesDirectory, inDomains: NSSearchPathDomainMask.UserDomainMask).last as NSURL
    let fileurl =  dir.URLByAppendingPathComponent("log.txt")
    var err:NSError?
    // until we find a way to append stuff to files
    if let current_content_of_file = NSString(contentsOfURL: fileurl, encoding: NSUTF8StringEncoding, error: &err) {
        "\(current_content_of_file)\n\(NSDate()) -> \(object)".writeToURL(fileurl, atomically: true, encoding: NSUTF8StringEncoding, error: &err)
    }else {
        "\(NSDate()) -> \(object)".writeToURL(fileurl, atomically: true, encoding: NSUTF8StringEncoding, error: &err)
    }
    if err != nil{
        println("CANNOT LOG: \(err)")
    }

回答1:

You should use NSFileHandle, it can seek to the end of the file

let dir:NSURL = NSFileManager.defaultManager().URLsForDirectory(NSSearchPathDirectory.CachesDirectory, inDomains: NSSearchPathDomainMask.UserDomainMask).last as NSURL
let fileurl =  dir.URLByAppendingPathComponent("log.txt")

let string = "\(NSDate())\n"
let data = string.dataUsingEncoding(NSUTF8StringEncoding, allowLossyConversion: false)!

if NSFileManager.defaultManager().fileExistsAtPath(fileurl.path!) {
    var err:NSError?
    if let fileHandle = NSFileHandle(forWritingToURL: fileurl, error: &err) {
        fileHandle.seekToEndOfFile()
        fileHandle.writeData(data)
        fileHandle.closeFile()
    }
    else {
        println("Can't open fileHandle \(err)")
    }
}
else {
    var err:NSError?
    if !data.writeToURL(fileurl, options: .DataWritingAtomic, error: &err) {
        println("Can't write \(err)")
    }
}

回答2:

Here's an update for PointZeroTwo's answer in Swift 3.0, with one quick note - in the playground testing using a simple filepath works, but in my actual app I needed to build the URL using .documentDirectory (or which ever directory you chose to use for reading and writing - make sure it's consistent throughout your app):

extension String {
    func appendLineToURL(fileURL: URL) throws {
         try (self + "\n").appendToURL(fileURL: fileURL)
     }

     func appendToURL(fileURL: URL) throws {
         let data = self.data(using: String.Encoding.utf8)!
         try data.append(fileURL: fileURL)
     }
 }

 extension Data {
     func append(fileURL: URL) throws {
         if let fileHandle = FileHandle(forWritingAtPath: fileURL.path) {
             defer {
                 fileHandle.closeFile()
             }
             fileHandle.seekToEndOfFile()
             fileHandle.write(self)
         }
         else {
             try write(to: fileURL, options: .atomic)
         }
     }
 }
 //test
 do {
     let dir: URL = FileManager.default.urls(for: .documentDirectory, in: .userDomainMask).last! as URL
     let url = dir.appendingPathComponent("logFile.txt")
     try "Test \(Date())".appendLineToURL(fileURL: url as URL)
     let result = try String(contentsOf: url as URL, encoding: String.Encoding.utf8)
 }
 catch {
     print("Could not write to file")
 }

Thanks PointZeroTwo.

回答3:

Here's a version for Swift 2, using extension methods on String and NSData.

//: Playground - noun: a place where people can play

import UIKit

extension String {
    func appendLineToURL(fileURL: NSURL) throws {
        try self.stringByAppendingString("\n").appendToURL(fileURL)
    }

    func appendToURL(fileURL: NSURL) throws {
        let data = self.dataUsingEncoding(NSUTF8StringEncoding)!
        try data.appendToURL(fileURL)
    }
}

extension NSData {
    func appendToURL(fileURL: NSURL) throws {
        if let fileHandle = try? NSFileHandle(forWritingToURL: fileURL) {
            defer {
                fileHandle.closeFile()
            }
            fileHandle.seekToEndOfFile()
            fileHandle.writeData(self)
        }
        else {
            try writeToURL(fileURL, options: .DataWritingAtomic)
        }
    }
}

// Test
do {
    let url = NSURL(fileURLWithPath: "test.log")
    try "Test \(NSDate())".appendLineToURL(url)
    let result = try String(contentsOfURL: url)
}
catch {
    print("Could not write to file")
}

回答4:

Here is a way to update a file in a much more efficient way.

let monkeyLine = "\nAdding a