Append text or data to text file in Swift

Question

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)")
    }

davidrynn · Answer 1 · 2016-11-21T19:37:01.530

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.

score 38 · Answer 2 · answered Dec 06 '14 at 01:45

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)")
    }
}

You would be amazed at how easy it is to convert this to swift 3 if you just click on those little red circles with the white dots in Xcode. — Chris, Mar 18 '17 at 03:02

score 34 · Answer 3 · answered Nov 04 '19 at 16:08

A variation over some of the posted answers, with following characteristics:

based on Swift 5
accessible as a static function
appends new entries to the end of the file, if it exists
creates the file, if it doesn't exist
no cast to NS objects (more Swiftly)

fails silently if the text cannot be encoded or the path does not exist

class Logger {

    static var logFile: URL? {
        guard let documentsDirectory = FileManager.default.urls(for: .documentDirectory, in: .userDomainMask).first else { return nil }
        let formatter = DateFormatter()
        formatter.dateFormat = "dd-MM-yyyy"
        let dateString = formatter.string(from: Date())
        let fileName = "\(dateString).log"
        return documentsDirectory.appendingPathComponent(fileName)
    }

    static func log(_ message: String) {
        guard let logFile = logFile else {
            return
        }

        let formatter = DateFormatter()
        formatter.dateFormat = "HH:mm:ss"
        let timestamp = formatter.string(from: Date())
        guard let data = (timestamp + ": " + message + "\n").data(using: String.Encoding.utf8) else { return }

        if FileManager.default.fileExists(atPath: logFile.path) {
            if let fileHandle = try? FileHandle(forWritingTo: logFile) {
                fileHandle.seekToEndOfFile()
                fileHandle.write(data)
                fileHandle.closeFile()
            }
        } else {
            try? data.write(to: logFile, options: .atomicWrite)
        }
    }
}

Hi @atineoSE do you have any information on what to use after Write on FileManager is deprecated? Looks like var writeabilityHandler: ((FileHandle) -> Void)? is async version — Yigit Alp Ciray, Oct 16 '20 at 16:33

Hiền Đỗ · Answer 4 · 2020-10-13T04:49:11.827

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

let monkeyLine = "\nAdding a  to the end of the file via FileHandle"

if let fileUpdater = try? FileHandle(forUpdating: newFileUrl) {

    // Function which when called will cause all updates to start from end of the file
    fileUpdater.seekToEndOfFile()

    // Which lets the caller move editing to any position within the file by supplying an offset
    fileUpdater.write(monkeyLine.data(using: .utf8)!)

    // Once we convert our new content to data and write it, we close the file and that’s it!
    fileUpdater.closeFile()
}

score 16 · Answer 5 · answered Sep 25 '15 at 17:23

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")
}

Luc-Olivier · Answer 6 · 2020-07-21T19:59:09.887

In order to stay in the spirit of @PointZero Two. Here an update of his code for Swift 4.1

extension String {
    func appendLine(to url: URL) throws {
        try self.appending("\n").append(to: url)
    }
    func append(to url: URL) throws {
        let data = self.data(using: String.Encoding.utf8)
        try data?.append(to: url)
    }
}

extension Data {
    func append(to url: URL) throws {
        if let fileHandle = try? FileHandle(forWritingTo: url) {
            defer {
                fileHandle.closeFile()
            }
            fileHandle.seekToEndOfFile()
            fileHandle.write(self)
        } else {
            try write(to: url)
        }
    }
}

jason z · Answer 7 · 2018-05-16T19:38:51.777

Update: I wrote a blog post on this, which you can find here!

Keeping things Swifty, here is an example using a FileWriter protocol with default implementation (Swift 4.1 at the time of this writing):

To use this, have your entity (class, struct, enum) conform to this protocol and call the write function (fyi, it throws!).
Writes to the document directory.
Will append to the text file if the file exists.
Will create a new file if the text file doesn't exist.

Note: this is only for text. You could do something similar to write/append Data.

import Foundation

enum FileWriteError: Error {
    case directoryDoesntExist
    case convertToDataIssue
}

protocol FileWriter {
    var fileName: String { get }
    func write(_ text: String) throws
}

extension FileWriter {
    var fileName: String { return "File.txt" }

    func write(_ text: String) throws {
        guard let dir = FileManager.default.urls(for: .documentDirectory, in: .userDomainMask).first else {
            throw FileWriteError.directoryDoesntExist
        }

        let encoding = String.Encoding.utf8

        guard let data = text.data(using: encoding) else {
            throw FileWriteError.convertToDataIssue
        }

        let fileUrl = dir.appendingPathComponent(fileName)

        if let fileHandle = FileHandle(forWritingAtPath: fileUrl.path) {
            fileHandle.seekToEndOfFile()
            fileHandle.write(data)
        } else {
            try text.write(to: fileUrl, atomically: false, encoding: encoding)
        }
    }
}

i'm trying to use this but I don't know how to do this... "have your entity (class, struct, enum) conform to this protocol and call the write function (fyi, it throws!)". How do I actually call this to save the text from a text view in a VC? — nc14, Jun 06 '19 at 14:30
This ties two concepts together: conforming to a protocol & using protocol extensions to provide a default functionality. For starters, your entity should conform to the protocol (e.g., class MyClass: FileWriter). Now since there is an extension with default implementation for the protocol requirements on the `FileWriter` protocol, your entity, MyClass in this example, gets the write functionality for free! So, you can just call that function on an instance of MyClass. (e.g., let myClassInstance = MyClass(); try! myClassInstance.write("hello")). — jason z, Jun 07 '19 at 15:11
Also, if you want a more detailed explanation with examples, check out my blog post that I included a link to in my answer above :). — jason z, Jun 07 '19 at 15:12

score 3 · Answer 8 · answered Nov 17 '21 at 14:48

All answers (as of now) recreate the FileHandle for every write operation. This may be fine for most applications, but this is also rather inefficient: A syscall is made, and the filesystem is accessed each time you create the FileHandle.

To avoid creating the filehandle multiple times, use something like:

final class FileHandleBuffer {
    let fileHandle: FileHandle
    let size: Int
    private var buffer: Data

    init(fileHandle: FileHandle, size: Int = 1024 * 1024) {
        self.fileHandle = fileHandle
        self.size = size
        self.buffer = Data(capacity: size)
    }

    deinit { try! flush() }

    func flush() throws {
        try fileHandle.write(contentsOf: buffer)
        buffer = Data(capacity: size)
    }

    func write(_ data: Data) throws {
        buffer.append(data)
        if buffer.count > size {
            try flush()
        }
    }
}

// USAGE

// Create the file if it does not yet exist
FileManager.default.createFile(atPath: fileURL.path, contents: nil)

let fileHandle = try FileHandle(forWritingTo: fileURL)

// Seek will make sure to not overwrite the existing content
// Skip the seek to overwrite the file
try fileHandle.seekToEnd()


let buffer = FileHandleBuffer(fileHandle: fileHandle)
for i in 0..<count {
    let data = getData() // Your implementation
    try buffer.write(data)
    print(i)
}

Append text or data to text file in Swift

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