An efficient way of writing to file, swift

Question

I'm getting data from sensors using bluetooth, I want to append the string of data I get to the end of file.

When I tried the regular approach

if let dir = NSSearchPathForDirectoriesInDomains(NSSearchPathDirectory.DocumentDirectory, NSSearchPathDomainMask.AllDomainsMask, true).first {
        let path = NSURL(fileURLWithPath: dir).URLByAppendingPathComponent(self.file)

        do {
            try text.writeToURL(path, atomically: false, encoding: NSUTF8StringEncoding)
        }
        catch {/* error handling here */}

My app started to slow down until even labels were not updating anymore.

Tried using dispatch_async to do in background thread but still it was slowing down my app.

What approach should I use? I read sth about stream but failed to find some solutions in swift I could rely on

Why are you reading all the contents of the file on each write? You could just write at the end of the file — redent84, Aug 01 '16 at 11:51
I'd be happy if you could privide an example how exactly I should do that — theDC, Aug 01 '16 at 11:52
You should try this library, makes it much more simple: https://github.com/nvzqz/FileKit — MCMatan, Aug 01 '16 at 12:07

score 9 · Accepted Answer · edited May 23 '17 at 12:08

Probably your bluetooth is reading data faster than you are performing your file operations. You can optimize it by appending the text to the file instead of reading all the content on each write operation. You could also reuse the file handler between writes and keep the file open.

This sample is extracted from this answer:

struct MyStreamer: OutputStreamType {
    lazy var fileHandle: NSFileHandle? = {
        let fileHandle = NSFileHandle(forWritingAtPath: self.logPath)
        return fileHandle
    }()

    lazy var logPath: String = {
        let path : NSString = NSSearchPathForDirectoriesInDomains(NSSearchPathDirectory.DocumentDirectory, NSSearchPathDomainMask.AllDomainsMask, true).first!
        let filePath = (path as NSString).stringByAppendingPathComponent("log.txt")

        if !NSFileManager.defaultManager().fileExistsAtPath(filePath) {
            NSFileManager.defaultManager().createFileAtPath(filePath, contents: nil, attributes: nil)
        }
        print(filePath)
        return filePath

    }()

    mutating func write(string: String) {
        print(fileHandle)
        fileHandle?.seekToEndOfFile()
        fileHandle?.writeData(string.dataUsingEncoding(NSUTF8StringEncoding)!)
    }
}

Then, you can create a single streamer and reuse it in different writes:

var myStream = MyStreamer()
myStream.write("First of all")
myStream.write("Then after")
myStream.write("And, finally")

In this case, you have the bonus that MyStreamer is also a OutputStreamType, so you can use it like this:

var myStream = MyStreamer()
print("First of all", toStream: &myStream )
print("Then after", toStream: &myStream)
print("And, finally", toStream: &myStream)

Finally I'd recommend you to move 'log.txt' string to a instance variable and pass it as a constructor parameter:

var myStream = MyStreamer("log.txt")

More info about file handler in the Apple Docs.

thanks a lot, will check it later on and get back to you with the outcome! :) — theDC, Aug 01 '16 at 12:16

score 3 · Answer 2 · answered May 10 '20 at 12:00

update @redent84's, to work in Swift 5

Code:

struct MyStreamer{
    lazy var fileHandle = FileHandle(forWritingAtPath: logPath)


    lazy var logPath: String = {
        let path = NSSearchPathForDirectoriesInDomains(.documentDirectory, .allDomainsMask, true)[0]
        let filePath = path + "/log.txt"
        if FileManager.default.fileExists(atPath: filePath) == false{
            FileManager.default.createFile(atPath: filePath, contents: nil, attributes: nil)
        }
        print(filePath)
        return filePath

    }()

    mutating func write(_ string: String) {
        print(fileHandle?.description ?? "呵呵")
        fileHandle?.seekToEndOfFile()
        if let data = string.data(using: String.Encoding.utf8){
            fileHandle?.write(data)
        }
    }
}

Usage:

var myStream = MyStreamer()
myStream.write("First of all")
myStream.write("Then after")
myStream.write("And, finally")

score -1 · Answer 3 · answered Aug 01 '16 at 12:05

try to write file like this..

var paths: [AnyObject] = NSSearchPathForDirectoriesInDomains(.DocumentDirectory, .UserDomainMask, true)
let filePath = paths[0].stringByAppendingString("/filename.mov")

do 
   {
     try NSFileManager.defaultManager().removeItemAtURL(outputURL)
   }
catch  
  {

            error as NSError
   }

do {
    try text.writeToURL(path, atomically: false, encoding: NSUTF8StringEncoding)
    }

I mean to say at last is you have to remove first.. If any query you can ask me

An efficient way of writing to file, swift

3 Answers3

Linked