Buffered writing to a file with Swift OutputStreamType and GCD i/o channel (Original version in Objective-C: https://gist.github.com/algal/54467df650c875827f2b)

FileOutputStream.swift

// Ref:
// https://gist.github.com/algal/54467df650c875827f2b
// https://developer.apple.com/documentation/dispatch/dispatchio/2892309-init

class FileOutputStream: OutputStream {
    private var filepath: URL!
    private var channel: DispatchIO!
    
    // Initialized an output stream that writes to `filepath`.
    // - requires: `filepath` must be a file path URL, for a file that can be created.
    init?(filepath: URL) {
        super.init(url: filepath, append: true)
        self.filepath = filepath
        if let cpath = filepath.path.cString(using: .utf8) {
            let outputflag: Int32 = O_CREAT | O_WRONLY
            let mode: mode_t = S_IRUSR | S_IWUSR | S_IRGRP | S_IROTH
            self.channel = DispatchIO(type: .stream,
                                      path: cpath,
                                      oflag: outputflag,
                                      mode: mode,
                                      queue: .global(qos: .background)) { errorCode in
                if errorCode != 0 {
                    print("FileOutputStream: error creating io channel")
                }
            }
        } else {
            self.channel = nil
            return nil
        }
    }
    
    //  Initializes output stream to write to `filename` in the user's Documents directory.
    convenience init?(filename: String) {
        if let pathURL = FileManager.default.urls(for: .documentDirectory, in: .userDomainMask).first?.appendingPathComponent(filename) {
            self.init(filepath: pathURL)
        } else {
            return nil
        }
    }
    
    func write(string: String) {
        if let dataString = string.data(using: .utf8) {
            let dispatchData = dataString.withUnsafeBytes { DispatchData(bytes: $0)}
            channel.write(offset: 0, data: dispatchData, queue: .global(qos: .background)) { done, data, errorCode in
                if errorCode != 0 {
                    print("FileOutputStream: error creating io channel")
                }
            }
        }
    }
}