How to open file and append a string in it, swift

If you want to be able to control whether to append or not, consider using OutputStream. For example:

do {
    let fileURL = try FileManager.default.url(for: .documentDirectory, in: .userDomainMask, appropriateFor: nil, create: false)
        .appendingPathComponent("votes.txt")
    
    guard let outputStream = OutputStream(url: fileURL, append: true) else {
        print("Unable to open file")
        return
    }

    outputStream.open()
    let text = "some text\n"
    try outputStream.write(text)
    outputStream.close()
} catch {
    print(error)
}

By the way, this is an extension that lets you easily write a String (or Data) to an OutputStream:

extension OutputStream {
    enum OutputStreamError: Error {
        case stringConversionFailure
        case bufferFailure
        case writeFailure
    }

    /// Write `String` to `OutputStream`
    ///
    /// - parameter string:                The `String` to write.
    /// - parameter encoding:              The `String.Encoding` to use when writing the string. This will default to `.utf8`.
    /// - parameter allowLossyConversion:  Whether to permit lossy conversion when writing the string. Defaults to `false`.

    func write(_ string: String, encoding: String.Encoding = .utf8, allowLossyConversion: Bool = false) throws {
        guard let data = string.data(using: encoding, allowLossyConversion: allowLossyConversion) else {
            throw OutputStreamError.stringConversionFailure
        }
        try write(data)
    }

    /// Write `Data` to `OutputStream`
    ///
    /// - parameter data:                  The `Data` to write.

    func write(_ data: Data) throws {
        try data.withUnsafeBytes { (buffer: UnsafeRawBufferPointer) throws in
            guard var pointer = buffer.baseAddress?.assumingMemoryBound(to: UInt8.self) else {
                throw OutputStreamError.bufferFailure
            }

            var bytesRemaining = buffer.count

            while bytesRemaining > 0 {
                let bytesWritten = write(pointer, maxLength: bytesRemaining)
                if bytesWritten < 0 {
                    throw OutputStreamError.writeFailure
                }

                bytesRemaining -= bytesWritten
                pointer += bytesWritten
            }
        }
    }
}

For Swift 2 rendition, see previous revision of this answer.