如果您希望能够控制是否追加，请考虑使用OutputStream。例如：

SWIFT 3

let fileURL = try! FileManager.default.url(for: .documentDirectory, in: .userDomainMask, appropriateFor: nil, create: false)
    .appendingPathComponent("votes.txt")

if let outputStream = OutputStream(url: fileURL, append: true) {
    outputStream.open()
    let text = "some text\n"
    let bytesWritten = outputStream.write(text)
    if bytesWritten < 0 { print("write failure") }
    outputStream.close()
} else {
    print("Unable to open file")
}

顺便说一句，这是一个扩展，可以让你轻松地编写String到OutputStream：

extension OutputStream {

    /// 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`.
    ///
    /// - returns:                         Return total number of bytes written upon success. Return `-1` upon failure.

    func write(_ string: String, encoding: String.Encoding = .utf8, allowLossyConversion: Bool = false) -> Int {

        if let data = string.data(using: encoding, allowLossyConversion: allowLossyConversion) {
            return data.withUnsafeBytes { (bytes: UnsafePointer<UInt8>) -> Int in
                var pointer = bytes
                var bytesRemaining = data.count
                var totalBytesWritten = 0

                while bytesRemaining > 0 {
                    let bytesWritten = self.write(pointer, maxLength: bytesRemaining)
                    if bytesWritten < 0 {
                        return -1
                    }

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

                return totalBytesWritten
            }
        }

        return -1
    }

}

或者，在 Swift 2中 使用NSOutputStream：

let documents = try! NSFileManager.defaultManager().URLForDirectory(.DocumentDirectory, inDomain: .UserDomainMask, appropriateForURL: nil, create: false)
let path = documents.URLByAppendingPathComponent("votes").path!

if let outputStream = NSOutputStream(toFileAtPath: path, append: true) {
    outputStream.open()
    let text = "some text"
    outputStream.write(text)

    outputStream.close()
} else {
    print("Unable to open file")
}

和

extension NSOutputStream {

    /// Write `String` to `NSOutputStream`
    ///
    /// - parameter string:                The string to write.
    /// - parameter encoding:              The NSStringEncoding 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`.
    ///
    /// - returns:                         Return total number of bytes written upon success. Return -1 upon failure.

    func write(string: String, encoding: NSStringEncoding = NSUTF8StringEncoding, allowLossyConversion: Bool = false) -> Int {
        if let data = string.dataUsingEncoding(encoding, allowLossyConversion: allowLossyConversion) {
            var bytes = UnsafePointer<UInt8>(data.bytes)
            var bytesRemaining = data.length
            var totalBytesWritten = 0

            while bytesRemaining > 0 {
                let bytesWritten = self.write(bytes, maxLength: bytesRemaining)
                if bytesWritten < 0 {
                    return -1
                }

                bytesRemaining -= bytesWritten
                bytes += bytesWritten
                totalBytesWritten += bytesWritten
            }

            return totalBytesWritten
        }

        return -1
    }

}