let data = dataToWrite.first!
self.outputStream.write(&data, maxLength: data.count)

Cannot convert value of type 'Data' to expected argument type 'UInt8'

let data = ... // a Data value
let bytesWritten = data.withUnsafeBytes { outputStream.write($0, maxLength: data.count) }

/// Access the bytes in the data.
///
/// - warning: The byte pointer argument should not be stored and used outside of the lifetime of the call to the closure.
public func withUnsafeBytes<ResultType, ContentType>(_ body: @noescape (UnsafePointer<ContentType>) throws -> ResultType) rethrows -> ResultType

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

    func write(_ string: String, encoding: String.Encoding = String.Encoding.utf8, allowLossyConversion: Bool = true) -> Int {
        if let data = string.data(using: encoding, allowLossyConversion: allowLossyConversion) {
            var bytesRemaining = data.count
            var totalBytesWritten = 0

            while bytesRemaining > 0 {
                let bytesWritten = data.withUnsafeBytes {
                    self.write(
                        $0.advanced(by: totalBytesWritten),
                        maxLength: bytesRemaining
                    )
                }
                if bytesWritten < 0 {
                    // "Can not OutputStream.write(): (self.streamError?.localizedDescription)"
                    return -1
                } else if bytesWritten == 0 {
                    // "OutputStream.write() returned 0"
                    return totalBytesWritten
                }

                bytesRemaining -= bytesWritten
                totalBytesWritten += bytesWritten
            }

            return totalBytesWritten
        }

        return -1
    }
}

let data: NSData = dataToWrite.first!
self.outputStream.write(UnsafePointer<UInt8>(data.bytes), maxLength: data.length)

extension OutputStream {
  func write(data: Data) -> Int {
    return data.withUnsafeBytes { write($0, maxLength: data.count) }
  }
}

extension InputStream {
  func read(data: inout Data) -> Int {
    return data.withUnsafeMutableBytes { read($0, maxLength: data.count) }
  }
}