SerialHandler.cs

using UnityEngine;
using System.Collections;
using System.IO.Ports;
using System.Threading;

public class SerialHandler : MonoBehaviour
{
    public delegate void SerialDataReceivedEventHandler(string message);
    public event SerialDataReceivedEventHandler OnDataReceived;

    public string portName = "/dev/tty.usbmodem1421";
    public int baudRate    = 57600;

    private SerialPort serialPort_;
    private Thread thread_;
    private bool isRunning_ = false;

    private string message_;
    private bool isNewMessageReceived_ = false;

    void Awake()
    {
		if (!Application.isEditor) {
			LoadPortFromFile();
		}
        Open();
    }

    void Update()
    {
        if (isNewMessageReceived_) {
			if (OnDataReceived != null && OnDataReceived.GetInvocationList().Length > 0) {
            	OnDataReceived(message_);
			}
        }
    }

    void OnDestroy()
    {
        Close();
    }

	[ContextMenu("Load port from file")]
	private void LoadPortFromFile()
	{
		var path = System.IO.Path.Combine(Application.streamingAssetsPath, "Serial.txt");
		using (var sr = System.IO.File.OpenText( path )) {
			portName = sr.ReadLine();
		}
	}

    private void Open()
    {
		try {
	        serialPort_ = new SerialPort(portName, baudRate, Parity.None, 8, StopBits.One);
	        serialPort_.Open();

	        isRunning_ = true;

	        thread_ = new Thread(Read);
	        thread_.Start();

			Debug.Log("Connected!");
		} catch (System.Exception e) {
			Debug.LogError(e.Message);
		}
    }

    private void Close()
    {
        isRunning_ = false;

        if (thread_ != null && thread_.IsAlive) {
            thread_.Join();
        }

        if (serialPort_ != null && serialPort_.IsOpen) {
            serialPort_.Close();
            serialPort_.Dispose();
        }
    }

    private void Read()
    {
        while (isRunning_ && serialPort_ != null && serialPort_.IsOpen) {
            try {
                if (serialPort_.BytesToRead > 0) {
                    message_ = serialPort_.ReadLine();
                   	isNewMessageReceived_ = true;
                }
            } catch (System.Exception e) {
                Debug.LogWarning(e.Message);
            }
        }
    }

    public void Write(string message)
    {
        try {
            serialPort_.Write(message);
        } catch (System.Exception e) {
            Debug.LogWarning(e.Message);
        }
    }
}