Android 藍牙連接通訊實作心得

最近因為專案需求，進行了一些藍牙通訊的研究。
首先來看官方線上文件：Bluetooth overview - Connect devices

前面描述開啟藍牙或搜尋、被搜尋……等較為基本的部份先跳過。從連接裝置（Connect devices）這一節開始，首先要將兩個裝置配對，然後使用 BluetoothAdapter 的 getBondedDevices() 方法取得已配對的裝置資訊，並從中取出 MAC address。

	Set<BluetoothDevice> pairedDevices = mBluetoothAdapter.getBondedDevices();
	// If there are paired devices
	if (pairedDevices.size() > 0) {
	// Loop through paired devices
	for (BluetoothDevice device : pairedDevices) {
	// Add the name and address to an array adapter to show in a ListView
	mArrayAdapter.add(device.getName() + "\n" + device.getAddress());
	}
	}

view raw BluetoothFragment.java hosted with ❤ by GitHub

接下來我們需要一組 UUID （通用唯一識別號）作為藍牙服務的識別碼。在網路上有許多 UUID 產生器，例如：https://www.uuidgenerator.net/。找一個喜歡的使用即可。

	private final String NAME = "MyBTService";
	private final UUID MY_UUID = UUID.fromString("00001101-0000-1000-8000-00805F9B34FB");

view raw MyBluetoothService.java hosted with ❤ by GitHub

因為藍牙的連接模式是由一個 Client 端主動發起連線給被動監聽的 Server 端，因此程式碼需要分別撰寫 Server 與 Client 端的動作。

首先來看接收藍牙連接的 Server 端程式碼：

	private class AcceptThread extends Thread {
	private final BluetoothServerSocket mmServerSocket;
	public AcceptThread() {
	// Use a temporary object that is later assigned to mmServerSocket
	// because mmServerSocket is final.
	BluetoothServerSocket tmp = null;
	try {
	// MY_UUID is the app's UUID string, also used by the client code.
	tmp = mBluetoothAdapter.listenUsingRfcommWithServiceRecord(NAME, MY_UUID);
	} catch (IOException e) {
	Log.e(TAG, "Socket's listen() method failed", e);
	}
	mmServerSocket = tmp;
	}
	public void run() {
	BluetoothSocket socket = null;
	// Keep listening until exception occurs or a socket is returned.
	while (true) {
	try {
	socket = mmServerSocket.accept();
	} catch (IOException e) {
	Log.e(TAG, "Socket's accept() method failed", e);
	break;
	}
	if (socket != null) {
	// A connection was accepted. Perform work associated with
	// the connection in a separate thread.
	manageMyConnectedSocket(socket);
	mmServerSocket.close();
	break;
	}
	}
	}
	// Closes the connect socket and causes the thread to finish.
	public void cancel() {
	try {
	mmServerSocket.close();
	} catch (IOException e) {
	Log.e(TAG, "Could not close the connect socket", e);
	}
	}
	}

view raw MyBluetoothService_server.java hosted with ❤ by GitHub

接受連接的步驟為：

1. 調用 listenUsingRfcommWithServiceRecord(String, UUID) 取得 BluetoothServerSocket
2. 調用 accept() 開始監聽連線請求
3. 在 manageConnectedSocket(socket) 副程式中管理 socket 連線。這部份後面再詳述。
4. 調用 close() 關閉監聽連線請求

然後是藍牙連接的 Client 端程式碼：

	private class ConnectThread extends Thread {
	private final BluetoothSocket mmSocket;
	private final BluetoothDevice mmDevice;
	public ConnectThread(BluetoothDevice device) {
	// Use a temporary object that is later assigned to mmSocket
	// because mmSocket is final.
	BluetoothSocket tmp = null;
	mmDevice = device;
	try {
	// Get a BluetoothSocket to connect with the given BluetoothDevice.
	// MY_UUID is the app's UUID string, also used in the server code.
	tmp = device.createRfcommSocketToServiceRecord(MY_UUID);
	} catch (IOException e) {
	Log.e(TAG, "Socket's create() method failed", e);
	}
	mmSocket = tmp;
	}
	public void run() {
	// Cancel discovery because it otherwise slows down the connection.
	mBluetoothAdapter.cancelDiscovery();
	try {
	// Connect to the remote device through the socket. This call blocks
	// until it succeeds or throws an exception.
	mmSocket.connect();
	} catch (IOException connectException) {
	// Unable to connect; close the socket and return.
	try {
	mmSocket.close();
	} catch (IOException closeException) {
	Log.e(TAG, "Could not close the client socket", closeException);
	}
	return;
	}
	// The connection attempt succeeded. Perform work associated with
	// the connection in a separate thread.
	manageMyConnectedSocket(mmSocket);
	}
	// Closes the client socket and causes the thread to finish.
	public void cancel() {
	try {
	mmSocket.close();
	} catch (IOException e) {
	Log.e(TAG, "Could not close the client socket", e);
	}
	}
	}

view raw MyBluetoothService_client.java hosted with ❤ by GitHub

發起連接的步驟為：

1. 調用 createRfcommSocketToServiceRecord(UUID) 取得 BluetoothSocket。
2. 調用 connect() 發起連接
3. 在 manageConnectedSocket(socket) 副程式中管理 socket 連線。

到這邊可以發現到，Client 和 Server 都在第三步驟調用了 manageConnectedSocket(socket) 副程式，但是官方線上文件並沒有進一步說明這個副程式的內容。
所幸我在官方範例程式中找到了藍牙連接的範例程式：BluetoothChat
從中得知，原來是要啟動另一個 Thread：ConnectedThread

	private void manageConnectedSocket(BluetoothSocket socket) {
	connectedThread = new ConnectedThread(socket);
	connectedThread.start();
	}

view raw MyBluetoothService_manageConnectedSocket.java hosted with ❤ by GitHub

而 ConnectedThread 的程式碼如下：

	private class ConnectedThread extends Thread {
	private final BluetoothSocket mmSocket;
	private final InputStream mmInStream;
	private final OutputStream mmOutStream;
	public ConnectedThread(BluetoothSocket socket) {
	mmSocket = socket;
	InputStream tmpIn = null;
	OutputStream tmpOut = null;
	// Get the input and output streams, using temp objects because
	// member streams are final
	try {
	tmpIn = socket.getInputStream();
	tmpOut = socket.getOutputStream();
	} catch (IOException e) { }
	mmInStream = tmpIn;
	mmOutStream = tmpOut;
	}
	public void run() {
	byte[] buffer = new byte[1024]; // buffer store for the stream
	int bytes; // bytes returned from read()
	// Keep listening to the InputStream until an exception occurs
	while (true) {
	try {
	// Read from the InputStream
	bytes = mmInStream.read(buffer);
	// Send the obtained bytes to the UI activity
	mHandler.obtainMessage(MESSAGE_READ, bytes, -1, buffer)
	.sendToTarget();
	} catch (IOException e) {
	break;
	}
	}
	}
	/* Call this from the main activity to send data to the remote device */
	public void write(byte[] bytes) {
	try {
	mmOutStream.write(bytes);
	} catch (IOException e) { }
	}
	/* Call this from the main activity to shutdown the connection */
	public void cancel() {
	try {
	mmSocket.close();
	} catch (IOException e) { }
	}
	}

view raw MyBluetoothService_connect.java hosted with ❤ by GitHub

ConnectedThread 透過 BluetoothSocket 取得了 InputStream 和 OutputStream 來處理數據傳輸，然後再透過 read(byte[]) 和 write(byte[]) 方法來讀取和寫入數據。

之後只要在 Activity（或 Fragment）中宣告一個 Handler，就可以用來接收對方傳來的數據並更新 UI；也能直接調用 ConnectedThread 的 write(byte[]) 來寫入數據給對方。

	private class UIHandler extends Handler {
	@Override
	public void handleMessage(Message msg) {
	super.handleMessage(msg);
	switch (msg.what) {
	case MESSAGE_READ:
	byte[] writeBuf = (byte[]) msg.obj;
	String writeMessage = new String(writeBuf);
	msgList.add(writeMessage);
	break;
	default:
	break;
	}
	}
	}

view raw BluetoothFragment_handler.java hosted with ❤ by GitHub

最後附上一張傳輸成功的截圖