低功耗蓝牙（BLE）开发——如何妥善处理包大小（MTU）限制

最近出于项目需要，花了几天时间，研究了一下低功耗蓝牙（BLE）的开发，为了让有需要的小伙伴们少走弯路，现将我所遇到的问题分享出来。

刚开始，我被低功耗蓝牙（BLE）的基础概念所困扰，想当然的以为低功耗蓝牙仅仅是传统蓝牙的升级版本，只要传统蓝牙能做的，低功耗蓝牙理应可以做到。这从一开始就陷入误区了。此外，也想当然地把低功耗蓝牙外设和中心的概念直接往C/S模式硬套，认为低功耗蓝牙的外设就是client，而中心就是server。这当然是不对的。蓝牙的外设在往外发广播，中心搜索到广播之后，可以发起并建立连接，外设和外设，中心和中心都无法直接连接，只有外设和中心搭配才可以建立连接。仔细分析，其实外设更像是server端。

为什么要做低功耗蓝牙开发？

很简单，由于ios的同事提出，无法用传统蓝牙的socket接口与我们的设备通讯，ios仅有关于BLE的开发库（sdk）。

低功耗蓝牙是什么？

从传统蓝牙（或说标准蓝牙）4.0的版本开始，新开发了低功耗蓝牙的分支，后来的蓝牙基本都实现了双模，也就是既支持传统蓝牙，又支持低功耗蓝牙。潜台词是，有的模块可能仅支持低功耗蓝牙，或者传统蓝牙。其实，传统蓝牙和低功耗蓝牙，不仅协议不同，通讯交互的流程不同，连硬件芯片也是不一样的。对我来说，最直观的体验就是，传统蓝牙需要配对，而低功耗蓝牙不需要配对，此外传统蓝牙的数据传输率还过的去（没仔细分析过，大概是超过1024字节的数据，会被自动分包吧），而低功耗蓝牙的数据传输率就不得不当当做一个问题来看待。（这里仅按照我的理解陈述，如有不妥还请指正。）

低功耗蓝牙适用于什么场景？

低功耗蓝牙据说一颗纽扣电池，就能供电好几个月甚至几年，可想而知，它在设计之初就极力的压缩能量消耗。所以，低功耗蓝牙适用于低速率、低频次、低功耗的短距离蓝牙通讯模块(如手环、鼠标、心率监测仪等)。

Android对低功耗蓝牙的支持如何？

Android系统从4.3(API 18)开始支持BLE，且从5.1(API 21)才开始支持MTU修改(默认MTU仅为23字节，而且传输本身用掉3字节)，但是实际测试结果显示，安卓手机对低功耗蓝牙的适配性并不好，很多机型都不支持低功耗蓝牙连接，比如：华为荣耀6x（后刷的Android 7.0），华为mate 8等。实测leMax2（Android 6.0）是支持低功耗蓝牙连接的，只不过需要申请位置相关的权限<uses-permission android:name="android.permission.ACCESS_COARSE_LOCATION"/>不做此申明就无法建立连接。

低功耗蓝牙的通讯流程是怎样的？

首先开发外设（peripheral），随后开发中心（centre），比较简单，一步一步基本都是回调机制。详情参照Android官方文档说明和示例。

开发外设，用到一个java文件足矣，广播的关键代码如下：

bluetoothGattServer = mBluetoothManager.openGattServer(context, bluetoothGattServerCallback);BluetoothGattService service = new BluetoothGattService(UUID_SERVER, BluetoothGattService.SERVICE_TYPE_PRIMARY);//添加一个可读、可通知的特征值，用于远端接收信息；//注意需要descriptor，远端才可实现通知；characteristicRead = new BluetoothGattCharacteristic(UUID_CHARREAD, BluetoothGattCharacteristic.PROPERTY_READ| BluetoothGattCharacteristic.PROPERTY_NOTIFY,BluetoothGattCharacteristic.PERMISSION_READ);BluetoothGattDescriptor descriptor = new BluetoothGattDescriptor(UUID_DESCRIPTOR, BluetoothGattCharacteristic.PERMISSION_WRITE);characteristicRead.addDescriptor(descriptor);service.addCharacteristic(characteristicRead);//添加一个可写的特征值，用于远端发送信息；BluetoothGattCharacteristic characteristicWrite = new BluetoothGattCharacteristic(UUID_CHARWRITE,BluetoothGattCharacteristic.PROPERTY_WRITE | BluetoothGattCharacteristic.PROPERTY_READ,//BluetoothGattCharacteristic.PROPERTY_WRITE//| BluetoothGattCharacteristic.PROPERTY_READ//| BluetoothGattCharacteristic.PROPERTY_NOTIFY,BluetoothGattCharacteristic.PERMISSION_WRITE);service.addCharacteristic(characteristicWrite);bluetoothGattServer.addService(service);

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关键的 BluetoothGattServerCallback 代码如下：

/*** 服务事件的回调*/private BluetoothGattServerCallback bluetoothGattServerCallback = new BluetoothGattServerCallback() {/*** 1.连接状态发生变化时* @param device* @param status* @param newState*/@Overridepublic void onConnectionStateChange(BluetoothDevice device, int status, int newState) {logd(String.format("1.onConnectionStateChange：device name = %s, address = %sstatus = %s, newState =%s ", device.getName(), device.getAddress(), status, newState));if(newState == 2){curDevice = device;} else {curDevice = null;}super.onConnectionStateChange(device, status, newState);}@Overridepublic void onServiceAdded(int status, BluetoothGattService service) {super.onServiceAdded(status, service);logd(String.format("onServiceAdded：status = %s", status));}@Overridepublic void onCharacteristicReadRequest(BluetoothDevice device, int requestId, int offset, BluetoothGattCharacteristic characteristic) {logd(String.format("%s,onCharacteristicReadRequest：device name = %s, address = %s, requestId = %s, offset = %s",characteristic.getUuid().toString(), device.getName(), device.getAddress(), requestId, offset));bluetoothGattServer.sendResponse(device, requestId, BluetoothGatt.GATT_SUCCESS, offset, characteristic.getValue());}/*** 3. onCharacteristicWriteRequest,接收具体的字节* @param device* @param requestId* @param characteristic* @param preparedWrite* @param responseNeeded* @param offset* @param requestBytes*/@Overridepublic void onCharacteristicWriteRequest(BluetoothDevice device, int requestId, BluetoothGattCharacteristic characteristic,boolean preparedWrite, boolean responseNeeded, int offset, byte[] requestBytes) {logd(String.format("%s,onCharacteristicWriteRequest：device name = %s, address = %s, requestId = %s, " +"preparedWrite=%s, responseNeeded=%s, offset=%s, value=%s", characteristic.getUuid().toString(),device.getName(), device.getAddress(), requestId, preparedWrite, responseNeeded, offset,OutputStringUtil.toHexString(requestBytes)));bluetoothGattServer.sendResponse(device, requestId, BluetoothGatt.GATT_SUCCESS, offset, requestBytes);String msg = OutputStringUtil.transferForPrint(requestBytes);logd("4.收到:" + msg.getBytes().length + "=" + msg);receiveMsg.append(new String(requestBytes));if (receiveMsg.toString().contains(BluetoothUtil.END_FLAG)&& receiveMsg.toString().endsWith(BluetoothUtil.END_FLAG)) {String[] msgs = receiveMsg.toString().split(BluetoothUtil.END_FLAG);for (String s : msgs) {//4.处理响应内容logd("length="+s.length() + ", cmd=" + s);dealwithReceivedMsg(s,device);}receiveMsg = new StringBuilder();}}/*** 2.描述被写入时，在这里执行 bluetoothGattServer.sendResponse(device, requestId, BluetoothGatt.GATT_SUCCESS... 收，触发 onCharacteristicWriteRequest* @param device* @param requestId* @param descriptor* @param preparedWrite* @param responseNeeded* @param offset* @param value*/@Overridepublic void onDescriptorWriteRequest(BluetoothDevice device, int requestId, BluetoothGattDescriptor descriptor, boolean preparedWrite, boolean responseNeeded, int offset, byte[] value) {logd(String.format("%s,onDescriptorWriteRequest：device name = %s, address = %s, requestId = %s, preparedWrite = %s, responseNeeded = %s, " +"offset = %s, value = %s,", descriptor.getUuid().toString(), device.getName(), device.getAddress(), requestId, preparedWrite,responseNeeded, offset, OutputStringUtil.toHexString(value)));// now tell the connected device that this was all successfullbluetoothGattServer.sendResponse(device, requestId, BluetoothGatt.GATT_SUCCESS, offset, value);}/*** 5.特征被读取。当回复响应成功后，客户端会读取然后触发本方法* @param device* @param requestId* @param offset* @param descriptor*/@Overridepublic void onDescriptorReadRequest(BluetoothDevice device, int requestId, int offset, BluetoothGattDescriptor descriptor) {logd(String.format("%s,onDescriptorReadRequest：device name = %s, address = %s, requestId = %s", descriptor.getUuid().toString(),device.getName(), device.getAddress(), requestId));// super.onDescriptorReadRequest(device, requestId, offset, descriptor);bluetoothGattServer.sendResponse(device, requestId, BluetoothGatt.GATT_SUCCESS, offset, null);}@Overridepublic void onNotificationSent(BluetoothDevice device, int status) {super.onNotificationSent(device, status);logd(String.format("5.onNotificationSent：device name = %s, address = %s, status = %s", device.getName(), device.getAddress(), status));}@Overridepublic void onMtuChanged(BluetoothDevice device, int mtu) {super.onMtuChanged(device, mtu);logd(String.format("onMtuChanged：mtu = %s", mtu));MTU = mtu - 3;}@Overridepublic void onExecuteWrite(BluetoothDevice device, int requestId, boolean execute) {super.onExecuteWrite(device, requestId, execute);logd(String.format("onExecuteWrite：requestId = %s", requestId));}};

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注意上述代码的 onMtuChanged 可以发挥关键作用。MTU默认取的是23，当收到 onMtuChanged 后，会根据传递的值修改MTU，注意由于传输用掉3字节，因此传递的值需要减3。为什么专门记录MTU，接下看看实际传输的函数，你就会明白。实际传输的代码如下：

private void doRealTransfer(String msg, BluetoothDevice device){if (bluetoothGattServer != null && characteristicRead != null && device != null) {msg += BluetoothUtil.END_FLAG;byte[] temp, data = msg.getBytes();int i = 0, j = 0;for (; i < data.length / MTU; i++) {temp = new byte[MTU];//src:源数组, srcPos:源数组要复制的起始位置,//dest:目的数组,destPos:目的数组放置的起始位置,length:要复制的长度System.arraycopy(data, j, temp, 0, MTU);logd("send: "+new String(temp));characteristicRead.setValue(temp);bluetoothGattServer.notifyCharacteristicChanged(device, characteristicRead, false);j += MTU;}if (j < data.length) {temp = new byte[data.length - j];System.arraycopy(data, j, temp, 0, data.length - j);logd("send: "+new String(temp));characteristicRead.setValue(temp);bluetoothGattServer.notifyCharacteristicChanged(device, characteristicRead, false);}} else {logd("can not send message out by BLE.");}}

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这了是按照MTU的大小严格约束每次发送的数据包大小，如果不这么做，很可能远端接收就会出错。除非你的数据包大小本身就很小。同时可以看到，我还是用到了结束符（END_FLAG）这么个小技巧，每句话的结尾一定有且仅有一个结束符，这样对方在解析时，就可根据这个特征把接收到的数据拼接完整。

开发中心（当然实际项目中，BLE中心就是在IOS端实现了），首先是搜索与选择：

private void scanLeDevice(final boolean enable) {logd("scanLeDevice:"+enable);if (enable) {// Stops scanning after a pre-defined scan period.mHandler.postDelayed(new Runnable() {@Overridepublic void run() {mScanning = false;mBluetoothAdapter.stopLeScan(mLeScanCallback);}}, SCAN_PERIOD);mScanning = true;mBluetoothAdapter.startLeScan(mLeScanCallback);} else {mScanning = false;mBluetoothAdapter.stopLeScan(mLeScanCallback);}}// 搜索到了之后就添加到界面进行显示.private BluetoothAdapter.LeScanCallback mLeScanCallback =new BluetoothAdapter.LeScanCallback() {@Overridepublic void onLeScan(final BluetoothDevice device, int rssi, byte[] scanRecord) {logd("BLE:" + device.getName() + "," + device.getAddress());if(isShowWaitNotice){mHandler.sendEmptyMessage(HIDDEN_WAIT);}if(!TextUtils.isEmpty(device.getName()) && device.getName().startsWith("TGT")) {runOnUiThread(new Runnable() {@Overridepublic void run() {mLeDeviceListAdapter.addDevice(device);mLeDeviceListAdapter.notifyDataSetChanged();}});}}};

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选中了某个设备后，就进行连接，连接成功就发现服务，解析服务、注册通知：

public boolean connect(final String address) {if (mBluetoothAdapter == null || address == null) {logd("BluetoothAdapter not initialized or unspecified address.");return false;}// Previously connected device. Try to reconnect.if (mBluetoothDeviceAddress != null && address.equals(mBluetoothDeviceAddress)&& mBluetoothGatt != null) {logd("Trying to use an existing mBluetoothGatt for connection.");if (mBluetoothGatt.connect()) {mConnectionState = STATE_CONNECTING;return true;} else {return false;}}final BluetoothDevice device = mBluetoothAdapter.getRemoteDevice(address);if (device == null) {logd("Device not found. Unable to connect.");return false;}// We want to directly connect to the device, so we are setting the autoConnect// parameter to false.mBluetoothGatt = device.connectGatt(this, false, mGattCallback);logd("Trying to create a new connection.");mBluetoothDeviceAddress = address;mConnectionState = STATE_CONNECTING;curDevice = device;return true;}private final BluetoothGattCallback mGattCallback = new BluetoothGattCallback() {@Overridepublic void onConnectionStateChange(BluetoothGatt gatt, int status, int newState) {String intentAction;if (newState == BluetoothProfile.STATE_CONNECTED) {if (Build.VERSION.SDK_INT >= Build.VERSION_CODES.LOLLIPOP) {int mtu = 185;logd("request " + mtu + " mtu:" + mBluetoothGatt.requestMtu(mtu));}intentAction = ACTION_GATT_CONNECTED;mConnectionState = STATE_CONNECTED;broadcastUpdate(intentAction);logd("Connected to GATT server.");// Attempts to discover services after successful connection.logd("Attempting to start service discovery:" + mBluetoothGatt.discoverServices());setStatus(getString(R.string.title_connected_to) + " " + getCurDeviceName());} else if (newState == BluetoothProfile.STATE_DISCONNECTED) {intentAction = ACTION_GATT_DISCONNECTED;mConnectionState = STATE_DISCONNECTED;logd("Disconnected from GATT server.");broadcastUpdate(intentAction);setStatus(R.string.title_not_connected);}}@Overridepublic void onServicesDiscovered(BluetoothGatt gatt, int status) {if (status == BluetoothGatt.GATT_SUCCESS) {parseGattServices(getSupportedGattServices());} else {logd("onServicesDiscovered received: " + status);}}@Overridepublic void onCharacteristicRead(BluetoothGatt gatt, BluetoothGattCharacteristic characteristic,int status) {logd("onCharacteristicRead. status = " + status + ", value = "+ new String(characteristic.getValue()));if (status == BluetoothGatt.GATT_SUCCESS) {//broadcastUpdate(ACTION_DATA_AVAILABLE, characteristic);}}@Overridepublic void onCharacteristicWrite(BluetoothGatt gatt, BluetoothGattCharacteristic characteristic, int status) {super.onCharacteristicWrite(gatt, characteristic, status);logd(String.format("onCharacteristicWrite: characteristic = %s, status = %s, value = %s",characteristic.getUuid(), status, new String(characteristic.getValue())));}@Overridepublic void onCharacteristicChanged(BluetoothGatt gatt, BluetoothGattCharacteristic characteristic) {logd(String.format("onCharacteristicChanged: characteristic = %s", characteristic.getUuid()));//broadcastUpdate(ACTION_DATA_AVAILABLE, characteristic);receiveMsg.append(new String(mReadCharacteristic.getValue()));if (receiveMsg.toString().contains(BluetoothChatService.END_FLAG)&& receiveMsg.toString().endsWith(BluetoothChatService.END_FLAG)) {String[] msgs = receiveMsg.toString().split(BluetoothChatService.END_FLAG);for (String s : msgs) {logd(s.length() + ":" + s);dealwithReceivedMsg(s);}receiveMsg = new StringBuilder();}}@Overridepublic void onDescriptorRead(BluetoothGatt gatt, BluetoothGattDescriptor descriptor, int status) {super.onDescriptorRead(gatt, descriptor, status);logd(String.format("onDescriptorRead: descriptor = %s, status = %s", descriptor.getUuid(), status));}@Overridepublic void onDescriptorWrite(BluetoothGatt gatt, BluetoothGattDescriptor descriptor, int status) {super.onDescriptorWrite(gatt, descriptor, status);logd(String.format("onDescriptorWrite: descriptor = %s, status = %s", descriptor.getUuid(), status));}@Overridepublic void onMtuChanged(BluetoothGatt gatt, int mtu, int status) {super.onMtuChanged(gatt, mtu, status);logd(String.format("onMtuChanged：mtu = %s", mtu));MTU = mtu-3;}};

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中心发送数据，基本上和外设类似：

public void sendMessageBLE(String msg){if (mBluetoothGatt != null && mWriteCharacteristic != null) {msg += BluetoothChatService.END_FLAG;byte[] temp, data = msg.getBytes();int i = 0, j = 0;for (; i < data.length / MTU; i++) {temp = new byte[MTU];System.arraycopy(data, j, temp, 0, MTU);logd("send: "+new String(temp));mWriteCharacteristic.setValue(temp);mWriteCharacteristic.setWriteType(BluetoothGattCharacteristic.WRITE_TYPE_NO_RESPONSE);logd(mBluetoothGatt.writeCharacteristic(mWriteCharacteristic));j += MTU;}if (j < data.length) {temp = new byte[data.length - j];System.arraycopy(data, j, temp, 0, data.length -j);logd("send: "+new String(temp));mWriteCharacteristic.setValue(temp);mWriteCharacteristic.setWriteType(BluetoothGattCharacteristic.WRITE_TYPE_NO_RESPONSE);logd(mBluetoothGatt.writeCharacteristic(mWriteCharacteristic));}} else {logd("can not send message out by BLE.");}}

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这里在顺带提一下，拼接数据包的技巧：

receiveMsg.append(new String(mReadCharacteristic.getValue()));if (receiveMsg.toString().contains(BluetoothChatService.END_FLAG)&& receiveMsg.toString().endsWith(BluetoothChatService.END_FLAG)) {String[] msgs = receiveMsg.toString().split(BluetoothChatService.END_FLAG);for (String s : msgs) {logd(s.length() + ":" + s);dealwithReceivedMsg(s);}receiveMsg = new StringBuilder();}

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总结：

经过实测，低功耗蓝牙的功耗影响很小，设备增加BLE的广播后（并持续对外发送），对于设备的待机时长几乎没有影响。不过谷歌的demo中，是设置了10分钟关闭广播的。BLE外设持续开启广播会有什么其他影响，暂待发掘。

我在开发的过程中也参考了许多网上的资料，感谢众多小伙伴无私的分享。若有对我的具体实现感兴趣的，可联系我的邮箱：suyux8@。

Android低功耗蓝牙介绍的官方资料：

/guide/topics/connectivity/bluetooth-le.html

/reference/android/bluetooth/BluetoothGatt.html

--------------------- 本文来自 Android-大雄 的CSDN 博客 ，全文地址请点击：/qiandaxiong/article/details/78903969?utm_source=copy

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