BLE 即 Bluetooth Low Energy，蓝牙低功耗技术，是蓝牙4.0引入的新技术。现在越来越多的智能设备使用了BLE，像满大街的智能手环。
Android在4.3（API 18）中引进了对BLE central role的支持，同时提供API供App来扫描设备、查询服务、读写特征值等。
关键术语和概念
- Generic Attribute Profile (GATT)—GATT概述（profile）是一个通用的通过BLE连接来发送和接受短的被称为“属性”的数据的规范。所有现在的低功耗应用概述（profile）都基于GATT。
- 蓝牙标准协会为低功耗设备定义了很多 profiles。概述（profile）是设备在特定应用场景下如何工作的规范。注意设备能实现不止一种概述，例如，一个设备可以包含心律监测和电池电量检测。
Attribute Protocol (ATT)—GATT建立在属性协议（ATT）上。通常他们一起被叫做GATT/ATT。ATT针对在BLE设备上运行做了优化。为了这个目的，它使用尽可能少的字节。每个属性通过一个标准的128位格式的字符串ID作为唯一标识信息的通用唯一识别码（Universally Unique Identifier 即UUID）来唯一的标识。属性被ATT格式化为特征和服务来传输。
Characteristic—一个特征包含一个单一的值（value）和0-n个描述信息块（descriptor）来描述特征的值。特征可以被认为是一种类型（type），类似一个类（class）。
Descriptor—描述信息块定义了特征值（characteristic value）的属性，一个描述信息块可能指定一个可读的描述，一个特征值可接受的范围，或者指定一个特征值的计量单位。
Service—服务是特征的集合。例如，你可以有个叫做“心率检测器”且包含“心律测量方式”特征的服务。你可以在bluetooth.org上找到一份现有的基于GATT的概述（profile）和服务列表。
角色和职责
中心 vs. 外围。你必须同时有这两种设备才能建立他们之间的连接。两个中心设备或者两个外围设备都不能建立连接。
GATT服务端 vs. GATT客户端。这取决于他们之间是怎么交流的。
BLE权限
检查设备是否支持BLE：
// Use this check to determine whether BLE is supported on the device. Then
// you can selectively disable BLE-related features.
if (!getPackageManager().hasSystemFeature(PackageManager.FEATURE_BLUETOOTH_LE)) {
Toast.makeText(this, R.string.ble_not_supported, Toast.LENGTH_SHORT).show();
finish();
}
设置BLE
整个设备只有一个BluetoothAdapter
- 获取BluetoothAdapter
整个设备只有一个BluetoothAdapter，它代表了设备自己的蓝牙适配器，应用通过这个对象与设备进行交互。
// Initializes Bluetooth adapter.
final BluetoothManager bluetoothManager =
(BluetoothManager) getSystemService(Context.BLUETOOTH_SERVICE);
mBluetoothAdapter = bluetoothManager.getAdapter();
- 开启蓝牙
private BluetoothAdapter mBluetoothAdapter;
...
// Ensures Bluetooth is available on the device and it is enabled. If not,
// displays a dialog requesting user permission to enable Bluetooth.
if (mBluetoothAdapter == null || !mBluetoothAdapter.isEnabled()) {
Intent enableBtIntent = new Intent(BluetoothAdapter.ACTION_REQUEST_ENABLE);
startActivityForResult(enableBtIntent, REQUEST_ENABLE_BT);
}
查找BLE设备
要查找BLE设备，需要使用startLeScan()方法，它的参数是BluetoothAdapter.LeScanCallback。你必须实现这个callbak，因为这是扫描结果返回的方式。蓝牙扫描是一个电量敏感的操作，你需要遵守以下指导：
- 只要找到了要找的设备，停止扫描
- 不要在循环里扫描，设置一个扫描时间。之前的设备可能已经移动到范围外，持续扫描会耗尽电量。
如何开始和停止扫描：
/**
* Activity for scanning and displaying available BLE devices.
*/
public class DeviceScanActivity extends ListActivity {
private BluetoothAdapter mBluetoothAdapter;
private boolean mScanning;
private Handler mHandler;
// Stops scanning after 10 seconds.
private static final long SCAN_PERIOD = 10000;
...
private void scanLeDevice(final boolean enable) {
if (enable) {
// Stops scanning after a pre-defined scan period.
mHandler.postDelayed(new Runnable() {
@Override
public void run() {
mScanning = false;
mBluetoothAdapter.stopLeScan(mLeScanCallback);
}
}, SCAN_PERIOD);
mScanning = true;
mBluetoothAdapter.startLeScan(mLeScanCallback);
} else {
mScanning = false;
mBluetoothAdapter.stopLeScan(mLeScanCallback);
}
...
}
...
}
如果只想扫描指定类型的外围设备，你可以调用startLeScan(UUID[], BluetoothAdapter.LeScanCallback)，提供一个指定了你app支持的GATT服务的UUID数组对象。
下面是一个用来传送BLE扫描结果的接口BluetoothAdapter.LeScanCallback的实现：
private LeDeviceListAdapter mLeDeviceListAdapter;
...
// Device scan callback.
private BluetoothAdapter.LeScanCallback mLeScanCallback =
new BluetoothAdapter.LeScanCallback() {
@Override
public void onLeScan(final BluetoothDevice device, int rssi,
byte[] scanRecord) {
runOnUiThread(new Runnable() {
@Override
public void run() {
mLeDeviceListAdapter.addDevice(device);
mLeDeviceListAdapter.notifyDataSetChanged();
}
});
}
};
注意：你只能扫描BLE设备或传统蓝牙设备二者之一，就像 Bluetooth描述的那样。你不能同时扫描两种设备。
连接GATT服务端
与BLE设备交互第一步就是要连接它—更确切的说，是连接到设备上的GATT服务端。要连接到BLE设备的GATT服务端，使用[connectGatt()](https://developer.android.com/reference/android/bluetooth/BluetoothDevice.html#connectGatt(android.content.Context, boolean, android.bluetooth.BluetoothGattCallback))方法。这个方法需要三个参数，一个Context对象，autoConnect（一个指示是否自动连接到BLE设备--当它一旦可用的时候--的布尔值），和一个 BluetoothGattCallback的引用：
mBluetoothGatt = device.connectGatt(this, false, mGattCallback);
返回的BluetoothGatt实例之后你可以用它来管理GATT客户端的操作。调用者（Android app)是GATT客户端。 BluetoothGattCallback
用于传递结果给用户，例如连接状态，以及任何进一步GATT客户端操作。
在这个例子中，这个BLE APP提供了一个activity（DeviceControlActivity)来连接，显示数据，显示该设备支持的GATT services和characteristics。根据用户的输入，这个activity与一个叫做BluetoothLeService的 Service通信，它通过Android BLE API实现与BLE设备交互：
// A service that interacts with the BLE device via the Android BLE API.
public class BluetoothLeService extends Service {
private final static String TAG = BluetoothLeService.class.getSimpleName();
private BluetoothManager mBluetoothManager;
private BluetoothAdapter mBluetoothAdapter;
private String mBluetoothDeviceAddress;
private BluetoothGatt mBluetoothGatt;
private int mConnectionState = STATE_DISCONNECTED;
private static final int STATE_DISCONNECTED = 0;
private static final int STATE_CONNECTING = 1;
private static final int STATE_CONNECTED = 2;
public final static String ACTION_GATT_CONNECTED =
"com.example.bluetooth.le.ACTION_GATT_CONNECTED";
public final static String ACTION_GATT_DISCONNECTED =
"com.example.bluetooth.le.ACTION_GATT_DISCONNECTED";
public final static String ACTION_GATT_SERVICES_DISCOVERED =
"com.example.bluetooth.le.ACTION_GATT_SERVICES_DISCOVERED";
public final static String ACTION_DATA_AVAILABLE =
"com.example.bluetooth.le.ACTION_DATA_AVAILABLE";
public final static String EXTRA_DATA =
"com.example.bluetooth.le.EXTRA_DATA";
public final static UUID UUID_HEART_RATE_MEASUREMENT =
UUID.fromString(SampleGattAttributes.HEART_RATE_MEASUREMENT);
// Various callback methods defined by the BLE API.
private final BluetoothGattCallback mGattCallback =
new BluetoothGattCallback() {
@Override
public void onConnectionStateChange(BluetoothGatt gatt, int status,
int newState) {
String intentAction;
if (newState == BluetoothProfile.STATE_CONNECTED) {
intentAction = ACTION_GATT_CONNECTED;
mConnectionState = STATE_CONNECTED;
broadcastUpdate(intentAction);
Log.i(TAG, "Connected to GATT server.");
Log.i(TAG, "Attempting to start service discovery:" +
mBluetoothGatt.discoverServices());
} else if (newState == BluetoothProfile.STATE_DISCONNECTED) {
intentAction = ACTION_GATT_DISCONNECTED;
mConnectionState = STATE_DISCONNECTED;
Log.i(TAG, "Disconnected from GATT server.");
broadcastUpdate(intentAction);
}
}
@Override
// New services discovered
public void onServicesDiscovered(BluetoothGatt gatt, int status) {
if (status == BluetoothGatt.GATT_SUCCESS) {
broadcastUpdate(ACTION_GATT_SERVICES_DISCOVERED);
} else {
Log.w(TAG, "onServicesDiscovered received: " + status);
}
}
@Override
// Result of a characteristic read operation
public void onCharacteristicRead(BluetoothGatt gatt,
BluetoothGattCharacteristic characteristic,
int status) {
if (status == BluetoothGatt.GATT_SUCCESS) {
broadcastUpdate(ACTION_DATA_AVAILABLE, characteristic);
}
}
...
};
...
}
当一个特定的回调被触发的时候，它会调用相应的broadcastUpdate()辅助方法并且传递给它一个action。注意在该部分中的数据解析按照蓝牙心率测量配置文件规格profile specifications进行。
private void broadcastUpdate(final String action) {
final Intent intent = new Intent(action);
sendBroadcast(intent);
}
private void broadcastUpdate(final String action,
final BluetoothGattCharacteristic characteristic) {
final Intent intent = new Intent(action);
// This is special handling for the Heart Rate Measurement profile. Data
// parsing is carried out as per profile specifications.
if (UUID_HEART_RATE_MEASUREMENT.equals(characteristic.getUuid())) {
int flag = characteristic.getProperties();
int format = -1;
if ((flag & 0x01) != 0) {
format = BluetoothGattCharacteristic.FORMAT_UINT16;
Log.d(TAG, "Heart rate format UINT16.");
} else {
format = BluetoothGattCharacteristic.FORMAT_UINT8;
Log.d(TAG, "Heart rate format UINT8.");
}
final int heartRate = characteristic.getIntValue(format, 1);
Log.d(TAG, String.format("Received heart rate: %d", heartRate));
intent.putExtra(EXTRA_DATA, String.valueOf(heartRate));
} else {
// For all other profiles, writes the data formatted in HEX.
final byte[] data = characteristic.getValue();
if (data != null && data.length > 0) {
final StringBuilder stringBuilder = new StringBuilder(data.length);
for(byte byteChar : data)
stringBuilder.append(String.format("%02X ", byteChar));
intent.putExtra(EXTRA_DATA, new String(data) + "\n" +
stringBuilder.toString());
}
}
sendBroadcast(intent);
}
返回到DeviceControlActivity, 这些事件由一个BroadcastReceiver来处理:
// Handles various events fired by the Service.
// ACTION_GATT_CONNECTED: connected to a GATT server.
// ACTION_GATT_DISCONNECTED: disconnected from a GATT server.
// ACTION_GATT_SERVICES_DISCOVERED: discovered GATT services.
// ACTION_DATA_AVAILABLE: received data from the device. This can be a
// result of read or notification operations.
private final BroadcastReceiver mGattUpdateReceiver = new BroadcastReceiver() {
@Override
public void onReceive(Context context, Intent intent) {
final String action = intent.getAction();
if (BluetoothLeService.ACTION_GATT_CONNECTED.equals(action)) {
mConnected = true;
updateConnectionState(R.string.connected);
invalidateOptionsMenu();
} else if (BluetoothLeService.ACTION_GATT_DISCONNECTED.equals(action)) {
mConnected = false;
updateConnectionState(R.string.disconnected);
invalidateOptionsMenu();
clearUI();
} else if (BluetoothLeService.
ACTION_GATT_SERVICES_DISCOVERED.equals(action)) {
// Show all the supported services and characteristics on the
// user interface.
displayGattServices(mBluetoothLeService.getSupportedGattServices());
} else if (BluetoothLeService.ACTION_DATA_AVAILABLE.equals(action)) {
displayData(intent.getStringExtra(BluetoothLeService.EXTRA_DATA));
}
}
};
读取BLE属性
一旦你的Android app已经连接到GATT服务端连接且发现services后，就可以读、写那些支持的属性。例如，这段代码遍历服务端的services和 characteristics，并且将它们显示在UI上。
public class DeviceControlActivity extends Activity {
...
// Demonstrates how to iterate through the supported GATT
// Services/Characteristics.
// In this sample, we populate the data structure that is bound to the
// ExpandableListView on the UI.
private void displayGattServices(List gattServices) {
if (gattServices == null) return;
String uuid = null;
String unknownServiceString = getResources().
getString(R.string.unknown_service);
String unknownCharaString = getResources().
getString(R.string.unknown_characteristic);
ArrayList> gattServiceData =
new ArrayList>();
ArrayList>> gattCharacteristicData
= new ArrayList>>();
mGattCharacteristics =
new ArrayList>();
// Loops through available GATT Services.
for (BluetoothGattService gattService : gattServices) {
HashMap currentServiceData =
new HashMap();
uuid = gattService.getUuid().toString();
currentServiceData.put(
LIST_NAME, SampleGattAttributes.
lookup(uuid, unknownServiceString));
currentServiceData.put(LIST_UUID, uuid);
gattServiceData.add(currentServiceData);
ArrayList> gattCharacteristicGroupData =
new ArrayList>();
List gattCharacteristics =
gattService.getCharacteristics();
ArrayList charas =
new ArrayList();
// Loops through available Characteristics.
for (BluetoothGattCharacteristic gattCharacteristic :
gattCharacteristics) {
charas.add(gattCharacteristic);
HashMap currentCharaData =
new HashMap();
uuid = gattCharacteristic.getUuid().toString();
currentCharaData.put(
LIST_NAME, SampleGattAttributes.lookup(uuid,
unknownCharaString));
currentCharaData.put(LIST_UUID, uuid);
gattCharacteristicGroupData.add(currentCharaData);
}
mGattCharacteristics.add(charas);
gattCharacteristicData.add(gattCharacteristicGroupData);
}
...
}
...
}
接收GATT通知
常见的需求是当设备上的特性改变时通知BLE应用程序。这段代码展示了如何使用 [setCharacteristicNotification()](https://developer.android.com/reference/android/bluetooth/BluetoothGatt.html#setCharacteristicNotification(android.bluetooth.BluetoothGattCharacteristic, boolean)) 给一个特性设置通知。
private BluetoothGatt mBluetoothGatt;
BluetoothGattCharacteristic characteristic;
boolean enabled;
...
mBluetoothGatt.setCharacteristicNotification(characteristic, enabled);
...
BluetoothGattDescriptor descriptor = characteristic.getDescriptor(
UUID.fromString(SampleGattAttributes.CLIENT_CHARACTERISTIC_CONFIG));
descriptor.setValue(BluetoothGattDescriptor.ENABLE_NOTIFICATION_VALUE);
mBluetoothGatt.writeDescriptor(descriptor);
一旦对一个特性启用了通知，当远程蓝牙设备特性发生变化时，回调函数[onCharacteristicChanged( )](http://developer.android.com/reference/android/bluetooth/BluetoothGattCallback.html#onCharacteristicChanged(android.bluetooth.BluetoothGatt, android.bluetooth.BluetoothGattCharacteristic))就会被触发：
@Override
// Characteristic notification
public void onCharacteristicChanged(BluetoothGatt gatt,
BluetoothGattCharacteristic characteristic) {
broadcastUpdate(ACTION_DATA_AVAILABLE, characteristic);
}
关闭客户端App
一旦你的app完成了对BLE设备的使用，需要调用close()以便系统能适当地释放资源：
public void close() {
if (mBluetoothGatt == null) {
return;
}
mBluetoothGatt.close();
mBluetoothGatt = null;
}
参考链接：
https://developer.android.com/guide/topics/connectivity/bluetooth-le.html