BLESSED for Android - BLE made easy

BLESSED is a very compact Bluetooth Low Energy (BLE) library for Android 5 and higher, that makes working with BLE on Android very easy. It takes care of many aspects of working with BLE you would normally have to take care of yourself like:

• Queueing commands, so you can don't have to wait anymore for the completion of a command before issueing the next command
• Bonding, so you don't have to do anything in order to robustly bond devices
• Easy scanning, so you don't have to setup complex scan filters
• Managing threading, so you don't have to worry about on which thread to issue commands or receive results
• Threadsafe, so you don't see weird threading related issues anymore
• Workarounds for some known Android bugs, so you don't have to research any workarounds
• Higher abstraction methods for convenience, so that you don't have to do a lot of low-level management to get stuff done

The library consists of 5 core classes and corresponding callback abstract classes:

1. BluetoothCentralManager, and it companion abstract class BluetoothCentralManagerCallback
2. BluetoothPeripheral, and it's companion abstract class BluetoothPeripheralCallback
3. BluetoothPeripheralManager, and it's companion abstract class BluetoothPeripheralManagerCallback
4. BluetoothCentral, which has no callback class
5. BluetoothBytesParser

The BluetoothCentralManager class is used to scan for devices and manage connections. The BluetoothPeripheral class is a replacement for the standard Android BluetoothDevice and BluetoothGatt classes. It wraps all GATT related peripheral functionality.

The BluetoothPeripheralManager class is used to create your own peripheral running on an Android phone. You can add service, control advertising and deal with requests from remote centrals, represented by the BluetoothCentral class. For more about creating your own peripherals see the separate guide: creating your own peripheral

The BluetoothBytesParser class is a utility class that makes parsing byte arrays easy. You can also use it construct your own byte arrays by adding integers, floats or strings.

The BLESSED library was inspired by CoreBluetooth on iOS and provides the same level of abstraction, but at the same time it also stays true to Android by keeping most methods the same and allowing you to work with the standard classes for Services, Characteristics and Descriptors. If you already have developed using CoreBluetooth you can very easily port your code to Android using this library.

BLESSED is written in Java but has been optimized for Kotlin as well. When using the library in Kotlin, all types will be correct in (non-)nullability. This library is also available for Linux, see blessed-bluez

Scanning

The BluetoothCentralManager class has several differrent scanning methods:

public void scanForPeripherals()
public void scanForPeripheralsWithServices(UUID[] serviceUUIDs)
public void scanForPeripheralsWithNames(String[] peripheralNames)
public void scanForPeripheralsWithAddresses(String[] peripheralAddresses)
public void scanForPeripheralsUsingFilters(List<ScanFilter> filters)

They all work in the same way and take an array of either service UUIDs, peripheral names or mac addresses. When a peripheral is found you will get a callback on onDiscoveredPeripheral with the BluetoothPeripheral object and a ScanResult object that contains the scan details. So in order to setup a scan for a device with the Bloodpressure service and connect to it, you do:

private final BluetoothCentralManagerCallback bluetoothCentralManagerCallback = new BluetoothCentralManagerCallback() {
        @Override
        public void onDiscoveredPeripheral(BluetoothPeripheral peripheral, ScanResult scanResult) {
            central.stopScan();
            central.connectPeripheral(peripheral, peripheralCallback);
        }
};

// Create BluetoothCentral and receive callbacks on the main thread
BluetoothCentralManager central = BluetoothCentralManager(getApplicationContext(), bluetoothCentralCallback, new Handler(Looper.getMainLooper()));

// Define blood pressure service UUID
UUID BLOODPRESSURE_SERVICE_UUID = UUID.fromString("00001810-0000-1000-8000-00805f9b34fb");

// Scan for peripherals with a certain service UUID
central.scanForPeripheralsWithServices(new UUID[]{BLOODPRESSURE_SERVICE_UUID});

Note Only 1 of these 4 types of scans can be active at one time! So call stopScan() before calling another scan.

The method scanForPeripheralsUsingFilters is for scanning using your own list of filters. See Android documentation for more info on the use of ScanFilters.

Connecting to devices

There are 3 ways to connect to a device:

public void connectPeripheral(BluetoothPeripheral peripheral, BluetoothPeripheralCallback peripheralCallback)
public void autoConnectPeripheral(BluetoothPeripheral peripheral, BluetoothPeripheralCallback peripheralCallback)
public void autoConnectPeripheralsBatch(Map<BluetoothPeripheral, BluetoothPeripheralCallback> batch) 

The method connectPeripheral will try to immediately connect to a device that has already been found using a scan. This method will time out after 30 seconds or less depending on the device manufacturer. Note that there can be only 1 outstanding connectPeripheral. So if it is called multiple times only 1 will succeed.

The method autoConnectPeripheral is for re-connecting to known devices for which you already know the device's mac address. The BLE stack will automatically connect to the device when it sees it in its internal scan. Therefore, it may take longer to connect to a device but this call will never time out! So you can issue the autoConnect command and the device will be connected whenever it is found. This call will also work when the device is not cached by the Android stack, as BLESSED takes care of it! In contrary to connectPeripheral, there can be multiple outstanding autoConnectPeripheral requests.

The method autoConnectPeripheralsBatch is for re-connecting to multiple peripherals in one go. Since the normal autoConnectPeripheral may involve scanning, if peripherals are uncached, it is not suitable for calling very fast after each other, since it may trigger scanner limitations of Android. So use autoConnectPeripheralsBatch if the want to re-connect to many known peripherals.

If you know the mac address of your peripheral you can obtain a BluetoothPeripheral object using:

BluetoothPeripheral peripheral = central.getPeripheral("CF:A9:BA:D9:62:9E");

After issuing a connect call, you will receive one of the following callbacks:

public void onConnectedPeripheral(BluetoothPeripheral peripheral)
public void onConnectionFailed(BluetoothPeripheral peripheral, HciStatus status)
public void onDisconnectedPeripheral(BluetoothPeripheral peripheral, HciStatus status)

To disconnect or to cancel an outstanding connectPeripheral() or autoConnectPeripheral(), you call:

public void cancelConnection(BluetoothPeripheral peripheral)

In all cases, you will get a callback on onDisconnectedPeripheral when the disconnection has been completed.

Service discovery

The BLESSED library will automatically do the service discovery for you and once it is completed you will receive the following callback:

public void onServicesDiscovered(BluetoothPeripheral peripheral)

In order to get the services you can use methods like getServices() or getService(UUID). In order to get hold of characteristics you can call getCharacteristic(UUID) on the BluetoothGattService object or call getCharacteristic() on the BluetoothPeripheral object.

This callback is the proper place to start enabling notifications or read/write characteristics.

Reading and writing

Reading and writing to characteristics is done using the following methods:

public boolean readCharacteristic(BluetoothGattCharacteristic characteristic)
public boolean writeCharacteristic(BluetoothGattCharacteristic characteristic, byte[] value, WriteType writeType)

Both methods are asynchronous and will be queued up. So you can just issue as many read/write operations as you like without waiting for each of them to complete. You will receive a callback once the result of the operation is available. For read operations you will get a callback on:

public void onCharacteristicUpdate(BluetoothPeripheral peripheral, byte[] value, BluetoothGattCharacteristic characteristic, GattStatus status)

If you want to write to a characteristic, you need to provide a value and a writeType. The writeType is usually WITH_RESPONSE or WITHOUT_RESPONSE. If the write type you specify is not supported by the characteristic you will see an error in your log. For write operations you will get a callback on:

public void onCharacteristicWrite(BluetoothPeripheral peripheral, byte[] value, BluetoothGattCharacteristic characteristic, final GattStatus status)

In these callbacks, the value parameter is the threadsafe byte array that was received. Use this value instead of the value that is part of the BluetoothGattCharacteristic object, since that one may have changed in the mean time because of incoming notifications or write operations.

Turning notifications on/off

BLESSED provides a convenience method setNotify to turn notifications/indications on or off. It will perform all the necessary operations like writing to the Client Characteristic Configuration descriptor for you. So all you need to do is:

BluetoothGattCharacteristic currentTimeCharacteristic = peripheral.getCharacteristic(CTS_SERVICE_UUID, CURRENT_TIME_CHARACTERISTIC_UUID);
if (currentTimeCharacteristic != null) {
     peripheral.setNotify(currentTimeCharacteristic, true);
}

Since this is an asynchronous operation you will receive a callback that indicates success or failure. You can use the method isNotifying to check if the characteristic is currently notifying or not:

@Override
public void onNotificationStateUpdate(BluetoothPeripheral peripheral, BluetoothGattCharacteristic characteristic, GattStatus status) {
     if (status == GattStatus.SUCCESS) {
          if(peripheral.isNotifying(characteristic)) {
               Log.i(TAG, String.format("SUCCESS: Notify set to 'on' for %s", characteristic.getUuid()));
          } else {
               Log.i(TAG, String.format("SUCCESS: Notify set to 'off' for %s", characteristic.getUuid()));
          }
     } else {
          Log.e(TAG, String.format("ERROR: Changing notification state failed for %s", characteristic.getUuid()));
     }
}

When notifications arrive, you will receive a callback on:

public void onCharacteristicUpdate(BluetoothPeripheral peripheral, byte[] value, BluetoothGattCharacteristic characteristic, GattStatus status)

Bonding

BLESSED handles bonding for you and will make sure all bonding variants work smoothly. During the process of bonding, you will be informed of the process via a number of callbacks:

public void onBondingStarted(final BluetoothPeripheral peripheral)
public void onBondingSucceeded(final BluetoothPeripheral peripheral)
public void onBondingFailed(final BluetoothPeripheral peripheral) 
public void onBondLost(final BluetoothPeripheral peripheral) 

In most cases, the peripheral will initiate bonding either at the time of connection, or when trying to read/write protected characteristics. However, if you want you can also initiate bonding yourself by calling createBond on a peripheral. There are two ways to do this:

• Calling createBond when not yet connected to a peripheral. In this case, a connection is made and bonding is requested.
• Calling createBond when already connected to a peripheral. In this case, only the bond is created.

It is also possible to remove a bond by calling removeBond. Note that this method uses a hidden Android API and may stop working in the future. When calling the removeBond method, the peripheral will also disappear from the settings menu on the phone.

Lastly, it is also possible to automatically issue a PIN code when pairing. Use the method setPinCodeForPeripheral to register a 6 digit PIN code. Once bonding starts, BLESSED will automatically issue the PIN code and the UI dialog to enter the PIN code will not appear anymore.

Requesting a higher MTU to increase throughput

The default MTU is 23 bytes, which allows you to send and receive byte arrays of MTU - 3 = 20 bytes at a time. The 3 bytes overhead are used by the ATT packet. If your peripheral supports a higher MTU, you can request that by calling:

public void requestMtu(int mtu)

You will get a callback on:

public void onMtuChanged(BluetoothPeripheral peripheral, int mtu, GattStatus status)

This callback will tell you what the negotiated MTU value is. Note that you may not get the value you requested if the peripheral doesn't accept your offer. If you simply want the highest possible MTU, you can call peripheral.requestMtu(BluetoothPeripheral.MAX_MTU) and that will lead to receiving the highest possible MTU your peripheral supports.

Once the MTU has been set, you can always access it by calling getCurrentMtu(). If you want to know the maximum length of the byte arrays that you can write, you can call the method getMaximumWriteValueLength(). Note that the maximum value depends on the write type you want to use.

Long reads and writes

The library also supports so called 'long reads/writes'. You don't need to do anything special for them. Just read a characteristic or descriptor as you normally do, and if the characteristic's value is longer than MTU - 1, then a series of reads will be done by the Android BLE stack. But you will simply receive the 'long' characteristic value in the same way as normal reads.

Similarly, for long writes, you just write to a characteristic or descriptor and the Android BLE stack will take care of the rest. But keep in mind that long writes only work with WriteType.WITH_RESPONSE and the maximum length of your byte array should be 512 or less. Note that not all peripherals support long reads/writes so this is not guaranteed to work always.

Status codes

When connecting or disconnecting, the callback methods will contain a parameter HciStatus status. This enum class will have the value SUCCESS if the operation succeeded and otherwise it will provide a value indicating what went wrong.

Similarly, when doing GATT operations, the callbacks methods contain a parameter GattStatus status. These two enum classes replace the int status parameter that Android normally passes.

Bluetooth 5 support

As of Android 8, Bluetooth 5 is natively supported. One of the things that Bluetooth 5 brings, is new physical layer options, called Phy that either give more speed or longer range. The options you can choose are:

• LE_1M, 1 mbit PHY, compatible with Bluetooth 4.0, 4.1, 4.2 and 5.0
• LE_2M, 2 mbit PHY for higher speeds, requires Bluetooth 5.0
• LE_CODED, Coded PHY for long range connections, requires Bluetooth 5.0

You can set a preferred Phy by calling:

public boolean setPreferredPhy(PhyType txPhy, PhyType rxPhy, PhyOptions phyOptions)

By calling setPreferredPhy() you indicate what you would like to have but it is not guaranteed that you get what you ask for. That depends on what the peripheral will actually support and give you. If you are requesting LE_CODED you can also provide PhyOptions which has 3 possible values:

• NO_PREFERRED, for no preference (use this when asking for LE_1M or LE_2M)
• S2, for 2x long range
• S8, for 4x long range

The result of this negotiation will be received on:

public void onPhyUpdate(PhyType txPhy, PhyType rxPhy, GattStatus status)

As you can see the Phy for sending and receiving can be different but most of the time you will see the same Phy for both. Note that onPhyUpdate will also be called by the Android stack when a connection is established or when the Phy changes for other reasons. If you don't call setPreferredPhy(), Android seems to pick PHY_LE_2M if the peripheral supports Bluetooth 5. So in practice you only need to call setPreferredPhy if you want to use PHY_LE_CODED.

You can request the current values at any point by calling:

public boolean readPhy()

The result will be again delivered on onPhyUpdate()

Example application

An example application is provided in the repo. It shows how to connect to Blood Pressure meters, Heart Rate monitors, Weight scales, Glucose Meters, Pulse Oximeters and Thermometers, read the data and show it on screen. It only works with peripherals that use the Bluetooth SIG services. Working peripherals include:

• Beurer FT95 thermometer
• GRX Thermometer (TD-1241)
• Masimo MightySat
• Nonin 3230
• Indiehealth scale
• A&D 352BLE scale
• A&D 651BLE blood pressure meter
• Beurer BM57 blood pressure meter
• Soehnle Connect 300/400 blood pressure meter
• Polar H7/H10/OH1 heartrate monitors
• Contour Next One glucose meter
• Accu-Chek Instant glucose meter

Acknowledgements

BLESSED is the result of learning from many others about how to do BLE. Here are some references to material that helped me develop BLESSED:

• Stuart Kent's presentation on BLE
• Emil's posts on StackOverflow
• The Android BLE Library by Nordic Semiconductor
• The RxAndroidBle library by Polidea