BLESSED for Android with Coroutines - BLE made easy

BLESSED is a very compact Bluetooth Low Energy (BLE) library for Android 8 and higher, that makes working with BLE on Android very easy. It is powered by Kotlin's Coroutines and turns asynchronous GATT methods into synchronous methods! It is based on the Blessed Java library and has been rewritten in Kotlin using Coroutines.

Installation

This library is available on Jitpack. Include the following in your gradle file:

allprojects {
    repositories {
        ...
        maven { url 'https://jitpack.io' }
    }
}

dependencies {
    implementation "com.github.weliem:blessed-android-coroutines:$version"
}

where $version is the latest published version in Jitpack

Overview of classes

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

1. BluetoothCentralManager, for scanning and connecting peripherals
2. BluetoothPeripheral, for all peripheral related methods
3. BluetoothPeripheralManager, and its companion abstract class BluetoothPeripheralManagerCallback
4. BluetoothCentral
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 to construct your own byte arrays by adding integers, floats, or strings.

Scanning

The BluetoothCentralManager class has several differrent scanning methods:

fun scanForPeripherals(resultCallback: (BluetoothPeripheral, ScanResult) -> Unit, scanError: (ScanFailure) -> Unit )
fun scanForPeripheralsWithServices(serviceUUIDs: Array<UUID>, resultCallback: (BluetoothPeripheral, ScanResult) -> Unit, scanError: (ScanFailure) -> Unit)
fun scanForPeripheralsWithNames(peripheralNames: Array<String>, resultCallback: (BluetoothPeripheral, ScanResult) -> Unit,  scanError: (ScanFailure) -> Unit)
fun scanForPeripheralsWithAddresses(peripheralAddresses: Array<String>, resultCallback: (BluetoothPeripheral, ScanResult) -> Unit, scanError: (ScanFailure) -> Unit)
fun scanForPeripheralsUsingFilters(filters: List<ScanFilter>,resultCallback: (BluetoothPeripheral, ScanResult) -> Unit, scanError: (ScanFailure) -> Unit)

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 your callback lambda will be called with the BluetoothPeripheral object and a ScanResult object that contains the scan details. The method scanForPeripheralsUsingFilters is for scanning using your own list of filters. See Android documentation for more info on the use of ScanFilter. A second lambda is used to deliver any scan failures.

So in order to setup a scan for a device with the Bloodpressure service or HeartRate service, you do:

val BLP_SERVICE_UUID = UUID.fromString("00001810-0000-1000-8000-00805f9b34fb")
val HRS_SERVICE_UUID = UUID.fromString("0000180D-0000-1000-8000-00805f9b34fb")

central.scanForPeripheralsWithServices(arrayOf(BLP_SERVICE_UUID, HRS_SERVICE_UUID)            
    { peripheral, scanResult ->
        Timber.i("Found peripheral '${peripheral.name}' with RSSI ${scanResult.rssi}")
        central.stopScan()
        connectPeripheral(peripheral)
    },
    { scanFailure -> Timber.e("scan failed with reason $scanFailure") })

The scanning functions are not suspending functions and simply use a lambda function to receive the results.

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

Connecting to devices

There are 3 ways to connect to a device:

suspend fun connectPeripheral(peripheral: BluetoothPeripheral): Unit
fun autoConnectPeripheral(peripheral: BluetoothPeripheral)
fun autoConnectPeripheralsBatch(batch: Set<BluetoothPeripheral>)

The method connectPeripheral is a suspending function that 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, and a ConnectionFailedException will be thrown. Note that there can be only 1 outstanding connectPeripheral. So if it is called multiple times only 1 will succeed.

scope.launch {
    try {
        central.connectPeripheral(peripheral)
    } catch (connectionFailed: ConnectionFailedException) {
        Timber.e("connection failed")
    }
}

The method autoConnectPeripheral will not suspend and 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 you want to re-connect to many known peripherals.

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

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

After issuing a connect call, you can observe the connection state of peripherals:

central.observeConnectionState { peripheral, state ->
    Timber.i("Peripheral ${peripheral.name} has $state")
}

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

suspend fun cancelConnection(peripheral: BluetoothPeripheral): Unit

The function will suspend until the peripheral is disconnected.

Service discovery

The BLESSED library will automatically do the service discovery for you. When the CONNECTED state is reached, the services have also been discovered.

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/descriptors is done using the following methods:

suspend fun readCharacteristic(serviceUUID: UUID, characteristicUUID: UUID): ByteArray
suspend fun readCharacteristic(characteristic: BluetoothGattCharacteristic): ByteArray
suspend fun writeCharacteristic(serviceUUID: UUID, characteristicUUID: UUID, value: ByteArray, writeType: WriteType): ByteArray
suspend fun writeCharacteristic(characteristic: BluetoothGattCharacteristic, value: ByteArray, writeType: WriteType): ByteArray

suspend fun readDescriptor(descriptor: BluetoothGattDescriptor): ByteArray
suspend fun writeDescriptor(descriptor: BluetoothGattDescriptor, value: ByteArray): ByteArray

All methods are suspending and will return the result of the operation. The method readCharacteristic will return the ByteArray that has been read. It will throw IllegalArgumentException if the characteristic you provide is not readable, and it will throw GattException if the read was not successful.

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 it will throw IllegalArgumentException. The method will return the bytes that were written or an empty byte array in case something went wrong.

There are 2 ways to specify which characteristic to use in the read/write method:

• Using its serviceUUID and characteristicUUID
• Using the BluetoothGattCharacteristic reference directly

For example:

peripheral.getCharacteristic(DIS_SERVICE_UUID, MANUFACTURER_NAME_CHARACTERISTIC_UUID)?.let {
    val manufacturerName = peripheral.readCharacteristic(it).asString()
    Timber.i("Received: $manufacturerName")
}

val model = peripheral.readCharacteristic(DIS_SERVICE_UUID, MODEL_NUMBER_CHARACTERISTIC_UUID).asString()
Timber.i("Received: $model")

Note that there are also some extension methods like asString() and asUInt8() to quickly turn byte arrays in Strings or UInt8s.

Turning notifications on/off

You can observe notifications/indications and receive them in the callback lambda. All the necessary operations like writing to the Client Characteristic Configuration descriptor are handled by Blessed. So all you need to do is:

peripheral.getCharacteristic(BLP_SERVICE_UUID, BLOOD_PRESSURE_MEASUREMENT_CHARACTERISTIC_UUID)?.let {
    peripheral.observe(it) { value ->
        val measurement = BloodPressureMeasurement.fromBytes(value)
        ...
    }
}

To stop observing notifications you call peripheral.stopObserving(characteristic: BluetoothGattCharacteristic)

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:

peripheral.observeBondState {
    Timber.i("Bond state is $it")
}

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 central.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:

val mtu = peripheral.requestMtu(185)

The method will return the negotiated MTU value. 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 peripheral.currentMtu. If you want to know the maximum length of the byte arrays that you can write, you can call the method peripheral.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:

suspend fun setPreferredPhy(txPhy: PhyType, rxPhy: PhyType, phyOptions: PhyOptions): Phy

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 as a Phy object that is returned by setPrefferedPhy

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. 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:

suspend fun readPhy(): Phy

It will return the current Phy

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