BleGattCoroutines

Functional Bluetooth GATT This library allows easy and safer usage of BluetoothGatt in Android. Instead of having callbacks to manage, you just need to call functions. It has also been tested successfully on Wear OS, with the sample included in this repository. It should work similarly on other Android variants such as Android TV.

It does so by taking advantage of the excellent coroutines feature in the Kotlin programming language that allows to write asynchronous code in a sequential/synchronous style, which means, without the callback hell, and without blocking any thread (which would waste memory and decrease performances).

This library makes it possible to have readable and debuggable code that interacts with Bluetooth Low Energy GATT (General Attribute), that is, the connection part of the Bluetooth Low Energy standard.

Why this library ?

As we needed to have an Android app interact with a Bluetooth Low Energy device, we found the Android BluetoothGatt API and a few RxJava libraries built on top of it. Unfortunately, none suited our needs:

• The vanilla Android BluetoothGatt API is extremely hard to get right, because you have to provide a single instance of what we call a "God Callback", that is, an instance of a class with overridable methods that asynchronously receive the results off all operations of a given type (for example, the result of a characteristic you requested to read, or the result of whether a characteristic write went well or failed. You're on your own to make readable code with this, where unrelated characteristic reads, writes or other operation types are dispatched in the same callback method (like onCharacteristicRead(…)).
• The RxJava libraries would mean we'd have to learn RxJava, which is known to have a steep learning curve, steeper than learning another programming language like Kotlin from Java experience, and steeper than learning Kotlin coroutines plus understanding the kotlinx.coroutines library guide. Also, RxJava is a big library, even bigger if you have to use both version 1 and version 2 in the same project. RxJava2 methods count is higher than the sum of Kotlin's stdlib and kotlinx.coroutines.

Experimental API

We are expecting to make a few API changes based on your feedback and real world usages to improve this library. You can help by sharing your experience or feedback in the issues having a green "help wanted" tag.

Since the API design it not final at the moment, we're very open to feedback while you're using this library.

Please, open an issue if something can be improved. If you just want to tell the author what you're doing with this library, feel free to reach out via Twitter DM, public tweet.

You can also join the discussion on Kotlin's Slack in the #beepiz-libraries channel (you can get an invitation here).

Usage

As usual, scan BLE devices using BluetoothLeScanner to find you BluetoothDevice, or create an instance from the MAC address of your target device.

With this BluetoothDevice instance, you can create a GattConnection object, which will be key to perform Bluetooth GATT operations using coroutines.

On this GattConnection object, call connect() to initiate the connection. Immediately after It will suspend until the connection is established, then you use the connection:

The currently supported GATT operations on the GattConnection class are:

• Services discovery, using discoverServices() which returns and cache the list of the services on the connected device.
• Characteristic read, using readCharacteristic(…). Services discovery has to be completed before, as usual.
• Characteristic write, using writeCharacteristic(…). Services discovery has to be completed before, as usual.
• ReliableWrite, with reliableWrite { … }. Implemented, but could not be tested yet. Open an issue if your device supports it
• Descriptor read, using readDescriptor(…). Services discovery has to be completed before, as usual.
• Descriptor write, using writeDescriptor(…). Services discovery has to be completed before, as usual.
• RSSI read, using readRemoteRssi(…).
• NOTIFY characteristics with the notifyChannel. These haven't been tested yet. Feedback wanted.
• Toggling characteristic update notifications with setCharacteristicNotificationsEnabled(…). Tied to NOTIFY feature.
• PHY, using readPhy(). Only supported in Android O. Has not been tested. We don't know what this is either, to be honest.

When you're done with the BLE device (you need to be done before the device's battery runs out, unless you're dealing with an always on wearable that the user didn't disconnect), call close().

If you want to reconnect within seconds, or a few minutes to the same device, you can call disconnect() instead, which will allow to call connect() again later.

Examples

Here's a basic example that just logs the characteristics (using the print() method defined here ):

suspend fun BluetoothDevice.logGattServices(tag: String = "BleGattCoroutines") {
    val deviceConnection = GattConnection(bluetoothDevice = this@logGattServices)
    try {
        deviceConnection.connect() // Suspends until connection is established
        val gattServices = deviceConnection.discoverServices() // Suspends until completed
        gattServices.forEach {
            it.characteristics.forEach {
                try { 
                    deviceConnection.readCharacteristic(it) // Suspends until characteristic is read
                } catch (e: Exception) {
                    Log.e(tag, "Couldn't read characteristic with uuid: ${it.uuid}", e)
                }
            }
            Log.d(tag, it.print(printCharacteristics = true))
        }
    } finally {
        deviceConnection.close() // Close when no longer used. Also triggers disconnect by default. 
    }
}

The snippet below is the example you can find in the sample, powered by two extension methods for brevity (deviceFor(…) and useBasic { device, services -> … }). It also uses the GenericAccess object, which is the definition of the standard Bluetooth GATT "Generic access". It includes extension functions and properties for easy and readable usage. You can write a similar specification for any BLE device or BluetoothGattService you want.

private val myEddystoneUrlBeaconMacAddress = "F2:D6:43:93:70:7A"
private val defaultDeviceMacAddress = myEddystoneUrlBeaconMacAddress

suspend fun logNameAndAppearance(deviceMacAddress: String = defaultDeviceMacAddress) {
    deviceFor(deviceMacAddress).useBasic { device, services ->
        services.forEach { Timber.d("Service found with UUID: ${it.uuid}") }
        with(GenericAccess) {
            device.readAppearance()
            Timber.d("Device appearance: ${device.appearance}")
            device.readDeviceName()
            Timber.d("Device name: ${device.deviceName}")
        }
    }
}

When connected to my Google Beacon, the code above outputs the following in logcat:

I/MainViewModel$logNameAndAppearance: Connected!
I/MainViewModel$logNameAndAppearance: Services discovered!
D/MainViewModel$logNameAndAppearance: Service found with UUID: 00001800-0000-1000-8000-00805f9b34fb
D/MainViewModel$logNameAndAppearance: Service found with UUID: 00001801-0000-1000-8000-00805f9b34fb
D/MainViewModel$logNameAndAppearance: Service found with UUID: ee0c2080-8786-40ba-ab96-99b91ac981d8
D/MainViewModel$logNameAndAppearance: Device appearance: 512
D/MainViewModel$logNameAndAppearance: Device name: eddystone Config
I/MainViewModel$logNameAndAppearance: Disconnected!
I/MainViewModel$logNameAndAppearance: Closed!

This proves our library is working and that Bluetooth GATT can be functional.

Download

Gradle instructions

Make sure you have jcenter() in the repositories defined in your project's (root) build.gradle file (default for new Android Studio projects).

Add the version of the library to not repeat yourself if you use multiple artifacts, and make sure their versions are in sync by adding an ext property into your root project build.gradle file:

allProjects {
    ext {
        blegattcoroutines_version = '0.4.1'
    }
}

Here are all the artifacts of this library. Just use the ones you need:

implementation("com.beepiz.blegattcoroutines:blegattcoroutines-core:$blegattcoroutines_version")
implementation("com.beepiz.blegattcoroutines:blegattcoroutines-genericaccess:$blegattcoroutines_version")

Dev versions

Let's say you need a new feature or a fix that did not make it to a release yet:

You can grab it in the latest dev version by adding the corresponding repository and changing the library version to the dev version you need in your root project build.gradle file:

allProjects {
    repositories {
        google()
        jcenter() // Add dev versions repo below
        maven { url 'https://dl.bintray.com/louiscad/splitties-dev' }
    }
    ext {
        splitties_version = '0.5.0-dev-001' // Change this line
    }
}

Other build systems

For maven and alternative build-systems, check the Bintray page.

New versions notifications

Releases are announced on GitHub, you can subscribe byclicking on "Watch", then "Releases only".