BleGattCoroutines

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Language
Kotlin
Version
N/A
Created
Dec 26, 2017
Updated
Feb 8, 2018
Owner
Beepiz
Contributor
Louis CAD (LouisCAD)
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Blurb

BleGattCoroutines

Make Gatt Great Again! This library allows easy and safer usage of BluetoothGatt in Android. It has also been tested successfully on Android Wear, with the sample included in this repository. It should work similarly on other Android variants such as Android Things.

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 overrideable 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. In fact, RxJava2 methods count is higher than the sum of Kotlin's stdlib and kotlinx.coroutines.

Experimental status

This library is based on the coroutines feature of Kotlin, as well as the kotlinx.coroutines library, which are both under the experimental status. Consequently, this library inherits this experimental status. Also, we are expecting to make a few API changes based on your feedback and real world usages to improve this library.

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, or public tweet.

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 attempt. If you want your code to suspend until the connection is established, call await() on the connect() result. Immediately after calling connect(), you can perform the operations you want. If you didn't await the connection before attempting your operations, they will just suspend until the connection is established and then will be executed in a first-come first-served basis (just make sure connect() is called before any operation, or asynchronously to avoid dead coroutines).

The currently supported GATT operations on the GattConnection class are:

  • Services discovery, using disoverServices() which returns and cache the list of the services on the connected device.
  • Characteristic read, using readCharacteristic(…). Services disovery has to be completed before, as usual.
  • Characteristic write, using writeCharacteristic(…). Services disovery has to be completed before, as usual.
  • ReliableWrite, with reliableWrite { … }. Implemented, but couldn't be tested yet. Open an issue if your device supports it
  • Descriptor read, using readDescriptor(…). Services disovery has to be completed before, as usual.
  • Descriptor write, using writeDescriptor(…). Services disovery 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. Hasn't 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. Note that just like connect(), disconnect() returns a Deferred<Unit> which you can await is needed by calling the obvious await() fun.

Examples

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

fun BluetoothDevice.logGattServices(tag: String = "BleGattCoroutines") = launch(UI) {
    val deviceConnection = GattConnection(bluetoothDevice = this@logGattServices)
    deviceConnection.connect().await() // Await is optional
    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.v(tag, it.print(printCharacteristics = true))
    }
    deviceConnection.disconnect().await() // Disconnection is optional. Useful if you don't close and reconnect later.
    deviceConnection.close() // Close when no longer used it NOT optional 
}

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 defition 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

fun logNameAndAppearance(deviceMacAddress: String = defaultDeviceMacAddress) = launch(UI) {
    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 WE MADE GATT GREAT AGAIN!

Download

This library is not published on jcenter yet (you can subscribe to the issue #4 to get notified when it is), but it's made of only 4 Kotlin files, and here's a good way to try the library, and then, to add it to your project:

  1. Download/clone the "whole" project from GitHub on your computer.
  2. Try the sample from your watch, phone or tablet (does it work on Chromebooks too?) on your connectable BLE device to understand how it works.
  3. Import the blegattcoroutines module in your project (this will take care of adding the few dependencies).
  4. In the module(s) where you need to use BleGattCoroutines, add the module dependency like it's done in the sample's build.gradle file.
  5. Success! You can use BLE with coroutines in your project.