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JuulLabs/ kable (0.2.0) about 1 month ago
Kotlin JavaScript kotlin-coroutines bluetooth-low-energy kotlin-multiplatform
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JuulLabs
173 Stars 14 Forks Apache License 2.0 36 Commits 18 Opened issues

Description

Kotlin Asynchronous Bluetooth Low-Energy

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Travis Wyatt twyatt
# 187,598
Android
Kotlin
Shell
cbor
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Kable

Kotlin Asynchronous Bluetooth Low Energy provides a simple Coroutines-powered API for interacting with Bluetooth Low Energy devices.

Usage is demonstrated with the SensorTag sample app.

Scanning

To scan for nearby peripherals, the 

Scanner
provides an 
advertisements
 which is a stream of 
Advertisement
 objects representing advertisements seen from nearby peripherals. 
Advertisement
 objects contain information such as the peripheral's name and RSSI (signal strength).

Scanning begins when the 

advertisements
is collected and stops when the 
Flow
 collection is terminated. A 
Flow
 terminal operator (such as 
first
) may be used to scan until an advertisement is found that matches a desired predicate. 

val advertisement = Scanner()
    .advertisements
    .first { it.name?.startsWith("Example") }

JavaScript: Scanning for nearby peripherals is supported, but only available on Chrome 79+ with "Experimental Web Platform features" enabled via: 

chrome://flags/#enable-experimental-web-platform-features

Peripheral

Once an 

Advertisement
is obtained, it can be converted to a 
Peripheral
 via the 
CoroutineScope.peripheral
 extension function. 
Peripheral
 objects represent actions that can be performed against a remote peripheral, such as connection handling and I/O operations.

val peripheral = scope.peripheral(advertisement)

JavaScript

On JavaScript, rather than processing a stream of advertisements, a specific peripheral can be requested using the 

CoroutineScope.requestPeripheral
extension function. Criteria (
Options
) such as expected service UUIDs on the peripheral and/or the peripheral's name may be specified. When 
requestPeripheral
 is called with the specified options, the browser shows the user a list of peripherals matching the criteria. The peripheral chosen by the user is then returned (as a 
Peripheral
 object).

val options = Options(
    optionalServices = arrayOf(
        "f000aa80-0451-4000-b000-000000000000",
        "f000aa81-0451-4000-b000-000000000000"
    ),
    filters = arrayOf(
        NamePrefix("Example")
    )
)
val peripheral = scope.requestPeripheral(options).await()

Connectivity

Once a 

Peripheral
object is acquired, a connection can be established via the 
connect
 function. The 
connect
 method suspends until a connection is established and ready (or a failure occurs). A connection is considered ready when connected, services have been discovered, and observations (if any) have been re-wired. Service discovery occurs automatically upon connection.

Multiple concurrent calls to 

connect
will all suspend until connection is ready.

peripheral.connect()

To disconnect, the 

disconnect
function will disconnect an active connection, or cancel an in-flight connection attempt. The 
disconnect
 function suspends until the peripheral has settled on a disconnected state.

peripheral.disconnect()

If the underlying subsystem fails to deliver the disconnected state then the 

disconnect
call could potentially stall indefinitely. To prevent this (and ensure underlying resources are cleaned up in a timely manner) it is recommended that 
disconnect
 be wrapped with a timeout, for example:

// Allow 5 seconds for graceful disconnect before forcefully closing `Peripheral`.
withTimeoutOrNull(5_000L) {
    peripheral.disconnect()
}

State

The connection state of a 

Peripheral
can be monitored via its 
state
.

peripheral.state.collect { state ->
    // Display and/or process the connection state.
}

The 

state
will typically transition through the following 
State
s:

Connection states

Disconnecting
state only occurs on Android platform. JavaScript and Apple-based platforms transition directly from 
Connected
 to 
Disconnected
.

I/O

Bluetooth Low Energy devices are organized into a tree-like structure of services, characteristics and descriptors; whereas characteristics and descriptors have the capability of being read from, or written to.

For example, a peripheral might have the following structure:

  • Service S1 (
    00001815-0000-1000-8000-00805f9b34fb
    )
    • Characteristic C1
      • Descriptor D1
      • Descriptor D2
    • Characteristic C2 (
      00002a56-0000-1000-8000-00805f9b34fb
      )
      • Descriptor D3 (
        00002902-0000-1000-8000-00805f9b34fb
        )
  • Service S2
    • Characteristic C3

To access a characteristic or descriptor, use the 

charactisticOf
or 
descriptorOf
 functions, respectively.

In the above example, to access "Descriptor D2":

val descriptor = descriptorOf(
    service = "00001815-0000-1000-8000-00805f9b34fb",
    characteristic = "00002a56-0000-1000-8000-00805f9b34fb",
    descriptor = "00002902-0000-1000-8000-00805f9b34fb"
)

Once connected, data can be read from, or written to, characteristics and/or descriptors via 

read
and 
write
 functions.

The 

read
and 
write
 functions throw 
NotReadyException
 until a connection is established.

val data = peripheral.read(characteristic)


peripheral.write(descriptor, byteArrayOf(1, 2, 3))

Notifications

Bluetooth Low Energy provides the capability of subscribing to characteristic changes by means of notifications, whereas a characteristic change on a connected peripheral is "pushed" to the central via a characteristic notification which carries the new value of the characteristic.

Characteristic change notifications can be observed/subscribed to via the 

observe
function which returns a 
Flow
 of the new characteristic data.

val observation = peripheral.observe(characteristic)
observation.collect { data ->
    // Process data.
}

The 

observe
function can be called (and its returned 
Flow
 can be collected) prior to a connection being established. Once a connection is established then characteristic changes will stream from the 
Flow
. If the connection drops, the 
Flow
 will remain active, and upon reconnecting it will resume streaming characteristic changes.

Failures related to notifications are propagated via 

connect
if the 
observe
 is collected prior to a connection being established. If a connection is already established when an 
observe
 is beginning to be collected, then notification failures are propagated via the 
observe
.

Structured Concurrency

Peripheral objects/connections are scoped to a Coroutine scope. When creating a 

Peripheral
, the 
CoroutineScope.peripheral
 extension function is used, which scopes the returned 
Peripheral
 to the 
CoroutineScope
 receiver. If the 
CoroutineScope
 receiver is cancelled then the 
Peripheral
 will disconnect and be disposed.

Scanner()
    .advertisements
    .filter { advertisement -> advertisement.name?.startsWith("Example") }
    .map { advertisement -> scope.peripheral(advertisement) }
    .onEach { peripheral -> peripheral.connect() }
    .launchIn(scope)


delay(60_000L)
scope.cancel() // All peripherals will implicitly disconnect and be disposed.


Peripheral.disconnect
is the preferred method of disconnecting peripherals, but disposal via Coroutine scope cancellation is provided to prevent connection leaks.

Setup

Gradle

Maven Central

Kable can be configured via Gradle Kotlin DSL as follows:

Multiplatform

plugins {
    id("com.android.application") // or id("com.android.library")
    kotlin("multiplatform")
}


repositories {
    jcenter() // or mavenCentral()
}


kotlin {
    android()
    js().browser() // and/or js().node()
    macosX64()


sourceSets {
    val commonMain by getting {
        dependencies {
            implementation("com.juul.kable:core:$version")
        }
    }
}


}


android {
    // ...
}

Note that Apple-based targets (e.g. 

macosX64
) require Coroutines with multithread support for Kotlin/Native. Kable is configured to use 
-native-mt
as a transitive dependency for Apple-based targets.

Platform-specific

repositories {
    jcenter() // or mavenCentral()
}


dependencies {
    implementation("com.juul.kable:core-$platform:$version")
}

Where 

$platform
represents (should be replaced with) the desired platform dependency (e.g. 
android
).

License

Copyright 2020 JUUL Labs, Inc.


Licensed under the Apache License, Version 2.0 (the "License");
you may not use this file except in compliance with the License.
You may obtain a copy of the License at


   http://www.apache.org/licenses/LICENSE-2.0


Unless required by applicable law or agreed to in writing, software
distributed under the License is distributed on an "AS IS" BASIS,
WITHOUT WARRANTIES OR CONDITIONS OF ANY KIND, either express or implied.
See the License for the specific language governing permissions and
limitations under the License.

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