Observe many times from same Observable (RxAndroidBle) - kotlin

Question

I'm using the RxAndroidBle library with RxJava2 to read from a BLE Characteristic. I think this question is just an RxJava question, but including the detail that I'm using RxAndroidBle in case that is useful.
I get connection, and then use it to call readCharacteristic(), which itself returns a Single<ByteArray>. At this point, I don't just want to just get the one ByteArray though. I need to read from this characteristic several times, because the BLE device is set up to let me get a small file back, and characteristics can only send 20 bytes back at a time, hence my need to read repeatedly.
Is it possible to modify this code so that the switchMap() below returns an Observable that will emit many ByteArrays, instead of just the single one?
I'm new to RxJava.
val connection: Observable<RxBleConnection> = selectedDevice.record.bleDevice.establishConnection(false, Timeout(30, TimeUnit.SECONDS))
return connection
.subscribeOn(Schedulers.io())
.switchMap {
// I want to get an Observable that can read multiple times here.
it.readCharacteristic(serverCertCharacteristicUUID).toObservable()
}
.doOnNext {
Timber.e("Got Certificate bytes")
}
.map {
String(it as ByteArray)
}
.doOnNext {
Timber.e("Got certificate: $it")
}
.singleOrError()

Answer 1

To repeat a read multiple times until a specific value is emitted one needs to change this part:
// I want to get an Observable that can read multiple times here.
it.readCharacteristic(serverCertCharacteristicUUID).toObservable()
to something like what was suggested by the RxJava author in the first answer that google gives for phrase rxjava single repeat:
// this will repeat until a `checkRepeatIf` returns false
Observable.defer {
val successValue = AtomicReference<ByteArray>()
connection.readCharacteristic(serverCertCharacteristicUUID)
.doOnSuccess { successValue.lazySet(it) }
.repeatWhen { completes -> completes.takeWhile { checkRepeatIf(successValue.get()) } }
}

Answer 2

I was able to get this working by sending a signal to stop both the connectionObservable, and the read on the Bluetooth characteristic. Of note is that you need to call toObservable() AFTER repeat() or this doesn't work, although I don't know why exactly.
override fun readMultipartCharacteristic(macAddress: String): Single<String> {
val CERTIFICATE_TERMINATOR = 0x30.toByte()
val device = bluetoothService.getBleDevice(macAddress)
if (connectionObservable == null || !device.connectionState.equals(RxBleConnection.RxBleConnectionState.CONNECTED)) {
connectionObservable = device.establishConnection(false, Timeout(30, TimeUnit.SECONDS))
}
val stop: PublishSubject<Unit> = PublishSubject.create()
return connectionObservable!!
.subscribeOn(Schedulers.io())
.takeUntil(stop)
.switchMap {
it.readCharacteristic(UUID("my-uuid"))
.repeat()
.toObservable()
.takeUntil(stop)
}
.collectInto(ByteArrayOutputStream(), { buffer, byteArray ->
// Watch for the signal of the end of the stream
if (byteArray.size == 1 && byteArray.get(0).equals(CERTIFICATE_TERMINATOR)) {
stop.onComplete()
} else {
buffer.write(byteArray)
}
})
.map {
String(it.toByteArray())
}
}

Answer 3

You can use the notification to buffer your data.
device.establishConnection(false)
.flatMap(rxBleConnection -> rxBleConnection.setupNotification(characteristicUuid))
.flatMap(notificationObservable -> notificationObservable) // <-- Notification has been set up, now observe value changes.
.subscribe(
bytes -> {
// Given characteristic has been changes, here is the value.
},
throwable -> {
// Handle an error here.
}
);