RxJava – Reactive Extensions for the JVM – a library for composing asynchronous and event-based programs using observable sequences for the Java VM.
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Hello people, is there a way with RXJava2 to have a cancellationToken to cancel a single ? What I’m trying to achieve is a long series of actions, at some point the user want to cancel it, so I want to stop my series of actions, but to cancel my actions I need to excecute some asynchronous task and I don’t want my single to finish until the cancellation is done. Is such a thing possible and how ? I have no clue how to implement this :/
@jaumard Ok so first I don't know if this is a great idea. I am just going to assume you have your reasons why just unsubscribing isn't good enough. Now on to how you would do this. If you wanted a cancellation token like C#, you could do it fairly easily if you used Observable.create. C# cancellation tokens are glorified boolean holders anyway. You could also use Maybe.create, Flowable.create, etc. I don't think you could do this with Single unless you just say it was an error on cancel. What you would do is in your tight loop you just check your cancellation token. My example will be in Kotlin (sorry I am primarily a Android programmer)
Observable.create { emitter ->
for (val action : actions) ->
if (token.cancelled()) {
break
}
emitter.onNext(action())
}
emitter.onComplete()
}Depending on what you are trying to do this might work. If you have a chain of Maybes, you could also just decide to not return the next step
fun Maybe<T>.ifNotCancelled(defered: () -> Maybe<R>): Maybe<R> {
return this.filter { !token.isCancelled() }
.flatMap(defered)
}
step1()
.ifNotCancelled { step2() }
.ifNotCancelled { step3() }
.ifNotCancelled { step4() }There are quite a few ways to do this and without more info I am just taking shots in the dark, but I hope that help gives you an idea of what you can do. The reason this is a little strange is probably because you don't want to model something as part of the stream. Usually it is just easier to keep all the information as part of the stream then you can do whatever you want via more RxJava operators like filter, flatMap, map etc. One of the ways we do this in my project is with switchMap. When you don't stay inside the stream, your kind of just using RxJava as a glorified Functional library / Thread replacement which isn't horrible but kind of leaves all these great ideas behind.
@dmstocking thanks for the feedback ! No problem with kotlin I’m also an Android programmer pimarily ^^
That’s the path I was thinking to take (and put the cancel as an error on my single); Just wanted some feedbacks on how was my solutions as I’m not expert in RX ^^. The full story is that we were given a proprietary SDK to interact with a BT device, this SDK work based on one listener with around 40 methods to override and implements, and we can put only one listener on the SDK. I want to hide this stuff by wrapping it into RX calls to be more readable/maintenable once used on the app (actually it’s a Flutter plugin but same problem). And the SDK doesn’t allow simultanous calls so once you cancel something you need to wait until it’s successfuly cancelled and you have receive the right callback from the SDK.
I know the solution of the token inside the create is not perfect, but I have to comply with the SDK I have and make it work ^^
I’ll go that way and see how it goes :)
That’s the path I was thinking to take (and put the cancel as an error on my single); Just wanted some feedbacks on how was my solutions as I’m not expert in RX ^^. The full story is that we were given a proprietary SDK to interact with a BT device, this SDK work based on one listener with around 40 methods to override and implements, and we can put only one listener on the SDK. I want to hide this stuff by wrapping it into RX calls to be more readable/maintenable once used on the app (actually it’s a Flutter plugin but same problem). And the SDK doesn’t allow simultanous calls so once you cancel something you need to wait until it’s successfuly cancelled and you have receive the right callback from the SDK.
I know the solution of the token inside the create is not perfect, but I have to comply with the SDK I have and make it work ^^
I’ll go that way and see how it goes :)
Still trying to understand how to scan for a limited time then return a list to work with. I am using the RxAndroidBle to scan for all BLE devices. I am trying to look for all devices starting with ABC for example. When I create a test by emitting values using Observable.intervalRange, everything works perfectly. If I don't try to constrain the RxAndroidBle.scanBleDevices function by using a .take() and just subscribe to the results, that works as expected. When I try to build up a list of BLE devices it finds and then return it though, it doesn't work (nothing ever gets returned).
I call the function like this:
List<String> bleDevicesWithMyPrefix = Scanner.create(context).scanForPrefix("ABC").blockingGet(); private fun scanAllDevices(): Observable<ScanResult>
= rxBleClient.scanBleDevices(
ScanSettings.Builder()
.setScanMode(ScanSettings.SCAN_MODE_LOW_LATENCY)
.setCallbackType(ScanSettings.CALLBACK_TYPE_ALL_MATCHES)
.build(),
ScanFilter.Builder()
.build())
The test I set up works perfectly. It looks like this:
//Begin test code
var emittedList: List<String> = listOf("MMM", "OOO", "ABC", "ABCDEF", "ZZZ", "ABC", "RRRRRRR", "ZZABC")
private fun scanAllDevicesTEST(): Observable<ScanResult> {
return Observable
.intervalRange(0, emittedList.size.toLong(), 0, 200, TimeUnit.MILLISECONDS, timeoutScheduler)
.map { index -> ScanResult(MyDevice(emittedList[index.toInt()]), 4, 10, ScanCallbackType.CALLBACK_TYPE_ALL_MATCHES, MyRecord()) }
}
So I am confused why it works for the test which emits values just like the real thing does, and not for the real thing. Again, if I don't do a take, and subscribe instead, then the scanAllDevices code does scan correctly. I just can't allow it to scan forever and I need one complete list. Not something that gets sent back an element at a time.
For example, doing this I do get the BLE devices scanned successfully. I just cannot use them coming back async. I need a complete list or as much of a list as can be found in a specified time period. Note that the scanner takes less than a second to get all of the values when it is working, but I gave it up to 15 just to take out that variable. Here is the code that does at least scan.
private fun scanAllDevices(context: Context) {
val rxBleClient = RxBleClient.create(context);
rxBleClient.scanBleDevices(
ScanSettings.Builder ()
.setScanMode(ScanSettings.SCAN_MODE_LOW_POWER) // change if needed
.setCallbackType(ScanSettings.CALLBACK_TYPE_ALL_MATCHES) // change if needed
.build(),
ScanFilter.Builder()
.build()
)
.observeOn(Schedulers.io())
.subscribe{scanResult->
val rxBleDevice = rxBleClient.getBleDevice(scanResult.getBleDevice().getMacAddress());
Logger.d(TAG, "The device has name ${rxBleDevice.name}")
}
}
Thanks for the suggestion. I did try that, and didn't see any .doOnNext methods ever get hit anywhere. Anyway, I figured out how to do what I want, but I still am not certain why what I had did not work. What I came up with looks like this:
button_scan3.setOnClickListener {
Log.d(TAG, "\n\nFiring up Scanner3")
val scanner = SuiteScanner.create(this.applicationContext)
val set = mutableSetOf<String>()
scanner.scanForAllBLEDevices3("ABC")
.doOnComplete { Log.d(TAG, "Completed...Scanner3")
for(name in set ) {
Log.d(TAG, "BLE Device: $name")
}
}
.subscribe { scanResult ->
val rxBleDevice = scanResult.getBleDevice()
Log.d(TAG, "The device has name ${rxBleDevice.name}")
val name = rxBleDevice.name
if (name != null) {
set.add(name)
}
}
}
fun scanForAllBLEDevices3(prefix: String): Observable<ScanResult> {
Log.w(TAG, "We are in scanForAllBLEDevices3.")
return rxBleScanner()
.take(2, TimeUnit.SECONDS, timeoutScheduler)
.observeOn(Schedulers.io())
.filter { device -> inspectScanResult(device.bleDevice.name, prefix) }
}
private fun inspectScanResult(foundDevice: String?, prefix: String): Boolean {
Log.d(TAG, " Inspecting: $foundDevice")
if(foundDevice == null) {
return false
}
return (foundDevice.startsWith(prefix))
}
So I'll just do what needs to be done in the onComplete()
I need that because i send multiple request concurrently (flatMap) but i only care about the first response coming back. Still i want the subsequent request to be performed. Any idea ?
Sadly, Flowable.amb() is also cancelling remaining requests
Also not completing and not cancelling may leak resources.
Using compose:
.compose(f -> {
final AtomicBoolean isFirst = new AtomicBoolean();
return f.filter(r -> isFirst.compareAndSet(false, true));
})
Flowable
.create(emitter -> {
int i = 0;
while (i++ < 10) {
emitter.onNext(RandomUtils.nextInt() + ":" + i);
Thread.sleep(100);
}
emitter.onComplete();
}, BackpressureStrategy.BUFFER)
//.subscribeOn(Schedulers.computation())
.observeOn(Schedulers.io())
.buffer(1, TimeUnit.SECONDS, Schedulers.trampoline())
.subscribe(list -> {
System.out.println("BlockedBufferRxTest.main:" + list.toString());
});
Thread.sleep(1000000);
Isn't there a backpressure problem here ?
Using RxJava 1.1.9, I have a problem as follows:
import rx.Observable;
public enum ZipWithTest {;
public static void main(String[] args) {
Observable
.just(1, 2, 3)
.doOnNext(next -> System.out.format("next: %d%n", next))
.zipWith(
Observable.just(10, 100, 1000),
(next, multiplier) -> next * multiplier)
.doOnNext(multiplied -> System.out.format("multiplied: %d%n", multiplied))
.toBlocking()
.last();
}
/*
next: 1
next: 2
next: 3
multiplied: 10
multiplied: 200
multiplied: 3000
*/
}I was expecting to get the following:
next: 1
multiplied: 10
next: 2
multiplied: 200
next: 3
multiplied: 3000What am I missing?
so that it invokes an action when it calls {@code onNext}
DoOnNext is called after ever onNext op from every element of source
@vy you should really upgrade to rxjava 2, but as rx1 observable has backpressure this might work as you said, but the problem is that zipWith has a buffer, so the 3 next call are juts filling the zip buffer, not sure if in rx1 exists the zipWith operator overload to indicate the buffer size, but you can do it in rx2, in that case you need to use Flowable instead of Observable to support backpressure
@ylecaillez you can use publish and merge the take(1) with a ignoreElements, so the source observable keep subscribe untils it completes (you can exgract the compose lamda to reuse the operator)
Observable.just(1, 2, 3).doOnNext(n -> out.println("emmit " + n))
.compose(o -> o.publish(p -> p.take(1)))
.subscribe(n -> out.println("final " + n));
// emmit 1 ⏎ final 1
Observable.just(1, 2, 3).doOnNext(n -> out.println("emmit " + n))
.compose(o -> o.publish(p -> p.take(1).mergeWith(p.ignoreElements())))
.subscribe(n -> out.println("final " + n));
// emmit 1 ⏎ final 1 ⏎ emmit 2 ⏎ emmit 3
'ello. i'm wondering if there's a recommended way to achieve the following on 1.x: i have a hot observable (a publishsubject) but i'd like to buffer the last produced item so that new subscribers always see the most recently published event when they subscribe (and then see any new events when i publish them after that)
i'm currently stuck on 1.x due to an incompatible third-party dependency in the project i'm working on, unfortunately
sorry, that should read "buffer the most recently produced value"
sorry for the noise!
I am trying to get a grasp of RxJava. How would i chain following situation in calls with RxJava: i retrieve a Single<User> from my database, loading a user by id for example, this should happen async. After loading the user object, i put some of his specific data in an arraylist. After that finished I want to listen on this array for changes, so that when an element gets added or removed I write to the database async to update the user. I have following code so far, also implemented the piece you have suggested