Hello guys! Please help me with Coroutines. I need...

# coroutines

[Oleh Ponomarenko]

Hello guys! Please help me with Coroutines. I need to implement this:
1. Run my task in background mode
2. Stop the task in 30 seconds
3. Delay 10 seconds.
4. Repeat my cycle (1-3 points) infinitely

https://prntscr.com/mu5xin (detailed on a picture)

[gildor]

https://kotlinlang.slack.com/archives/C0B8M7BUY/p1551891564163100?thread_ts=1551890794.162200&cid=C0B8M7BUY

[Oleh Ponomarenko]

Here is my code:

CoroutineScope(Dispatchers.Default).launch {
            BluetoothAdapter.getDefaultAdapter()?.takeIf { !it.isEnabled }?.enable()

            if (bluetoothScanner == null) {
                delay(3000)
                bluetoothScanner = BluetoothAdapter.getDefaultAdapter()?.bluetoothLeScanner
            }
            try {
                bluetoothScanner?.startScan(filters, settings, callback)
            } catch (e: IllegalStateException) {
                Log.e(TAG, e.localizedMessage)
            }
        }

Can you help to implement this due to my picture?
I need to scan for 30 seconds, then stop for 10 seconds, then resume to scanning for 30 seconds. This cycle should be endless.

[gildor]

To make it endless just wrap it to while(true)

[Oleh Ponomarenko]

How can I stop my task in 30 seconds?

[gildor]

See my first comment

[gildor]

Use withTimeout and wrap it to try/catch to catch cancellation exception, this is probably the easiest way, but you can check implementation and write own similar coroutine builder that doesn't throw exception on timeout

[gildor]

It also depends on how this scanning works

[gildor]

I see that you just start it and pass callback, in this case you probably have some stopScanning method

[Oleh Ponomarenko]

yes, i have

[gildor]

So just use delay(30, Seconds) after start

[Oleh Ponomarenko]

Is it correct way:

CoroutineScope(Dispatchers.Default).launch {
            while (true) {
                val timeout = 20000L
                try {
                    Log.d(TAG, "starting")
                    bluetoothScanner?.startScan(filters, settings, callback)
                    withTimeout(timeout) {
                        Log.d(TAG, "stop scan")
                        bluetoothScanner?.stopScan(callback)
                    }
                    delay(10000)
                    Log.d(TAG, "Pause works fine")
                } catch (e: Exception) {
                    Log.e(TAG, e.localizedMessage)
                }
            }
        }

?

[gildor]

You don't need withTimeout in this case

[gildor]

Delay is enough

[gildor]

withTimeout make sense only if you have some suspend operation that should be cancelled on timeout, in this case you just call non-suspend method

[gildor]

Just a note about your code: explicitly passing default dispatcher is most probably redundant

[gildor]

Just want to add that correctness of this code highly depends on how bluetoothScanner works, but if it common call back API this approach should work

[Oleh Ponomarenko]

Everything works fine, thanks

[sitepodmatt]

if bluetooth scanner becomes null which you seem to suggest is possible it will just continue looping ?

[bdawg.io]

Something to note is that withTimeout is collaborative cancellation. If stopScan is a blocking method that takes 25 seconds, it won’t be cancelled at the 20 second timeout.

[Oleh Ponomarenko]

Yes, sometimes I use cancellation, when I switch between running in Service and usual mode.

[gildor]

this method doesn’t look as blocking, because callback is passed to startScan and to stopScan, most probably it’s some non-blocking method that returns immediately