Sometimes, ConflatedBroadcastChannel fires recent value without any action

[theapache64]

In Google's official codelab about advanced-coroutines-codelab sample, they've used ConflatedBroadcastChannel to watch a variable/object change.

https://github.com/googlecodelabs/kotlin-coroutines/blob/b71b981f8354a92fb3ebda37eceb7461e783d0bf/advanced-coroutines-codelab/finished_code/src/main/java/com/example/android/advancedcoroutines/PlantListViewModel.kt#L91-L103

I've used same technique in one of my side projects, and when resuming the listening activity, sometimes ConflatedBroadcastChannel fires it's recent value, causing the execution of flatMapLatest body without any change.

I think this is happening while the system collects the garbage since I can reproduce this issue by calling System.gc() from another activity.

MainActivity.kt

class MainActivity : AppCompatActivity() {
    
    override fun onCreate(savedInstanceState: Bundle?) {
        super.onCreate(savedInstanceState)
        setContentView(R.layout.activity_main)

        val viewModel = ViewModelProvider(this).get(MainViewModel::class.java)
        val tvCount = findViewById<TextView>(R.id.tv_count)

        viewModel.count.observe(this, Observer {
            tvCount.text = it
            Toast.makeText(this, "Incremented", Toast.LENGTH_LONG).show();
        })

        findViewById<Button>(R.id.b_inc).setOnClickListener {
            viewModel.increment()
        }

        findViewById<Button>(R.id.b_detail).setOnClickListener {
            startActivity(Intent(this, DetailActivity::class.java))
        }
        
    }
}

MainViewModel.kt

class MainViewModel : ViewModel() {

    companion object {
        val TAG = MainViewModel::class.java.simpleName
    }

    class IncrementRequest

    private var tempCount = 0
    private val requestChannel = ConflatedBroadcastChannel<IncrementRequest>()

    val count = requestChannel
        .asFlow()
        .flatMapLatest {
            tempCount++
            Log.d(TAG, "Incrementing number to $tempCount")
            flowOf("Number is $tempCount")
        }
        .asLiveData()

    fun increment() {
        requestChannel.offer(IncrementRequest())
    }
}

DetailActivity.kt

class DetailActivity : AppCompatActivity() {

    override fun onCreate(savedInstanceState: Bundle?) {
        super.onCreate(savedInstanceState)
        setContentView(R.layout.activity_detail)
        val button = findViewById<Button>(R.id.b_gc)


        val timer = object : CountDownTimer(5000, 1000) {
            override fun onFinish() {
                button.isEnabled = true
                button.text = "CALL SYSTEM.GC() AND CLOSE ACTIVITY"
            }

            override fun onTick(millisUntilFinished: Long) {
                button.text = "${TimeUnit.MILLISECONDS.toSeconds(millisUntilFinished)} second(s)"
            }
        }

        button.setOnClickListener {
            System.gc()
            finish()
        }

        timer.start()

    }
}

Here's the full source code :
CoroutinesFlowTest.zip

Why is this happening? What am I missing?

[elizarov]

Thanks a lot for a detailed report! The problem here is that you are trying to use ConflatedBroadcastChannel for events, while it is designed to represent current state as shown in the codelab. Every time the downstream LiveData is reactivated it receives the most recent state and performs the incrementing action. Don't use ConflatedBroadcastChannel for events.

To fix it, you can replace ConflatedBroadcastChannel with BroadcastChannel<IncrementRequest>(1) (non-conflated channel, which is Ok for events to use) and it'll work as you expect it too.

Does it help?

[theapache64]

Yes. Absolutely. Thanks for responding @elizarov