why flow collect call more than twice in kotlin?

Issue

Hey I am working in kotlin flow in android. I noticed that my kotlin flow collectLatest is calling twice and sometimes even more. I tried this answer but it didn't work for me. I printed the log inside my collectLatest function it print the log. I am adding the code

MainActivity.kt

class MainActivity : AppCompatActivity(), CustomManager {

    private val viewModel by viewModels<ActivityViewModel>()
    private lateinit var binding: ActivityMainBinding
    private var time = 0

    override fun onCreate(savedInstanceState: Bundle?) {
        super.onCreate(savedInstanceState)
        binding = ActivityMainBinding.inflate(layoutInflater)
        setContentView(binding.root)
        setupView()
    }

    private fun setupView() {
        viewModel.fetchData()

        lifecycleScope.launchWhenStarted {
            repeatOnLifecycle(Lifecycle.State.STARTED) {
                viewModel.conversationMutableStateFlow.collectLatest { data ->
                    Log.e("time", "${time++}")
                   ....
                }
            }
        }
    }
}

ActivityViewModel.kt

class ActivityViewModel(app: Application) : AndroidViewModel(app) {

    var conversationMutableStateFlow = MutableStateFlow<List<ConversationDate>>(emptyList())

    fun fetchData() {
        viewModelScope.launch {
            val response = ApiInterface.create().getResponse()
            conversationMutableStateFlow.value = response.items
        }
    }

   .....
}

I don't understand why this is calling two times. I am attaching logs

2022-01-17 22:02:15.369 8248-8248/com.example.fragmentexample E/time: 0
2022-01-17 22:02:15.629 8248-8248/com.example.fragmentexample E/time: 1

As you can see it call two times. But I load more data than it call more than twice. I don't understand why it is calling more than once. Can someone please guide me what I am doing wrong. If you need whole code, I am adding my project link.

Solution

You are using a MutableStateFlow which derives from StateFlow, StateFlow has initial value, you are specifying it as an emptyList:

var conversationMutableStateFlow = MutableStateFlow<List<String>>(emptyList())

So the first time you get data in collectLatest block, it is an empty list. The second time it is a list from the response.

When you call collectLatest the conversationMutableStateFlow has only initial value, which is an empty list, that's why you are receiving it first.

You can change your StateFlow to SharedFlow, it doesn't have an initial value, so you will get only one call in collectLatest block. In ActivityViewModel class:

var conversationMutableStateFlow = MutableSharedFlow<List<String>>()

fun fetchData() {
    viewModelScope.launch {
        val response = ApiInterface.create().getResponse()
        conversationMutableStateFlow.emit(response.items)
    }
}

Or if you want to stick to StateFlow you can filter your data:

viewModel.conversationMutableStateFlow.filter { data ->
                data.isNotEmpty()
            }.collectLatest { data ->
                // ...
            }

Answered By - Sergey