I'm subscribed to an observable in my Fragment, the observable listens for some user input from three different sources.

The main issue is that once I navigate to another Fragment and return to the one with the subscription, the data is duplicated as the observable is handled twice.

What is the correct way to handle a situation like this?

I've migrated my application to a Single-Activity and before it, the subscription was made in the activity without any problem.

Here is my Fragment code:

@AndroidEntryPoint
class ProductsFragment : Fragment() {
    @Inject
    lateinit var sharedPreferences: SharedPreferences

    private var _binding: FragmentProductsBinding? = null
    private val binding get() = _binding!!

    private val viewModel: ProductsViewModel by viewModels()
    private val scanner: CodeReaderViewModel by activityViewModels()

    private fun observeBarcode() {
        scanner.barcode.observe(viewLifecycleOwner) { barcode ->
            if (barcode.isNotEmpty()) {
                if (binding.searchView.isIconified) {
                    addProduct(barcode) // here if the fragment is resumed from a backstack the data is duplicated.
                }

                if (!binding.searchView.isIconified) {
                    binding.searchView.setQuery(barcode, true)
                }
            }
        }
    }

    private fun addProduct(barcode: String) {
        if (barcode.isEmpty()) {
            return
        }

        viewModel.insert(barcode)
    }

    override fun onViewCreated(view: View, savedInstanceState: Bundle?) {
        super.onViewCreated(view, savedInstanceState)

        viewModel.start(args.documentId)

        if (args.documentType == "Etichette") {
            binding.cvLabels.visibility = View.VISIBLE
        }

        initUI()
        observe()
    }

    private fun observe() {
        observeBarcode()
        observeProducts()
        observeLoading()
        observeLast()
    }
}

CodePudding user response：

Every time you call

scanner.barcode.observe(viewLifecycleOwner){
}

You are creating a new anonymous observer. So every new call to observe will add another observer that will get onChanged callbacks. You could move this observer out to be a property. With this solution observe won't register new observers.

Try

class property
val observer = Observer<String> { onChanged() }

inside your method
scanner.barcode.observe(viewLifecycleOwner, observer)

Alternatively you could keep your observe code as is but move it to a Fragment's callback that only gets called once fex. onCreate(). onCreate gets called only once per fragment instance whereas onViewCreated gets called every time the fragment's view is created.

CodePudding user response：

Unfortunately, LiveData is a terribly bad idea (the way it was designed), Google insisted till they kinda phased it out (but not really since it's still there) that "it's just a value holder"...

Anyway... not to rant too much, the solution you have to use can be:

1. Use The "SingleLiveEvent" (method is officially "deprecated now" but... you can read more about it here).

2. Follow the "official guidelines" and use a Flow instead, as described in the official guideline for handling UI Events.