Android ViewModel LiveData 文档

Question

I read the documentation but became confuse on some part.

ViewModel objects are designed to outlive specific instantiations of views or LifecycleOwners. This design also means you can write tests to cover a ViewModel more easily as it doesn't know about view and Lifecycle objects. ViewModel objects can contain LifecycleObservers, such as LiveData objects. However ViewModel objects must never observe changes to lifecycle-aware observables, such as LiveData objects. If the ViewModel needs the Application context, for example to find a system service, it can extend the AndroidViewModel class and have a constructor that receives the Application in the constructor, since Application class extends Context.

This part is somewhat confusing for me

However ViewModel objects must never observe changes to lifecycle-aware observables, such as LiveData objects.

Is this referring to this kind of implementation?

Fragment

class AboutFragment : Fragment() {

    private lateinit var aboutViewModel: AboutViewModel
    private var _binding: FragmentAboutBinding? = null

    // This property is only valid between onCreateView and
    // onDestroyView.
    private val binding get() = _binding!!

    override fun onCreateView(
            inflater: LayoutInflater,
            container: ViewGroup?,
            savedInstanceState: Bundle?
    ): View {
        aboutViewModel =
                ViewModelProvider(this).get(AboutViewModel::class.java)

        _binding = FragmentAboutBinding.inflate(inflater, container, false)
        val root: View = binding.root

        val textView = binding.aboutTxt

        //Observe changes
        aboutViewModel.text.observe(viewLifecycleOwner, {
            textView.text = it
        })
        return root
    }

    override fun onDestroyView() {
        super.onDestroyView()
        _binding = null
    }
}

ViewModel

class AboutViewModel : ViewModel() {

private val _text = MutableLiveData<String>().apply {
    value = "Foobar......"
           
}

fun setText(text: String){ _text.value = text}

val text: LiveData<String> = _text

}

Or this is what they mean LiveData<LiveData<Object>> not to do?

Answer 1

it means you should never use observe on a livedata object in your viewmodel, only in activity/fragment like you are already doing