LiveData不会更新数据

Question

我想向您寻求帮助。我正在编写使用MVVM和LiveData体系结构的应用程序。在ViewPager内，我有3个片段来显示来自ViewModel的数据。而且我注意到，将viewModel连接到活动和片段之后，仅在活动启动时才更新数据，但是即使数据已更改，Observe也不会更新数据。在调用服务器的下一个查询之后，在onDataSet内部，我发送了适当的时间并从服务器获取JSON数据，该数据将解析并传递给ViewModel。为什么Fragment一开始只更新一次数据，之后什么都没改变？

这是承载片段的活动

class MainActivity : AppCompatActivity(), DatePickerDialog.OnDateSetListener {

    private lateinit var currencyViewModel: CurrencyViewModel
    private lateinit var viewPager: ViewPager2
    private lateinit var tabLayout: TabLayout
    private lateinit var navigationView: NavigationView
    private lateinit var floatingActionButton: FloatingActionButton

    override fun onCreate(savedInstanceState: Bundle?) {
        super.onCreate(savedInstanceState)
        setContentView(R.layout.activity_main)
        val fm = supportFragmentManager

        currencyViewModel = ViewModelProvider
            .AndroidViewModelFactory(application)
            .create(CurrencyViewModel::class.java)

        viewPager = findViewById(R.id.viewPager)
        tabLayout = findViewById(R.id.tabLayout)
        navigationView = findViewById(R.id.navigationView)
        floatingActionButton = findViewById(R.id.floatingActionButton)

        val viewPagerAdapter = CurrencyViewPagerAdapter(this)
        viewPager.adapter = viewPagerAdapter

        TabLayoutMediator(tabLayout
            ,viewPager
            ,TabLayoutMediator.TabConfigurationStrategy {
                    tab, position ->
                when(position){
                    0 -> tab.text = "Tabela A"
                    1 -> tab.text = "Tabela B"
                    2 -> tab.text = "Tabela C"
                }
            }).attach()
        floatingActionButton.setOnClickListener {
            val dialog = CalendarFragment()
            dialog.show(fm, "DatePickerDialog")
        }
    }
    override fun onDateSet(view: DatePicker?, year: Int, month: Int, dayOfMonth: Int) {
        //Convert year,month,day to millisecounds
        val c = Calendar.getInstance()
        c.set(year,month,dayOfMonth)
        val dayInMillis = c.time.time
        val today = Calendar.getInstance()

        if(checkIsDateAfterToday(today, c)){
            CoroutineScope(Dispatchers.Main).launch {
                currencyViewModel.setTableA(dayInMillis)
            }
        }

    }

这是活动和片段通用的ViewModel

class CurrencyViewModel : ViewModel() {

    private val repository = CurrencyRepository()
    val tableA: MutableLiveData<Array<TableA>> by lazy {
        MutableLiveData<Array<TableA>>().also {
            loadTableA(Date().time)
        }
    }

    private fun loadTableA(time: Long) {
            CoroutineScope(Dispatchers.Main).launch {
               val loadedData = CoroutineScope(Dispatchers.IO).async {
                    repository.getTableA(time)
                }.await()
                tableA.value = loadedData
            }
    }

   fun setTableA(time: Long){
       loadTableA(time)
   }

}

这就是在recyclerView中显示数据的片段

class TableAFragment : Fragment() {

    private lateinit var currencyViewModel: CurrencyViewModel
    private lateinit var recyclerViewA: RecyclerView

    override fun onCreateView(inflater: LayoutInflater, container: ViewGroup?, savedInstanceState: Bundle?): View? {

        return inflater.inflate(R.layout.fragment_table_a, container, false)
    }

    override fun onCreate(savedInstanceState: Bundle?) {
        super.onCreate(savedInstanceState)

        currencyViewModel = ViewModelProvider.AndroidViewModelFactory
            .getInstance(requireActivity().application)
            .create(CurrencyViewModel::class.java)

    }

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

        recyclerViewA = view.findViewById(R.id.recyclerView_A)
        recyclerViewA.layoutManager = LinearLayoutManager(requireContext())

        currencyViewModel.tableA.observe(viewLifecycleOwner, androidx.lifecycle.Observer{
            val nbpAdapter = NBPAdapter(it)
            recyclerViewA.adapter = nbpAdapter
        })

    }

}

Answer 1

您的ViewModel实例化不正确。

应该是

currencyViewModel = ViewModelProvider(this).get<CurrencyViewModel>() // lifecycle-ktx

和片段中：

currencyViewModel = ViewModelProvider(requireActivity()).get<CurrencyViewModel>() // lifecycle-ktx