未调用 LiveData 观察器

Question

我有一个 activity，TabBarActivity 承载一个片段 EquipmentRecyclerViewFragment。该片段接收到 LiveData 回调，但 Activity 没有（在调试模式下使用断点进行了验证）。奇怪的是，如果我调用 ViewModel 的 initData 方法，Activity 回调会触发。以下是上述组件的相关部分：

TabBarActivity

override fun onCreate(savedInstanceState: Bundle?) {
    super.onCreate(savedInstanceState)
    initVM()
    setContentView(R.layout.activity_nav)
    val equipmentRecyclerViewFragment = EquipmentRecyclerViewFragment()
    supportFragmentManager
            .beginTransaction()
            .replace(R.id.frameLayout, equipmentRecyclerViewFragment, equipmentRecyclerViewFragment.TAG)
            .commit()
    navigation.setOnNavigationItemSelectedListener(mOnNavigationItemSelectedListener)

}

var eVM : EquipmentViewModel? = null
private fun initVM() {
    eVM = ViewModelProviders.of(this).get(EquipmentViewModel::class.java)
    eVM?.let { lifecycle.addObserver(it) } //Add ViewModel as an observer of this fragment's lifecycle
    eVM?.equipment?.observe(this, loadingObserver)//        eVM?.initData() //TODO: Not calling this causes Activity to never receive the observed ∆
}
val loadingObserver = Observer<List<Gun>> { equipment ->
    ...}

EquipmentRecyclerViewFragment

override fun onCreate(savedInstanceState: Bundle?) {
    super.onCreate(savedInstanceState)
    columnCount = 2
    initVM()
}

//MARK: ViewModel Methods
var eVM : EquipmentViewModel? = null
private fun initVM() {
    eVM = ViewModelProviders.of(this).get(EquipmentViewModel::class.java)
    eVM?.let { lifecycle.addObserver(it) } //Add ViewModel as an observer of this fragment's lifecycle
    eVM?.equipment?.observe(this, equipmentObserver)
    eVM?.initData()
}
override fun onCreateView(inflater: LayoutInflater, container: ViewGroup?, savedInstanceState: Bundle?): View? {
    val view = inflater.inflate(R.layout.fragment_equipment_list, container, false)
    if (view is RecyclerView) { // Set the adapter
        val context = view.getContext()
        view.layoutManager = GridLayoutManager(context, columnCount)
        view.adapter = adapter
    }
    return view
}

EquipmentViewModel

class EquipmentViewModel(application: Application) : AndroidViewModel(application), LifecycleObserver {
var equipment = MutableLiveData<List<Gun>>()
var isLoading = MutableLiveData<Boolean>()

fun initData() {
    isLoading.setValue(true)
    thread { Thread.sleep(5000) //Simulates async network call
        var gunList = ArrayList<Gun>()
        for (i in 0..100){
            gunList.add(Gun("Gun "+i.toString()))
        }
        equipment.postValue(gunList)
        isLoading.postValue(false)
    }
}

最终目标是让 activity 只观察 isLoading MutableLiveData 布尔值，但由于那不起作用，我将 activity 更改为只观察设备 LiveData 到尽量减少变量的数量。

Answer 1

要获得对 Activity 的 ViewModel 的相同引用，您需要传递相同的 Activity 实例，您应该使用 ViewModelProviders.of(getActivity)。当您将 this 作为参数传递时，您会收到与您的 Fragment.

关联的 ViewModel 实例

有两个重载方法：

ViewModelProvider.of(Fragment fragment)

ViewModelProvider.of(FragmentActivity activity)

更多信息Share data between fragments

Answer 2

我把这段代码放在了onActivityCreated片段里面，别小看getActivity ;)

if (activity != null) {            
     globalViewModel = ViewModelProvider(activity!!).get(GlobalViewModel::class.java)
    }


globalViewModel.onStop.observe(viewLifecycleOwner, Observer { status ->
            Log.d("Parent Viewmodel", status.toString())
        })

此代码帮助我监听片段中父 ViewModel 的变化。

Answer 3

Kotlin 答案

Remove these two points in your function if you are using:

= viewModelScope.launch { }
暂停

Answer 4

当您创建片段而不是通过 viewModels() 获取 viewModel 对象时，从 activityViewModels()

获取它

import androidx.fragment.app.activityViewModels

class WeatherFragment : Fragment(R.layout.fragment_weather) {

    private lateinit var binding: FragmentWeatherBinding
    private val viewModel: WeatherViewModel by activityViewModels() // Do not use viewModels()

    override fun onCreateView(inflater: LayoutInflater, container: ViewGroup?, savedInstanceState: Bundle?): View {
        binding = FragmentWeatherBinding.inflate(inflater, container, false)

        binding.viewModel = viewModel

        // Observing for testing & Logging
        viewModel.cityName.observe(viewLifecycleOwner, Observer {
            Log.d(TAG, "onCreateView() | City name changed $it")
        })
        return binding.root
    }
}

Answer 5

仅供那些在 SharedViewModel 定义与使两个片段使用一个视图模型之间感到困惑的人：

SharedViewModel 用于共享 'DATA'（假设正在创建两个新实例并将视图模型中的数据发送到两个片段）它不用于可观察对象，因为可观察对象寻找 'SAME'采取行动的实例。这意味着您需要为两个片段创建一个视图模型实例。

IMO：Google 应该在他们的文档中以某种方式提及这一点，因为我自己认为在引擎盖下它们是相同的实例，但它基本上不是，现在实际上是有道理的。

编辑： Kotlin 中的解决方案： 11/25/2021

在你的 activity -> val viewModel : YourViewModel by viewModels()

片段 1 - >

val fragmentViewModel =
                ViewModelProvider(requireActivity() as YourActivity)[YourViewModel::class.java]

片段 2 - >

val fragmentViewModel =
                ViewModelProvider(requireActivity() as YourActivity)[YourViewModel::class.java]

This Way 2 个片段共享一个 Activity 视图模型实例，两个片段都可以使用侦听器来观察它们之间的变化。

未调用 LiveData 观察器

LiveData Observer not Called

android

mvvm

android-fragments

kotlin

android-livedata