LiveData 观察者未调用

我有一个活动，TabBarActivity承载一个片段，EquipmentRecyclerViewFragment。片段收到 LiveData 回调，但 Activity 没有（在调试模式下使用断点进行证明）。奇怪的是，如果我调用 ViewModel 的 Activity 回调就会触发initData方法。以下是上述组件的相关部分：

TabBar活动

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
}

设备视图模型

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)
    }
}

最终目标是让活动只是观察isLoadingMutableLiveData 布尔值，但由于这不起作用，我更改了活动以仅观察设备 LiveData，以最大限度地减少起作用的变量数量。

获得相同的参考ViewModel你的Activity你需要通过同样的Activity例如，你应该使用ViewModelProviders.of(getActivity)。当你通过时this作为参数，您收到的实例ViewModel与你相关的Fragment.

有两个重载方法：

ViewModelProvider.of(Fragment fragment)

ViewModelProvider.of(FragmentActivity activity)

欲了解更多信息在片段之间共享数据 https://developer.android.com/topic/libraries/architecture/viewmodel.html#sharing