领域结果的 LiveData Observer 第一次没有被触发

Question

我正在通过 Guide to app architecture and trying to implement MVVM and LiveData in one of my apps. I am using realm and I am using this 创建如下所示的 RealmLiveData

class RealmLiveData<T : RealmModel>(private val results: RealmResults<T>) : MutableLiveData<RealmResults<T>>() {
private val listener = RealmChangeListener<RealmResults<T>> { results -> value = results }
override fun onActive() {
    results.addChangeListener(listener)
}

override fun onInactive() {
    results.removeChangeListener(listener)
}
}

这就是我将列表更新到 recyclerview 的方式

var mList:ArrayList<Notes> = ArrayList()

lateinit var historyViewModel: HistoryViewModel

override fun onCreateView(
    inflater: LayoutInflater, container: ViewGroup?,
    savedInstanceState: Bundle?
): View? {
    val view = inflater.inflate(R.layout.fragment_history, container, false)
    mRCview = view.findViewById(R.id.list)
    historyViewModel = ViewModelProviders.of(activity!!).get(HistoryViewModel::class.java)

    // this is how I observe
    historyViewModel.getList().observe(this, Observer{
        (mRCview.adapter as MyHistoryRecyclerViewAdapter).setData(it)
    })

    with(mRCview) {
        setHasFixedSize(true)
        layoutManager = LinearLayoutManager(mContext)
        mList = ArrayList()
        adapter = MyHistoryRecyclerViewAdapter(
            mContext as OnListFragmentInteractionListener
        )
    }
    return view
}

这就是我在存储库中获取数据的方式class

class HistoryRepository {

fun getHistory(): RealmLiveData<Notes> {
    val realmInstance = Realm.getDefaultInstance()
    val realmResults = realmInstance
        .where(Notes::class.java)
        .findAll()
        .sort("lastUpdatedTimeStamp", Sort.DESCENDING)
    return realmResults.asLiveData()
}

fun <T:RealmModel> RealmResults<T>.asLiveData() = RealmLiveData(this)
}

编辑

这是视图模型

class HistoryViewModel: ViewModel() {

val repository = HistoryRepository()

fun getList(): RealmLiveData<Notes> {
    return repository.getHistory()
}
}

问题是观察者没有第一次被触发。如果我更新领域结果，将调用实时数据更新并更新我的列表。请告诉我如何解决这个问题。

Answer 1

我们需要将现有数据通知观察者。当第一个观察者注册到 historyViewModel.getList() 时，您正在注册领域回调。这个时候我们需要触发一个change只是为了通知这个Observer已经有数据了。

类似

class RealmLiveData<T : RealmModel>(private val results: RealmResults<T>) : MutableLiveData<RealmResults<T>>() {
private val listener = RealmChangeListener<RealmResults<T>> { results -> value = results }
override fun onActive() {
    results.addChangeListener(listener)
    listener.onChange(results) // notify the added Observer of the existing data.
}

override fun onInactive() {
    results.removeChangeListener(listener)
}
}

领域结果的 LiveData Observer 第一次没有被触发

LiveData Observer for realmresults not getting triggered first time

android

mvvm

realm

kotlin

android-livedata