观察MediatorLiveData问题

Question

我有以下LiveData变量在我的ViewModel(简化的例子):

val currentUser : LiveData<UserObject>
val allSites : LiveData<ArrayList<SiteObject>>
val filterSitesForUser : LiveData<Boolean>
val orderSitesByField : LiveData<String>
val orderSitesDirection : LiveData<Query.Direction>
val searchFilterSitesText : LiveData<String>

我试图使用MediatorLiveData有一个数据流连接到我的RecyclerView。

因此，我在ViewModel中也有以下代码，在Fragment中可以看到:

fun sitesList() : LiveData<ArrayList<SiteObject>> {
val result = MediatorLiveData<ArrayList<SiteObject>>()

result.addSource(currentUser) {
result.value = combineSitesData(currentUser, allSites, filterSitesForUser, orderSitesByField, orderSitesDirection, searchFilterSitesText)
}
result.addSource(allSites) {
result.value = combineSitesData(currentUser, allSites, filterSitesForUser, orderSitesByField, orderSitesDirection, searchFilterSitesText)
}
result.addSource(filterSitesForUser) {
result.value = combineSitesData(currentUser, allSites, filterSitesForUser, orderSitesByField, orderSitesDirection, searchFilterSitesText)
}

result.addSource(orderSitesByField) {
result.value = combineSitesData(currentUser, allSites, filterSitesForUser, orderSitesByField, orderSitesDirection, searchFilterSitesText)
}
result.addSource(orderSitesDirection) {
result.value = combineSitesData(currentUser, allSites, filterSitesForUser, orderSitesByField, orderSitesDirection, searchFilterSitesText)
}
result.addSource(searchFilterSitesText) {
result.value = combineSitesData(currentUser, allSites, filterSitesForUser, orderSitesByField, orderSitesDirection, searchFilterSitesText)
}

return result
}

. .与此同时，也在ViewModel:

private fun combineSitesData (
currentUser: LiveData<UserObject>,
allSites: LiveData<ArrayList<SiteObject>>,
filterSitesForUser: LiveData<Boolean>,
orderSitesByField: LiveData<String>,
orderSitesDirection: LiveData<Query.Direction>,
searchFilterSitesText: LiveData<String>
) : ArrayList<SiteObject> {
var sitesList = ArrayList<SiteObject>()
val userId = currentUser.value?.userID
if (userId == null || allSites.value == null) {
Log.d(TAG, "combineSitesData() - currentUser or allSites value null")
return sitesList
}
when (filterSitesForUser.value) {
true -> sitesList.addAll(allSites.value!!.filter { site -> site.users.contains(userId) })
false -> sitesList.addAll(allSites.value!!)
}
if (orderSitesDirection.value == Query.Direction.ASCENDING){
when (orderSitesByField.value) {
DATE_CREATED -> sitesList.sortBy { it.dateCreatedTimestamp }
DATE_EDITED -> sitesList.sortBy { it.dateEditedTimestamp }
SITE_TASKS -> sitesList.sortBy { it.siteTask }
SITE_RATING -> sitesList.sortBy { it.siteRating }
else -> sitesList.sortBy {it.siteReference}
}
}
if (orderSitesDirection.value == Query.Direction.DESCENDING){
when (orderSitesByField.value) {
DATE_CREATED -> sitesList.sortByDescending { it.dateCreatedTimestamp }
DATE_EDITED -> sitesList.sortByDescending { it.dateEditedTimestamp }
SITE_TASKS -> sitesList.sortByDescending { it.siteTask }
SITE_RATING -> sitesList.sortByDescending { it.siteRating }
else -> sitesList.sortByDescending {it.siteReference}
}
}
if (!searchFilterSitesText.value.isNullOrEmpty()) {
var filteredList = ArrayList<SiteObject>()
var filterPattern = searchFilterSitesText.value.toString().toLowerCase().trim()
for (site in sitesList) {
if(site.siteReference.toLowerCase().contains(filterPattern) || site.siteAddress.toLowerCase().contains(filterPattern)) {
filteredList.add(site)
}
}
Log.d(TAG, "combineSitesData() - returned filteredList (size = ${filteredList.size})")
return filteredList
}
Log.d(TAG, "combineSitesData() - returned sitesList (size = ${sitesList.size})")
return sitesList
}

这是Fragment中观察站点列表的代码:

// Observe ViewModel SitesList
businessViewModel.sitesList().observe(viewLifecycleOwner, Observer { sitesList ->
if (sitesList != null) {
businessViewModel.currentUser.value?.userID?.let { sitesAdapter.setList(it, sitesList) }
sitesAdapter.notifyDataSetChanged()
Log.d (TAG, "setupViewModelObservers(): businessViewModel.sitesList().size = ${sitesList.size}" )
}
})

它似乎是工作除了事实，当它第一次观察，它发射了六次，因此更新我的RecylcerView六次-当这是一个大的列表，我认为这是一个有点问题!

初始化之后，它只在LiveData的任何更改时更新一次，所以这是可以的。

我不知道如何防止这种情况发生，没有在Fragment或ViewModel中持有列表来比较观察到的列表，我试图避免(试图遵循MVVM架构)..

编辑:为了清晰起见，我添加了RecyclerViewAdapter来显示setList函数的作用:

class SitesRecyclerViewAdapter(
private var currentUserId: String,
private val onItemClickedListener: (SiteObject) -> Unit,
private val onItemLongClickedListener: (SiteObject) -> Boolean
) : RecyclerView.Adapter<RecyclerView.ViewHolder>() {
private var sitesList = ArrayList<SiteObject>()
class SiteViewHolder(itemView: View) : RecyclerView.ViewHolder(itemView) {
private val listItemBinding = SitesListItemBinding.bind(itemView)
fun bind(
userId: String,
site: SiteObject,
clickListener: (SiteObject) -> Unit,
longClickListener: (SiteObject) -> Boolean
) {
// Set text fields
listItemBinding.sitesItemTitleText.text = site.siteReference
//            listItemBinding.sitesItemProjectsText.text = site.recentProjectsText TODO: Create way of showing recent projects with arrays
//Reset Icons visibility
listItemBinding.sitesItemTaskImageView.visibility = View.INVISIBLE
listItemBinding.sitesItemRating1ImageView.visibility = View.INVISIBLE
listItemBinding.sitesItemRating2ImageView.visibility = View.INVISIBLE
listItemBinding.sitesItemRating3ImageView.visibility = View.INVISIBLE
listItemBinding.sitesItemFavouriteImageView.visibility = View.GONE
//Set sitePriority Icon visibility
if (site.siteTask) listItemBinding.sitesItemTaskImageView.visibility = View.VISIBLE
//Set siteRating Icon visibility
when(site.siteRating){
1 -> listItemBinding.sitesItemRating1ImageView.visibility = View.VISIBLE
2 -> listItemBinding.sitesItemRating2ImageView.visibility = View.VISIBLE
3 -> listItemBinding.sitesItemRating3ImageView.visibility = View.VISIBLE
}
//Set siteFavourite Icon visibility
if (site.users.contains(userId)) listItemBinding.sitesItemFavouriteImageView.visibility = View.VISIBLE
// Set Listeners
listItemBinding.sitesItemCardview.setOnClickListener { clickListener(site) }
listItemBinding.sitesItemCardview.setOnLongClickListener { longClickListener(site) }
// Not set map listener?
}
}
override fun onCreateViewHolder(parent: ViewGroup, viewType: Int): RecyclerView.ViewHolder {
// LayoutInflater: takes ID from layout defined in XML.
// Instantiates the layout XML into corresponding View objects.
// Use context from main app -> also supplies theme layout values!
val inflater = LayoutInflater.from(parent.context)
// Inflate XML. Last parameter: don't immediately attach new view to the parent view group
val view = inflater.inflate(R.layout.sites_list_item, parent, false)
return SiteViewHolder(view)
}
override fun getItemCount(): Int = sitesList.size
override fun onBindViewHolder(holder: RecyclerView.ViewHolder, position: Int) {
// Populate ViewHolder with data that corresponds to the position in the list
// which we are told to load
(holder as SiteViewHolder).bind(
currentUserId,
sitesList[position],
onItemClickedListener,
onItemLongClickedListener
)
}
fun setList(userID: String, observedSites: ArrayList<SiteObject>) {
currentUserId = userID
sitesList = observedSites
notifyDataSetChanged()  
Log.d(TAG, "setList(), observedSites size = ${observedSites.size}")
}
}

Answer 1

可以用postValue代替setValue

如果在主线程执行已发布的任务之前多次调用此方法，则只能最后一个值将被调度。