如何将 MutableLiveData <String> 从函数传递给寻呼机？

Question

我正在尝试将我的搜索功能与 Paging 3 一起使用。我检查了我的 addQuery() 函数，当我进行搜索时，我的 _repoQuery 值不为空。但是当我尝试将值传递给 Pager {} 时，它始终为空。我知道，因为它总是会显示与默认值（“花”）相对应的书籍列表。有谁知道我在搜索时如何将 _repoQuery 值传递给 Pager {}？

class MainViewModel () : ViewModel() {

    // Queries
    private val _repoQuery = MutableLiveData<String?>()
    private val _filter = MutableLiveData<String?>()
    val reusableQuery = MutableLiveData<String?>()
    private val repo = Repo

    fun addFilter(filter: String?) {
        _filter.value = filter
    }

    fun addQuery(query: String) {
        _repoQuery.value = query // Here _repoQuery.value exists
        reusableQuery.value = _repoQuery.value
    }

    val books: Flow<PagingData<Volume>> = Pager(PagingConfig(pageSize = 40)) {
        BookListSource(repo, bookFilter = _filter.value, query = _repoQuery.value?:"flowers")
    }.flow.cachedIn(viewModelScope) // Here _repoQuery.value is null
}

解决方案

这最终奏效了：

val repoQuery = _repoQuery.switchMap {
        liveData {
            emit(Pager(PagingConfig(pageSize = 40)) {
                BookListSource(repo, bookFilter = _filter.value, query = _repoQuery.value)
            }.flow.cachedIn(viewModelScope))
        }
    } 

我的理解是 liveData 在到达时会发出新的结果...

Answer 1

它为空，因为在创建 Pager 时，您的 filter/query 没有任何值。当您更改这些值时，您需要重新制作Pager，因为您提供的BookListSource 不再有效。这不是世界上最漂亮的东西，但它应该让你开始。

//Make it once at the start to avoid lateinit 
var books : Flow<PagingData<Volume>> = Pager(PagingConfig(pageSize = 40)) {
          BookListSource(repo, bookFilter = _filter.value, query = _repoQuery.value?:"flowers")
     }.flow.cachedIn(viewModelScope) 

fun addFilter(filter: String?) {
    _filter.value = filter
    //filter changes so the BookListSource needs to be different
    books = Pager(PagingConfig(pageSize = 40)) {
         BookListSource(repo, bookFilter = _filter.value, query = _repoQuery.value?:"flowers")
    }.flow.cachedIn(viewModelScope) 
}

fun addQuery(query: String) {
    _repoQuery.value = query // Here _repoQuery.value exists
    reusableQuery.value = _repoQuery.value
    //query changes so the BookListSource needs to be different
    books = Pager(PagingConfig(pageSize = 40)) {
         BookListSource(repo, bookFilter = _filter.value, query = _repoQuery.value?:"flowers")
    }.flow.cachedIn(viewModelScope) 
}