Android 后台线程上的实时数据转换

Question

我看到了this但我不确定如何实现它，或者如果这是同一个问题，我有一个调解器实时数据，当其 2 个源实时数据更新或底层数据(Room db)更新时，它会更新，它似乎可以工作很好，但是如果数据更新很多，它会快速连续刷新很多，并且我会收到错误

Cannot run invalidation tracker. Is the db closed?
Cannot access database on the main thread since it may potentially lock the UI for a long period of time

这种情况不会每次都会发生，只有当数据库连续快速更新时才会发生，这就是 View 模型的问题部分，

var search: MutableLiveData<String> = getSearchState()
val filters: MutableLiveData<MutableSet<String>> = getCurrentFiltersState()
val searchPokemon: LiveData<PagingData<PokemonWithTypesAndSpeciesForList>>

val isFiltersLayoutExpanded: MutableLiveData<Boolean> = getFiltersLayoutExpanded()

init {

    val combinedValues =
        MediatorLiveData<Pair<String?, MutableSet<String>?>?>().apply {
            addSource(search) {
                value = Pair(it, filters.value)
            }
            addSource(filters) {
                value = Pair(search.value, it)
            }
        }

    searchPokemon = Transformations.switchMap(combinedValues) { pair ->
        val search = pair?.first
        val filters = pair?.second
        if (search != null && filters != null) {
            searchAndFilterPokemonPager(search, filters.toList())
        } else null
    }.distinctUntilChanged()
}

@SuppressLint("DefaultLocale")
private fun searchAndFilterPokemonPager(search: String, filters: List<String>): LiveData<PagingData<PokemonWithTypesAndSpeciesForList>> {
    return Pager(
        config = PagingConfig(
            pageSize = 20,
            enablePlaceholders = false,
            maxSize = 60
        )
    ) {
        if (filters.isEmpty()){
            searchPokemonForPaging(search)
        } else {
            searchAndFilterPokemonForPaging(search, filters)
        }
    }.liveData.cachedIn(viewModelScope)
}

@SuppressLint("DefaultLocale")
private fun getAllPokemonForPaging(): PagingSource<Int, PokemonWithTypesAndSpecies> {
    return repository.getAllPokemonWithTypesAndSpeciesForPaging()
}

@SuppressLint("DefaultLocale")
private fun searchPokemonForPaging(search: String): PagingSource<Int, PokemonWithTypesAndSpeciesForList> {
    return repository.searchPokemonWithTypesAndSpeciesForPaging(search)
}

@SuppressLint("DefaultLocale")
private fun searchAndFilterPokemonForPaging(search: String, filters: List<String>): PagingSource<Int, PokemonWithTypesAndSpeciesForList> {
    return repository.searchAndFilterPokemonWithTypesAndSpeciesForPaging(search, filters)
}

该错误实际上是由函数 searchPokemonForPaging 引发的

例如，当应用程序启动时会发生这种情况，该应用程序会执行大约 300 次写入，但如果我通过暂停所有内容并使用 runBlocking 返回寻呼机来强制调用关闭主线程，它确实可以工作，并且我不再收到错误，但是它显然会阻塞用户界面，那么有没有办法使 switchmap 异步或使 searchAndFilterPokemonPager 方法异步返回寻呼机？我知道第二个在技术上是可能的(从异步返回)，但也许协程有办法解决这个问题
感谢您的帮助

Answer 1

您可以使用 combineTuple (which is available as a library that I wrote for this specific purpose) 简化组合(可选)

之后，您可以使用 liveData { 协程构建器将执行移至后台线程

现在您的代码将如下所示

val search: MutableLiveData<String> = getSearchState()
val filters: MutableLiveData<Set<String>> = getCurrentFiltersState()
val searchPokemon: LiveData<PagingData<PokemonWithTypesAndSpeciesForList>>

val isFiltersLayoutExpanded: MutableLiveData<Boolean> = getFiltersLayoutExpanded()

init {
    searchPokemon = combineTuple(search, filters).switchMap { (search, filters) -> 
        liveData {
            val search = search ?: return@liveData
            val filters = filters ?: return@liveData

            withContext(Dispatchers.IO) {
                emit(searchAndFilterPokemonPager(search, filters.toList()))
            }
        }
    }.distinctUntilChanged()
}