android - Android MVVM + Room通过其他LiveData对象创建LiveData RecyclerViewItem对象

Question

我有 session 室实体类“症状”，其名称为“症状”及其ID。

@Entity(tableName = "symptoms")
data class Symptom(
    @PrimaryKey @NonNull val id: Int,
    val name: String) {

    override fun toString(): String {
        return "Symptom $id: $name"
    }
}

我在以下类(class)中得到它:
症状道

@Dao
interface SymptomDao {

    @Query("SELECT * FROM symptoms WHERE id=:id LIMIT 1")
    fun getSymptom(id: Int): Symptom

    @Query("SELECT * FROM symptoms")
    fun getAllSymptoms(): LiveData<List<Symptom>>
}

症状存储库

class SymptomRepository(private val symptomDao: SymptomDao) {

    fun getSymptom(id: Int) = symptomDao.getSymptom(id)

    fun getAllSymptoms() = symptomDao.getAllSymptoms()
}

症状ViewModel

class SymptomsViewModel(symptomRepository: SymptomRepository): ViewModel() {

    private val symptomsList = symptomRepository.getAllSymptoms()
    private val symptomsItemsList: MutableLiveData<List<SymptomItem>> = MutableLiveData()

    fun getAllSymptoms(): LiveData<List<Symptom>> {
        return symptomsList
    }

    fun getAllSymptomsItems(): LiveData<List<SymptomItem>> {
        return symptomsItemsList
    }
}

我的RecyclerView带有 SymptomItem 列表和复选框，以记住用户选择列表的哪些症状:

data class SymptomItem(
    val symptom: Symptom,
    var checked: Boolean = false)

问题
我的问题是如何通过 LiveData<List<SymptomItem>>获得 LiveData<List<Symptom>>？我刚刚开始学习MVVM，但找不到简单的答案来做到这一点。我已经尝试过以各种方式填写此列表，但是每次旋转手机时，它都会丢失 checked变量。如有任何提示，我将不胜感激。

Answer 1

您需要通过将其ID存储在ViewModel的列表中来存储要检查的项目。然后，您将合并Symptom对象的列表和检查哪些项目的列表，并生成SymptomItem对象的列表。
我将使用Kotlin Flow来实现这一目标。

@Dao
interface SymptomDao {

    @Query("SELECT * FROM symptoms")
    fun flowAllSymptoms(): Flow<List<Symptom>>
}

class SymptomRepository(private val symptomDao: SymptomDao) {

    fun flowAllSymptoms() = symptomDao.flowAllSymptoms()
}

class SymptomsViewModel(
    private val symptomRepository: SymptomRepository
) : ViewModel() {

    private val symptomsListFlow = symptomRepository.flowAllSymptoms()

    private val symptomsItemsList: MutableLiveData<List<SymptomItem>> = MutableLiveData()

    private var checkedIdsFlow = MutableStateFlow(emptyList<Int>())

    init {
        viewModelScope.launch {
            collectSymptomsItems()
        }
    }

    private suspend fun collectSymptomsItems() =
        flowSymptomsItems().collect { symptomsItems ->
            symptomsItemsList.postValue(symptomsItems)
        }

    private fun flowSymptomsItems() =
        symptomsListFlow
        .combine(checkedIdsFlow) { list, checkedIds ->
            list.map { SymptomItem(it, checkedIds.contains(it.id)) }
        }

    fun checkItem(id: Int) {
        (checkedIdsFlow.value as MutableList<Int>).add(id)
        checkedIdsFlow.value = checkedIdsFlow.value
    }

    fun uncheckItem(id: Int) {
        (checkedIdsFlow.value as MutableList<Int>).remove(id)
        checkedIdsFlow.value = checkedIdsFlow.value
    }

    fun getSymptomsItems(): LiveData<List<SymptomItem>> {
        return symptomsItemsList
    }
}

在您的片段中，观察 getSymptomsItems()并更新您的适配器数据。
该代码未经测试，您可能需要进行一些小的调整才能编译。