在 Android 应用程序中将从改造 (MutableLiveData) 获取的数据与来自 Room DB (LiveData) 的数据同步答案

【问题标题】：Syncing data fetched from retrofit (MutableLiveData) with data from Room DB (LiveData) in an android app在 Android 应用程序中将从改造 (MutableLiveData) 获取的数据与来自 Room DB (LiveData) 的数据同步
【发布时间】：2021-04-15 15:52:09
【问题描述】：

该应用程序的基本思想如下：每当服务器启动时使用改造从服务器获取数据，每当服务器无法访问时回退到本地房间数据库。但是，我在视图模型中保存数据的方式存在问题。我使用LiveData 获取从Room 获取的数据，使用MutableLiveData 获取从服务器获取的数据。不过，不确定如何为数据提供单一来源。 Retrofit API 将我的实体 (Recipe) 返回为 List<Recipe>。我知道坚持使用MutableLiveData的唯一方法：

var readAllIngredients = MutableLiveData<List<Ingredient>>().apply { value = emptyList() }
...
readAllIngredients.postValue(NetworkService.service.getIngredients())

将readAllIngredients 更改为LiveData<List<Ingredient>>，然后尝试设置列表的value 字段不起作用，因为显然value 不可分配给。

也无法使 DAO 返回 MutableLiveData<List<Ingredient>>（出现一些编译错误）。而且我听说尝试将其中一种类型转换为另一种并不是最佳实践。所以我不确定我还能尝试什么。

class InventoryViewModel(application: Application): AndroidViewModel(application) {
    var readAllIngredients = MutableLiveData<List<Ingredient>>().apply { value = emptyList() }
    private val ingredientRepository: IngredientRepository

    init {
        val ingredientDAO = ShoppingAppDatabase.getDatabase(application).ingredientDAO()
        ingredientRepository = IngredientRepository(ingredientDAO)
        viewModelScope.launch(Dispatchers.IO) {
            // logic moved from repository in order to make example more concise 
            if(NetworkService.serverReachable()) {
                readAllIngredients.postValue(NetworkService.service.getIngredients())
            }
            else {
                //save the data from the database somehow; used to do it like
                //readAllIngredients = ingredientRepository.allIngredients 
                //when readAllIngredients was of type `LiveData<List<Ingredient>>`
            }
        }
    }

【问题讨论】：

标签： kotlin retrofit android-room

【解决方案1】：

MediatorLiveData 在您想要组合多个来源时使用。示例：

class InventoryViewModel(application: Application): AndroidViewModel(application) {
    var readAllIngredients = MediatorLiveData<List<Ingredient>>().apply { value = emptyList() }
    private val ingredientRepository: IngredientRepository
    private var gotNetworkIngredients = false

    init {
        val ingredientDAO = ShoppingAppDatabase.getDatabase(application).ingredientDAO()
        ingredientRepository = IngredientRepository(ingredientDAO)
        readAllIngredients.addSource(ingredientRepository.allIngredients) { repoValue ->
            if (!gotNetworkIngredients) {
                readAllIngredients.value = repoValue
            }
        }

        viewModelScope.launch(Dispatchers.IO) {
            // logic moved from repository in order to make example more concise 
            if(NetworkService.serverReachable()) {
                gotNetworkIngredients = true
                readAllIngredients.postValue(NetworkService.service.getIngredients())
            }
            else {
                //save the data from the database somehow; used to do it like
                //readAllIngredients = ingredientRepository.allIngredients 
                //when readAllIngredients was of type `LiveData<List<Ingredient>>`
            }
        }
    }
}

【讨论】：