android - 使用 Kotlin 协程的 NetworkBoundResource
问题描述
您对如何使用NetworkBoundResource和 Kotlin 协程实现存储库模式有任何想法吗?我知道我们可以使用 GlobalScope 启动协程,但这可能会导致协程泄漏。我想将viewModelScope作为参数传递,但在实现方面有点棘手(因为我的存储库不知道任何 ViewModel 的 CoroutineScope)。
abstract class NetworkBoundResource<ResultType, RequestType>
@MainThread constructor(
private val coroutineScope: CoroutineScope
) {
private val result = MediatorLiveData<Resource<ResultType>>()
init {
result.value = Resource.loading(null)
@Suppress("LeakingThis")
val dbSource = loadFromDb()
result.addSource(dbSource) { data ->
result.removeSource(dbSource)
if (shouldFetch(data)) {
fetchFromNetwork(dbSource)
} else {
result.addSource(dbSource) { newData ->
setValue(Resource.success(newData))
}
}
}
}
@MainThread
private fun setValue(newValue: Resource<ResultType>) {
if (result.value != newValue) {
result.value = newValue
}
}
private fun fetchFromNetwork(dbSource: LiveData<ResultType>) {
val apiResponse = createCall()
result.addSource(dbSource) { newData ->
setValue(Resource.loading(newData))
}
result.addSource(apiResponse) { response ->
result.removeSource(apiResponse)
result.removeSource(dbSource)
when (response) {
is ApiSuccessResponse -> {
coroutineScope.launch(Dispatchers.IO) {
saveCallResult(processResponse(response))
withContext(Dispatchers.Main) {
result.addSource(loadFromDb()) { newData ->
setValue(Resource.success(newData))
}
}
}
}
is ApiEmptyResponse -> {
coroutineScope.launch(Dispatchers.Main) {
result.addSource(loadFromDb()) { newData ->
setValue(Resource.success(newData))
}
}
}
is ApiErrorResponse -> {
onFetchFailed()
result.addSource(dbSource) { newData ->
setValue(Resource.error(response.errorMessage, newData))
}
}
}
}
}
}
解决方案
更新(2020-05-27):
一种比我之前的示例更符合 Kotlin 语言的方式,使用 Flow API,并借用 Juan 的答案,可以表示为一个独立的函数,如下所示:
inline fun <ResultType, RequestType> networkBoundResource(
crossinline query: () -> Flow<ResultType>,
crossinline fetch: suspend () -> RequestType,
crossinline saveFetchResult: suspend (RequestType) -> Unit,
crossinline onFetchFailed: (Throwable) -> Unit = { Unit },
crossinline shouldFetch: (ResultType) -> Boolean = { true }
) = flow<Resource<ResultType>> {
emit(Resource.Loading(null))
val data = query().first()
val flow = if (shouldFetch(data)) {
emit(Resource.Loading(data))
try {
saveFetchResult(fetch())
query().map { Resource.Success(it) }
} catch (throwable: Throwable) {
onFetchFailed(throwable)
query().map { Resource.Error(throwable, it) }
}
} else {
query().map { Resource.Success(it) }
}
emitAll(flow)
}
上面的代码可以从一个类中调用,例如一个 Repository,像这样:
fun getItems(request: MyRequest): Flow<Resource<List<MyItem>>> {
return networkBoundResource(
query = { dao.queryAll() },
fetch = { retrofitService.getItems(request) },
saveFetchResult = { items -> dao.insert(items) }
)
}
原答案:
这就是我使用livedata-ktx
工件的方式;无需传入任何 CoroutineScope。该类也只使用一种类型而不是两种类型(例如 ResultType/RequestType),因为我最终总是在其他地方使用适配器来映射它们。
import androidx.lifecycle.LiveData
import androidx.lifecycle.liveData
import androidx.lifecycle.map
import nihk.core.Resource
// Adapted from: https://developer.android.com/topic/libraries/architecture/coroutines
abstract class NetworkBoundResource<T> {
fun asLiveData() = liveData<Resource<T>> {
emit(Resource.Loading(null))
if (shouldFetch(query())) {
val disposable = emitSource(queryObservable().map { Resource.Loading(it) })
try {
val fetchedData = fetch()
// Stop the previous emission to avoid dispatching the saveCallResult as `Resource.Loading`.
disposable.dispose()
saveFetchResult(fetchedData)
// Re-establish the emission as `Resource.Success`.
emitSource(queryObservable().map { Resource.Success(it) })
} catch (e: Exception) {
onFetchFailed(e)
emitSource(queryObservable().map { Resource.Error(e, it) })
}
} else {
emitSource(queryObservable().map { Resource.Success(it) })
}
}
abstract suspend fun query(): T
abstract fun queryObservable(): LiveData<T>
abstract suspend fun fetch(): T
abstract suspend fun saveFetchResult(data: T)
open fun onFetchFailed(exception: Exception) = Unit
open fun shouldFetch(data: T) = true
}
然而,就像@CommonsWare 在评论中所说,最好只公开一个Flow<T>
. 这是我尝试过的方法。请注意,我没有在生产中使用此代码,所以买家要小心。
import kotlinx.coroutines.flow.*
import nihk.core.Resource
abstract class NetworkBoundResource<T> {
fun asFlow(): Flow<Resource<T>> = flow {
val flow = query()
.onStart { emit(Resource.Loading<T>(null)) }
.flatMapConcat { data ->
if (shouldFetch(data)) {
emit(Resource.Loading(data))
try {
saveFetchResult(fetch())
query().map { Resource.Success(it) }
} catch (throwable: Throwable) {
onFetchFailed(throwable)
query().map { Resource.Error(throwable, it) }
}
} else {
query().map { Resource.Success(it) }
}
}
emitAll(flow)
}
abstract fun query(): Flow<T>
abstract suspend fun fetch(): T
abstract suspend fun saveFetchResult(data: T)
open fun onFetchFailed(throwable: Throwable) = Unit
open fun shouldFetch(data: T) = true
}
推荐阅读
- python - TypeError:“NoneType”和“float”的实例之间不支持“<”
- javascript - 有没有办法使用 JavaScript 中的 for 循环将 JSON 数据转换为嵌套数组?
- javascript - 我可以通过 jquery 编辑“类 id”的继承属性吗?
- perl - 获取变量“@xml_files”将不会在 ... 行保持共享
- types - Common Lisp 中的动态类型规范?
- javascript - JS React onpress 短路
- python - 如何在 Python 中使用 .writer 填充行中的空白/空单元格?
- python - 使用 df1 作为 df2 的查找表,在 Python 中 df2 比 df1 有更多的唯一值
- c# - Xamarin 中的事件和委托表单父页面和子页面
- mysql - 如何在 SQL 中制作百分比列?