PageKeyedDataSource loadAfter 连续调用

Question

我有一个PageKeyedDataSource，不断调用loadAfter，在Recyclerview中多次添加所有item。从 API 方面来看，一个空 lastEvaluatedKey 意味着给我第一页，这对于为什么它一直调用以获取第一页但 A 有点道理。如果没有更多数据要获取，它是否应该停止（又名 params.key == null ? 和 B. 适配器中的 COMPARATOR 不应该不允许多次添加相同的项目吗？我错过了什么？

PageKeyedDataSource.kt

class ReservationsPageKeyedDataSource(private val retryExecutor: Executor) : PageKeyedDataSource<String, Reservation?>() {

    private var retry: (() -> Any)? = null

    val initialLoad = MutableLiveData<PagingNetworkState>()

    fun retryAllFailed() {
        val prevRetry = retry
        retry = null
        prevRetry?.let {
            retryExecutor.execute {
                it.invoke()
            }
        }
    }

    override fun loadInitial(
        params: LoadInitialParams<String>,
        callback: LoadInitialCallback<String, Reservation?>
    ) {
        val request = Api.reservationsService.getReservations(dateType = RERVATIONS_DATE_TYPE.future, last = null)

        initialLoad.postValue(PagingNetworkState.LOADING)

        // triggered by a refresh, execute in sync
        try {
            val response = request.execute()
            val originalData = response.body()?.result?.reservations
            val data = mutableListOf<Reservation>()
            // some data munipulation
            retry = null
            initialLoad.postValue(PagingNetworkState.LOADED)

            callback.onResult(
                data.toList(),
                null,
                response.body()?.result?.lastEvaluatedKey.toString()
            )
        } catch (ioException: IOException) {
            retry = {
                loadInitial(params, callback)
            }
            val error = PagingNetworkState.error(ioException.message ?: "unknown error")
            initialLoad.postValue(error)
        }
    }

    override fun loadBefore(
        params: LoadParams<String>,
        callback: LoadCallback<String, Reservation?>
    ) {
        // no-op
    }

    override fun loadAfter(
        params: LoadParams<String>,
        callback: LoadCallback<String, Reservation?>
    ) {

        // I tried adding an if statement here to check if the params.key is null or not but that didn't help

        Api.reservationsService.getReservations(dateType = RERVATIONS_DATE_TYPE.future, last = params.key)
            .enqueue(object : Callback<ReservationListResponse> {
                override fun onFailure(call: Call<ReservationListResponse>, t: Throwable) {
                    retry = { loadAfter(params, callback) }
                }

                override fun onResponse(
                    call: Call<ReservationListResponse>,
                    response: Response<ReservationListResponse>
                ) {
                    if (response.isSuccessful) {
                        val data = response.body()?.result?.reservations
                        retry = null
                        callback.onResult(
                            data.orEmpty(),
                            response.body()?.result?.lastEvaluatedKey.toString()
                        )
                    } else {
                        retry = { loadAfter(params, callback) }
                    }
                }
            })
    }
}

PagedListAdapter 中的比较器：

companion object {
        val COMPARATOR = object : DiffUtil.ItemCallback<Reservation>() {
            override fun areContentsTheSame(oldItem: Reservation, newItem: Reservation): Boolean =
                oldItem == newItem

            override fun areItemsTheSame(oldItem: Reservation, newItem: Reservation): Boolean =
                oldItem.id == newItem.id
        }
    }

Answer 1

另一个可能的原因是，如果您在 ScrollView 内部使用 Recycleview，loadAfter 将被无限调用，直到数据完成。 Recycleview 会自动处理滚动，所以不要在 scrollView 中使用它。

Answer 2

代码看起来大部分都很好，但是您通过 toString 将响应的下一个标记部分转换为字符串并将其用作密钥似乎很奇怪。

试试PageKeyedDataSource<KeyType, Reservation>，而不是PageKeyedDataSource<String, Reservation?>（从上面的代码中不确定该键的类型）。

然后您可以直接从 API 中获取下一个令牌，并将其传递到您的 API 的 last 参数中，而无需修改它。

您还应该使用不可为空的Reservation - Paging 库期望加载的项目是非空的，因为空值是为表示占位符而保留的：https://developer.android.com/reference/androidx/paging/DataSource#implementing-a-datasource