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/*
* Copyright 2019 The Android Open Source Project
*
* Licensed under the Apache License, Version 2.0 (the "License");
* you may not use this file except in compliance with the License.
* You may obtain a copy of the License at
*
* http://www.apache.org/licenses/LICENSE-2.0
*
* Unless required by applicable law or agreed to in writing, software
* distributed under the License is distributed on an "AS IS" BASIS,
* WITHOUT WARRANTIES OR CONDITIONS OF ANY KIND, either express or implied.
* See the License for the specific language governing permissions and
* limitations under the License.
*/
package androidx.paging
import androidx.paging.RemoteMediator.InitializeAction.LAUNCH_INITIAL_REFRESH
import kotlinx.coroutines.ExperimentalCoroutinesApi
import kotlinx.coroutines.FlowPreview
import kotlinx.coroutines.channels.ConflatedBroadcastChannel
import kotlinx.coroutines.channels.SendChannel
import kotlinx.coroutines.flow.Flow
import kotlinx.coroutines.flow.asFlow
import kotlinx.coroutines.flow.channelFlow
import kotlinx.coroutines.flow.collect
import kotlinx.coroutines.flow.filterNotNull
import kotlinx.coroutines.flow.mapLatest
import kotlinx.coroutines.flow.onStart
import kotlinx.coroutines.flow.scan
@OptIn(ExperimentalCoroutinesApi::class, FlowPreview::class)
internal class PageFetcher<Key : Any, Value : Any>(
private val pagingSourceFactory: () -> PagingSource<Key, Value>,
private val initialKey: Key?,
private val config: PagingConfig,
@OptIn(ExperimentalPagingApi::class)
remoteMediator: RemoteMediator<Key, Value>? = null
) {
private val remoteMediatorAccessor = remoteMediator?.let { RemoteMediatorAccessor(it) }
/**
* Channel of refresh signals that would trigger a new instance of [PageFetcherSnapshot].
* Signals sent to this channel should be `true` if a remote REFRESH load should be triggered,
* `false` otherwise.
*
* NOTE: This channel is conflated, which means it has a buffer size of 1, and will always
* broadcast the latest value received.
*/
private val refreshChannel = ConflatedBroadcastChannel<Boolean>()
private val retryChannel = ConflatedBroadcastChannel<Unit>()
// The object built by paging builder can maintain the scope so that on rotation we don't stop
// the paging.
val flow: Flow<PagingData<Value>> = channelFlow {
refreshChannel.asFlow()
.onStart {
@OptIn(ExperimentalPagingApi::class)
emit(remoteMediatorAccessor?.initialize() == LAUNCH_INITIAL_REFRESH)
}
.scan(null) { previousGeneration: PageFetcherSnapshot<Key, Value>?,
triggerRemoteRefresh ->
var pagingSource = generateNewPagingSource(previousGeneration?.pagingSource)
while (pagingSource.invalid) {
pagingSource = generateNewPagingSource(previousGeneration?.pagingSource)
}
@OptIn(ExperimentalPagingApi::class)
val initialKey: Key? = previousGeneration?.refreshKeyInfo()
?.let { pagingSource.getRefreshKey(it) }
?: initialKey
previousGeneration?.close()
PageFetcherSnapshot<Key, Value>(
initialKey = initialKey,
pagingSource = pagingSource,
config = config,
retryFlow = retryChannel.asFlow(),
// Only trigger remote refresh on refresh signals that do not originate from
// initialization or PagingSource invalidation.
triggerRemoteRefresh = triggerRemoteRefresh,
remoteMediatorAccessor = remoteMediatorAccessor,
invalidate = this@PageFetcher::refresh
)
}
.filterNotNull()
.mapLatest { generation ->
PagingData(generation.pageEventFlow, PagerUiReceiver(generation, retryChannel))
}
.collect { send(it) }
}
fun refresh() {
refreshChannel.offer(true)
}
private fun invalidate() {
refreshChannel.offer(false)
}
private fun generateNewPagingSource(
previousPagingSource: PagingSource<Key, Value>?
): PagingSource<Key, Value> {
val pagingSource = pagingSourceFactory()
// Ensure pagingSourceFactory produces a new instance of PagingSource.
check(pagingSource !== previousPagingSource) {
"""
An instance of PagingSource was re-used when Pager expected to create a new
instance. Ensure that the pagingSourceFactory passed to Pager always returns a
new instance of PagingSource.
""".trimIndent()
}
// Hook up refresh signals from PagingSource.
pagingSource.registerInvalidatedCallback(::invalidate)
previousPagingSource?.unregisterInvalidatedCallback(::invalidate)
previousPagingSource?.invalidate() // Note: Invalidate is idempotent.
return pagingSource
}
inner class PagerUiReceiver<Key : Any, Value : Any> constructor(
private val pageFetcherSnapshot: PageFetcherSnapshot<Key, Value>,
private val retryChannel: SendChannel<Unit>
) : UiReceiver {
override fun accessHint(viewportHint: ViewportHint) {
pageFetcherSnapshot.accessHint(viewportHint)
}
override fun retry() {
retryChannel.offer(Unit)
}
override fun refresh() = this@PageFetcher.refresh()
}
}