| /* |
| * Copyright 2021 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. |
| */ |
| |
| @file:RestrictTo(RestrictTo.Scope.LIBRARY) |
| |
| package androidx.paging |
| |
| import androidx.annotation.RestrictTo |
| import androidx.paging.LoadType.APPEND |
| import androidx.paging.LoadType.PREPEND |
| import kotlinx.coroutines.channels.BufferOverflow |
| import kotlinx.coroutines.flow.Flow |
| import kotlinx.coroutines.flow.MutableSharedFlow |
| import java.util.concurrent.locks.ReentrantLock |
| import kotlin.concurrent.withLock |
| |
| /** |
| * Helper class to handle UI hints. |
| * It processes incoming hints and keeps a min/max (prepend/append) values and provides them as a |
| * flow to [PageFetcherSnapshot]. |
| */ |
| internal class HintHandler { |
| private val state = State() |
| |
| /** |
| * Latest call to [processHint]. Note that this value might be ignored wrt prepend and append |
| * hints if it is not expanding the range. |
| */ |
| val lastAccessHint: ViewportHint.Access? |
| get() = state.lastAccessHint |
| |
| /** |
| * Returns a flow of hints for the given [loadType]. |
| */ |
| fun hintFor(loadType: LoadType): Flow<ViewportHint> = when (loadType) { |
| PREPEND -> state.prependFlow |
| APPEND -> state.appendFlow |
| else -> throw IllegalArgumentException("invalid load type for hints") |
| } |
| |
| /** |
| * Resets the hint for the given [loadType]. |
| * Note that this won't update [lastAccessHint] or the other load type. |
| */ |
| fun forceSetHint( |
| loadType: LoadType, |
| viewportHint: ViewportHint |
| ) { |
| require( |
| loadType == PREPEND || loadType == APPEND |
| ) { |
| "invalid load type for reset: $loadType" |
| } |
| state.modify( |
| accessHint = null |
| ) { prependHint, appendHint -> |
| if (loadType == PREPEND) { |
| prependHint.value = viewportHint |
| } else { |
| appendHint.value = viewportHint |
| } |
| } |
| } |
| |
| /** |
| * Processes the hint coming from UI. |
| */ |
| fun processHint(viewportHint: ViewportHint) { |
| state.modify(viewportHint as? ViewportHint.Access) { prependHint, appendHint -> |
| if (viewportHint.shouldPrioritizeOver( |
| previous = prependHint.value, |
| loadType = PREPEND |
| ) |
| ) { |
| prependHint.value = viewportHint |
| } |
| if (viewportHint.shouldPrioritizeOver( |
| previous = appendHint.value, |
| loadType = APPEND |
| ) |
| ) { |
| appendHint.value = viewportHint |
| } |
| } |
| } |
| |
| private inner class State { |
| private val prepend = HintFlow() |
| private val append = HintFlow() |
| var lastAccessHint: ViewportHint.Access? = null |
| private set |
| val prependFlow |
| get() = prepend.flow |
| val appendFlow |
| get() = append.flow |
| private val lock = ReentrantLock() |
| |
| /** |
| * Modifies the state inside a lock where it gets access to the mutable values. |
| */ |
| fun modify( |
| accessHint: ViewportHint.Access?, |
| block: (prepend: HintFlow, append: HintFlow) -> Unit |
| ) { |
| lock.withLock { |
| if (accessHint != null) { |
| lastAccessHint = accessHint |
| } |
| block(prepend, append) |
| } |
| } |
| } |
| |
| /** |
| * Like a StateFlow that holds the value but does not do de-duping. |
| * Note that, this class is not thread safe. |
| */ |
| private inner class HintFlow { |
| var value: ViewportHint? = null |
| set(value) { |
| field = value |
| if (value != null) { |
| _flow.tryEmit(value) |
| } |
| } |
| private val _flow = MutableSharedFlow<ViewportHint>( |
| replay = 1, |
| onBufferOverflow = BufferOverflow.DROP_OLDEST |
| ) |
| val flow: Flow<ViewportHint> |
| get() = _flow |
| } |
| } |
| |
| internal fun ViewportHint.shouldPrioritizeOver( |
| previous: ViewportHint?, |
| loadType: LoadType |
| ): Boolean { |
| return when { |
| previous == null -> true |
| // Prioritize Access hints over Initialize hints |
| previous is ViewportHint.Initial && this is ViewportHint.Access -> true |
| this is ViewportHint.Initial && previous is ViewportHint.Access -> false |
| // Prioritize hints from most recent presenter state |
| // not that this it not a gt/lt check because we would like to prioritize any |
| // change in available pages, not necessarily more or less as drops can have an impact. |
| this.originalPageOffsetFirst != previous.originalPageOffsetFirst -> true |
| this.originalPageOffsetLast != previous.originalPageOffsetLast -> true |
| // Prioritize hints that would load the most items |
| previous.presentedItemsBeyondAnchor(loadType) <= presentedItemsBeyondAnchor(loadType) -> |
| false |
| else -> true |
| } |
| } |
