FlowExt.kt
/*
* 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.
*/
package androidx.paging
import kotlinx.coroutines.channels.SendChannel
import kotlinx.coroutines.flow.Flow
import kotlinx.coroutines.flow.FlowCollector
import kotlinx.coroutines.flow.collect
import kotlinx.coroutines.flow.collectLatest
import kotlinx.coroutines.flow.emitAll
import kotlinx.coroutines.flow.flow
/**
* This File includes custom flow operators that we implement to avoid using experimental APIs
* from coroutines. Eventually, this file should be deleted once those APIs become stable.
*/
private val NULL = Any()
/**
* Temporary `scan` operator on Flow without experimental APIs.
*/
internal fun <T, R> Flow<T>.simpleScan(
initial: R,
operation: suspend (accumulator: R, value: T) -> R
): Flow<R> = flow {
var accumulator: R = initial
emit(accumulator)
collect { value ->
accumulator = operation(accumulator, value)
emit(accumulator)
}
}
/**
* Temporary `runningReduce` operator on Flow without experimental APIs.
*/
internal fun <T> Flow<T>.simpleRunningReduce(
operation: suspend (accumulator: T, value: T) -> T
): Flow<T> = flow {
var accumulator: Any? = NULL
collect { value ->
accumulator = if (accumulator === NULL) {
value
} else {
@Suppress("UNCHECKED_CAST")
operation(accumulator as T, value)
}
@Suppress("UNCHECKED_CAST")
emit(accumulator as T)
}
}
/**
* This is a similar implementation to transformLatest using a channel Flow.
*/
internal fun <T, R> Flow<T>.simpleTransformLatest(
transform: suspend FlowCollector<R>.(value: T) -> Unit
): Flow<R> = simpleChannelFlow {
val origin = this@simpleTransformLatest
val collector = ChannelFlowCollector(this@simpleChannelFlow)
origin.collectLatest { value ->
collector.transform(value)
}
}
/**
* flatMapLatest without experimental APIs
*/
internal inline fun <T, R> Flow<T>.simpleFlatMapLatest(
crossinline transform: suspend (value: T) -> Flow<R>
): Flow<R> = simpleTransformLatest { emitAll(transform(it)) }
/**
* mapLatest without experimental APIs
*/
internal inline fun <T, R> Flow<T>.simpleMapLatest(
crossinline transform: suspend (value: T) -> R
): Flow<R> = simpleTransformLatest { emit(transform(it)) }
internal class ChannelFlowCollector<T>(
val channel: SendChannel<T>
) : FlowCollector<T> {
override suspend fun emit(value: T) {
channel.send(value)
}
}