SingleRunner.kt
/*
* Copyright 2020 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.CancellationException
import kotlinx.coroutines.Job
import kotlinx.coroutines.coroutineScope
import kotlinx.coroutines.sync.Mutex
import kotlinx.coroutines.sync.withLock
/**
* Class which guarantees single execution of blocks passed to [runInIsolation] by cancelling the
* previous call. [runInIsolation] is backed by a [Mutex], which is fair, so concurrent callers of
* [runInIsolation] will trigger in order, with the last call winning (by cancelling previous calls)
*
* When priorities are used, if the currently running block has a higher priority, the new one is
* cancelled. If the currently running block has lower priority, currently running block is
* cancelled.
* If they have equal priority:
* * if cancelPreviousInEqualPriority == true, existing block is cancelled
* * if cancelPreviousInEqualPriority == false, new block is cancelled
*
* Note: When a block is cancelled, the outer scope (which called runInIsolation) is NOT cancelled.
*/
internal class SingleRunner(
cancelPreviousInEqualPriority: Boolean = true
) {
private val holder = Holder(this, cancelPreviousInEqualPriority)
suspend fun runInIsolation(
priority: Int = DEFAULT_PRIORITY,
block: suspend () -> Unit
) {
try {
coroutineScope {
val myJob = checkNotNull(coroutineContext[Job]) {
"Internal error. coroutineScope should've created a job."
}
val run = holder.tryEnqueue(
priority = priority,
job = myJob
)
if (run) {
try {
block()
} finally {
holder.onFinish(myJob)
}
}
}
} catch (cancelIsolatedRunner: CancelIsolatedRunnerException) {
// if i was cancelled by another caller to this SingleRunner, silently cancel me
if (cancelIsolatedRunner.runner !== this@SingleRunner) {
throw cancelIsolatedRunner
}
}
}
/**
* Internal exception which is used to cancel previous instance of an isolated runner.
* We use this special class so that we can still support regular cancelation coming from the
* `block` but don't cancel its coroutine just to cancel the block.
*/
private class CancelIsolatedRunnerException(val runner: SingleRunner) : CancellationException()
private class Holder(
private val singleRunner: SingleRunner,
private val cancelPreviousInEqualPriority: Boolean
) {
private val mutex = Mutex()
private var previous: Job? = null
private var previousPriority: Int = 0
suspend fun tryEnqueue(
priority: Int,
job: Job
): Boolean {
mutex.withLock {
val prev = previous
return if (prev == null ||
!prev.isActive ||
previousPriority < priority ||
(previousPriority == priority && cancelPreviousInEqualPriority)
) {
prev?.cancel(CancelIsolatedRunnerException(singleRunner))
prev?.join()
previous = job
previousPriority = priority
true
} else {
false
}
}
}
suspend fun onFinish(job: Job) {
mutex.withLock {
if (job === previous) {
previous = null
}
}
}
}
companion object {
const val DEFAULT_PRIORITY = 0
}
}