BroadcastFrameClock.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.compose.runtime
import androidx.compose.runtime.snapshots.fastForEach
import kotlinx.coroutines.CancellationException
import kotlinx.coroutines.suspendCancellableCoroutine
import kotlin.coroutines.Continuation
import kotlin.coroutines.resumeWithException
/**
* A simple frame clock.
*
* This implementation is intended for low-contention environments involving
* low total numbers of threads in a pool on the order of ~number of CPU cores available for UI
* recomposition work, while avoiding additional allocation where possible.
*
* [onNewAwaiters] will be invoked whenever the number of awaiters has changed from 0 to 1.
* If [onNewAwaiters] **fails** by throwing an exception it will permanently fail this
* [BroadcastFrameClock]; all current and future awaiters will resume with the thrown exception.
*/
class BroadcastFrameClock(
private val onNewAwaiters: (() -> Unit)? = null
) : MonotonicFrameClock {
private class FrameAwaiter<R>(val onFrame: (Long) -> R, val continuation: Continuation<R>) {
fun resume(timeNanos: Long) {
continuation.resumeWith(runCatching { onFrame(timeNanos) })
}
}
private val lock = Any()
private var failureCause: Throwable? = null
private var awaiters = mutableListOf<FrameAwaiter<*>>()
private var spareList = mutableListOf<FrameAwaiter<*>>()
/**
* `true` if there are any callers of [withFrameNanos] awaiting to run for a pending frame.
*/
val hasAwaiters: Boolean get() = synchronized(lock) { awaiters.isNotEmpty() }
/**
* Send a frame for time [timeNanos] to all current callers of [withFrameNanos].
* The `onFrame` callback for each caller is invoked synchronously during the call to
* [sendFrame].
*/
fun sendFrame(timeNanos: Long) {
synchronized(lock) {
// Rotate the lists so that if a resumed continuation on an immediate dispatcher
// bound to the thread calling sendFrame immediately awaits again we don't disrupt
// iteration of resuming the rest.
val toResume = awaiters
awaiters = spareList
spareList = toResume
for (i in 0 until toResume.size) {
toResume[i].resume(timeNanos)
}
toResume.clear()
}
}
override suspend fun <R> withFrameNanos(
onFrame: (Long) -> R
): R = suspendCancellableCoroutine { co ->
lateinit var awaiter: FrameAwaiter<R>
val hasNewAwaiters = synchronized(lock) {
val cause = failureCause
if (cause != null) {
co.resumeWithException(cause)
return@suspendCancellableCoroutine
}
awaiter = FrameAwaiter(onFrame, co)
val hadAwaiters = awaiters.isNotEmpty()
awaiters.add(awaiter)
!hadAwaiters
}
co.invokeOnCancellation {
synchronized(lock) {
awaiters.remove(awaiter)
}
}
// Wake up anything that was waiting for someone to schedule a frame
if (hasNewAwaiters && onNewAwaiters != null) {
try {
// BUG: Kotlin 1.4.21 plugin doesn't smart cast for a direct onNewAwaiters() here
onNewAwaiters.invoke()
} catch (t: Throwable) {
// If onNewAwaiters fails, we permanently fail the BroadcastFrameClock.
fail(t)
}
}
}
private fun fail(cause: Throwable) {
synchronized(lock) {
if (failureCause != null) return
failureCause = cause
awaiters.fastForEach { awaiter ->
awaiter.continuation.resumeWithException(cause)
}
awaiters.clear()
}
}
/**
* Permanently cancel this [BroadcastFrameClock] and cancel all current and future
* awaiters with [cancellationException].
*/
fun cancel(
cancellationException: CancellationException = CancellationException("clock cancelled")
) {
fail(cancellationException)
}
}