Move frame encoding to separate thread when possible (#829)

## Proposed Changes If no CATransaction sync is needed, perform Picture recorded commands execution on a separate thread. Fix a freeze on `waitUntilScheduled` if no transaction is available by moving synchronization from interop scope (any UIView is present in the composition) to per-frame scope (any UIView transaction is issued by composition in current frame). ## Testing Test: N/A ## Issues Fixed Removes work from main thread, when possible, allowing Compose to run there without waiting for CPU to finish encoding GPU commands. A freeze on `waitUntilScheduled` during interop synchronization if no transaction is available. <img width="324" alt="Screenshot 2023-09-19 at 12 13 00" src="https://github.com/JetBrains/compose-multiplatform-core/assets/4167681/e54d83e3-ca58-43aa-a4a7-4764dd8ce841"> <img width="371" alt="Screenshot 2023-09-19 at 12 13 05" src="https://github.com/JetBrains/compose-multiplatform-core/assets/4167681/8e4fcf0b-2b64-473e-a930-fae97a086a12"> ## Depends on #820

Author: elijah-semyonov

compose/ui/ui/src/uikitMain/kotlin/androidx/compose/ui/interop/LocalUIKitInteropContext.kt

@@ -131,7 +131,7 @@ internal class UIKitInteropContext(
  }
 }
 
-private inline fun <T> NSLock.doLocked(block: () -> T): T {
+internal inline fun <T> NSLock.doLocked(block: () -> T): T {
  lock()
 
  try {

compose/ui/ui/src/uikitMain/kotlin/androidx/compose/ui/window/MetalRedrawer.kt

@@ -18,6 +18,7 @@ package androidx.compose.ui.window
 
 import androidx.compose.ui.interop.UIKitInteropState
 import androidx.compose.ui.interop.UIKitInteropTransaction
+import androidx.compose.ui.interop.doLocked
 import androidx.compose.ui.interop.isNotEmpty
 import androidx.compose.ui.util.fastForEach
 import kotlin.math.roundToInt
@@ -34,6 +35,7 @@ import platform.UIKit.UIApplicationWillEnterForegroundNotification
 import platform.darwin.*
 import kotlin.math.roundToInt
 import org.jetbrains.skia.Rect
+import platform.Foundation.NSLock
 import platform.Foundation.NSTimeInterval
 import platform.UIKit.UIApplication
 import platform.UIKit.UIApplicationState
@@ -165,6 +167,27 @@ internal interface MetalRedrawerCallbacks {
  fun retrieveInteropTransaction(): UIKitInteropTransaction
 }
 
+internal class InflightCommandBuffers(
+ private val maxInflightCount: Int
+) {
+ private val lock = NSLock()
+ private val list = mutableListOf<MTLCommandBufferProtocol>()
+
+ fun waitUntilAllAreScheduled() = lock.doLocked {
+ list.fastForEach {
+ it.waitUntilScheduled()
+ }
+ }
+
+ fun add(commandBuffer: MTLCommandBufferProtocol) = lock.doLocked {
+ if (list.size == maxInflightCount) {
+ list.removeAt(0)
+ }
+
+ list.add(commandBuffer)
+ }
+}
+
 internal class MetalRedrawer(
  private val metalLayer: CAMetalLayer,
  private val callbacks: MetalRedrawerCallbacks,
@@ -177,13 +200,14 @@ internal class MetalRedrawer(
  private val queue = device.newCommandQueue()
  ?: throw IllegalStateException("Couldn't create Metal command queue")
  private val context = DirectContext.makeMetal(device.objcPtr(), queue.objcPtr())
- private val inflightCommandBuffers = mutableListOf<MTLCommandBufferProtocol>()
  private var lastRenderTimestamp: NSTimeInterval = CACurrentMediaTime()
  private val pictureRecorder = PictureRecorder()
 
  // Semaphore for preventing command buffers count more than swapchain size to be scheduled/executed at the same time
  private val inflightSemaphore =
  dispatch_semaphore_create(metalLayer.maximumDrawableCount.toLong())
+ private val inflightCommandBuffers =
+ InflightCommandBuffers(metalLayer.maximumDrawableCount.toInt())
 
  var isForcedToPresentWithTransactionEveryFrame = false
 
@@ -213,6 +237,7 @@ internal class MetalRedrawer(
  // If active, make metalLayer transparent, opaque otherwise.
  // Rendering into opaque CAMetalLayer allows direct-to-screen optimization.
  metalLayer.setOpaque(!value)
+ metalLayer.drawsAsynchronously = !value
  }
 
  /**
@@ -242,10 +267,7 @@ internal class MetalRedrawer(
  if (!isApplicationActive) {
  // If application goes background, synchronously schedule all inflightCommandBuffers, as per
  // https://developer.apple.com/documentation/metal/gpu_devices_and_work_submission/preparing_your_metal_app_to_run_in_the_background?language=objc
- inflightCommandBuffers.forEach {
- // Will immediately return for MTLCommandBuffer's which are not in `Commited` status
- it.waitUntilScheduled()
- }
+ inflightCommandBuffers.waitUntilAllAreScheduled()
  }
  }
 
@@ -271,8 +293,6 @@ internal class MetalRedrawer(
  caDisplayLink = null
 
  pictureRecorder.close()
-
- context.flush()
  context.close()
  }
 
@@ -353,67 +373,76 @@ internal class MetalRedrawer(
  return@autoreleasepool
  }
 
- surface.canvas.drawPicture(picture)
- picture.close()
- surface.flushAndSubmit()
-
  val interopTransaction = callbacks.retrieveInteropTransaction()
  if (interopTransaction.state == UIKitInteropState.BEGAN) {
  isInteropActive = true
  }
  val presentsWithTransaction =
- isForcedToPresentWithTransactionEveryFrame || isInteropActive
+ isForcedToPresentWithTransactionEveryFrame || interopTransaction.isNotEmpty()
  metalLayer.presentsWithTransaction = presentsWithTransaction
 
- // We only need to synchronize this specific frame if there are any pending changes or isForcedToPresentWithTransactionEveryFrame is true
- val synchronizePresentation = isForcedToPresentWithTransactionEveryFrame || (presentsWithTransaction && interopTransaction.isNotEmpty())
+ val mustEncodeAndPresentOnMainThread = presentsWithTransaction || waitUntilCompletion
 
- val commandBuffer = queue.commandBuffer()!!
- commandBuffer.label = "Present"
+ val encodeAndPresentBlock = {
+ surface.canvas.drawPicture(picture)
+ picture.close()
+ surface.flushAndSubmit()
 
- if (!synchronizePresentation) {
- // If there are no pending changes in UIKit interop, present the drawable ASAP
- commandBuffer.presentDrawable(metalDrawable)
- }
+ val commandBuffer = queue.commandBuffer()!!
+ commandBuffer.label = "Present"
 
- commandBuffer.addCompletedHandler {
- // Signal work finish, allow a new command buffer to be scheduled
- dispatch_semaphore_signal(inflightSemaphore)
- }
- commandBuffer.commit()
-
- if (synchronizePresentation) {
- // If there are pending changes in UIKit interop, [waitUntilScheduled](https://developer.apple.com/documentation/metal/mtlcommandbuffer/1443036-waituntilscheduled) is called
- // to ensure that transaction is available
- commandBuffer.waitUntilScheduled()
- metalDrawable.present()
- interopTransaction.actions.fastForEach {
- it.invoke()
+ if (!presentsWithTransaction) {
+ commandBuffer.presentDrawable(metalDrawable)
  }
 
- if (interopTransaction.state == UIKitInteropState.ENDED) {
- isInteropActive = false
+ commandBuffer.addCompletedHandler {
+ // Signal work finish, allow a new command buffer to be scheduled
+ dispatch_semaphore_signal(inflightSemaphore)
  }
+ commandBuffer.commit()
 
- CATransaction.commit()
- }
+ if (presentsWithTransaction) {
+ // If there are pending changes in UIKit interop, [waitUntilScheduled](https://developer.apple.com/documentation/metal/mtlcommandbuffer/1443036-waituntilscheduled) is called
+ // to ensure that transaction is available
+ commandBuffer.waitUntilScheduled()
+ metalDrawable.present()
 
- surface.close()
- renderTarget.close()
- // TODO manually release metalDrawable when K/N API arrives
+  interopTransaction.actions.fastForEach {
+  it.invoke()
+  }
 
- // Track current inflight command buffers to synchronously wait for their schedule in case app goes background
- if (inflightCommandBuffers.size == metalLayer.maximumDrawableCount.toInt()) {
- inflightCommandBuffers.removeAt(0)
- }
+ if (interopTransaction.state == UIKitInteropState.ENDED) {
+ isInteropActive = false
+ }
+ }
+
+ surface.close()
+ renderTarget.close()
 
- inflightCommandBuffers.add(commandBuffer)
+ // Track current inflight command buffers to synchronously wait for their schedule in case app goes background
+ inflightCommandBuffers.add(commandBuffer)
 
- if (waitUntilCompletion) {
- commandBuffer.waitUntilCompleted()
+ if (waitUntilCompletion) {
+ commandBuffer.waitUntilCompleted()
+ }
+ }
+
+ if (mustEncodeAndPresentOnMainThread) {
+ encodeAndPresentBlock()
+ } else {
+ dispatch_async(renderingDispatchQueue) {
+ autoreleasepool {
+ encodeAndPresentBlock()
+ }
+ }
  }
  }
  }
+
+ companion object {
+ private val renderingDispatchQueue =
+ dispatch_queue_create("RenderingDispatchQueue", null)
+ }
 }
 
 private class DisplayLinkProxy(