Optimize MetalRedrawer synchronization

compose/ui/ui/src/iosMain/kotlin/androidx/compose/ui/window/MetalRedrawer.ios.kt

@@ -25,14 +25,12 @@ import androidx.compose.ui.viewinterop.UIKitInteropTransaction
 import kotlin.math.roundToInt
 import kotlinx.cinterop.*
 import org.jetbrains.skia.*
+import org.jetbrains.skia.Rect
 import platform.Foundation.NSRunLoop
 import platform.Foundation.NSSelectorFromString
 import platform.Foundation.NSThread
-import platform.Metal.MTLCommandBufferProtocol
 import platform.QuartzCore.*
 import platform.darwin.*
-import org.jetbrains.skia.Rect
-import platform.Foundation.NSLock
 import platform.Foundation.NSRunLoopCommonModes
 import platform.Foundation.NSTimeInterval
 import platform.Metal.MTLCommandQueueProtocol
@@ -105,27 +103,6 @@ private class DisplayLinkConditions(
     }
 }
 
-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 var retrieveInteropTransaction: () -> UIKitInteropTransaction,
@@ -149,12 +126,8 @@ internal class MetalRedrawer(
     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 inflightCommandBuffersGroup = dispatch_group_create()
     private val drawCanvasSemaphore = dispatch_semaphore_create(1)
-    private val inflightCommandBuffers =
-        InflightCommandBuffers(metalLayer.maximumDrawableCount.toInt())
 
     var isForcedToPresentWithTransactionEveryFrame = false
 
@@ -256,7 +229,10 @@ internal class MetalRedrawer(
 
                 displayLinkConditions.isActive = newValue
                 if (!newValue) {
-                    inflightCommandBuffers.waitUntilAllAreScheduled()
+                    // If an 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
+                    // Set the expiration time to 1 second to ensure that the main thread does not get stuck when the app is suspended.
+                    dispatch_group_wait(inflightCommandBuffersGroup, dispatch_time(DISPATCH_TIME_NOW, 1L * NSEC_PER_SEC.toLong()))
                 }
             }
         }
@@ -305,7 +281,6 @@ internal class MetalRedrawer(
         if (caDisplayLink == null) {
             return
         }
-
         draw(waitUntilCompletion, CACurrentMediaTime())
     }
 
@@ -374,10 +349,6 @@ internal class MetalRedrawer(
                 currentFrameRate = Float.NaN
             }
 
-            trace("MetalRedrawer:draw:waitInflightSemaphore") {
-                dispatch_semaphore_wait(inflightSemaphore, DISPATCH_TIME_FOREVER)
-            }
-
             val metalDrawable = trace("MetalRedrawer:draw:nextDrawable") {
                 metalDrawablesHandler.nextDrawable()
             }
@@ -386,7 +357,6 @@ internal class MetalRedrawer(
                 // TODO: anomaly, log
                 // Logger.warn { "'metalLayer.nextDrawable()' returned null. 'metalLayer.allowsNextDrawableTimeout' should be set to false. Skipping the frame." }
                 picture.close()
-                dispatch_semaphore_signal(inflightSemaphore)
                 return@autoreleasepool
             }
 
@@ -411,7 +381,6 @@ internal class MetalRedrawer(
                 picture.close()
                 renderTarget.close()
                 metalDrawablesHandler.releaseDrawable(metalDrawable)
-                dispatch_semaphore_signal(inflightSemaphore)
                 return@autoreleasepool
             }
 
@@ -439,6 +408,9 @@ internal class MetalRedrawer(
                     picture.close()
                     surface.flushAndSubmit()
 
+                    surface.close()
+                    renderTarget.close()
+
                     if (useSeparateRenderThreadWhenPossible) {
                         dispatch_semaphore_signal(drawCanvasSemaphore)
                     }
@@ -452,9 +424,9 @@ internal class MetalRedrawer(
                         metalDrawablesHandler.scheduleDrawablePresentation(metalDrawable, commandBuffer)
                     }
 
+                    dispatch_group_enter(inflightCommandBuffersGroup)
                     commandBuffer.addCompletedHandler {
-                        // Signal work finish, allow a new command buffer to be scheduled
-                        dispatch_semaphore_signal(inflightSemaphore)
+                        dispatch_group_leave(inflightCommandBuffersGroup)
                     }
                     commandBuffer.commit()
 
@@ -478,12 +450,7 @@ internal class MetalRedrawer(
                         }
                     }
 
-                    surface.close()
-                    renderTarget.close()
-
                     // Track current inflight command buffers to synchronously wait for their schedule in case app goes background
-                    inflightCommandBuffers.add(commandBuffer)
-
                     if (waitUntilCompletion) {
                         trace("MetalRedrawer:draw:waitUntilCompleted") {
                             commandBuffer.waitUntilCompleted()
@@ -560,13 +527,3 @@ private class DisplayLinkProxy(
         callback()
     }
 }
-
-private inline fun <T> NSLock.doLocked(block: () -> T): T {
-    lock()
-
-    try {
-        return block()
-    } finally {
-        unlock()
-    }
-}