iOS avoid redundant compositing (#813)

## Proposed Changes

Only use `presentWithTransaction` and transparent `CAMetalLayer` when
UIKit interop is active.

## Testing

Test: N/A

## Issues Fixed

Fixes: occasional frame drops reported in
JetBrains/compose-multiplatform#3605 when iOS
compositor fails to meet frame deadline because of going slow off-screen
rendering path, instead of direct-to-screen.

Composited | Direct
---|---

![Composited](https://github.com/JetBrains/compose-multiplatform-core/assets/4167681/130ec472-1675-4eda-b2cc-fa1e1e84ec1d)
|
![Direct](https://github.com/JetBrains/compose-multiplatform-core/assets/4167681/65ebf09a-7896-4354-9c64-9c0e547a87f6)

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

@@ -19,6 +19,62 @@ package androidx.compose.ui.interop
 import androidx.compose.runtime.staticCompositionLocalOf
 import platform.Foundation.NSLock
 
+internal enum class UIKitInteropState {
+    BEGAN, UNCHANGED, ENDED
+}
+
+internal enum class UIKitInteropViewHierarchyChange {
+    VIEW_ADDED,
+    VIEW_REMOVED
+}
+
+/**
+ * Lambda containing changes to UIKit objects, which can be synchronized within [CATransaction]
+ */
+typealias UIKitInteropAction = () -> Unit
+
+internal interface UIKitInteropTransaction {
+    val actions: List<UIKitInteropAction>
+    val state: UIKitInteropState
+
+    companion object {
+        val empty = object : UIKitInteropTransaction {
+            override val actions: List<UIKitInteropAction>
+                get() = listOf()
+
+            override val state: UIKitInteropState
+                get() = UIKitInteropState.UNCHANGED
+        }
+    }
+}
+
+internal fun UIKitInteropTransaction.isEmpty() = actions.isEmpty() && state == UIKitInteropState.UNCHANGED
+internal fun UIKitInteropTransaction.isNotEmpty() = !isEmpty()
+
+private class UIKitInteropMutableTransaction: UIKitInteropTransaction {
+    override val actions = mutableListOf<UIKitInteropAction>()
+    override var state = UIKitInteropState.UNCHANGED
+        set(value) {
+            field = when (value) {
+                UIKitInteropState.UNCHANGED -> error("Can't assign UNCHANGED value explicitly")
+                UIKitInteropState.BEGAN -> {
+                    when (field) {
+                        UIKitInteropState.BEGAN -> error("Can't assign BEGAN twice in the same transaction")
+                        UIKitInteropState.UNCHANGED -> value
+                        UIKitInteropState.ENDED -> UIKitInteropState.UNCHANGED
+                    }
+                }
+                UIKitInteropState.ENDED -> {
+                    when (field) {
+                        UIKitInteropState.BEGAN -> UIKitInteropState.UNCHANGED
+                        UIKitInteropState.UNCHANGED -> value
+                        UIKitInteropState.ENDED -> error("Can't assign ENDED twice in the same transaction")
+                    }
+                }
+            }
+        }
+}
+
 /**
  * Class which can be used to add actions related to UIKit objects to be executed in sync with compose rendering,
  * Addding deferred actions is threadsafe, but they will be executed in the order of their submission, and on the main thread.
@@ -27,29 +83,52 @@ internal class UIKitInteropContext(
     val requestRedraw: () -> Unit
 ) {
     private val lock: NSLock = NSLock()
-    private val actions = mutableListOf<() -> Unit>()
+    private var transaction = UIKitInteropMutableTransaction()
+
+    /**
+     * Number of views, created by interop API and present in current view hierarchy
+     */
+    private var viewsCount = 0
+        set(value) {
+            require(value >= 0)
+
+            field = value
+        }
 
     /**
      * Add lambda to a list of commands which will be executed later in the same CATransaction, when the next rendered Compose frame is presented
      */
-    fun deferAction(action: () -> Unit) {
+    fun deferAction(hierarchyChange: UIKitInteropViewHierarchyChange? = null, action: () -> Unit) {
         requestRedraw()
 
         lock.doLocked {
-            actions.add(action)
+            if (hierarchyChange == UIKitInteropViewHierarchyChange.VIEW_ADDED) {
+                if (viewsCount == 0) {
+                    transaction.state = UIKitInteropState.BEGAN
+                }
+                viewsCount += 1
+            }
+
+            transaction.actions.add(action)
+
+            if (hierarchyChange == UIKitInteropViewHierarchyChange.VIEW_REMOVED) {
+                viewsCount -= 1
+                if (viewsCount == 0) {
+                    transaction.state = UIKitInteropState.ENDED
+                }
+            }
         }
     }
 
     /**
-     * Return a copy of the list of [actions] and clear it.
+     * Return an object containing pending changes and reset internal storage
      */
-    internal fun getActionsAndClear(): List<() -> Unit> {
-        return lock.doLocked {
-            val result = actions.toList()
-            actions.clear()
+    internal fun retrieve(): UIKitInteropTransaction =
+        lock.doLocked {
+            val result = transaction
+            transaction = UIKitInteropMutableTransaction()
             result
         }
-    }
 }
 
 private inline fun <T> NSLock.doLocked(block: () -> T): T {

compose/ui/ui/src/uikitMain/kotlin/androidx/compose/ui/interop/UIKitView.uikit.kt

@@ -43,13 +43,13 @@ import androidx.compose.ui.unit.IntRect
 import androidx.compose.ui.unit.round
 import kotlinx.atomicfu.atomic
 import kotlinx.cinterop.CValue
+import kotlinx.cinterop.useContents
 import platform.CoreGraphics.CGRect
 import platform.CoreGraphics.CGRectMake
 import platform.Foundation.NSThread
+import platform.UIKit.NSStringFromCGRect
 import platform.UIKit.UIColor
 import platform.UIKit.UIView
-import platform.darwin.dispatch_async
-import platform.darwin.dispatch_get_main_queue
 
 private val STUB_CALLBACK_WITH_RECEIVER: Any.() -> Unit = {}
 private val NoOpUpdate: UIView.() -> Unit = STUB_CALLBACK_WITH_RECEIVER
@@ -122,17 +122,19 @@ fun <T : UIView> UIKitView(
 
     DisposableEffect(Unit) {
         componentInfo.component = factory()
-        componentInfo.updater = Updater(componentInfo.component, update, interopContext::deferAction)
+        componentInfo.updater = Updater(componentInfo.component, update) {
+            interopContext.deferAction(action = it)
+        }
 
-        interopContext.deferAction {
+        interopContext.deferAction(UIKitInteropViewHierarchyChange.VIEW_ADDED) {
             componentInfo.container = UIView().apply {
                 addSubview(componentInfo.component)
             }
             root.insertSubview(componentInfo.container, 0)
         }
 
         onDispose {
-            interopContext.deferAction {
+            interopContext.deferAction(UIKitInteropViewHierarchyChange.VIEW_REMOVED) {
                 componentInfo.container.removeFromSuperview()
                 componentInfo.updater.dispose()
                 onRelease(componentInfo.component)
@@ -234,4 +236,4 @@ private class Updater<T : UIView>(
         snapshotObserver.clear()
         isDisposed = true
     }
-}
+}

compose/ui/ui/src/uikitMain/kotlin/androidx/compose/ui/window/ComposeWindow.uikit.kt

@@ -32,7 +32,9 @@ import androidx.compose.ui.input.pointer.toCompose
 import androidx.compose.ui.interop.LocalLayerContainer
 import androidx.compose.ui.interop.LocalUIKitInteropContext
 import androidx.compose.ui.interop.LocalUIViewController
+import androidx.compose.ui.interop.UIKitInteropAction
 import androidx.compose.ui.interop.UIKitInteropContext
+import androidx.compose.ui.interop.UIKitInteropTransaction
 import androidx.compose.ui.platform.*
 import androidx.compose.ui.text.input.PlatformTextInputService
 import androidx.compose.ui.uikit.*
@@ -131,6 +133,7 @@ private class AttachedComposeContext(
             view.bottomAnchor.constraintEqualToAnchor(parentView.bottomAnchor)
         )
     }
+
     fun dispose() {
         scene.close()
         view.dispose()
@@ -146,9 +149,11 @@ internal actual class ComposeWindow : UIViewController {
     private var isInsideSwiftUI = false
     private val safeAreaState = mutableStateOf(IOSInsets())
     private val layoutMarginsState = mutableStateOf(IOSInsets())
-    private val interopContext = UIKitInteropContext(requestRedraw = {
-        attachedComposeContext?.view?.needRedraw()
-    })
+    private val interopContext = UIKitInteropContext(
+        requestRedraw = {
+            attachedComposeContext?.view?.needRedraw()
+        }
+    )
 
     /*
      * Initial value is arbitarily chosen to avoid propagating invalid value logic
@@ -634,8 +639,8 @@ internal actual class ComposeWindow : UIViewController {
                 )
             }
 
-            override fun retrieveCATransactionCommands(): List<() -> Unit> =
-                interopContext.getActionsAndClear()
+            override fun retrieveInteropTransaction(): UIKitInteropTransaction =
+                interopContext.retrieve()
 
             override fun draw(surface: Surface, targetTimestamp: NSTimeInterval) {
                 // The calculation is split in two instead of
@@ -644,7 +649,8 @@ internal actual class ComposeWindow : UIViewController {
                 val integral = floor(targetTimestamp)
                 val fractional = targetTimestamp - integral
                 val secondsToNanos = 1_000_000_000L
-                val nanos = integral.roundToLong() * secondsToNanos + (fractional * 1e9).roundToLong()
+                val nanos =
+                    integral.roundToLong() * secondsToNanos + (fractional * 1e9).roundToLong()
 
                 scene.render(surface.canvas, nanos)
             }
@@ -679,26 +685,26 @@ internal actual class ComposeWindow : UIViewController {
 }
 
 private fun UIViewController.checkIfInsideSwiftUI(): Boolean {
-        var parent = parentViewController
-
-        while (parent != null) {
-            val isUIHostingController = parent.`class`()?.let {
-                val className = NSStringFromClass(it)
-                // SwiftUI UIHostingController has mangled name depending on generic instantiation type,
-                // It always contains UIHostingController substring though
-                return className.contains("UIHostingController")
-            } ?: false
-
-            if (isUIHostingController) {
-                return true
-            }
-
-            parent = parent.parentViewController
+    var parent = parentViewController
+
+    while (parent != null) {
+        val isUIHostingController = parent.`class`()?.let {
+            val className = NSStringFromClass(it)
+            // SwiftUI UIHostingController has mangled name depending on generic instantiation type,
+            // It always contains UIHostingController substring though
+            return className.contains("UIHostingController")
+        } ?: false
+
+        if (isUIHostingController) {
+            return true
         }
 
-        return false
+        parent = parent.parentViewController
     }
 
+    return false
+}
+
 private fun UIUserInterfaceStyle.asComposeSystemTheme(): SystemTheme {
     return when (this) {
         UIUserInterfaceStyle.UIUserInterfaceStyleLight -> SystemTheme.Light