Avoid deallocations of the view controllers on background threads

If the view controller is deallocated on the background thread, it can cause the data races in UIKit. Trying to avoid that.

Author: Gleb Tarasov

PerformanceSuite/Sources/ViewControllerObserver.swift

@@ -33,6 +33,16 @@ final class ViewControllerObserverFactory<T: ViewControllerObserver, S: ScreenMe
     private let metricsReceiver: S
     private let observerMaker: (S.ScreenIdentifier) -> T
 
+    private func ensureDeallocationOnTheMainThread(viewController: UIViewController) {
+        DispatchQueue.main.async {
+            // Make sure viewController is deallocated on the main thread, because
+            // if the last access is on the background thread, it will be deallocated
+            // in background, and it can cause data races in UIKit.
+            // Calling `hash` to make sure this call is not removed by the compilation optimizer
+            _ = viewController.hash
+        }
+    }
+
     private func observer(for viewController: UIViewController) -> T? {
         if PerformanceMonitoring.experiments.observersOnBackgroundQueue {
             dispatchPrecondition(condition: .onQueue(PerformanceMonitoring.queue))
@@ -66,6 +76,7 @@ final class ViewControllerObserverFactory<T: ViewControllerObserver, S: ScreenMe
         if PerformanceMonitoring.experiments.observersOnBackgroundQueue {
             PerformanceMonitoring.queue.async {
                 self.observer(for: viewController)?.beforeInit(viewController: viewController)
+                self.ensureDeallocationOnTheMainThread(viewController: viewController)
             }
         } else {
             observer(for: viewController)?.beforeInit(viewController: viewController)
@@ -76,6 +87,7 @@ final class ViewControllerObserverFactory<T: ViewControllerObserver, S: ScreenMe
         if PerformanceMonitoring.experiments.observersOnBackgroundQueue {
             PerformanceMonitoring.queue.async {
                 self.observer(for: viewController)?.beforeViewDidLoad(viewController: viewController)
+                self.ensureDeallocationOnTheMainThread(viewController: viewController)
             }
         } else {
             observer(for: viewController)?.beforeViewDidLoad(viewController: viewController)
@@ -86,6 +98,7 @@ final class ViewControllerObserverFactory<T: ViewControllerObserver, S: ScreenMe
         if PerformanceMonitoring.experiments.observersOnBackgroundQueue {
             PerformanceMonitoring.queue.async {
                 self.observer(for: viewController)?.afterViewDidAppear(viewController: viewController)
+                self.ensureDeallocationOnTheMainThread(viewController: viewController)
             }
         } else {
             observer(for: viewController)?.afterViewDidAppear(viewController: viewController)
@@ -96,6 +109,7 @@ final class ViewControllerObserverFactory<T: ViewControllerObserver, S: ScreenMe
         if PerformanceMonitoring.experiments.observersOnBackgroundQueue {
             PerformanceMonitoring.queue.async {
                 self.observer(for: viewController)?.beforeViewWillDisappear(viewController: viewController)
+                self.ensureDeallocationOnTheMainThread(viewController: viewController)
             }
         } else {
             observer(for: viewController)?.beforeViewWillDisappear(viewController: viewController)
@@ -106,6 +120,7 @@ final class ViewControllerObserverFactory<T: ViewControllerObserver, S: ScreenMe
         if PerformanceMonitoring.experiments.observersOnBackgroundQueue {
             PerformanceMonitoring.queue.async {
                 self.observer(for: viewController)?.afterViewWillAppear(viewController: viewController)
+                self.ensureDeallocationOnTheMainThread(viewController: viewController)
             }
         } else {
             observer(for: viewController)?.afterViewWillAppear(viewController: viewController)
@@ -116,6 +131,7 @@ final class ViewControllerObserverFactory<T: ViewControllerObserver, S: ScreenMe
         if PerformanceMonitoring.experiments.observersOnBackgroundQueue {
             PerformanceMonitoring.queue.async {
                 self.observer(for: viewController)?.beforeViewDidDisappear(viewController: viewController)
+                self.ensureDeallocationOnTheMainThread(viewController: viewController)
             }
         } else {
             observer(for: viewController)?.beforeViewDidDisappear(viewController: viewController)

PerformanceSuite/Tests/TTIObserverExtensionTests.swift

@@ -19,6 +19,7 @@ class TTIObserverExtensionTests: XCTestCase {
 
     override func setUpWithError() throws {
         try super.setUpWithError()
+        PerformanceMonitoring.experiments = Experiments(observersOnBackgroundQueue: true)
         defaultTimeProvider = timeProvider
         metricsReceiver = TTIMetricsReceiverStub()
         try PerformanceMonitoring.enable(config: [.screenLevelTTI(metricsReceiver)], experiments: Experiments(observersOnBackgroundQueue: true))
@@ -244,7 +245,8 @@ class TTIObserverExtensionTests: XCTestCase {
         window.rootViewController = tabbar
         window.makeKeyAndVisible()
 
-        waitForExpectations(timeout: 3, handler: nil)
+        wait(for: [exp1], timeout: 3)
+        PerformanceMonitoring.queue.sync {}
 
         // for the first controller we should calculate TTI between `init` and `screenIsReady` -> 1 second
         vc1.screenIsReady()
@@ -334,7 +336,8 @@ class TTIObserverExtensionTests: XCTestCase {
         window.rootViewController = vc1
         window.makeKeyAndVisible()
 
-        waitForExpectations(timeout: 3, handler: nil)
+        wait(for: [exp1], timeout: 3)
+        PerformanceMonitoring.queue.sync {}
 
         // we call screenIsReady for the child, but it should work for the parent
         vc2.screenIsReady()
@@ -403,6 +406,10 @@ private class MyViewController: UIViewController {
         super.viewDidLayoutSubviews()
         output += "viewDidLayoutSubviews\n"
     }
+
+    deinit {
+        XCTAssertTrue(Thread.isMainThread)
+    }
 }
 
 private struct MyView: View {

PerformanceSuite/Tests/ViewControllerObserverTests.swift

@@ -12,6 +12,18 @@ import XCTest
 
 class ViewControllerObserverTests: XCTestCase {
 
+    override func setUp() {
+        super.setUp()
+        PerformanceMonitoring.experiments = Experiments(observersOnBackgroundQueue: true)
+    }
+
+    override func tearDown() {
+        super.tearDown()
+        PerformanceMonitoring.consumerQueue.sync { }
+        PerformanceMonitoring.queue.sync { }
+        PerformanceMonitoring.experiments = Experiments()
+    }
+
     func testObserversCollection() {
         let o1 = Observer()
         let o2 = Observer()
@@ -159,4 +171,8 @@ private class MetricsConsumerForSwiftUITest: ScreenMetricsReceiver {
     func appRenderingMetricsReceived(metrics: RenderingMetrics) {}
 }
 
-private class MyViewController: UIViewController { }
+private class MyViewController: UIViewController {
+    deinit {
+        XCTAssertTrue(Thread.isMainThread)
+    }
+}