Fix rare crashes in IOSBackgroundTaskScheduler with on entering callbacks (#3751)

Author: laevandus

CHANGELOG.md

@@ -15,6 +15,8 @@ The format is based on [Keep a Changelog](https://keepachangelog.com/en/1.0.0/).
 - Add `Components.isReactionPushEmojisEnabled` to control whether an emoji should be set when adding a reaction [#3738](https://github.com/GetStream/stream-chat-swift/pull/3738)
 - Add `MessageNotificationContent.reaction` that can be used by Notification Service Extension [#3738](https://github.com/GetStream/stream-chat-swift/pull/3738)
 - Add average message response time for users [#3739](https://github.com/GetStream/stream-chat-swift/pull/3739)
+### 🐞 Fixed
+- Fix rare crashes in `IOSBackgroundTaskScheduler` with on entering callbacks [#3751](https://github.com/GetStream/stream-chat-swift/pull/3751)
 
 ## StreamChatUI
 ### 🐞 Fixed

Sources/StreamChat/WebSocketClient/BackgroundTaskScheduler.swift

@@ -82,8 +82,10 @@ class IOSBackgroundTaskScheduler: BackgroundTaskScheduler {
         onEnteringBackground: @escaping () -> Void,
         onEnteringForeground: @escaping () -> Void
     ) {
-        self.onEnteringForeground = onEnteringForeground
-        self.onEnteringBackground = onEnteringBackground
+        queue.sync {
+            self.onEnteringForeground = onEnteringForeground
+            self.onEnteringBackground = onEnteringBackground
+        }
 
         NotificationCenter.default.addObserver(
             self,
@@ -101,8 +103,10 @@ class IOSBackgroundTaskScheduler: BackgroundTaskScheduler {
     }
 
     func stopListeningForAppStateUpdates() {
-        onEnteringForeground = {}
-        onEnteringBackground = {}
+        queue.sync {
+            self.onEnteringForeground = {}
+            self.onEnteringBackground = {}
+        }
 
         NotificationCenter.default.removeObserver(
             self,
@@ -118,11 +122,13 @@ class IOSBackgroundTaskScheduler: BackgroundTaskScheduler {
     }
 
     @objc private func handleAppDidEnterBackground() {
-        onEnteringBackground()
+        let callback = queue.sync { onEnteringBackground }
+        callback()
     }
 
     @objc private func handleAppDidBecomeActive() {
-        onEnteringForeground()
+        let callback = queue.sync { onEnteringForeground }
+        callback()
     }
 
     deinit {

Tests/StreamChatTests/WebSocketClient/BackgroundTaskScheduler_Tests.swift

@@ -75,6 +75,17 @@ final class IOSBackgroundTaskScheduler_Tests: XCTestCase {
         XCTAssertEqual(3, endTaskCallCount)
     }
 
+    func test_callingAppStateUpdatesConcurretly() {
+        let scheduler = IOSBackgroundTaskScheduler()
+        DispatchQueue.concurrentPerform(iterations: 100) { index in
+            if index.quotientAndRemainder(dividingBy: 2).remainder == 0 {
+                scheduler.startListeningForAppStateUpdates(onEnteringBackground: {}, onEnteringForeground: {})
+            } else {
+                scheduler.stopListeningForAppStateUpdates()
+            }
+        }
+    }
+    
     // MARK: - Mocks
 
     class IOSBackgroundTaskSchedulerMock: IOSBackgroundTaskScheduler {