fix(replay): avoid throttling and locking on main thread

CHANGELOG.md

@@ -1,5 +1,7 @@
 ## Next
 
+- fix: session replay perforamnce improvements ([#364](https://github.com/PostHog/posthog-ios/pull/364))
+
 ## 3.28.1 - 2025-06-23
 
 - fix: surveys decoding error ([#363](https://github.com/PostHog/posthog-ios/pull/363))

PostHog/ApplicationViewLayoutPublisher.swift

@@ -95,10 +95,10 @@
 
         var onViewLayoutCallbacks: [UUID: ThrottledHandler] = [:]
 
-        static let dispatchQueue = DispatchQueue(label: "com.posthog.PostHogReplayIntegration",
-                                                 target: .global(qos: .utility))
-
         final class ThrottledHandler {
+            static let throttleQueue = DispatchQueue(label: "com.posthog.ThrottledHandler",
+                                                     target: .global(qos: .utility))
+
             let interval: TimeInterval
             let handler: ApplicationViewLayoutHandler
 
@@ -110,23 +110,13 @@
             }
 
             func throttleHandler() {
-                let runThrottle = { [weak self] in
-                    guard let self else { return }
-                    let now = now()
-                    let timeSinceLastFired = now.timeIntervalSince(lastFired)
-
-                    if timeSinceLastFired >= interval {
-                        lastFired = now
-                        handler()
-                    }
-                }
+                let now = now()
+                let timeSinceLastFired = now.timeIntervalSince(lastFired)
 
-                if Thread.isMainThread {
-                    runThrottle()
-                } else {
-                    DispatchQueue.main.async {
-                        runThrottle()
-                    }
+                if timeSinceLastFired >= interval {
+                    lastFired = now
+                    // notify on main
+                    DispatchQueue.main.async(execute: handler)
                 }
             }
         }
@@ -150,9 +140,12 @@
         }
 
         func notifyHandlers() {
-            let handlers = registrationLock.withLock { onViewLayoutCallbacks.values }
-            for handler in handlers {
-                handler.throttleHandler()
+            ThrottledHandler.throttleQueue.async {
+                // Don't lock on main
+                let handlers = self.registrationLock.withLock { self.onViewLayoutCallbacks.values }
+                for handler in handlers {
+                    handler.throttleHandler()
+                }
             }
         }
     }

PostHog/PostHogSessionManager.swift

@@ -31,6 +31,7 @@ import Foundation
         registerApplicationSendEvent()
     }
 
+    private let queue = DispatchQueue(label: "com.posthog.PostHogSessionManager", target: .global(qos: .utility))
     private var sessionId: String?
     private var sessionStartTimestamp: TimeInterval?
     private var sessionActivityTimestamp: TimeInterval?
@@ -240,7 +241,9 @@ import Foundation
             applicationEventToken = applicationEventPublisher.onApplicationEvent { [weak self] _, _ in
                 // update "last active" session
                 // we want to keep track of the idle time, so we need to maintain a timestamp on the last interactions of the user with the app. UIEvents are a good place to do so since it means that the user is actively interacting with the app (e.g not just noise background activity)
-                self?.touchSession()
+                self?.queue.async {
+                    self?.touchSession()
+                }
             }
         #endif
     }

PostHog/Replay/PostHogReplayIntegration.swift

@@ -240,11 +240,6 @@
                 return
             }
 
-            // always make sure we have a fresh session id as early as possible
-            guard let sessionId = PostHogSessionManager.shared.getSessionId(at: date) else {
-                return
-            }
-
             // capture necessary touch information on the main thread before performing any asynchronous operations
             // - this ensures that UITouch associated objects like UIView, UIWindow, or [UIGestureRecognizer] are still valid.
             // - these objects may be released or erased by the system if accessed asynchronously, resulting in invalid/zeroed-out touch coordinates
@@ -253,6 +248,11 @@
             }
 
             PostHogReplayIntegration.dispatchQueue.async { [touchInfo, weak postHog = postHog] in
+                // always make sure we have a fresh session id as early as possible
+                guard let sessionId = PostHogSessionManager.shared.getSessionId(at: date) else {
+                    return
+                }
+
                 // captured weakly since integration may have uninstalled by now
                 guard let postHog else { return }
 
@@ -312,11 +312,6 @@
                 windowViews.object(forKey: window) ?? ViewTreeSnapshotStatus()
             }
 
-            // always make sure we have a fresh session id at correct timestamp
-            guard let sessionId = PostHogSessionManager.shared.getSessionId(at: timestampDate) else {
-                return
-            }
-
             var snapshotsData: [Any] = []
 
             if !snapshotStatus.sentMetaEvent {
@@ -345,6 +340,11 @@
             // TODO: IncrementalSnapshot, type=2
 
             PostHogReplayIntegration.dispatchQueue.async {
+                // always make sure we have a fresh session id at correct timestamp
+                guard let sessionId = PostHogSessionManager.shared.getSessionId(at: timestampDate) else {
+                    return
+                }
+
                 var wireframes: [Any] = []
                 wireframes.append(wireframe.toDict())
                 let initialOffset = ["top": 0, "left": 0]