Fix iOS StateObservable data race when updating cached state. (#38)

Author: aMarquisMirego

CHANGELOG.md

@@ -6,6 +6,9 @@ The format is based on [Keep a Changelog](https://keepachangelog.com/en/1.0.0/).
 
 ## Upcoming
 
+### Fixed
+- iOS `StateObservable` no longer races on its cached state: the flow/state-flow collection now mutates `lastState` on the main actor instead of the background collection context, preventing a crash (`objc_opt_isKindOfClass` via SKIE `onEnum`) when the value is read during SwiftUI body evaluation
+
 ### Updates
 
 ### Breaking Changes

viewmodel/ios/NullableStateObservable.swift

@@ -32,18 +32,7 @@ public class NullableStateObservable<State>: ObservableObject {
 
         task = Task { [weak self] in
             for await newState in stateFlow.dropFirst() {
-                if let animation = self?.animation?(self?.lastState, newState) {
-                    DispatchQueue.main.async {
-                        withAnimation(animation) {
-                            self?.objectWillChange.send()
-                        }
-                    }
-                } else {
-                    DispatchQueue.main.async {
-                        self?.objectWillChange.send()
-                    }
-                }
-                self?.lastState = newState
+                await self?.apply(newState)
             }
         }
     }
@@ -55,22 +44,23 @@ public class NullableStateObservable<State>: ObservableObject {
 
         task = Task { [weak self] in
             for await newState in flow {
-                if let animation = self?.animation?(self?.lastState, newState) {
-                    DispatchQueue.main.async {
-                        withAnimation(animation) {
-                            self?.objectWillChange.send()
-                        }
-                    }
-                } else {
-                    DispatchQueue.main.async {
-                        self?.objectWillChange.send()
-                    }
-                }
-                self?.lastState = newState
+                await self?.apply(newState)
             }
         }
     }
 
+    @MainActor
+    private func apply(_ newState: State?) {
+        if let animation = animation?(lastState, newState) {
+            withAnimation(animation) {
+                objectWillChange.send()
+            }
+        } else {
+            objectWillChange.send()
+        }
+        lastState = newState
+    }
+
     deinit {
         task?.cancel()
     }

viewmodel/ios/StateObservable.swift

@@ -32,18 +32,7 @@ public class StateObservable<State>: ObservableObject {
 
         task = Task { [weak self] in
             for await newState in stateFlow.dropFirst() {
-                if let lastState = self?.lastState, let animation = self?.animation?(lastState, newState) {
-                    DispatchQueue.main.async {
-                        withAnimation(animation) {
-                            self?.objectWillChange.send()
-                        }
-                    }
-                } else {
-                    DispatchQueue.main.async {
-                        self?.objectWillChange.send()
-                    }
-                }
-                self?.lastState = newState
+                await self?.apply(newState)
             }
         }
     }
@@ -55,22 +44,23 @@ public class StateObservable<State>: ObservableObject {
 
         task = Task { [weak self] in
             for await newState in flow {
-                if let lastState = self?.lastState, let animation = self?.animation?(lastState, newState) {
-                    DispatchQueue.main.async {
-                        withAnimation(animation) {
-                            self?.objectWillChange.send()
-                        }
-                    }
-                } else {
-                    DispatchQueue.main.async {
-                        self?.objectWillChange.send()
-                    }
-                }
-                self?.lastState = newState
+                await self?.apply(newState)
             }
         }
     }
 
+    @MainActor
+    private func apply(_ newState: State) {
+        if let animation = animation?(lastState, newState) {
+            withAnimation(animation) {
+                objectWillChange.send()
+            }
+        } else {
+            objectWillChange.send()
+        }
+        lastState = newState
+    }
+
     deinit {
         task?.cancel()
     }