Make 'didSet' main actor. (#3206)

* Make 'didSet' main actor.

* tests for AppStorageKey

* wip

* tests

* clean up

* Update Sources/ComposableArchitecture/SharedState/PersistenceKey/AppStorageKey.swift

* Update Sources/ComposableArchitecture/SharedState/PersistenceKey/AppStorageKeyPathKey.swift

* try working around CI problem

* Added NB

* wip

* Update MigratingTo1.11.md

---------

Co-authored-by: Stephen Celis <stephen@stephencelis.com>

Sources/ComposableArchitecture/Documentation.docc/Articles/MigrationGuides/MigratingTo1.11.md

@@ -12,6 +12,10 @@ APIs and deprecated 1 API.
 > Important: Before following this migration guide be sure you have fully migrated to the newest
 > tools of version 1.10. See <doc:MigrationGuides> for more information.
 
+* [Mutating shared state concurrently](#Mutating-shared-state-concurrently)
+* [Supplying mock read-only state to previews](#Supplying-mock-read-only-state-to-previews)
+* [Migrating to 1.11.2](#Migrating-to-1112)
+
 ## Mutating shared state concurrently
 
 Version 1.10 of the Composable Architecture introduced a powerful tool for 

Sources/ComposableArchitecture/Internal/DispatchQueue.swift

@@ -0,0 +1,27 @@
+import Dispatch
+
+func mainActorASAP(execute block: @escaping @MainActor @Sendable () -> Void) {
+  if DispatchQueue.getSpecific(key: key) == value {
+    assumeMainActorIsolated {
+      block()
+    }
+  } else {
+    DispatchQueue.main.async {
+      block()
+    }
+  }
+}
+
+private let key: DispatchSpecificKey<UInt8> = {
+  let key = DispatchSpecificKey<UInt8>()
+  DispatchQueue.main.setSpecific(key: key, value: value)
+  return key
+}()
+private let value: UInt8 = 0
+
+// NB: Currently we can't use 'MainActor.assumeIsolated' on CI, but we can approximate this in
+//     the meantime.
+@MainActor(unsafe)
+private func assumeMainActorIsolated(_ block: @escaping @MainActor @Sendable () -> Void) {
+  block()
+}

Sources/ComposableArchitecture/SharedState/PersistenceKey.swift

@@ -35,7 +35,7 @@ public protocol PersistenceReaderKey<Value> {
   ///   deinitialized, the `didSet` closure will no longer be invoked.
   func subscribe(
     initialValue: Value?,
-    didSet: @Sendable @escaping (_ newValue: Value?) -> Void
+    didSet: @escaping @Sendable (_ newValue: Value?) -> Void
   ) -> Shared<Value>.Subscription
 }
 
@@ -46,7 +46,7 @@ extension PersistenceReaderKey where ID == Self {
 extension PersistenceReaderKey {
   public func subscribe(
     initialValue: Value?,
-    didSet: @Sendable @escaping (_ newValue: Value?) -> Void
+    didSet: @escaping @Sendable (_ newValue: Value?) -> Void
   ) -> Shared<Value>.Subscription {
     Shared.Subscription {}
   }

Sources/ComposableArchitecture/SharedState/PersistenceKey/AppStorageKey.swift

@@ -287,7 +287,7 @@ extension AppStorageKey: PersistenceKey {
 
   public func subscribe(
     initialValue: Value?,
-    didSet: @Sendable @escaping (_ newValue: Value?) -> Void
+    didSet: @escaping @Sendable (_ newValue: Value?) -> Void
   ) -> Shared<Value>.Subscription {
     let previousValue = LockIsolated(initialValue)
     let userDefaultsDidChange = NotificationCenter.default.addObserver(

Sources/ComposableArchitecture/SharedState/PersistenceKey/AppStorageKeyPathKey.swift

@@ -48,7 +48,7 @@ extension AppStorageKeyPathKey: PersistenceKey, Hashable {
 
   public func subscribe(
     initialValue: Value?,
-    didSet: @Sendable @escaping (_ newValue: Value?) -> Void
+    didSet: @escaping @Sendable (_ newValue: Value?) -> Void
   ) -> Shared<Value>.Subscription {
     let observer = self.store.observe(self.keyPath, options: .new) { _, change in
       guard

Sources/ComposableArchitecture/SharedState/PersistenceKey/FileStorageKey.swift

@@ -79,7 +79,7 @@ public final class FileStorageKey<Value: Codable & Sendable>: PersistenceKey, Se
 
   public func subscribe(
     initialValue: Value?,
-    didSet: @Sendable @escaping (_ newValue: Value?) -> Void
+    didSet: @escaping @Sendable (_ newValue: Value?) -> Void
   ) -> Shared<Value>.Subscription {
     let cancellable = LockIsolated<AnyCancellable?>(nil)
     @Sendable func setUpSources() {

Sources/ComposableArchitecture/SharedState/PersistenceKey/PersistenceKeyDefault.swift

@@ -41,7 +41,7 @@ public struct PersistenceKeyDefault<Base: PersistenceReaderKey>: PersistenceRead
 
   public func subscribe(
     initialValue: Base.Value?,
-    didSet: @Sendable @escaping (Base.Value?) -> Void
+    didSet: @escaping @Sendable (Base.Value?) -> Void
   ) -> Shared<Base.Value>.Subscription {
     self.base.subscribe(initialValue: initialValue, didSet: didSet)
   }

Sources/ComposableArchitecture/SharedState/References/ValueReference.swift

@@ -399,8 +399,10 @@ final class ValueReference<Value, Persistence: PersistenceReaderKey<Value>>: Ref
           self._$perceptionRegistrar.willSet(self, keyPath: \.value)
           defer { self._$perceptionRegistrar.didSet(self, keyPath: \.value) }
         #endif
-        self.lock.withLock {
-          self._value = value ?? initialValue
+        mainActorASAP {
+          self.lock.withLock {
+            self._value = value ?? initialValue
+          }
         }
       }
     }

Tests/ComposableArchitectureTests/AppStorageTests.swift

@@ -200,6 +200,100 @@ final class AppStorageTests: XCTestCase {
     store.setValue(2, forKey: "other-count")
     XCTAssertEqual(values.value, [1])
   }
+
+  @MainActor
+  func testUpdateStoreFromBackgroundThread() async throws {
+    @Dependency(\.defaultAppStorage) var store
+    @Shared(.appStorage("count")) var count = 0
+
+    let publisherExpectation = expectation(description: "publisher")
+    let cancellable = $count.publisher.sink { _ in
+      XCTAssertTrue(Thread.isMainThread)
+      publisherExpectation.fulfill()
+    }
+    defer { _ = cancellable }
+
+    await withUnsafeContinuation { continuation in
+      DispatchQueue.global().async { [store = UncheckedSendable(store)] in
+        XCTAssertFalse(Thread.isMainThread)
+        store.wrappedValue.setValue(1, forKey: "count")
+        continuation.resume()
+      }
+    }
+
+    await fulfillment(of: [publisherExpectation], timeout: 0)
+  }
+
+  @MainActor
+  func testUpdateStoreFromMainThread() async throws {
+    @Dependency(\.defaultAppStorage) var store
+    @Shared(.appStorage("count")) var count = 0
+    let isInStackFrame = LockIsolated(false)
+
+    let publisherExpectation = expectation(description: "publisher")
+    let cancellable = $count.publisher.sink { _ in
+      XCTAssertTrue(Thread.isMainThread)
+      XCTAssertTrue(isInStackFrame.value)
+      publisherExpectation.fulfill()
+    }
+    defer { _ = cancellable }
+
+    await withUnsafeContinuation { continuation in
+      XCTAssertTrue(Thread.isMainThread)
+      isInStackFrame.withValue { $0 = true }
+      store.setValue(1, forKey: "count")
+      isInStackFrame.withValue { $0 = false }
+      continuation.resume()
+    }
+
+    await fulfillment(of: [publisherExpectation], timeout: 0)
+  }
+
+  @MainActor
+  func testWillEnterForegroundFromBackgroundThread() async throws {
+    @Shared(.appStorage("count")) var count = 0
+
+    let publisherExpectation = expectation(description: "publisher")
+    let cancellable = $count.publisher.sink { _ in
+      XCTAssertTrue(Thread.isMainThread)
+      publisherExpectation.fulfill()
+    }
+    defer { _ = cancellable }
+
+    await withUnsafeContinuation { continuation in
+      DispatchQueue.global().async {
+        XCTAssertFalse(Thread.isMainThread)
+        NotificationCenter.default.post(name: willEnterForegroundNotificationName!, object: nil)
+        continuation.resume()
+      }
+    }
+
+    await fulfillment(of: [publisherExpectation], timeout: 0)
+  }
+
+  @MainActor
+  func testUpdateStoreFromBackgroundThread_KeyPath() async throws {
+    @Dependency(\.defaultAppStorage) var store
+    @Shared(.appStorage(\.count)) var count = 0
+
+    let publisherExpectation = expectation(description: "publisher")
+    publisherExpectation.expectedFulfillmentCount = 2
+    let cancellable = $count.publisher.sink { _ in
+      XCTAssertTrue(Thread.isMainThread)
+      publisherExpectation.fulfill()
+    }
+    defer { _ = cancellable }
+
+    await withUnsafeContinuation { continuation in
+      DispatchQueue.global().async { [store = UncheckedSendable(store)] in
+        XCTAssertFalse(Thread.isMainThread)
+        store.wrappedValue.count = 1
+        continuation.resume()
+      }
+    }
+
+    await fulfillment(of: [publisherExpectation], timeout: 0)
+  }
 }
 
 extension UserDefaults {

Tests/ComposableArchitectureTests/FileStorageTests.swift

@@ -434,6 +434,34 @@ final class FileStorageTests: XCTestCase {
       XCTAssertEqual(count, max * (max + 1) / 2)
     }
   }
+
+  @MainActor
+  func testUpdateFileSystemFromBackgroundThread() async throws {
+    await withDependencies {
+      $0.defaultFileStorage = .fileSystem
+    } operation: {
+      try? FileManager.default.removeItem(at: .fileURL)
+
+      @Shared(.fileStorage(.fileURL)) var count = 0
+
+      let publisherExpectation = expectation(description: "publisher")
+      let cancellable = $count.publisher.sink { _ in
+        XCTAssertTrue(Thread.isMainThread)
+        publisherExpectation.fulfill()
+      }
+      defer { _ = cancellable }
+
+      await withUnsafeContinuation { continuation in
+        DispatchQueue.global().async {
+          XCTAssertFalse(Thread.isMainThread)
+          try! Data("1".utf8).write(to: .fileURL)
+          continuation.resume()
+        }
+      }
+
+      await fulfillment(of: [publisherExpectation], timeout: 0.1)
+    }
+  }
 }
 
 extension URL {