Bring back _printChanges queue.

Sources/ComposableArchitecture/Reducer/Reducers/DebugReducer.swift

@@ -1,3 +1,6 @@
+import Combine
+import Dispatch
+
 extension Reducer {
   #if swift(>=5.8)
     /// Enhances a reducer with debug logging of received actions and state mutations for the given
@@ -26,13 +29,19 @@ extension Reducer {
   #endif
 }
 
+private let printQueue = DispatchQueue(label: "co.pointfree.swift-composable-architecture.printer")
+
 public struct _ReducerPrinter<State, Action> {
   private let _printChange: (_ receivedAction: Action, _ oldState: State, _ newState: State) -> Void
+  @usableFromInline
+  let queue: DispatchQueue
 
   public init(
-    printChange: @escaping (_ receivedAction: Action, _ oldState: State, _ newState: State) -> Void
+    printChange: @escaping (_ receivedAction: Action, _ oldState: State, _ newState: State) -> Void,
+    queue: DispatchQueue? = nil
   ) {
     self._printChange = printChange
+    self.queue = queue ?? printQueue
   }
 
   public func printChange(receivedAction: Action, oldState: State, newState: State) {
@@ -81,8 +90,13 @@ public struct _PrintChangesReducer<Base: Reducer>: Reducer {
         let oldState = state
         let effects = self.base.reduce(into: &state, action: action)
         return effects.merge(
-          with: .run { [newState = state] _ in
-            printer.printChange(receivedAction: action, oldState: oldState, newState: newState)
+          with: .publisher { [newState = state, queue = printer.queue] in
+            Deferred<Empty<Action, Never>> {
+              queue.async {
+                printer.printChange(receivedAction: action, oldState: oldState, newState: newState)
+              }
+              return Empty()
+            }
           }
         )
       }

Tests/ComposableArchitectureTests/DebugTests.swift

@@ -98,8 +98,8 @@
     }
 
     @MainActor
-    func testDebugReducer() async {
-      struct DebuggedReducer: Reducer {
+    func testDebugReducer() async throws {
+      struct Feature: Reducer {
         typealias State = Int
         typealias Action = Bool
         func reduce(into state: inout Int, action: Bool) -> Effect<Bool> {
@@ -108,8 +108,76 @@
         }
       }
 
-      let store = TestStore(initialState: 0) { DebuggedReducer()._printChanges() }
-      await store.send(true) { $0 = 1 }
+      let logs = LockIsolated<String>("")
+      let printer = _ReducerPrinter<Feature.State, Feature.Action>(
+        printChange: { action, oldState, newState in
+          logs.withValue { _ = dump(action, to: &$0) }
+        }
+      )
+
+      let store = Store(initialState: 0) { Feature()._printChanges(printer) }
+      store.send(true)
+      try await Task.sleep(nanoseconds: 300_000_000)
+      XCTAssertNoDifference(
+        logs.value,
+        """
+        - true
+
+        """
+      )
+    }
+
+    func testDebugReducer_Order() {
+      struct Feature: Reducer {
+        typealias State = Int
+        typealias Action = Bool
+        func reduce(into state: inout Int, action: Bool) -> Effect<Bool> {
+          state += action ? 1 : -1
+          return .run { _ in await Task.yield() }
+        }
+      }
+
+      let logs = LockIsolated<String>("")
+      let printer = _ReducerPrinter<Feature.State, Feature.Action>(
+        printChange: { action, oldState, newState in
+          logs.withValue { _ = dump(action, to: &$0) }
+        }
+      )
+
+      let store = Store(initialState: 0) {
+        Feature()
+          ._printChanges(printer)
+          ._printChanges(printer)
+          ._printChanges(printer)
+          ._printChanges(printer)
+      }
+      store.send(true)
+      store.send(false)
+      store.send(true)
+      store.send(false)
+      _ = XCTWaiter.wait(for: [self.expectation(description: "wait")], timeout: 0.3)
+      XCTAssertNoDifference(
+        logs.value,
+        """
+        - true
+        - true
+        - true
+        - true
+        - false
+        - false
+        - false
+        - false
+        - true
+        - true
+        - true
+        - true
+        - false
+        - false
+        - false
+        - false
+
+        """
+      )
     }
   }
 #endif