Runtime warn when stack integration is missing from store's reducer (#…

…3048)

* Runtime warn when reducer is missing stack integration

If SwiftUI writes to a binding and the state is invalid (it is not
inserting or removing elements from the stack), then the most likely
cause is a missing `forEach` in the reducer (or a missing integration in
some parent reducer).

This commit adds a runtime warning to detect this issue.

* wip

Sources/ComposableArchitecture/Observation/NavigationStack+Observation.swift

@@ -109,13 +109,15 @@ import SwiftUI
     public init<State, Action, Destination: View, R>(
       path: Binding<Store<StackState<State>, StackAction<State, Action>>>,
       root: () -> R,
-      @ViewBuilder destination: @escaping (Store<State, Action>) -> Destination
+      @ViewBuilder destination: @escaping (Store<State, Action>) -> Destination,
+      fileID: StaticString = #fileID,
+      line: UInt = #line
     )
     where
       Data == StackState<State>.PathView,
       Root == ModifiedContent<R, _NavigationDestinationViewModifier<State, Action, Destination>>
     {
-      self.init(path: path[]) {
+      self.init(path: path[fileID: "\(fileID)", line: line]) {
         root()
           .modifier(
             _NavigationDestinationViewModifier(store: path.wrappedValue, destination: destination)
@@ -285,11 +287,40 @@ import SwiftUI
   }
 
   extension Store {
-    fileprivate subscript<ElementState, ElementAction>() -> StackState<ElementState>.PathView
+    @_spi(Internals) public subscript<ElementState, ElementAction>(
+      fileID fileID: String,
+      line line: UInt
+    ) -> StackState<ElementState>.PathView
     where State == StackState<ElementState>, Action == StackAction<ElementState, ElementAction> {
       get { self.currentState.path }
       set {
         let newCount = newValue.count
+        guard newCount != self.currentState.count else {
+          runtimeWarn(
+            """
+            SwiftUI wrote to a "NavigationStack" binding at "\(fileID):\(line)" with a path that \
+            has the same number of elements that already exist in the store. SwiftUI should only \
+            write to this binding with a path that has pushed a new element onto the stack, or \
+            popped one or more elements from the stack.
+
+            This usually means the "forEach" has not been integrated with the reducer powering the \
+            store, and this reducer is responsible for handling stack actions.
+
+            To fix this, ensure that "forEach" is invoked from the reducer's "body":
+
+                Reduce { state, action in
+                  // ...
+                }
+                .forEach(\\.path, action: \\.path) {
+                  Path()
+                }
+
+            And ensure that every parent reducer is integrated into the root reducer that powers \
+            the store.
+            """
+          )
+          return
+        }
         if newCount > self.currentState.count, let component = newValue.last {
           self.send(.push(id: component.id, state: component.element))
         } else {

Sources/ComposableArchitecture/SwiftUI/Binding.swift

@@ -785,7 +785,7 @@ extension WithViewStore where ViewState: Equatable, Content: View {
         customDump(self.value, to: &value, maxDepth: 0)
         runtimeWarn(
           """
-          A binding action sent from a view store \
+          A binding action sent from a store \
           \(self.context == .bindingState ? "for binding state defined " : "")at \
           "\(self.fileID):\(self.line)" was not handled. …
 

Sources/ComposableArchitecture/TestStore.swift

@@ -536,7 +536,6 @@ public final class TestStore<State, Action> {
     self.timeout = 1 * NSEC_PER_SEC
     self.sharedChangeTracker = sharedChangeTracker
     self.useMainSerialExecutor = true
-    let dismiss = self.reducer.dependencies.dismiss.dismiss
     self.reducer.store = self
   }
 

Tests/ComposableArchitectureTests/RuntimeWarningTests.swift

@@ -1,6 +1,6 @@
 #if DEBUG
   import Combine
-  import ComposableArchitecture
+  @_spi(Internals) import ComposableArchitecture
   import XCTest
 
   final class RuntimeWarningTests: BaseTCATestCase {
@@ -193,7 +193,7 @@
         ViewStore(store, observe: { $0 }).$value.wrappedValue = 42
       } issueMatcher: {
         $0.compactDescription == """
-          A binding action sent from a view store for binding state defined at \
+          A binding action sent from a store for binding state defined at \
           "\(#fileID):\(line)" was not handled. …
 
             Action:
@@ -219,7 +219,7 @@
         ViewStore(store, observe: { $0 }).$value.wrappedValue = 42
       } issueMatcher: {
         $0.compactDescription == """
-          A binding action sent from a view store for binding state defined at \
+          A binding action sent from a store for binding state defined at \
           "\(#fileID):\(line)" was not handled. …
 
             Action:
@@ -229,5 +229,50 @@
           """
       }
     }
+
+    #if swift(>=5.9)
+      @Reducer
+      struct TestStorePath_NotIntegrated {
+        @ObservableState
+        struct State: Equatable {
+          var path = StackState<Int>()
+        }
+        enum Action {
+          case path(StackAction<Int, Void>)
+        }
+      }
+      @MainActor
+      func testStorePath_NotIntegrated() {
+        let store = Store(initialState: TestStorePath_NotIntegrated.State()) {
+          TestStorePath_NotIntegrated()
+        }
+
+        XCTExpectFailure {
+          store.scope(state: \.path, action: \.path)[fileID: "file.swift", line: 1] = .init()
+        } issueMatcher: {
+          $0.compactDescription == """
+            SwiftUI wrote to a "NavigationStack" binding at "file.swift:1" with a path that has \
+            the same number of elements that already exist in the store. SwiftUI should only write \
+            to this binding with a path that has pushed a new element onto the stack, or popped \
+            one or more elements from the stack.
+
+            This usually means the "forEach" has not been integrated with the reducer powering the \
+            store, and this reducer is responsible for handling stack actions.
+
+            To fix this, ensure that "forEach" is invoked from the reducer's "body":
+
+                Reduce { state, action in
+                  // ...
+                }
+                .forEach(\\.path, action: \\.path) {
+                  Path()
+                }
+
+            And ensure that every parent reducer is integrated into the root reducer that powers \
+            the store.
+            """
+        }
+      }
+    #endif
   }
 #endif