More docs on transformDependency and a fusion fix. (#1495)

* More docs on transformDependency and a fusion fix.

* wip

(cherry picked from commit 4a4d9cd119ed0eeda650acbde5a4d2a3741a003c)

Author: mbrandonw

Sources/ComposableArchitecture/Reducer/Reducers/DependencyKeyWritingReducer.swift

@@ -84,7 +84,7 @@ extension ReducerProtocol {
   /// value.
   ///
   /// For example, suppose you want to see when a particular endpoint of a dependency gets called
-  /// in your application. You can override that endpoint to insert a breakpoint or print state,
+  /// in your application. You can override that endpoint to insert a breakpoint or print statement,
   /// but still call out to the original endpoint:
   ///
   /// ```swift
@@ -97,6 +97,20 @@ extension ReducerProtocol {
   ///     }
   /// ```
   ///
+  /// You can also transform _all_ dependency values at once by using the `\.self` key path:
+  ///
+  /// ```swift
+  /// Feature()
+  ///   .transformDependency(\.self) { dependencyValues in
+  ///     // Access to all dependencies in here
+  ///   }
+  /// ```
+  ///
+  /// > Warning: The trailing closure of ``transformDependency(_:transform:)`` is called for every
+  /// action sent to the reducer, and so you can expect it to be called many times in an
+  /// application's lifecycle. This means you should typically not create dependencies in the
+  /// closure as that will cause a new dependency to be created everytime an action is sent.
+  ///
   /// - Parameters:
   ///   - keyPath: A key path that indicates the property of the `DependencyValues` structure to
   ///     transform.
@@ -155,6 +169,9 @@ public struct _DependencyKeyWritingReducer<Base: ReducerProtocol>: ReducerProtoc
     _ keyPath: WritableKeyPath<DependencyValues, V>,
     transform: @escaping (inout V) -> Void
   ) -> Self {
-    Self(base: self.base) { transform(&$0[keyPath: keyPath]) }
+    Self(base: self.base) { values in
+      transform(&values[keyPath: keyPath])
+      self.update(&values)
+    }
   }
 }

Tests/ComposableArchitectureTests/DependencyKeyWritingReducerTests.swift

@@ -6,12 +6,23 @@ final class DependencyKeyWritingReducerTests: XCTestCase {
   func testWritingFusion() async {
     let reducer: _DependencyKeyWritingReducer<Feature> = Feature()
       .dependency(\.myValue, 42)
-    let _: _DependencyKeyWritingReducer<Feature> =
-      reducer
       .dependency(\.myValue, 1729)
       .dependency(\.myValue, 1)
       .dependency(\.myValue, 2)
       .dependency(\.myValue, 3)
+
+    XCTAssertTrue((reducer as Any) is _DependencyKeyWritingReducer<Feature>)
+  }
+
+  func testTransformFusion() async {
+    let reducer: _DependencyKeyWritingReducer<Feature> = Feature()
+      .transformDependency(\.myValue) { $0 = 42 }
+      .transformDependency(\.myValue) { $0 = 1729 }
+      .transformDependency(\.myValue) { $0 = 1 }
+      .transformDependency(\.myValue) { $0 = 2 }
+      .transformDependency(\.myValue) { $0 = 3 }
+
+    XCTAssertTrue((reducer as Any) is _DependencyKeyWritingReducer<Feature>)
   }
 
   func testWritingFusionOrder() async {
@@ -29,12 +40,44 @@ final class DependencyKeyWritingReducerTests: XCTestCase {
     }
   }
 
+  func testTransformFusionOrder() async {
+    let reducer = Feature()
+      .transformDependency(\.myValue) { $0 = 42 }
+      .transformDependency(\.myValue) { $0 = 1729 }
+
+    let store = TestStore(
+      initialState: Feature.State(),
+      reducer: reducer
+    )
+
+    await store.send(.tap) {
+      $0.value = 42
+    }
+  }
+
   func testWritingOrder() async {
     let reducer = CombineReducers {
       Feature()
         .dependency(\.myValue, 42)
     }
-    .dependency(\.myValue, 1729)
+      .dependency(\.myValue, 1729)
+
+    let store = TestStore(
+      initialState: Feature.State(),
+      reducer: reducer
+    )
+
+    await store.send(.tap) {
+      $0.value = 42
+    }
+  }
+
+  func testTransformOrder() async {
+    let reducer = CombineReducers {
+      Feature()
+        .transformDependency(\.myValue) { $0 = 42 }
+    }
+      .transformDependency(\.myValue) { $0 = 1729 }
 
     let store = TestStore(
       initialState: Feature.State(),