Correctly show (no state changes) in diff. (#132)

* Correct show (no state changes) in diff.

* test for debug operator with no state changes

Author: mbrandonw

Sources/ComposableArchitecture/Debugging/ReducerDebugging.swift

@@ -75,7 +75,7 @@ extension Reducer {
             debugEnvironment.queue.async {
               let actionOutput = debugOutput(localAction).indent(by: 2)
               let stateOutput =
-                debugDiff(previousState, nextState).map { "\($0)\n" } ?? "  (No state changes)"
+                debugDiff(previousState, nextState).map { "\($0)\n" } ?? "  (No state changes)\n"
               debugEnvironment.printer(
                 """
                 \(prefix.isEmpty ? "" : "\(prefix): ")received action:

Sources/ComposableArchitecture/Internal/Diff.swift

@@ -71,7 +71,7 @@ func diff(_ first: String, _ second: String) -> String? {
     first.split(separator: "\n", omittingEmptySubsequences: false)[...],
     second.split(separator: "\n", omittingEmptySubsequences: false)[...]
   )
-  guard !differences.isEmpty else { return nil }
+  if differences.count == 1, case .both = differences[0].which { return nil }
   var string = differences.reduce(into: "") { string, diff in
     diff.elements.forEach { line in
       string += "\(diff.which.prefix) \(line)\n"

Tests/ComposableArchitectureTests/ReducerTests.swift

@@ -96,22 +96,24 @@ final class ReducerTests: XCTestCase {
   }
 
   func testPrint() {
-    struct Unit: Equatable {}
+    enum Action: Equatable { case incr, noop }
     struct State: Equatable { var count = 0 }
 
     var logs: [String] = []
 
-    let expectation = self.expectation(description: "printed")
-
-    let reducer = Reducer<State, Unit, Void> { state, _, _ in
-      state.count += 1
-      return .none
+    let reducer = Reducer<State, Action, Void> { state, action, _ in
+      switch action {
+      case .incr:
+        state.count += 1
+        return .none
+      case .noop:
+        return .none
+      }
     }
     .debug("[prefix]") { _ in
       DebugEnvironment(
         printer: {
           logs.append($0)
-          expectation.fulfill()
         }
       )
     }
@@ -122,22 +124,29 @@ final class ReducerTests: XCTestCase {
       environment: ()
     )
     store.assert(
-      .send(Unit()) { $0.count = 1 }
+      .send(.incr) { $0.count = 1 },
+      .send(.noop)
     )
 
-    self.wait(for: [expectation], timeout: 1)
+    _ = XCTWaiter.wait(for: [self.expectation(description: "wait")], timeout: 0.1)
 
     XCTAssertEqual(
       logs,
       [
         #"""
         [prefix]: received action:
-          Unit()
+          Action.incr
           State(
         −   count: 0
         +   count: 1
           )
 
+        """#,
+        #"""
+        [prefix]: received action:
+          Action.noop
+          (No state changes)
+
         """#
       ]
     )