fix(core): make StepPopped reducer immutable

core/src/activity-utils/makeActivityReducer.spec.ts

@@ -0,0 +1,58 @@
+import { makeEvent } from "../event-utils";
+import type { Activity } from "../Stack";
+import { makeActivityReducer } from "./makeActivityReducer";
+
+function makeActivity(): Activity {
+  const pushedEvent = makeEvent("Pushed", {
+    activityId: "a1",
+    activityName: "sample",
+    activityParams: { page: "1" },
+  });
+
+  const stepPushedEvent = makeEvent("StepPushed", {
+    stepId: "s2",
+    stepParams: { page: "2" },
+  });
+
+  return {
+    id: "a1",
+    name: "sample",
+    transitionState: "enter-done",
+    params: { page: "2" },
+    enteredBy: pushedEvent,
+    steps: [
+      {
+        id: "a1",
+        params: { page: "1" },
+        enteredBy: pushedEvent,
+        zIndex: -1,
+      },
+      {
+        id: "s2",
+        params: { page: "2" },
+        enteredBy: stepPushedEvent,
+        zIndex: -1,
+      },
+    ],
+    isTop: true,
+    isActive: true,
+    isRoot: true,
+    zIndex: 0,
+  };
+}
+
+test("makeActivityReducer - StepPopped는 activity.steps를 변경하지 않고 새 배열을 반환합니다", () => {
+  const activity = makeActivity();
+  const originalSteps = activity.steps;
+
+  const reducer = makeActivityReducer({ transitionDuration: 300, now: 0 });
+  const stepPoppedEvent = makeEvent("StepPopped", {});
+
+  const nextActivity = reducer(activity, stepPoppedEvent);
+
+  expect(activity.steps).toBe(originalSteps);
+  expect(activity.steps.length).toBe(2);
+  expect(nextActivity.steps).not.toBe(originalSteps);
+  expect(nextActivity.steps.length).toBe(1);
+  expect(nextActivity.params).toEqual({ page: "1" });
+});

core/src/activity-utils/makeActivityReducer.ts

@@ -108,13 +108,14 @@ export function makeActivityReducer(context: {
      * If there are params in the previous step, set them as the new params
      */
     StepPopped: (activity: Activity, event: StepPoppedEvent): Activity => {
-      activity.steps.pop();
+      const steps = activity.steps.slice(0, -1);
 
-      const beforeActivityParams = last(activity.steps)?.params;
+      const beforeActivityParams = last(steps)?.params;
 
       return {
         ...activity,
         params: beforeActivityParams ?? activity.params,
+        steps,
       };
     },