Test showStateChangeNotifications

src/Farmhand.js

@@ -274,18 +274,7 @@ export default class Farmhand extends Component {
   }
 
   componentDidUpdate(prevProps, prevState) {
-    const {
-      state: { purchasedCowPen },
-    } = this;
-
-    // TODO: Test this.
-    if (purchasedCowPen !== prevState.purchasedCowPen) {
-      const { cows } = PURCHASEABLE_COW_PENS.get(purchasedCowPen);
-
-      this.showNotification(
-        `Purchased a cow pen with capacity for ${cows} cows!`
-      );
-    }
+    this.showStateChangeNotifications(prevState);
   }
 
   clearPersistedData() {
@@ -307,6 +296,23 @@ export default class Farmhand extends Component {
     }));
   }
 
+  /**
+   * @param {farmhand.state} prevState
+   */
+  showStateChangeNotifications(prevState) {
+    const {
+      state: { purchasedCowPen },
+    } = this;
+
+    if (purchasedCowPen !== prevState.purchasedCowPen) {
+      const { cows } = PURCHASEABLE_COW_PENS.get(purchasedCowPen);
+
+      this.showNotification(
+        `Purchased a cow pen with capacity for ${cows} cows!`
+      );
+    }
+  }
+
   incrementDay() {
     const nextDayState = computeStateForNextDay(this.state);
     const pendingNotifications = [...nextDayState.newDayNotifications];

src/Farmhand.test.js

@@ -189,6 +189,24 @@ describe('instance methods', () => {
     });
   });
 
+  describe('showStateChangeNotifications', () => {
+    describe('cow pen purchasing', () => {
+      test('shows notification', () => {
+        const showNotification = jest.spyOn(
+          component.instance(),
+          'showNotification'
+        );
+
+        component.setState({ purchasedCowPen: 1 });
+        component
+          .instance()
+          .showStateChangeNotifications({ purchasedCowPen: 0 });
+
+        expect(showNotification).toHaveBeenCalled();
+      });
+    });
+  });
+
   describe('incrementDay', () => {
     beforeEach(() => {
       jest.spyOn(component.instance().localforage, 'setItem');