[animations] Add widget tests for card_swipe (#442)

animations/test/misc/card_swipe_test.dart

@@ -0,0 +1,66 @@
+// Copyright 2020 The Flutter team. All rights reserved.
+// Use of this source code is governed by a BSD-style license that can be
+// found in the LICENSE file.
+
+import 'package:animations/src/misc/card_swipe.dart';
+import 'package:flutter/gestures.dart';
+import 'package:flutter/material.dart';
+import 'package:flutter_test/flutter_test.dart';
+
+Widget createCardSwipeScreen() => MaterialApp(
+      home: CardSwipeDemo(),
+    );
+
+void main() {
+  group('Card Swipe Tests', () {
+    testWidgets('One card swiped out', (tester) async {
+      await tester.pumpWidget(createCardSwipeScreen());
+
+      // Get the total number of cards available.
+      var totalCards = tester.widgetList(find.byType(SwipeableCard)).length;
+
+      // Swipe out one card.
+      await tester.drag(find.byType(SwipeableCard).first, Offset(100.0, 0.0));
+      await tester.pumpAndSettle();
+
+      // Check if removed properly.
+      expect(tester.widgetList(find.byType(SwipeableCard)).length,
+          lessThan(totalCards));
+    });
+
+    testWidgets('All cards swiped out', (tester) async {
+      await tester.pumpWidget(createCardSwipeScreen());
+
+      // Get the total number of cards availabe.
+      var totalCards = tester.widgetList(find.byType(SwipeableCard)).length;
+
+      // Swipe out all cards.
+      for (var i = 0; i < totalCards; i++) {
+        // Swipe out one by one.
+        await tester.drag(find.byType(SwipeableCard).first, Offset(100.0, 0.0));
+        await tester.pumpAndSettle();
+      }
+
+      // Check if any card is remaining.
+      expect(find.byType(SwipeableCard), findsNothing);
+    });
+
+    testWidgets('Stack refilled with cards', (tester) async {
+      await tester.pumpWidget(createCardSwipeScreen());
+
+      // Get the total number of cards availabe.
+      var totalCards = tester.widgetList(find.byType(SwipeableCard)).length;
+
+      // Swipe out one card.
+      await tester.drag(find.byType(SwipeableCard).first, Offset(100.0, 0.0));
+      await tester.pumpAndSettle();
+
+      // Tap the Refill button.
+      await tester.tap(find.byType(RaisedButton));
+      await tester.pumpAndSettle();
+
+      // Check if the entire stack is refilled.
+      expect(find.byType(SwipeableCard), findsNWidgets(totalCards));
+    });
+  });
+}