Add test for interim state of the animation to make sure we are transitioning...

Add test for interim state of the animation to make sure we are transitioning the border color the right way. (#13499)

packages/flutter/test/material/expansion_tile_initial_state_test.dart → packages/flutter/test/material/expansion_tile_test.dart

@@ -7,6 +7,8 @@ import 'package:flutter/widgets.dart';
 import 'package:flutter/material.dart';
 
 void main() {
+  final Color _dividerColor = const Color(0x1f333333);
+
   testWidgets('ExpansionTile initial state', (WidgetTester tester) async {
     final Key topKey = new UniqueKey();
     final Key expandedKey = const PageStorageKey<String>('expanded');
@@ -16,6 +18,10 @@ void main() {
     final Key tileKey = new UniqueKey();
 
     await tester.pumpWidget(new MaterialApp(
+      theme: new ThemeData(
+        platform: TargetPlatform.iOS,
+        dividerColor: _dividerColor,
+      ),
       home: new Material(
         child: new SingleChildScrollView(
           child: new Column(
@@ -62,7 +68,6 @@ void main() {
       of: find.byKey(key),
       matching: find.byType(Container),
     ));
-    final Color dividerColor = Theme.of(tester.element(find.text('Collapsed'))).dividerColor;
 
     expect(getHeight(topKey), getHeight(expandedKey) - getHeight(tileKey) - 2.0);
     expect(getHeight(topKey), getHeight(collapsedKey) - 2.0);
@@ -70,8 +75,8 @@ void main() {
 
     BoxDecoration expandedContainerDecoration = getContainer(expandedKey).decoration;
     expect(expandedContainerDecoration.color, Colors.red);
-    expect(expandedContainerDecoration.border.top.color, dividerColor);
-    expect(expandedContainerDecoration.border.bottom.color, dividerColor);
+    expect(expandedContainerDecoration.border.top.color, _dividerColor);
+    expect(expandedContainerDecoration.border.bottom.color, _dividerColor);
 
     BoxDecoration collapsedContainerDecoration = getContainer(collapsedKey).decoration;
     expect(collapsedContainerDecoration.color, Colors.transparent);
@@ -83,6 +88,16 @@ void main() {
     await tester.tap(find.text('Default'));
 
     await tester.pump();
+
+    // Pump to the middle of the animation for expansion.
+    await tester.pump(const Duration(milliseconds: 100));
+    final BoxDecoration collapsingContainerDecoration = getContainer(collapsedKey).decoration;
+    expect(collapsingContainerDecoration.color, Colors.transparent);
+    // Opacity should change but color component should remain the same.
+    expect(collapsingContainerDecoration.border.top.color, const Color(0x15333333));
+    expect(collapsingContainerDecoration.border.bottom.color, const Color(0x15333333));
+
+    // Pump all the way to the end now.
     await tester.pump(const Duration(seconds: 1));
 
     expect(getHeight(topKey), getHeight(expandedKey) - 2.0);
@@ -98,7 +113,7 @@ void main() {
     // Collapsed should be expanded now.
     collapsedContainerDecoration = getContainer(collapsedKey).decoration;
     expect(collapsedContainerDecoration.color, Colors.transparent);
-    expect(collapsedContainerDecoration.border.top.color, dividerColor);
-    expect(collapsedContainerDecoration.border.bottom.color, dividerColor);
+    expect(collapsedContainerDecoration.border.top.color, _dividerColor);
+    expect(collapsedContainerDecoration.border.bottom.color, _dividerColor);
   });
 }