[CupertinoAlertDialog] Rewrite (#150410)

This PR rewrites `CupertinoAlertDialog` in a cleaner logic, mostly its layout.

There are two major difficulties to lay out `CupertinoAlertDialog`:
* Laying out the actions section, which switches between a horizontal mode (two buttons in a row) and a vertical mode (several buttons in a column). This PR rewrites it in a special layout widget, `_CupertinoAlertActionSection`.
* Allocating vertical space between the content section and the actions section. This reuses `_PriorityColumn`, which was created for the action sheet.

In a similar fashion to the action sheet, the management and configuration for dividers and background (overscrolls) were rewritten as well.

This PR tries to keep as much original code and behavior as possible since this PR is already very large. As a result, almost no tests are broken. Further improvement will be done in future PRs.
* The test that verifies painting overscrolls is replaced by a golden test, since the original test assumes that the background is painted in one rectangle.

packages/flutter/test/cupertino/dialog_test.dart

@@ -765,10 +765,7 @@ void main() {
 
     final RenderBox actionsSectionBox = findScrollableActionsSectionRenderBox(tester);
 
-    expect(
-      actionsSectionBox.size.height,
-      67.80000000000001,
-    );
+    expect(actionsSectionBox.size.height, 67.8);
   });
 
   testWidgets('Actions section height for 2 stacked buttons without enough room and large accessibility font is 50% of dialog height.', (WidgetTester tester) async {
@@ -858,166 +855,102 @@ void main() {
     );
   });
 
-  testWidgets('Actions section overscroll is painted white.', (WidgetTester tester) async {
-    final ScrollController scrollController = ScrollController();
-    addTearDown(scrollController.dispose);
+  testWidgets('Actions section correctly renders overscrolls', (WidgetTester tester) async {
+    // Verifies that when the actions section overscrolls, the overscroll part
+    // is correctly covered with background.
+    final ScrollController actionScrollController = ScrollController();
+    addTearDown(actionScrollController.dispose);
     await tester.pumpWidget(
       createAppWithButtonThatLaunchesDialog(
         dialogBuilder: (BuildContext context) {
           return CupertinoAlertDialog(
-            title: const Text('The Title'),
-            content: const Text('The message'),
-            actions: const <Widget>[
+            actions: List<Widget>.generate(12, (int i) =>
               CupertinoDialogAction(
-                child: Text('Option 1'),
+                onPressed: () {},
+                child: Text('Button ${'*' * i}'),
               ),
-              CupertinoDialogAction(
-                child: Text('Option 2'),
-              ),
-              CupertinoDialogAction(
-                child: Text('Option 3'),
-              ),
-            ],
-            scrollController: scrollController,
+            ),
           );
         },
       ),
     );
 
     await tester.tap(find.text('Go'));
+    await tester.pumpAndSettle();
+
+    final TestGesture gesture = await tester.startGesture(tester.getCenter(find.text('Button *')));
+    await tester.pumpAndSettle();
+    // The button should be pressed now, since the scrolling gesture has not
+    // taken over.
+    await expectLater(
+      find.byType(CupertinoAlertDialog),
+      matchesGoldenFile('cupertinoAlertDialog.overscroll.0.png'),
+    );
+    // The dragging gesture must be dispatched in at least two segments.
+    // After the first movement, the gesture is started, but the delta is still
+    // zero. The second movement gives the delta.
+    await gesture.moveBy(const Offset(0, 40));
+    await tester.pumpAndSettle();
+    await gesture.moveBy(const Offset(0, 100));
+    // Test the top overscroll. Use `pump` not `pumpAndSettle` to verify the
+    // rendering result of the immediate next frame.
     await tester.pump();
+    await expectLater(
+      find.byType(CupertinoAlertDialog),
+      matchesGoldenFile('cupertinoAlertDialog.overscroll.1.png'),
+    );
 
-    final RenderBox actionsSectionBox = findScrollableActionsSectionRenderBox(tester);
-
-    // The way that overscroll white is accomplished in a scrollable action
-    // section is that the custom RenderBox that lays out the buttons and draws
-    // the dividers also paints a white background the size of Rect.largest.
-    // That background ends up being clipped by the containing ScrollView.
-    //
-    // Here we test that the Rect(0.0, 0.0, renderBox.size.width, renderBox.size.height)
-    // is contained within the painted Path.
-    // We don't test for exclusion because for some reason the Path is reporting
-    // that even points beyond Rect.largest are within the Path. That's not an
-    // issue for our use-case, so we don't worry about it.
-    expect(actionsSectionBox, paints..path(
-      includes: <Offset>[
-        Offset.zero,
-        Offset(actionsSectionBox.size.width, actionsSectionBox.size.height),
-      ],
-    ));
+    await gesture.moveBy(const Offset(0, -300));
+    // Test the bottom overscroll. Use `pump` not `pumpAndSettle` to verify the
+    // rendering result of the immediate next frame.
+    await tester.pump();
+    await expectLater(
+      find.byType(CupertinoAlertDialog),
+      matchesGoldenFile('cupertinoAlertDialog.overscroll.2.png'),
+    );
+    await gesture.up();
   });
 
-  testWidgets('Pressed button changes appearance and dividers disappear.', (WidgetTester tester) async {
-    final ScrollController scrollController = ScrollController();
-    addTearDown(scrollController.dispose);
-    const double dividerThickness = 0.3;
+  testWidgets('Actions section correctly renders overscrolls with very far scrolls', (WidgetTester tester) async {
+    // When the scroll is really far, the overscroll might be longer than the
+    // actions section, causing overflow if not controlled.
+    final ScrollController actionScrollController = ScrollController();
+    addTearDown(actionScrollController.dispose);
     await tester.pumpWidget(
       createAppWithButtonThatLaunchesDialog(
         dialogBuilder: (BuildContext context) {
           return CupertinoAlertDialog(
-            title: const Text('The Title'),
-            content: const Text('The message'),
-            actions: const <Widget>[
-              CupertinoDialogAction(
-                child: Text('Option 1'),
+            content: Text('content' * 1000),
+            actions: List<Widget>.generate(4, (int i) =>
+              CupertinoActionSheetAction(
+                onPressed: () {},
+                child: Text('Button $i'),
               ),
-              CupertinoDialogAction(
-                child: Text('Option 2'),
-              ),
-              CupertinoDialogAction(
-                child: Text('Option 3'),
-              ),
-            ],
-            scrollController: scrollController,
+            ),
           );
         },
       ),
     );
 
     await tester.tap(find.text('Go'));
-    await tester.pump();
+    await tester.pumpAndSettle();
 
-    const Color normalButtonBackgroundColor = Color(0xCCF2F2F2);
-    const Color pressedButtonBackgroundColor = Color(0xFFE1E1E1);
-    final RenderBox firstButtonBox = findActionButtonRenderBoxByTitle(tester, 'Option 1');
-    final RenderBox secondButtonBox = findActionButtonRenderBoxByTitle(tester, 'Option 2');
-    final RenderBox actionsSectionBox = findScrollableActionsSectionRenderBox(tester);
-
-    final Offset pressedButtonCenter = Offset(
-      secondButtonBox.size.width / 2.0,
-      firstButtonBox.size.height + dividerThickness + (secondButtonBox.size.height / 2.0),
+    final TestGesture gesture = await tester.startGesture(tester.getCenter(find.text('Button 0')));
+    await tester.pumpAndSettle();
+    await gesture.moveBy(const Offset(0, 40)); // A short drag to start the gesture.
+    await tester.pumpAndSettle();
+    // The drag is far enough to make the overscroll longer than the section.
+    await gesture.moveBy(const Offset(0, 1000));
+    await tester.pumpAndSettle();
+    // The buttons should be out of the screen
+    expect(
+      tester.getTopLeft(find.text('Button 0')).dy,
+      greaterThan(tester.getBottomLeft(find.byType(ClipRRect)).dy)
     );
-    final Offset topDividerCenter = Offset(
-      secondButtonBox.size.width / 2.0,
-      firstButtonBox.size.height + (0.5 * dividerThickness),
+    await expectLater(
+      find.byType(CupertinoAlertDialog),
+      matchesGoldenFile('cupertinoAlertDialog.long-overscroll.0.png'),
     );
-    final Offset bottomDividerCenter = Offset(
-      secondButtonBox.size.width / 2.0,
-      firstButtonBox.size.height +
-          dividerThickness +
-          secondButtonBox.size.height +
-          (0.5 * dividerThickness),
-    );
-
-    // Before pressing the button, verify following expectations:
-    // - Background includes the button that will be pressed
-    // - Background excludes the divider above and below the button that will be pressed
-    // - Pressed button background does NOT include the button that will be pressed
-    expect(actionsSectionBox, paints
-      ..path(
-        color: normalButtonBackgroundColor,
-        includes: <Offset>[
-          pressedButtonCenter,
-        ],
-        excludes: <Offset>[
-          topDividerCenter,
-          bottomDividerCenter,
-        ],
-      )
-      ..path(
-        color: pressedButtonBackgroundColor,
-        excludes: <Offset>[
-          pressedButtonCenter,
-        ],
-      ),
-    );
-
-    // Press down on the button.
-    final TestGesture gesture = await tester.press(find.widgetWithText(CupertinoDialogAction, 'Option 2'));
-    await tester.pump();
-
-    // While pressing the button, verify following expectations:
-    // - Background excludes the pressed button
-    // - Background includes the divider above and below the pressed button
-    // - Pressed button background includes the pressed
-    expect(actionsSectionBox, paints
-      ..path(
-        color: normalButtonBackgroundColor,
-        // The background should contain the divider above and below the pressed
-        // button. While pressed, surrounding dividers disappear, which means
-        // they become part of the background.
-        includes: <Offset>[
-          topDividerCenter,
-          bottomDividerCenter,
-        ],
-        // The background path should not include the tapped button background...
-        excludes: <Offset>[
-          pressedButtonCenter,
-        ],
-      )
-      // For a pressed button, a dedicated path is painted with a pressed button
-      // background color...
-      ..path(
-        color: pressedButtonBackgroundColor,
-        includes: <Offset>[
-          pressedButtonCenter,
-        ],
-      ),
-    );
-
-    // We must explicitly cause an "up" gesture to avoid a crash.
-    // TODO(mattcarroll): remove this call, https://github.com/flutter/flutter/issues/19540
-    await gesture.up();
   });
 
   testWidgets('ScaleTransition animation for showCupertinoDialog()', (WidgetTester tester) async {