separate scroll until visible implementation for lower level accessibility (#62462)

packages/flutter_test/lib/src/controller.dart

@@ -817,26 +817,32 @@ abstract class WidgetController {
   /// Shorthand for `Scrollable.ensureVisible(element(finder))`
   Future<void> ensureVisible(Finder finder) => Scrollable.ensureVisible(element(finder));
 
-  /// Repeatedly scrolls the `scrollable` by `delta` in the
+  /// Repeatedly scrolls a [Scrollable] by `delta` in the
   /// [Scrollable.axisDirection] until `finder` is visible.
   ///
   /// Between each scroll, wait for `duration` time for settling.
   ///
+  /// If `scrollable` is `null`, this will find a [Scrollable].
+  ///
   /// Throws a [StateError] if `finder` is not found for maximum `maxScrolls`
   /// times.
   ///
   /// This is different from [ensureVisible] in that this allows looking for
   /// `finder` that is not built yet, but the caller must specify the scrollable
-  /// that will build child specified by `finder`.
+  /// that will build child specified by `finder` when there are multiple
+  ///[Scrollable]s.
+  ///
+  /// See also [dragUntilVisible].
   Future<void> scrollUntilVisible(
     Finder finder,
-    Finder scrollable,
     double delta, {
+      Finder scrollable,
       int maxScrolls = 50,
       Duration duration = const Duration(milliseconds: 50),
     }
   ) {
     assert(maxScrolls > 0);
+    scrollable ??= find.byType(Scrollable);
     return TestAsyncUtils.guard<void>(() async {
       Offset moveStep;
       switch(widget<Scrollable>(scrollable).axisDirection) {
@@ -853,10 +859,33 @@ abstract class WidgetController {
           moveStep = Offset(-delta, 0);
           break;
       }
-      while(maxScrolls > 0 && finder.evaluate().isEmpty) {
-        await drag(scrollable, moveStep);
+      await dragUntilVisible(
+        finder,
+        scrollable,
+        moveStep,
+        maxIteration: maxScrolls,
+        duration: duration);
+    });
+  }
+
+  /// Repeatedly drags the `view` by `moveStep` until `finder` is visible.
+  ///
+  /// Between each operation, wait for `duration` time for settling.
+  ///
+  /// Throws a [StateError] if `finder` is not found for maximum `maxIteration`
+  /// times.
+  Future<void> dragUntilVisible(
+    Finder finder,
+    Finder view,
+    Offset moveStep, {
+      int maxIteration = 50,
+      Duration duration = const Duration(milliseconds: 50),
+  }) {
+    return TestAsyncUtils.guard<void>(() async {
+      while(maxIteration > 0 && finder.evaluate().isEmpty) {
+        await drag(view, moveStep);
         await pump(duration);
-        maxScrolls -= 1;
+        maxIteration-= 1;
       }
       await Scrollable.ensureVisible(element(finder));
     });

packages/flutter_test/test/controller_test.dart

@@ -551,7 +551,7 @@ void main() {
   );
 
   testWidgets(
-    'ensureVisibl: scrolls to make widget visible',
+    'ensureVisible: scrolls to make widget visible',
     (WidgetTester tester) async {
       await tester.pumpWidget(
         MaterialApp(
@@ -596,7 +596,6 @@ void main() {
 
         await tester.scrollUntilVisible(
           find.text('Item 45', skipOffstage: false),
-          find.byType(Scrollable),
           100,
         );
         await tester.pumpAndSettle();
@@ -628,7 +627,6 @@ void main() {
 
         await tester.scrollUntilVisible(
           find.text('Item 45', skipOffstage: false),
-          find.byType(Scrollable),
           100,
         );
         await tester.pumpAndSettle();
@@ -656,7 +654,6 @@ void main() {
         try {
           await tester.scrollUntilVisible(
             find.text('Item 55', skipOffstage: false),
-            find.byType(Scrollable),
             100,
           );
         } on StateError catch (e) {
@@ -664,5 +661,48 @@ void main() {
         }
       },
     );
+
+    testWidgets('Drag Until Visible', (WidgetTester tester) async {
+      // when there are two implicit [Scrollable], `scrollUntilVisible` is hard
+      // to use.
+      await tester.pumpWidget(
+        MaterialApp(
+          home: Scaffold(
+            body: Column(
+              children: <Widget>[
+                Container(height: 200, child: ListView.builder(
+                  key: const Key('listView-a'),
+                  itemCount: 50,
+                  shrinkWrap: true,
+                  itemBuilder: (BuildContext context, int i) => ListTile(title: Text('Item a-$i')),
+                )),
+                const Divider(thickness: 5),
+                Expanded(child: ListView.builder(
+                  key: const Key('listView-b'),
+                  itemCount: 50,
+                  shrinkWrap: true,
+                  itemBuilder: (BuildContext context, int i) => ListTile(title: Text('Item b-$i')),
+                )),
+              ],
+            ),
+          ),
+        ),
+      );
+      await tester.pumpAndSettle();
+      expect(find.byType(Scrollable), findsNWidgets(2));
+
+      // Make sure widget isn't built yet.
+      expect(find.text('Item b-45', skipOffstage: false), findsNothing);
+
+      await tester.dragUntilVisible(
+        find.text('Item b-45', skipOffstage: false),
+        find.byKey(const ValueKey<String>('listView-b')),
+        const Offset(0, -100),
+      );
+      await tester.pumpAndSettle();
+
+      // Now the widget is on screen.
+      expect(find.text('Item b-45', skipOffstage: true), findsOneWidget);
+    });
   });
 }