Skip painting hidden children of a sliver list (#18465)

We have many hidden children because of iOS accessibility (see also #18410).

For example, in the tile screen of complex_layout app, there could be up to 20 children being painted without this PR. With this PR, only 12 get painted.

This should improve the performance of FL-53 by 40%.

packages/flutter/lib/src/rendering/sliver_multi_box_adaptor.dart

@@ -524,7 +524,9 @@ abstract class RenderSliverMultiBoxAdaptor extends RenderSliver
       );
       if (addExtent)
         childOffset += mainAxisUnit * paintExtentOf(child);
-      context.paintChild(child, childOffset);
+      final double mainAxisOffset = mainAxisUnit.dx != 0 ? childOffset.dx : childOffset.dy;
+      if (mainAxisOffset < constraints.remainingPaintExtent && mainAxisOffset + paintExtentOf(child) > 0)
+        context.paintChild(child, childOffset);
       child = childAfter(child);
     }
   }

packages/flutter/test/rendering/mock_canvas.dart

@@ -48,6 +48,11 @@ Matcher get paintsNothing => new _TestRecordingCanvasPaintsNothingMatcher();
 /// Matches objects or functions that assert when they try to paint.
 Matcher get paintsAssertion => new _TestRecordingCanvasPaintsAssertionMatcher();
 
+/// Matches objects or functions that draw `methodName` exactly `count` number of times
+Matcher paintsExactlyCountTimes(Symbol methodName, int count) {
+  return new _TestRecordingCanvasPaintsCountMatcher(methodName, count);
+}
+
 /// Signature for the [PaintPattern.something] and [PaintPattern.everything]
 /// predicate argument.
 ///
@@ -552,6 +557,34 @@ abstract class _TestRecordingCanvasMatcher extends Matcher {
   }
 }
 
+class _TestRecordingCanvasPaintsCountMatcher extends _TestRecordingCanvasMatcher {
+  final Symbol _methodName;
+  final int _count;
+
+  _TestRecordingCanvasPaintsCountMatcher(Symbol methodName, int count)
+      : _methodName = methodName, _count = count;
+
+  @override
+  Description describe(Description description) {
+    return description.add('Object or closure painting $_methodName exactly $_count times');
+  }
+
+  @override
+  bool _evaluatePredicates(Iterable<RecordedInvocation> calls,
+      StringBuffer description) {
+    int count = 0;
+    for(RecordedInvocation call in calls) {
+      if (call.invocation.isMethod && call.invocation.memberName == _methodName) {
+        count++;
+      }
+    }
+    if (count != _count) {
+      description.write('It painted $_methodName $count times instead of $_count times.');
+    }
+    return count == _count;
+  }
+}
+
 class _TestRecordingCanvasPaintsNothingMatcher extends _TestRecordingCanvasMatcher {
   @override
   Description describe(Description description) {

packages/flutter/test/widgets/keep_alive_test.dart

@@ -307,14 +307,14 @@ void main() {
       '                     │     constraints: BoxConstraints(w=800.0, h=400.0)\n'
       '                     │     size: Size(800.0, 400.0)\n'
       '                     │\n'
-      '                     └─child with index 2: RenderLimitedBox#00000\n'
+      '                     └─child with index 2: RenderLimitedBox#00000 NEEDS-PAINT\n'
       '                       │ parentData: index=2; layoutOffset=800.0\n'
       '                       │ constraints: BoxConstraints(w=800.0, h=400.0)\n'
       '                       │ size: Size(800.0, 400.0)\n'
       '                       │ maxWidth: 400.0\n'
       '                       │ maxHeight: 400.0\n'
       '                       │\n'
-      '                       └─child: RenderCustomPaint#00000\n'
+      '                       └─child: RenderCustomPaint#00000 NEEDS-PAINT\n'
       '                           parentData: <none> (can use size)\n'
       '                           constraints: BoxConstraints(w=800.0, h=400.0)\n'
       '                           size: Size(800.0, 400.0)\n'
@@ -412,14 +412,14 @@ void main() {
       '                     │   cacheExtent: 1100.0)\n'
       '                     │ currently live children: 4 to 7\n'
       '                     │\n'
-      '                     ├─child with index 4: RenderLimitedBox#00000\n'
+      '                     ├─child with index 4: RenderLimitedBox#00000 NEEDS-PAINT\n'
       '                     │ │ parentData: index=4; layoutOffset=1600.0\n'
       '                     │ │ constraints: BoxConstraints(w=800.0, h=400.0)\n'
       '                     │ │ size: Size(800.0, 400.0)\n'
       '                     │ │ maxWidth: 400.0\n'
       '                     │ │ maxHeight: 400.0\n'
       '                     │ │\n'
-      '                     │ └─child: RenderCustomPaint#00000\n'
+      '                     │ └─child: RenderCustomPaint#00000 NEEDS-PAINT\n'
       '                     │     parentData: <none> (can use size)\n'
       '                     │     constraints: BoxConstraints(w=800.0, h=400.0)\n'
       '                     │     size: Size(800.0, 400.0)\n'
@@ -448,14 +448,14 @@ void main() {
       '                     │     constraints: BoxConstraints(w=800.0, h=400.0)\n'
       '                     │     size: Size(800.0, 400.0)\n'
       '                     │\n'
-      '                     ├─child with index 7: RenderLimitedBox#00000\n'
+      '                     ├─child with index 7: RenderLimitedBox#00000 NEEDS-PAINT\n'
       '                     ╎ │ parentData: index=7; layoutOffset=2800.0\n'
       '                     ╎ │ constraints: BoxConstraints(w=800.0, h=400.0)\n'
       '                     ╎ │ size: Size(800.0, 400.0)\n'
       '                     ╎ │ maxWidth: 400.0\n'
       '                     ╎ │ maxHeight: 400.0\n'
       '                     ╎ │\n'
-      '                     ╎ └─child: RenderCustomPaint#00000\n'
+      '                     ╎ └─child: RenderCustomPaint#00000 NEEDS-PAINT\n'
       '                     ╎     parentData: <none> (can use size)\n'
       '                     ╎     constraints: BoxConstraints(w=800.0, h=400.0)\n'
       '                     ╎     size: Size(800.0, 400.0)\n'

packages/flutter/test/widgets/list_view_viewporting_test.dart

@@ -6,6 +6,8 @@ import 'package:flutter_test/flutter_test.dart';
 import 'package:flutter/material.dart';
 import 'package:flutter/rendering.dart';
 
+import '../rendering/mock_canvas.dart';
+
 import 'test_widgets.dart';
 
 void main() {
@@ -429,4 +431,33 @@ void main() {
     expect(position.viewportDimension, equals(600.0));
     expect(position.minScrollExtent, equals(0.0));
   });
+
+  testWidgets('ListView should not paint hidden children', (WidgetTester tester) async {
+    const Text text = const Text('test');
+    await tester.pumpWidget(
+        new Directionality(
+            textDirection: TextDirection.ltr,
+            child: new Center(
+              child: new Container(
+                  height: 200.0,
+                  child: new ListView(
+                    cacheExtent: 500.0,
+                    controller: new ScrollController(initialScrollOffset: 300.0),
+                    children: <Widget>[
+                      new Container(height: 140.0, child: text),
+                      new Container(height: 160.0, child: text),
+                      new Container(height: 90.0, child: text),
+                      new Container(height: 110.0, child: text),
+                      new Container(height: 80.0, child: text),
+                      new Container(height: 70.0, child: text),
+                    ],
+                  )
+              ),
+            )
+        )
+    );
+
+    final RenderSliverList list = tester.renderObject(find.byType(SliverList));
+    expect(list, paintsExactlyCountTimes(#drawParagraph, 2));
+  });
 }

packages/flutter/test/widgets/sliver_fill_viewport_test.dart

@@ -82,7 +82,7 @@ void main() {
         ' │ │ constraints: BoxConstraints(w=800.0, h=600.0)\n'
         ' │ │ layer: OffsetLayer#00000\n'
         ' │ │ size: Size(800.0, 600.0)\n'
-        ' │ │ metrics: 75.0% useful (1 bad vs 3 good)\n'
+        ' │ │ metrics: 66.7% useful (1 bad vs 2 good)\n'
         ' │ │ diagnosis: insufficient data to draw conclusion (less than five\n'
         ' │ │   repaints)\n'
         ' │ │\n'
@@ -103,9 +103,9 @@ void main() {
         ' └─child with index 1: RenderRepaintBoundary#00000\n'
         '   │ parentData: index=1; layoutOffset=600.0\n'
         '   │ constraints: BoxConstraints(w=800.0, h=600.0)\n'
-        '   │ layer: OffsetLayer#00000\n'
+        '   │ layer: OffsetLayer#00000 DETACHED\n'
         '   │ size: Size(800.0, 600.0)\n'
-        '   │ metrics: 75.0% useful (1 bad vs 3 good)\n'
+        '   │ metrics: 50.0% useful (1 bad vs 1 good)\n'
         '   │ diagnosis: insufficient data to draw conclusion (less than five\n'
         '   │   repaints)\n'
         '   │\n'