Fixed extent lists cannot handle infinite extent

Previously they tried to compute an integer target end index, but
integers can't represent infinity. Now we use null to represent
infinity.

Also, fix some similar issues with grids.

Fixes flutter#8398

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

@@ -120,13 +120,14 @@ abstract class RenderSliverFixedExtentBoxAdaptor extends RenderSliverMultiBoxAda
     );
 
     final int firstIndex = getMinChildIndexForScrollOffset(scrollOffset, itemExtent);
-    final int targetLastIndex = getMaxChildIndexForScrollOffset(targetEndScrollOffset, itemExtent);
+    final int targetLastIndex = targetEndScrollOffset.isFinite ?
+        getMaxChildIndexForScrollOffset(targetEndScrollOffset, itemExtent) : null;
 
     if (firstChild != null) {
       final int oldFirstIndex = indexOf(firstChild);
       final int oldLastIndex = indexOf(lastChild);
       final int leadingGarbage = (firstIndex - oldFirstIndex).clamp(0, childCount);
-      final int trailingGarbage = (oldLastIndex - targetLastIndex).clamp(0, childCount);
+      final int trailingGarbage = targetLastIndex == null ? 0 : (oldLastIndex - targetLastIndex).clamp(0, childCount);
       if (leadingGarbage + trailingGarbage > 0)
         collectGarbage(leadingGarbage, trailingGarbage);
     }
@@ -156,7 +157,7 @@ abstract class RenderSliverFixedExtentBoxAdaptor extends RenderSliverMultiBoxAda
       trailingChildWithLayout = firstChild;
     }
 
-    while (indexOf(trailingChildWithLayout) < targetLastIndex) {
+    while (targetLastIndex == null || indexOf(trailingChildWithLayout) < targetLastIndex) {
       RenderBox child = childAfter(trailingChildWithLayout);
       if (child == null) {
         child = insertAndLayoutChild(childConstraints, after: trailingChildWithLayout);
@@ -180,7 +181,7 @@ abstract class RenderSliverFixedExtentBoxAdaptor extends RenderSliverMultiBoxAda
     assert(firstIndex == 0 || childScrollOffset(firstChild) <= scrollOffset);
     assert(debugAssertChildListIsNonEmptyAndContiguous());
     assert(indexOf(firstChild) == firstIndex);
-    assert(lastIndex <= targetLastIndex);
+    assert(targetLastIndex == null || lastIndex <= targetLastIndex);
 
     final double estimatedMaxScrollOffset = estimateMaxScrollOffset(
       constraints,
@@ -201,7 +202,8 @@ abstract class RenderSliverFixedExtentBoxAdaptor extends RenderSliverMultiBoxAda
       paintExtent: paintExtent,
       maxPaintExtent: estimatedMaxScrollOffset,
       // Conservative to avoid flickering away the clip during scroll.
-      hasVisualOverflow: lastIndex >= targetLastIndex || constraints.scrollOffset > 0.0,
+      hasVisualOverflow: (targetLastIndex != null && lastIndex >= targetLastIndex)
+        || constraints.scrollOffset > 0.0,
     );
 
     assert(childManager.debugAssertChildListLocked());

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

@@ -172,13 +172,16 @@ class SliverGridRegularTileLayout extends SliverGridLayout {
 
   @override
   int getMinChildIndexForScrollOffset(double scrollOffset) {
-    return crossAxisCount * (scrollOffset ~/ mainAxisStride);
+    return mainAxisStride > 0.0 ? crossAxisCount * (scrollOffset ~/ mainAxisStride) : 0;
   }
 
   @override
   int getMaxChildIndexForScrollOffset(double scrollOffset) {
-    final int mainAxisCount = (scrollOffset / mainAxisStride).ceil();
-    return math.max(0, crossAxisCount * mainAxisCount - 1);
+    if (mainAxisStride > 0.0) {
+      final int mainAxisCount = (scrollOffset / mainAxisStride).ceil();
+      return math.max(0, crossAxisCount * mainAxisCount - 1);
+    }
+    return 0;
   }
 
   @override
@@ -486,13 +489,14 @@ class RenderSliverGrid extends RenderSliverMultiBoxAdaptor {
     final SliverGridLayout layout = _gridDelegate.getLayout(constraints);
 
     final int firstIndex = layout.getMinChildIndexForScrollOffset(scrollOffset);
-    final int targetLastIndex = layout.getMaxChildIndexForScrollOffset(targetEndScrollOffset);
+    final int targetLastIndex = targetEndScrollOffset.isFinite ?
+      layout.getMaxChildIndexForScrollOffset(targetEndScrollOffset) : null;
 
     if (firstChild != null) {
       final int oldFirstIndex = indexOf(firstChild);
       final int oldLastIndex = indexOf(lastChild);
       final int leadingGarbage = (firstIndex - oldFirstIndex).clamp(0, childCount);
-      final int trailingGarbage = (oldLastIndex - targetLastIndex).clamp(0, childCount);
+      final int trailingGarbage = targetLastIndex == null ? 0 : (oldLastIndex - targetLastIndex).clamp(0, childCount);
       if (leadingGarbage + trailingGarbage > 0)
         collectGarbage(leadingGarbage, trailingGarbage);
     }
@@ -532,7 +536,7 @@ class RenderSliverGrid extends RenderSliverMultiBoxAdaptor {
       trailingChildWithLayout = firstChild;
     }
 
-    for (int index = indexOf(trailingChildWithLayout) + 1; index <= targetLastIndex; ++index) {
+    for (int index = indexOf(trailingChildWithLayout) + 1; targetLastIndex == null || index <= targetLastIndex; ++index) {
       final SliverGridGeometry gridGeometry = layout.getGeometryForChildIndex(index);
       final BoxConstraints childConstraints = gridGeometry.getBoxConstraints(constraints);
       RenderBox child = childAfter(trailingChildWithLayout);
@@ -559,7 +563,7 @@ class RenderSliverGrid extends RenderSliverMultiBoxAdaptor {
     assert(childScrollOffset(firstChild) <= scrollOffset);
     assert(debugAssertChildListIsNonEmptyAndContiguous());
     assert(indexOf(firstChild) == firstIndex);
-    assert(lastIndex <= targetLastIndex);
+    assert(targetLastIndex == null || lastIndex <= targetLastIndex);
 
     final double estimatedTotalExtent = childManager.estimateMaxScrollOffset(
       constraints,

packages/flutter/test/widgets/grid_view_test.dart

@@ -328,6 +328,47 @@ void main() {
     expect(find.byType(GridView), isNot(paints..rect(color: green)..rect(color: green)..rect(color: green)));
   });
 
+  testWidgets('GridView in zero context', (WidgetTester tester) async {
+    await tester.pumpWidget(
+      new Center(
+        child: new SizedBox(
+          width: 0.0,
+          height: 0.0,
+          child: new GridView.count(
+            crossAxisCount: 4,
+            children: new List<Widget>.generate(20, (int i) {
+              return new Container(
+                child: new Text('$i'),
+              );
+            }),
+          ),
+        ),
+      ),
+    );
+
+    expect(find.text('0'), findsOneWidget);
+    expect(find.text('1'), findsNothing);
+  });
+
+  testWidgets('GridView in unbounded context', (WidgetTester tester) async {
+    await tester.pumpWidget(
+      new SingleChildScrollView(
+        child: new GridView.count(
+          crossAxisCount: 4,
+          shrinkWrap: true,
+          children: new List<Widget>.generate(20, (int i) {
+            return new Container(
+              child: new Text('$i'),
+            );
+          }),
+        ),
+      ),
+    );
+
+    expect(find.text('0'), findsOneWidget);
+    expect(find.text('19'), findsOneWidget);
+  });
+
   // TODO(ianh): can you tap a grid cell that is slightly off the bottom of the screen?
   // (try it with the flutter_gallery Grid demo)
 }

packages/flutter/test/widgets/list_view_test.dart

@@ -144,4 +144,23 @@ void main() {
     expect(find.text('4'), findsOneWidget);
     expect(find.text('5'), findsNothing);
   });
+
+  testWidgets('ListView with itemExtent in unbounded context', (WidgetTester tester) async {
+    await tester.pumpWidget(
+      new SingleChildScrollView(
+        child: new ListView(
+          itemExtent: 100.0,
+          shrinkWrap: true,
+          children: new List<Widget>.generate(20, (int i) {
+            return new Container(
+              child: new Text('$i'),
+            );
+          }),
+        ),
+      ),
+    );
+
+    expect(find.text('0'), findsOneWidget);
+    expect(find.text('19'), findsOneWidget);
+  });
 }