Support an empty PageableList

packages/flutter/lib/src/widgets/homogeneous_viewport.dart

@@ -256,7 +256,6 @@ class _HomogeneousPageViewportElement extends _ViewportBaseElement<HomogeneousPa
     // doing our own layout.)
     BuildableElement.lockState(() {
       double itemExtent = widget.direction == ScrollDirection.vertical ? constraints.maxHeight : constraints.maxWidth;
-      double contentExtent = itemExtent * widget.itemCount;
       double offset;
       if (widget.startOffset <= 0.0 && !widget.itemsWrap) {
         _layoutFirstIndex = 0;
@@ -265,9 +264,10 @@ class _HomogeneousPageViewportElement extends _ViewportBaseElement<HomogeneousPa
         _layoutFirstIndex = widget.startOffset.floor();
         offset = -((widget.startOffset * itemExtent) % itemExtent);
       }
-      if (itemExtent < double.INFINITY) {
-        _layoutItemCount = ((contentExtent - offset) / contentExtent).ceil();
-        if (widget.itemCount != null && !widget.itemsWrap)
+      if (itemExtent < double.INFINITY && widget.itemCount != null) {
+        final double contentExtent = itemExtent * widget.itemCount;
+        _layoutItemCount = contentExtent == 0.0 ? 0 : ((contentExtent - offset) / contentExtent).ceil();
+        if (!widget.itemsWrap)
           _layoutItemCount = math.min(_layoutItemCount, widget.itemCount - _layoutFirstIndex);
       } else {
         assert(() {

packages/flutter/lib/src/widgets/pageable_list.dart

@@ -148,15 +148,15 @@ class PageableListState<T, Config extends PageableList<T>> extends ScrollableSta
   bool get snapScrollOffsetChanges => config.itemsSnapAlignment == ItemsSnapAlignment.item;
 
   double snapScrollOffset(double newScrollOffset) {
-    double previousItemOffset = newScrollOffset.floorToDouble();
-    double nextItemOffset = newScrollOffset.ceilToDouble();
+    final double previousItemOffset = newScrollOffset.floorToDouble();
+    final double nextItemOffset = newScrollOffset.ceilToDouble();
     return (newScrollOffset - previousItemOffset < 0.5 ? previousItemOffset : nextItemOffset)
       .clamp(scrollBehavior.minScrollOffset, scrollBehavior.maxScrollOffset);
   }
 
   Future _flingToAdjacentItem(Offset velocity) {
-    double scrollVelocity = scrollDirectionVelocity(velocity);
-    double newScrollOffset = snapScrollOffset(scrollOffset + scrollVelocity.sign)
+    final double scrollVelocity = scrollDirectionVelocity(velocity);
+    final double newScrollOffset = snapScrollOffset(scrollOffset + scrollVelocity.sign)
       .clamp(snapScrollOffset(scrollOffset - 0.5), snapScrollOffset(scrollOffset + 0.5));
     return scrollTo(newScrollOffset, duration: config.duration, curve: config.curve)
       .then(_notifyPageChanged);
@@ -177,9 +177,9 @@ class PageableListState<T, Config extends PageableList<T>> extends ScrollableSta
   }
 
   List<Widget> buildItems(BuildContext context, int start, int count) {
-    List<Widget> result = new List<Widget>();
-    int begin = config.itemsWrap ? start : math.max(0, start);
-    int end = config.itemsWrap ? begin + count : math.min(begin + count, config.items.length);
+    final List<Widget> result = new List<Widget>();
+    final int begin = config.itemsWrap ? start : math.max(0, start);
+    final int end = config.itemsWrap ? begin + count : math.min(begin + count, itemCount);
     for (int i = begin; i < end; ++i)
       result.add(config.itemBuilder(context, config.items[i % itemCount], i));
     assert(result.every((Widget item) => item.key != null));

packages/unit/test/widget/pageable_list_test.dart

@@ -7,7 +7,7 @@ import 'package:flutter/widgets.dart';
 import 'package:test/test.dart';
 
 const Size pageSize = const Size(800.0, 600.0);
-const List<int> pages = const <int>[0, 1, 2, 3, 4, 5];
+const List<int> defaultPages = const <int>[0, 1, 2, 3, 4, 5];
 int currentPage = null;
 bool itemsWrap = false;
 
@@ -20,7 +20,7 @@ Widget buildPage(BuildContext context, int page, int index) {
   );
 }
 
-Widget buildFrame() {
+Widget buildFrame({ List<int> pages: defaultPages }) {
   // The test framework forces the frame (and so the PageableList)
   // to be 800x600. The pageSize constant reflects this.
   return new PageableList<int>(
@@ -66,7 +66,6 @@ void main() {
 
   test('PageableList with itemsWrap: true', () {
     testWidgets((WidgetTester tester) {
-      tester.pumpWidget(new Container());
       currentPage = null;
       itemsWrap = true;
       tester.pumpWidget(buildFrame());
@@ -79,4 +78,44 @@ void main() {
       expect(currentPage, equals(5));
     });
   });
+
+  test('PageableList with two items', () {
+    testWidgets((WidgetTester tester) {
+      currentPage = null;
+      itemsWrap = true;
+      tester.pumpWidget(buildFrame(pages: <int>[0, 1]));
+      expect(currentPage, isNull);
+      pageLeft(tester);
+      expect(currentPage, equals(1));
+      pageRight(tester);
+      expect(currentPage, equals(0));
+      pageRight(tester);
+      expect(currentPage, equals(1));
+    });
+  });
+
+  test('PageableList with one item', () {
+    testWidgets((WidgetTester tester) {
+      currentPage = null;
+      itemsWrap = true;
+      tester.pumpWidget(buildFrame(pages: <int>[0]));
+      expect(currentPage, isNull);
+      pageLeft(tester);
+      expect(currentPage, equals(0));
+      pageRight(tester);
+      expect(currentPage, equals(0));
+      pageRight(tester);
+      expect(currentPage, equals(0));
+    });
+  });
+
+  test('PageableList with no items', () {
+    testWidgets((WidgetTester tester) {
+      currentPage = null;
+      itemsWrap = true;
+      tester.pumpWidget(buildFrame(pages: null));
+      expect(currentPage, isNull);
+    });
+  });
+
 }