Add a pageSnapping parameter to PageView (#12596)

* Add a pageSnapping parameter to PageView

Setting the pageSnapping property allows extending the PageView scroll
behavior, such as custom scroll animations or custom scroll bars.

* Apply pageSnapping CR feedback

- Remove _kNonSnappingPhysics, use null instead.
- Minor code style fixes.
- It turns out that the forth state is Arkansas, not California.

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

@@ -344,6 +344,7 @@ class PageView extends StatefulWidget {
     this.reverse: false,
     PageController controller,
     this.physics,
+    this.pageSnapping: true,
     this.onPageChanged,
     List<Widget> children: const <Widget>[],
   }) : controller = controller ?? _defaultPageController,
@@ -368,6 +369,7 @@ class PageView extends StatefulWidget {
     this.reverse: false,
     PageController controller,
     this.physics,
+    this.pageSnapping: true,
     this.onPageChanged,
     @required IndexedWidgetBuilder itemBuilder,
     int itemCount,
@@ -383,6 +385,7 @@ class PageView extends StatefulWidget {
     this.reverse: false,
     PageController controller,
     this.physics,
+    this.pageSnapping: true,
     this.onPageChanged,
     @required this.childrenDelegate,
   }) : assert(childrenDelegate != null),
@@ -423,6 +426,9 @@ class PageView extends StatefulWidget {
   /// Defaults to matching platform conventions.
   final ScrollPhysics physics;
 
+  /// Set to false to disable page snapping, useful for custom scroll behavior.
+  final bool pageSnapping;
+
   /// Called whenever the page in the center of the viewport changes.
   final ValueChanged<int> onPageChanged;
 
@@ -463,6 +469,10 @@ class _PageViewState extends State<PageView> {
   @override
   Widget build(BuildContext context) {
     final AxisDirection axisDirection = _getDirection(context);
+    final ScrollPhysics physics = widget.pageSnapping
+        ? _kPagePhysics.applyTo(widget.physics)
+        : widget.physics;
+
     return new NotificationListener<ScrollNotification>(
       onNotification: (ScrollNotification notification) {
         if (notification.depth == 0 && widget.onPageChanged != null && notification is ScrollUpdateNotification) {
@@ -478,7 +488,7 @@ class _PageViewState extends State<PageView> {
       child: new Scrollable(
         axisDirection: axisDirection,
         controller: widget.controller,
-        physics: widget.physics == null ? _kPagePhysics : _kPagePhysics.applyTo(widget.physics),
+        physics: physics,
         viewportBuilder: (BuildContext context, ViewportOffset position) {
           return new Viewport(
             axisDirection: axisDirection,
@@ -502,5 +512,6 @@ class _PageViewState extends State<PageView> {
     description.add(new FlagProperty('reverse', value: widget.reverse, ifTrue: 'reversed'));
     description.add(new DiagnosticsProperty<PageController>('controller', widget.controller, showName: false));
     description.add(new DiagnosticsProperty<ScrollPhysics>('physics', widget.physics, showName: false));
+    description.add(new FlagProperty('pageSnapping', value: widget.pageSnapping, ifFalse: 'snapping disabled'));
   }
 }

packages/flutter/test/widgets/page_view_test.dart

@@ -368,6 +368,69 @@ void main() {
     expect(tester.getTopLeft(find.text('Idaho')), const Offset(790.0, 0.0));
   });
 
+  testWidgets('Page snapping disable and reenable', (WidgetTester tester) async {
+    final List<int> log = <int>[];
+
+    Widget build({ bool pageSnapping }) {
+      return new Directionality(
+        textDirection: TextDirection.ltr,
+        child: new PageView(
+          pageSnapping: pageSnapping,
+          onPageChanged: log.add,
+          children:
+              kStates.map<Widget>((String state) => new Text(state)).toList(),
+        ),
+      );
+    }
+
+    await tester.pumpWidget(build(pageSnapping: true));
+    expect(log, isEmpty);
+
+    // Drag more than halfway to the next page, to confirm the default behavior.
+    TestGesture gesture = await tester.startGesture(const Offset(100.0, 100.0));
+    // The page view is 800.0 wide, so this move is just beyond halfway.
+    await gesture.moveBy(const Offset(-420.0, 0.0));
+
+    expect(log, equals(const <int>[1]));
+    log.clear();
+
+    // Release the gesture, confirm that the page settles on the next.
+    await gesture.up();
+    await tester.pumpAndSettle();
+
+    expect(find.text('Alabama'), findsNothing);
+    expect(find.text('Alaska'), findsOneWidget);
+
+    // Disable page snapping, and try moving halfway. Confirm it doesn't snap.
+    await tester.pumpWidget(build(pageSnapping: false));
+    gesture = await tester.startGesture(const Offset(100.0, 100.0));
+    // Move just beyond halfway, again.
+    await gesture.moveBy(const Offset(-420.0, 0.0));
+
+    // Page notifications still get sent.
+    expect(log, equals(const <int>[2]));
+    log.clear();
+
+    // Release the gesture, confirm that both pages are visible.
+    await gesture.up();
+    await tester.pumpAndSettle();
+
+    expect(find.text('Alabama'), findsNothing);
+    expect(find.text('Alaska'), findsOneWidget);
+    expect(find.text('Arizona'), findsOneWidget);
+    expect(find.text('Arkansas'), findsNothing);
+
+    // Now re-enable snapping, confirm that we've settled on a page.
+    await tester.pumpWidget(build(pageSnapping: true));
+    await tester.pumpAndSettle();
+
+    expect(log, isEmpty);
+
+    expect(find.text('Alaska'), findsNothing);
+    expect(find.text('Arizona'), findsOneWidget);
+    expect(find.text('Arkansas'), findsNothing);
+  });
+
   testWidgets('PageView small viewportFraction', (WidgetTester tester) async {
     final PageController controller = new PageController(viewportFraction: 1/8);