Add a pageChanged callback to PageableList

Also includes changes to widget_tester sufficient to
test scrolling.

@abarth

packages/flutter/lib/widgets/scrollable.dart

@@ -542,6 +542,8 @@ class ScrollableList<T> extends ScrollableWidgetList {
   }
 }
 
+typedef void PageChangedCallback(int newPage);
+
 class PageableList<T> extends ScrollableList<T> {
   PageableList({
     Key key,
@@ -551,6 +553,7 @@ class PageableList<T> extends ScrollableList<T> {
     ItemBuilder<T> itemBuilder,
     bool itemsWrap: false,
     double itemExtent,
+    PageChangedCallback this.pageChanged,
     EdgeDims padding,
     this.duration: const Duration(milliseconds: 200),
     this.curve: ease
@@ -567,10 +570,12 @@ class PageableList<T> extends ScrollableList<T> {
 
   Duration duration;
   Curve curve;
+  PageChangedCallback pageChanged;
 
   void syncConstructorArguments(PageableList<T> source) {
     duration = source.duration;
     curve = source.curve;
+    pageChanged = source.pageChanged;
     super.syncConstructorArguments(source);
   }
 
@@ -592,12 +597,19 @@ class PageableList<T> extends ScrollableList<T> {
     double newScrollOffset = _snapScrollOffset(scrollOffset + velocity.sign * itemExtent)
       .clamp(_snapScrollOffset(scrollOffset - itemExtent / 2.0),
              _snapScrollOffset(scrollOffset + itemExtent / 2.0));
-    scrollTo(newScrollOffset, duration: duration, curve: curve);
+    scrollTo(newScrollOffset, duration: duration, curve: curve).then(_notifyPageChanged);
     return EventDisposition.processed;
   }
 
+  int get currentPage => (scrollOffset / itemExtent).floor();
+
+  void _notifyPageChanged(_) {
+    if (pageChanged != null)
+      pageChanged(currentPage);
+  }
+
   void settleScrollOffset() {
-    scrollTo(_snapScrollOffset(scrollOffset), duration: duration, curve: curve);
+    scrollTo(_snapScrollOffset(scrollOffset), duration: duration, curve: curve).then(_notifyPageChanged);
   }
 }
 

packages/unit/pubspec.yaml

@@ -3,6 +3,7 @@ dependencies:
   sky: any
   sky_tools: any
   test: any
+  quiver: any
 dependency_overrides:
   material_design_icons:
     path: ../packages/material_design_icons

packages/unit/test/widget/pageable_list_test.dart

+import 'package:sky/widgets.dart';
+import 'package:test/test.dart';
+import 'package:quiver/testing/async.dart';
+
+import 'widget_tester.dart';
+
+void main() {
+  test('Scrolling changes page', () {
+    WidgetTester tester = new WidgetTester();
+
+    List<int> pages = [0, 1, 2, 3, 4, 5];
+    Size pageSize = new Size(200.0, 200.0);
+    int currentPage;
+
+    Widget buildPage(int page) {
+      return new Container(
+        key: new StringKey(page.toString()),
+        width: pageSize.width,
+        height: pageSize.height,
+        child: new Text(page.toString())
+      );
+    }
+
+    Widget builder() {
+      return new Container(
+        height: pageSize.height,
+        child: new PageableList<int>(
+          padding: new EdgeDims.symmetric(horizontal: 10.0),
+          items: pages,
+          itemBuilder: buildPage,
+          scrollDirection: ScrollDirection.horizontal,
+          itemExtent: pageSize.width,
+          pageChanged: (int page) {
+            currentPage = page;
+          }
+        )
+      );
+    }
+
+    tester.pumpFrame(builder);
+    // TODO(abarth): We shouldn't need to pump a second frame here.
+    tester.pumpFrame(builder);
+
+    expect(currentPage, isNull);
+    new FakeAsync().run((async) {
+      tester.scroll(tester.findText('1'), new Offset(300.0, 0.0));
+      // One frame to start the animation, a second to complete it.
+      tester.pumpFrame(builder);
+      tester.pumpFrame(builder, 5000.0);
+      async.elapse(new Duration(seconds: 5));
+      expect(currentPage, equals(2));
+    });
+  });
+}

packages/unit/test/widget/widget_tester.dart

 import 'dart:sky' as sky;
 import 'package:sky/rendering.dart';
 import 'package:sky/widgets.dart';
+import 'package:sky/base/scheduler.dart' as scheduler;
 
 typedef Widget WidgetBuilder();
 
@@ -21,6 +22,63 @@ class TestApp extends App {
   }
 }
 
+class TestPointerEvent extends sky.PointerEvent {
+  TestPointerEvent({
+    this.type,
+    this.pointer,
+    this.kind,
+    this.x,
+    this.y,
+    this.dx,
+    this.dy,
+    this.velocityX,
+    this.velocityY,
+    this.buttons,
+    this.down,
+    this.primary,
+    this.obscured,
+    this.pressure,
+    this.pressureMin,
+    this.pressureMax,
+    this.distance,
+    this.distanceMin,
+    this.distanceMax,
+    this.radiusMajor,
+    this.radiusMinor,
+    this.radiusMin,
+    this.radiusMax,
+    this.orientation,
+    this.tilt
+  });
+
+  // These are all of the PointerEvent members, but not all of Event.
+  String type;
+  int pointer;
+  String kind;
+  double x;
+  double y;
+  double dx;
+  double dy;
+  double velocityX;
+  double velocityY;
+  int buttons;
+  bool down;
+  bool primary;
+  bool obscured;
+  double pressure;
+  double pressureMin;
+  double pressureMax;
+  double distance;
+  double distanceMin;
+  double distanceMax;
+  double radiusMajor;
+  double radiusMinor;
+  double radiusMin;
+  double radiusMax;
+  double orientation;
+  double tilt;
+}
+
 class TestGestureEvent extends sky.GestureEvent {
   TestGestureEvent({
     this.type,
@@ -33,6 +91,7 @@ class TestGestureEvent extends sky.GestureEvent {
     this.velocityY
   });
 
+  // These are all of the GestureEvent members, but not all of Event.
   String type;
   int primaryPointer;
   double x;
@@ -105,14 +164,25 @@ class WidgetTester {
     dispatchEvent(new TestGestureEvent(type: 'gesturetap'), getCenter(widget));
   }
 
+  void scroll(Widget widget, Offset offset) {
+    dispatchEvent(new TestGestureEvent(type: 'gesturescrollstart'), getCenter(widget));
+    dispatchEvent(new TestGestureEvent(
+      type: 'gesturescrollupdate',
+      dx: offset.dx,
+      dy: offset.dy), getCenter(widget));
+    // pointerup to trigger scroll settling in Scrollable<T>
+    dispatchEvent(new TestPointerEvent(
+      type: 'pointerup', down: false, primary: true), getCenter(widget));
+  }
+
   void dispatchEvent(sky.Event event, Point position) {
     HitTestResult result = SkyBinding.instance.hitTest(position);
     SkyBinding.instance.dispatchEvent(event, result);
   }
 
-  void pumpFrame(WidgetBuilder builder) {
+  void pumpFrame(WidgetBuilder builder, [double frameTimeMs = 0.0]) {
     _app.builder = builder;
-    SkyBinding.instance.beginFrame(0.0);
+    scheduler.beginFrame(frameTimeMs);
   }
 
 }