Report zero velocity at the end of a panning simulation (Resolves #43) (#44)

Author: matthew-carroll

.github/workflows/pr_validation.yaml

@@ -0,0 +1,24 @@
+name: Test project in a PR
+on: [pull_request]
+jobs:
+  test_linux:
+    runs-on: ubuntu-latest
+    defaults:
+      run:
+        working-directory: .
+    steps:
+      # Checkout the PR branch
+      - uses: actions/checkout@v3
+
+      # Setup Flutter environment
+      - uses: subosito/flutter-action@v2
+        with:
+          channel: "stable"
+
+      # Download all the packages that the app uses
+      - run: flutter pub get
+
+      # TODO: run static analysis here when we get to zero analysis warnings
+
+      # Run all tests
+      - run: flutter test

.run/Minimal Viewport (debug).run.xml

@@ -0,0 +1,6 @@
+<component name="ProjectRunConfigurationManager">
+  <configuration default="false" name="Minimal Viewport (debug)" type="FlutterRunConfigurationType" factoryName="Flutter">
+    <option name="filePath" value="$PROJECT_DIR$/example/lib/main_minimal_viewport.dart" />
+    <method v="2" />
+  </configuration>
+</component>

example/lib/main_minimal_viewport.dart

@@ -0,0 +1,55 @@
+import 'package:flutter/material.dart';
+import 'package:page_list_viewport/page_list_viewport.dart';
+
+void main() {
+  runApp(
+    MaterialApp(
+      home: _MinimalViewportDemo(),
+    ),
+  );
+}
+
+class _MinimalViewportDemo extends StatefulWidget {
+  const _MinimalViewportDemo({Key? key}) : super(key: key);
+
+  @override
+  State<_MinimalViewportDemo> createState() => _MinimalViewportDemoState();
+}
+
+class _MinimalViewportDemoState extends State<_MinimalViewportDemo> with TickerProviderStateMixin {
+  late final PageListViewportController _controller;
+
+  @override
+  void initState() {
+    super.initState();
+    _controller = PageListViewportController(vsync: this);
+  }
+
+  @override
+  void dispose() {
+    _controller.dispose();
+    super.dispose();
+  }
+
+  @override
+  Widget build(BuildContext context) {
+    return Scaffold(
+      body: PageListViewportGestures(
+        controller: _controller,
+        child: PageListViewport(
+          controller: _controller,
+          pageCount: 10,
+          naturalPageSize: const Size(8.5, 11) * 72,
+          builder: (context, index) {
+            return DecoratedBox(
+              decoration: BoxDecoration(
+                color: Colors.white,
+                border: Border.all(color: Colors.grey),
+              ),
+            );
+          },
+        ),
+      ),
+    );
+  }
+}

lib/src/page_list_viewport.dart

@@ -632,6 +632,11 @@ class PageListViewportController extends OrientationController {
     // Check if the simulation is close enough to complete for us to stop it.
     if ((_origin - _previousOrigin).distance.abs() < 0.01) {
       stopSimulation();
+
+      // Ensure that we always report a zero velocity at the end of the simulation.
+      _acceleration = Offset.zero;
+      _velocity = Offset.zero;
+      _scaleVelocity = 0;
     }
 
     // Update our previous-frame accounting, for the next simulation frame.

test/panning_simulation_test.dart

@@ -0,0 +1,61 @@
+import 'package:flutter/material.dart';
+import 'package:flutter_test/flutter_test.dart';
+import 'package:page_list_viewport/page_list_viewport.dart';
+
+void main() {
+  group("Panning simulation", () {
+    testWidgets("reports zero velocity when it completes", (widgetTester) async {
+      final controller = PageListViewportController(vsync: widgetTester);
+      await _pumpPageListViewport(widgetTester, controller: controller);
+
+      Offset? latestVelocity;
+      controller.addListener(() {
+        latestVelocity = controller.velocity;
+      });
+
+      // Fling up, to scroll down, and run a panning simulation.
+      await widgetTester.fling(find.byType(Scaffold), const Offset(0, -500), 4000);
+      await widgetTester.pumpAndSettle();
+
+      // Ensure that the final reported velocity is zero.
+      expect(latestVelocity, isNotNull);
+      expect(latestVelocity, Offset.zero);
+    });
+  });
+}
+
+Future<void> _pumpPageListViewport(
+  WidgetTester tester, {
+  PageListViewportController? controller,
+  int pageCount = 10,
+  Size? naturalPageSize,
+  PageBuilder? pageBuilder,
+}) async {
+  controller ??= PageListViewportController(vsync: tester);
+  naturalPageSize ??= const Size(8.5, 11) * 72;
+  pageBuilder ??= _defaultPageBuilder;
+
+  await tester.pumpWidget(
+    MaterialApp(
+      home: Scaffold(
+        body: PageListViewportGestures(
+          controller: controller,
+          child: PageListViewport(
+            controller: controller,
+            pageCount: pageCount,
+            naturalPageSize: naturalPageSize,
+            builder: pageBuilder,
+          ),
+        ),
+      ),
+    ),
+  );
+
+  await tester.pumpAndSettle();
+}
+
+Widget _defaultPageBuilder(BuildContext context, int pageIndex) {
+  return const ColoredBox(
+    color: Colors.white,
+  );
+}