Add an example for TapAndPanGestureRecognizer (#131873)

This adds an example for `TapAndPanGestureRecognizer` that demonstrates how to scale a widget using a double tap + vertical drag gesture.

https://github.com/flutter/flutter/assets/948037/4c6c5467-2157-4b6a-bc52-264a3b6303de

examples/api/lib/gestures/tap_and_drag/tap_and_drag.0.dart

@@ -0,0 +1,138 @@
+// Copyright 2014 The Flutter Authors. All rights reserved.
+// Use of this source code is governed by a BSD-style license that can be
+// found in the LICENSE file.
+
+import 'package:flutter/gestures.dart';
+import 'package:flutter/material.dart';
+
+/// Flutter code sample for [TapAndPanGestureRecognizer].
+
+void main() {
+  runApp(const TapAndDragToZoomApp());
+}
+
+class TapAndDragToZoomApp extends StatelessWidget {
+  const TapAndDragToZoomApp({super.key});
+
+  @override
+  Widget build(BuildContext context) {
+    return const MaterialApp(
+      home: Scaffold(
+        body: Center(
+          child: TapAndDragToZoomWidget(
+            child: MyBoxWidget(),
+          ),
+        ),
+      ),
+    );
+  }
+}
+
+class MyBoxWidget extends StatelessWidget {
+  const MyBoxWidget({super.key});
+
+  @override
+  Widget build(BuildContext context) {
+    return Container(
+      color: Colors.blueAccent,
+      height: 100.0,
+      width: 100.0,
+    );
+  }
+}
+
+// This widget will scale its child up when it detects a drag up, after a
+// double tap/click. It will scale the widget down when it detects a drag down,
+// after a double tap. Dragging down and then up after a double tap/click will
+// zoom the child in/out. The scale of the child will be reset when the drag ends.
+class TapAndDragToZoomWidget extends StatefulWidget {
+  const TapAndDragToZoomWidget({super.key, required this.child});
+
+  final Widget child;
+
+  @override
+  State<TapAndDragToZoomWidget> createState() => _TapAndDragToZoomWidgetState();
+}
+
+class _TapAndDragToZoomWidgetState extends State<TapAndDragToZoomWidget> {
+  final double scaleMultiplier = -0.0001;
+  double _currentScale = 1.0;
+  Offset? _previousDragPosition;
+
+  static double _keepScaleWithinBounds(double scale) {
+    const double minScale = 0.1;
+    const double maxScale = 30;
+    if (scale <= 0) {
+      return minScale;
+    }
+    if (scale >= 30) {
+      return maxScale;
+    }
+    return scale;
+  }
+
+  void _zoomLogic(Offset currentDragPosition) {
+    final double dx = (_previousDragPosition!.dx - currentDragPosition.dx).abs();
+    final double dy = (_previousDragPosition!.dy - currentDragPosition.dy).abs();
+
+    if (dx > dy) {
+      // Ignore horizontal drags.
+      _previousDragPosition = currentDragPosition;
+      return;
+    }
+
+    if (currentDragPosition.dy < _previousDragPosition!.dy) {
+      // Zoom out on drag up.
+      setState(() {
+        _currentScale += currentDragPosition.dy * scaleMultiplier;
+        _currentScale = _keepScaleWithinBounds(_currentScale);
+      });
+    } else {
+      // Zoom in on drag down.
+      setState(() {
+        _currentScale -= currentDragPosition.dy * scaleMultiplier;
+        _currentScale = _keepScaleWithinBounds(_currentScale);
+      });
+    }
+    _previousDragPosition = currentDragPosition;
+  }
+
+  @override
+  Widget build(BuildContext context) {
+    return RawGestureDetector(
+      gestures: <Type, GestureRecognizerFactory>{
+        TapAndPanGestureRecognizer: GestureRecognizerFactoryWithHandlers<TapAndPanGestureRecognizer>(
+          () => TapAndPanGestureRecognizer(),
+          (TapAndPanGestureRecognizer instance) {
+            instance
+              ..onTapDown = (TapDragDownDetails details) {
+                _previousDragPosition = details.globalPosition;
+              }
+              ..onDragStart = (TapDragStartDetails details) {
+                if (details.consecutiveTapCount == 2) {
+                  _zoomLogic(details.globalPosition);
+                }
+              }
+              ..onDragUpdate = (TapDragUpdateDetails details) {
+                if (details.consecutiveTapCount == 2) {
+                  _zoomLogic(details.globalPosition);
+                }
+              }
+              ..onDragEnd = (TapDragEndDetails details) {
+                if (details.consecutiveTapCount == 2) {
+                  setState(() {
+                    _currentScale = 1.0;
+                  });
+                  _previousDragPosition = null;
+                }
+              };
+          }
+        ),
+      },
+      child: Transform.scale(
+        scale: _currentScale,
+        child: widget.child,
+      ),
+    );
+  }
+}

examples/api/test/gestures/tap_and_drag/tap_and_drag.0_test.dart

@@ -0,0 +1,88 @@
+// Copyright 2014 The Flutter Authors. All rights reserved.
+// Use of this source code is governed by a BSD-style license that can be
+// found in the LICENSE file.
+
+import 'package:flutter/material.dart';
+import 'package:flutter_api_samples/gestures/tap_and_drag/tap_and_drag.0.dart'
+    as example;
+import 'package:flutter_test/flutter_test.dart';
+
+void main() {
+  testWidgets('Single tap + drag should not change the scale of child', (WidgetTester tester) async {
+    await tester.pumpWidget(
+      const example.TapAndDragToZoomApp(),
+    );
+
+    double getScale() {
+      final RenderBox box = tester.renderObject(find.byType(Container).first);
+      return box.getTransformTo(null)[0];
+    }
+
+    final Finder containerFinder = find.byType(Container).first;
+    final Offset centerOfChild = tester.getCenter(containerFinder);
+
+    expect(getScale(), 1.0);
+
+    // Single tap + drag down.
+    final TestGesture gesture = await tester.startGesture(centerOfChild);
+    await tester.pump();
+    await gesture.moveTo(centerOfChild + const Offset(0, 100.0));
+    await tester.pump();
+    expect(getScale(), 1.0);
+
+    // Single tap + drag up.
+    await gesture.moveTo(centerOfChild);
+    await tester.pump();
+    expect(getScale(), 1.0);
+  });
+
+  testWidgets('Double tap + drag should change the scale of the child', (WidgetTester tester) async {
+    await tester.pumpWidget(
+      const example.TapAndDragToZoomApp(),
+    );
+
+    double getScale() {
+      final RenderBox box = tester.renderObject(find.byType(Container).first);
+      return box.getTransformTo(null)[0];
+    }
+
+    final Finder containerFinder = find.byType(Container).first;
+    final Offset centerOfChild = tester.getCenter(containerFinder);
+
+    expect(getScale(), 1.0);
+
+    // Double tap + drag down to scale up.
+    final TestGesture gesture = await tester.startGesture(centerOfChild);
+    await tester.pump();
+    await gesture.up();
+    await tester.pump();
+
+    await gesture.down(centerOfChild);
+    await tester.pump();
+    await gesture.moveTo(centerOfChild + const Offset(0, 100.0));
+    await tester.pump();
+    expect(getScale(), greaterThan(1.0));
+
+    // Scale is reset on drag end.
+    await gesture.up();
+    await tester.pumpAndSettle();
+    expect(getScale(), 1.0);
+
+    // Double tap + drag up to scale down.
+    await gesture.down(centerOfChild);
+    await tester.pump();
+    await gesture.up();
+    await tester.pump();
+
+    await gesture.down(centerOfChild);
+    await tester.pump();
+    await gesture.moveTo(centerOfChild + const Offset(0, -100.0));
+    await tester.pump();
+    expect(getScale(), lessThan(1.0));
+
+    // Scale is reset on drag end.
+    await gesture.up();
+    await tester.pumpAndSettle();
+    expect(getScale(), 1.0);
+  });
+}

packages/flutter/lib/src/gestures/tap_and_drag.dart

@@ -684,6 +684,13 @@ mixin _TapStatusTrackerMixin on OneSequenceGestureRecognizer {
 /// pointer does travel enough distance then the recognizer that entered the arena
 /// first will win. The gesture detected in this case is a drag.
 ///
+/// {@tool dartpad}
+/// This example shows how to use the [TapAndPanGestureRecognizer] along with a
+/// [RawGestureDetector] to scale a Widget.
+///
+/// ** See code in examples/api/lib/gestures/tap_and_drag/tap_and_drag.0.dart **
+/// {@end-tool}
+///
 /// {@tool snippet}
 ///
 /// This example shows how to hook up [TapAndPanGestureRecognizer]s' to nested