Pan and zoom gallery demo (flutter#25164)

Adds the "2D Transformations" demo to the gallery, which shows how to do things such as navigate around a map a la Google Maps, or show a full screen zoomable photo.  The idea is to abstract this code into a first class widget soon.

examples/flutter_gallery/lib/demo/all.dart

@@ -12,5 +12,6 @@ export 'images_demo.dart';
 export 'material/material.dart';
 export 'pesto_demo.dart';
 export 'shrine_demo.dart';
+export 'transformations/transformations_demo.dart';
 export 'typography_demo.dart';
 export 'video_demo.dart';

examples/flutter_gallery/lib/demo/transformations/transformations_demo.dart

@@ -0,0 +1,151 @@
+import 'dart:ui' show Vertices;
+import 'package:flutter/material.dart';
+import 'transformations_demo_board.dart';
+import 'transformations_demo_edit_board_point.dart';
+import 'transformations_demo_gesture_transformable.dart';
+
+class TransformationsDemo extends StatefulWidget {
+  const TransformationsDemo({ Key key }) : super(key: key);
+
+  static const String routeName = '/transformations';
+
+  @override _TransformationsDemoState createState() => _TransformationsDemoState();
+}
+class _TransformationsDemoState extends State<TransformationsDemo> {
+  // The radius of a hexagon tile in pixels.
+  static const double _kHexagonRadius = 32.0;
+  // The margin between hexagons.
+  static const double _kHexagonMargin = 1.0;
+  // The radius of the entire board in hexagons, not including the center.
+  static const int _kBoardRadius = 8;
+
+  bool _reset = false;
+  Board _board = Board(
+    boardRadius: _kBoardRadius,
+    hexagonRadius: _kHexagonRadius,
+    hexagonMargin: _kHexagonMargin,
+  );
+
+  @override
+  Widget build (BuildContext context) {
+    final BoardPainter painter = BoardPainter(
+      board: _board,
+    );
+
+    // The scene is drawn by a CustomPaint, but user interaction is handled by
+    // the GestureTransformable parent widget.
+    return Scaffold(
+      appBar: AppBar(),
+      body: LayoutBuilder(
+        builder: (BuildContext context, BoxConstraints constraints) {
+          // Draw the scene as big as is available, but allow the user to
+          // translate beyond that to a visibleSize that's a bit bigger.
+          final Size size = Size(constraints.maxWidth, constraints.maxHeight);
+          final Size visibleSize = Size(size.width * 3, size.height * 2);
+          return GestureTransformable(
+            reset: _reset,
+            onResetEnd: () {
+              setState(() {
+                _reset = false;
+              });
+            },
+            child: CustomPaint(
+              painter: painter,
+            ),
+            boundaryRect: Rect.fromLTWH(
+              -visibleSize.width / 2,
+              -visibleSize.height / 2,
+              visibleSize.width,
+              visibleSize.height,
+            ),
+            // Center the board in the middle of the screen. It's drawn centered
+            // at the origin, which is the top left corner of the
+            // GestureTransformable.
+            initialTranslation: Offset(size.width / 2, size.height / 2),
+            onTapUp: _onTapUp,
+            size: size,
+          );
+        },
+      ),
+      floatingActionButton: _board.selected == null ? resetButton : editButton,
+    );
+  }
+
+  FloatingActionButton get resetButton {
+    return FloatingActionButton(
+      onPressed: () {
+        setState(() {
+          _reset = true;
+        });
+      },
+      tooltip: 'Reset Transform',
+      backgroundColor: Theme.of(context).primaryColor,
+      child: const Icon(Icons.home),
+    );
+  }
+
+  FloatingActionButton get editButton {
+    return FloatingActionButton(
+      onPressed: () {
+        if (_board.selected == null) {
+          return;
+        }
+        showModalBottomSheet<Widget>(context: context, builder: (BuildContext context) {
+          return Container(
+            width: double.infinity,
+            height: 150,
+            padding: const EdgeInsets.all(12.0),
+            child: EditBoardPoint(
+              boardPoint: _board.selected,
+              onColorSelection: (Color color) {
+                setState(() {
+                  _board = _board.copyWithBoardPointColor(_board.selected, color);
+                  Navigator.pop(context);
+                });
+              },
+            ),
+          );
+        });
+      },
+      tooltip: 'Edit Tile',
+      child: const Icon(Icons.edit),
+    );
+  }
+
+  void _onTapUp(TapUpDetails details) {
+    final Offset scenePoint = details.globalPosition;
+    final BoardPoint boardPoint = _board.pointToBoardPoint(scenePoint);
+    setState(() {
+      _board = _board.copyWithSelected(boardPoint);
+    });
+  }
+}
+
+// CustomPainter is what is passed to CustomPaint and actually draws the scene
+// when its `paint` method is called.
+class BoardPainter extends CustomPainter {
+  const BoardPainter({
+    this.board,
+  });
+
+  final Board board;
+
+  @override
+  void paint(Canvas canvas, Size size) {
+    void drawBoardPoint(BoardPoint boardPoint) {
+      final Color color = boardPoint.color.withOpacity(
+        board.selected == boardPoint ? 0.2 : 1.0,
+      );
+      final Vertices vertices = board.getVerticesForBoardPoint(boardPoint, color);
+      canvas.drawVertices(vertices, BlendMode.color, Paint());
+    }
+
+    board.forEach(drawBoardPoint);
+  }
+
+  // We should repaint whenever the board changes, such as board.selected.
+  @override
+  bool shouldRepaint(BoardPainter oldDelegate) {
+    return oldDelegate.board != board;
+  }
+}