add centering and focus capabilities

example/lib/app.dart

@@ -11,15 +11,21 @@ class App extends StatefulWidget {
 }
 
 class _AppState extends State<App> {
-  final CanvasController controller = CanvasController(debug: true, initialScale: 1);
+  final CanvasController controller = CanvasController(debug: true);
 
   @override
   void initState() {
     super.initState();
-    for (int i = 0; i < 10000; i++) {
+    for (int i = 0; i < 10; i++) {
       controller.addChild(
         Offset((i % 80) * 100.0, (i ~/ 80) * 100.0),
-        () => InfoContainer(color: Colors.primaries[i % Colors.primaries.length]),
+        (_) => GestureDetector(
+          onTap: () {
+            ScaffoldMessenger.of(context).showSnackBar(SnackBar(content: Text('Tapped a container')));
+            controller.focusOnChild(context, i);
+          },
+          child: InfoContainer(color: Colors.primaries[i % Colors.primaries.length]),
+        ),
       );
     }
   }
@@ -61,11 +67,6 @@ class InfoContainer extends StatelessWidget {
 
   @override
   Widget build(BuildContext context) {
-    return GestureDetector(
-      onTap: () {
-        ScaffoldMessenger.of(context).showSnackBar(SnackBar(content: Text('Tapped a container')));
-      },
-      child: Container(color: color, width: 50, height: 50),
-    );
+    return Container(color: color, width: 50, height: 50);
   }
 }

lib/core/controller/controller.dart

@@ -1,6 +1,10 @@
+import 'dart:collection';
+
 import 'package:flutter/material.dart';
-import 'package:infinite_lazy_2d_grid/core/spatial_hashing.dart';
-import 'package:infinite_lazy_2d_grid/utils/offset_extensions.dart';
+import 'package:infinite_lazy_2d_grid/utils/measure_size.dart';
+import '../spatial_hashing.dart';
+import '../../utils/offset_extensions.dart';
+import '../../utils/size_extensions.dart';
 import '../../utils/conversions.dart';
 import '../../utils/styles.dart';
 
@@ -11,29 +15,26 @@ part 'types.dart';
 class CanvasController with ChangeNotifier {
   int _nextId = 0; // surely we won't run out of IDs, Clueless
   Offset _gsTopLeftOffset = Offset.zero;
-  double _baseScale, _scale;
+  Offset? _lastProcessedOffset;
+  double _baseScale = 1, _scale = 1;
+  double? _lastProcessedScale;
   late Size _canvasSize;
-  final Map<int, _ChildInfo> _children = {}; // int for IDs
+  final HashMap<int, _ChildInfo> _children = HashMap<int, _ChildInfo>(); // int for IDs
   final Offset? _buildCacheExtent;
   late Offset _buildExtent;
   final Size _hashCellSize;
   bool _init = false;
   late final SpatialHashing<int> _spatialHash;
+  List<ChildInfo> _renderedChildren = [];
 
   bool debug;
 
-  CanvasController({
-    double initialScale = 1,
-    this.debug = false,
-    Offset? buildCacheExtent,
-    Size hashCellSize = const Size(100, 100),
-  }) : _scale = initialScale,
-       _baseScale = initialScale,
-       _hashCellSize = hashCellSize,
-       _buildCacheExtent = buildCacheExtent != null ? buildCacheExtent + Offset(50, 50) : null,
-       // only top left is considered to if a widget has long width, it'll not be rendered
-       // unless the cache extent is sufficient
-       assert(initialScale > 0, 'Initial scale must be greater than 0') {
+  CanvasController({this.debug = false, Offset? buildCacheExtent, Size hashCellSize = const Size(100, 100)})
+    : _hashCellSize = hashCellSize,
+      _buildCacheExtent = buildCacheExtent != null ? buildCacheExtent + Offset(50, 50) : null
+  // only top left is considered so if a widget has long width, it'll not be rendered
+  // unless the cache extent is sufficient
+  {
     _spatialHash = SpatialHashing<int>(cellSize: _hashCellSize);
   }
 
@@ -43,6 +44,7 @@ class CanvasController with ChangeNotifier {
   Size get canvasSize => _canvasSize;
   Offset get _ssCenter => Offset(_canvasSize.width / 2, _canvasSize.height / 2);
   Offset get _gsCenter => ssToGs(_ssCenter, _gsTopLeftOffset, _scale);
+  bool get _cleanRenderState => _init && _lastProcessedOffset == _gsTopLeftOffset && _lastProcessedScale == _scale;
 
   // ==================== Public Functions ====================
 
@@ -61,6 +63,10 @@ class CanvasController with ChangeNotifier {
     }
   }
 
+  void onChildSizeChange(int id, Size size) {
+    _children[id]!.lastRenderedSize = size;
+  }
+
   // ==================== Child Management ====================
 
   /// Add a child at a given position with a builder. Returns the child ID.
@@ -140,17 +146,24 @@ class CanvasController with ChangeNotifier {
   // ==================== Positioning Logic ====================
 
   /// Currently rendered widgets with their position info
-  List<ChildInfo> widgetsWithScreenPositions() {
+  List<ChildInfo> widgetsWithScreenPositions(BuildContext context, {bool forceRebuild = false}) {
     if (!_init) return [];
 
+    if (_cleanRenderState && !forceRebuild) {
+      return _renderedChildren;
+    }
+
+    _lastProcessedOffset = _gsTopLeftOffset;
+    _lastProcessedScale = _scale;
+
     final idsToBuild = _childrenWithinBuildArea(_gsCenter, _buildExtent);
 
-    return idsToBuild.map((id) {
+    return _renderedChildren = idsToBuild.map((id) {
       final item = _children[id]!;
       final ssPosition = gsToSs(item.gsPosition, _gsTopLeftOffset, _scale);
-      var child = item.builder();
+      var child = item.builder(context);
       if (debug) child = _Debug(id: id, gs: item.gsPosition, ss: ssPosition, child: child);
-      return ChildInfo(gsPosition: item.gsPosition, ssPosition: ssPosition, child: child);
+      return ChildInfo(id: id, gsPosition: item.gsPosition, ssPosition: ssPosition, child: child);
     }).toList();
   }
 
@@ -159,4 +172,68 @@ class CanvasController with ChangeNotifier {
     final items = _spatialHash.getPointsAround(center.toPoint(), halfExtent);
     return items.map((item) => item.data).toList(); // data is the child id here
   }
+
+  // ==================== Centering & Focus Functions ====================
+
+  /// Center the canvas so that the given screen-space offset is at the center of the viewport.
+  void centerOnScreenOffset(Offset ssOffset) {
+    centerOnGridOffset(ssToGs(ssOffset, _gsTopLeftOffset, _scale));
+  }
+
+  /// Center the canvas so that the given grid-space offset is at the center of the viewport.
+  void centerOnGridOffset(Offset gsOffset) {
+    // if 2x scale you need to adjust lesser
+    _gsTopLeftOffset = gsOffset + (canvasSize * (2 * scale)).toOffset();
+    notifyListeners();
+  }
+
+  /// Focus the viewport on a child by its ID, with a margin in screen-space.
+  /// If it's already rendered, size will be picked up from the child widget. If not
+  /// an offstage rendering will be used ( double render )
+  /// Preferred horizontal margin used for [ScalingMode.fitInViewport].
+  void focusOnChild(
+    BuildContext context,
+    int id, {
+    ScalingMode scalingMode = ScalingMode.keepScale,
+    double preferredHorizontalMargin = 16,
+    Size? childSize,
+    forceRedraw = false,
+  }) {
+    if (!_children.containsKey(id)) {
+      throw _ChildNotFoundException;
+    }
+
+    final childInfo = _children[id]!;
+
+    // try to figure out the size, take from render cache if available
+    // else do an offstage render
+    childSize ??= childInfo.lastRenderedSize != null && !forceRedraw
+        ? childInfo.lastRenderedSize
+        : measureWidgetSize(context, childInfo.builder);
+
+    /*
+    margin is symmatric on ltrb so
+    2mx + cx = screenWidth
+    2my + cy = screenHeight
+    where c is child size in screen space and m is margin
+    */
+
+    switch (scalingMode) {
+      case ScalingMode.keepScale:
+        // do nothing
+        break;
+      case ScalingMode.resetScale:
+        _scale = 1;
+      case ScalingMode.fitInViewport:
+        // the scale needs to be determined in this case
+        // and hence a margin is needed to constrain on x, to get the scale, we then center it along y
+        _scale = (canvasSize.width - 2 * preferredHorizontalMargin) / childSize!.width;
+        break;
+    }
+
+    final marginOffset = ((canvasSize.toOffset() - (childSize! * scale).toOffset()) / (2 * scale)).makeAtleast(0);
+    _gsTopLeftOffset = childInfo.gsPosition - marginOffset;
+
+    notifyListeners();
+  }
 }

lib/core/controller/types.dart

@@ -1,7 +1,5 @@
 part of 'controller.dart';
 
-typedef WidgetBuilder = Widget Function();
-
 // ------------------------------ Private Types ------------------------------
 
 // some simple exceptions
@@ -10,14 +8,18 @@ final _ChildNotFoundException = Exception('Child with the given ID does not exis
 
 class _ChildInfo {
   Offset gsPosition;
+  Size? lastRenderedSize;
   final WidgetBuilder builder;
 
   _ChildInfo({required this.gsPosition, required this.builder});
 }
 
 class ChildInfo {
+  int id;
   Offset gsPosition;
   Offset ssPosition;
   Widget child;
-  ChildInfo({required this.gsPosition, required this.ssPosition, required this.child});
+  ChildInfo({required this.id, required this.gsPosition, required this.ssPosition, required this.child});
 }
+
+enum ScalingMode { resetScale, keepScale, fitInViewport }

lib/core/render.dart

@@ -22,14 +22,17 @@ class CanvasView extends StatelessWidget {
       child: ListenableBuilder(
         listenable: controller,
         builder: (_, _) {
-          final childrenWithPositions = controller.widgetsWithScreenPositions();
+          final childrenWithPositions = controller.widgetsWithScreenPositions(context);
           final ssPositions = childrenWithPositions.map((e) => e.ssPosition).toList();
+          final childrenIds = childrenWithPositions.map((e) => e.id).toList();
           final children = childrenWithPositions.map((e) => e.child).toList();
           final canvas = _CanvasRenderObject(
+            childrenIds: childrenIds,
             canvasBackground: canvasBackground,
             ssPositions: ssPositions,
             scale: controller.scale,
             onCanvasSizeChange: controller.onCanvasSizeChange,
+            onChildSizeChange: controller.onChildSizeChange,
             children: children,
           );
 
@@ -59,16 +62,20 @@ class CanvasView extends StatelessWidget {
 /// A combined widget for all the render object of the children + background.
 /// Everything is in screen space here
 class _CanvasRenderObject extends MultiChildRenderObjectWidget {
+  final List<int> childrenIds;
   final List<Offset> ssPositions;
   final double scale;
   final CanvasBackground canvasBackground;
   final Function onCanvasSizeChange;
+  final Function onChildSizeChange;
 
   const _CanvasRenderObject({
+    required this.childrenIds,
     required this.ssPositions,
     required this.scale,
     required this.canvasBackground,
     required this.onCanvasSizeChange,
+    required this.onChildSizeChange,
     required super.children, // children go to the MultiChildRenderObjectWidget
   }) : assert(ssPositions.length == children.length, 'Children and positions must have the same length'),
        assert(scale != 0);
@@ -76,16 +83,19 @@ class _CanvasRenderObject extends MultiChildRenderObjectWidget {
   @override
   RenderObject createRenderObject(BuildContext context) {
     return _CanvasRenderBox(
+      childrenIds: childrenIds,
       ssPositions: ssPositions,
       scale: scale,
       canvasBackground: canvasBackground,
       onCanvasSizeChange: onCanvasSizeChange,
+      onChildSizeChange: onChildSizeChange,
     );
   }
 
   @override
   void updateRenderObject(BuildContext context, _CanvasRenderBox renderObject) {
     renderObject
+      ..childrenIds = childrenIds
       ..ssPositions = ssPositions
       ..canvasBackground = canvasBackground
       ..scale = scale;
@@ -94,6 +104,7 @@ class _CanvasRenderObject extends MultiChildRenderObjectWidget {
 
 class _CanvasWidgetParentData extends ContainerBoxParentData<RenderBox> {
   // there is already an "offset" defined in BoxParentdata that is exactly what I want
+  late int id;
   late double scale; // scale is same for all rn but this makes it trivial to expand to children with diff scales
 }
 
@@ -102,14 +113,24 @@ class _CanvasRenderBox extends RenderBox
         ContainerRenderObjectMixin<RenderBox, _CanvasWidgetParentData>,
         RenderBoxContainerDefaultsMixin<RenderBox, _CanvasWidgetParentData> {
   CanvasBackground _canvasBackground;
+  List<int> _childrenIds;
   List<Offset> _ssPositions;
   double _scale;
   Function onCanvasSizeChange;
+  Function onChildSizeChange;
 
-  _CanvasRenderBox({required ssPositions, required scale, required canvasBackground, required this.onCanvasSizeChange})
-    : _ssPositions = ssPositions,
-      _scale = scale,
-      _canvasBackground = canvasBackground;
+  _CanvasRenderBox({
+    required childrenIds,
+    required ssPositions,
+    required scale,
+    required canvasBackground,
+    required this.onCanvasSizeChange,
+    required this.onChildSizeChange,
+  }) : assert(childrenIds.length == ssPositions.length),
+       _childrenIds = childrenIds,
+       _ssPositions = ssPositions,
+       _scale = scale,
+       _canvasBackground = canvasBackground;
 
   @override
   void setupParentData(RenderBox child) {
@@ -128,6 +149,12 @@ class _CanvasRenderBox extends RenderBox
     }
   }
 
+  set childrenIds(List<int> childrenIds) {
+    if (_childrenIds != childrenIds) {
+      _childrenIds = childrenIds;
+    }
+  }
+
   set scale(double scale) {
     if (_scale != scale) {
       _scale = scale;
@@ -153,11 +180,16 @@ class _CanvasRenderBox extends RenderBox
     int index = 0;
     while (child != null) {
       final _CanvasWidgetParentData childParentData = child.parentData! as _CanvasWidgetParentData;
-      childParentData.offset = _ssPositions[index++];
+      childParentData.offset = _ssPositions[index];
+      childParentData.id = _childrenIds[index];
       childParentData.scale = _scale;
+      index++;
       // loosen so like a stack can take it's own size inside parent
-      // like a stack parent is not using size and is always expanded hence the second arg
-      child.layout(constraints.loosen(), parentUsesSize: false);
+      child.layout(constraints.loosen(), parentUsesSize: true);
+
+      // notify the controller about the size of the child
+      onChildSizeChange(childParentData.id, child.size);
+
       child = childParentData.nextSibling;
     }
   }

lib/utils/measure_size.dart

@@ -0,0 +1,28 @@
+import 'package:flutter/material.dart';
+
+/// Utility to synchronously measure the size of a widget by building it offscreen in an Overlay.
+/// Only works for widgets with deterministic, constraint-based sizing (no async/layout dependencies).
+Size measureWidgetSize(BuildContext context, WidgetBuilder builder, {BoxConstraints? constraints}) {
+  final key = GlobalKey();
+  final overlay = Overlay.of(context);
+  Size? measuredSize;
+
+  final widget = Offstage(
+    child: ConstrainedBox(
+      constraints: constraints ?? const BoxConstraints(),
+      child: Container(key: key, child: builder(context)),
+    ),
+  );
+
+  final entry = OverlayEntry(builder: (_) => widget);
+  overlay.insert(entry);
+
+  WidgetsBinding.instance.handleDrawFrame();
+
+  final ctx = key.currentContext;
+  if (ctx != null) {
+    measuredSize = (ctx.findRenderObject() as RenderBox).size;
+  }
+  entry.remove();
+  return measuredSize ?? Size.zero;
+}

lib/utils/offset_extensions.dart

@@ -8,10 +8,29 @@ extension OffsetCartesionOps on Offset {
     return dx * dx + dy * dy;
   }
 
+  Offset operator +(Offset other) {
+    return Offset(dx + other.dx, dy + other.dy);
+  }
+
   Offset operator -(Offset other) {
     return Offset(dx - other.dx, dy - other.dy);
   }
 
+  Offset operator *(double scalar) {
+    return Offset(dx * scalar, dy * scalar);
+  }
+
+  Offset makeAtleast(double value) {
+    return Offset(dx < value ? value : dx, dy < value ? value : dy);
+  }
+
+  Offset operator /(double scalar) {
+    if (scalar == 0) {
+      throw ArgumentError('Division by zero is not allowed.');
+    }
+    return Offset(dx / scalar, dy / scalar);
+  }
+
   Point toPoint() {
     return Point(dx, dy);
   }

lib/utils/size_extensions.dart

@@ -0,0 +1,18 @@
+import 'package:flutter/material.dart';
+
+extension SizeOps on Size {
+  Size operator /(double scalar) {
+    if (scalar == 0) {
+      throw ArgumentError('Division by zero is not allowed.');
+    }
+    return Size(width / scalar, height / scalar);
+  }
+
+  Size operator *(double scalar) {
+    return Size(width * scalar, height * scalar);
+  }
+
+  Offset toOffset() {
+    return Offset(width, height);
+  }
+}

test/canvas_view_test.dart

@@ -15,9 +15,9 @@ class TestChild extends StatelessWidget {
 
 void main() {
   testWidgets('CanvasView renders only visible children and reduces count on zoom out', (WidgetTester tester) async {
-    final controller = CanvasController(debug: true, initialScale: 1);
+    final controller = CanvasController(debug: true);
     for (int i = 0; i < 10000; i++) {
-      controller.addChild(Offset((i % 80) * 100.0, (i ~/ 80) * 100.0), () => TestChild(index: i));
+      controller.addChild(Offset((i % 80) * 100.0, (i ~/ 80) * 100.0), (_) => TestChild(index: i));
     }
     await tester.pumpWidget(
       MaterialApp(
@@ -46,9 +46,9 @@ void main() {
   });
 
   testWidgets('CanvasView scales as expected', (WidgetTester tester) async {
-    final controller = CanvasController(debug: true, initialScale: 1);
-    controller.addChild(const Offset(0, 0), () => TestChild(index: 0));
-    controller.addChild(const Offset(100, 100), () => TestChild(index: 1));
+    final controller = CanvasController(debug: true);
+    controller.addChild(const Offset(0, 0), (_) => TestChild(index: 0));
+    controller.addChild(const Offset(100, 100), (_) => TestChild(index: 1));
     await tester.pumpWidget(
       MaterialApp(
         home: Scaffold(
@@ -57,6 +57,7 @@ void main() {
       ),
     );
     await tester.pumpAndSettle();
+    final context = tester.element(find.byType(CanvasView));
 
     // Initial size check
     final finder = find.byType(TestChild);
@@ -67,7 +68,7 @@ void main() {
     controller.onScaleUpdate(ScaleUpdateDetails(focalPoint: const Offset(0, 0), scale: 2.0));
     await tester.pumpAndSettle();
     expect(controller.scale, 2.0);
-    final ssPositions = controller.widgetsWithScreenPositions().map((e) => e.ssPosition).toList();
+    final ssPositions = controller.widgetsWithScreenPositions(context).map((e) => e.ssPosition).toList();
     expect(ssPositions, [Offset.zero, const Offset(200, 200)]);
   });
 }