couple view and controller + offset working

lib/core/controller.dart

@@ -0,0 +1,78 @@
+import 'package:flutter/material.dart';
+import 'package:infinite_lazy_2d_grid/utils/conversions.dart';
+
+// ------------------------------ Public Types -------------------------------
+
+typedef WidgetBuilder = Widget Function();
+
+// ------------------------------ Private Types ------------------------------
+
+// some simple exceptions
+// ignore: non_constant_identifier_names
+final _ChildNotFoundException = Exception('Child with the given ID does not exist');
+
+class _CanvasItem {
+  Offset gsPosition;
+  final WidgetBuilder builder;
+
+  _CanvasItem({required this.gsPosition, required this.builder});
+}
+
+// ------------------------------ Controller ------------------------------
+
+/// Every canvas view needs one and this handles the positioning logic
+class CanvasController with ChangeNotifier {
+  int _nextId = 0; // surely we won't run out of IDs, Clueless
+  Offset _gsTopLeftOffset = Offset.zero;
+  final Map<int, _CanvasItem> _children = {}; // int for IDs
+
+  // -------------------- public functions --------------------
+
+  // -------- child manip --------
+
+  int addChild(Offset position, WidgetBuilder builder) {
+    _children[_nextId] = _CanvasItem(gsPosition: position, builder: builder);
+    return _nextId++;
+  }
+
+  void removeChild(int id) {
+    _children.remove(id);
+  }
+
+  int updatePosition(int id, Offset newPosition) {
+    if (_children.containsKey(id)) {
+      _children[id]!.gsPosition = newPosition;
+      return id;
+    } else {
+      throw _ChildNotFoundException;
+    }
+  }
+
+  Offset getPosition(int id) {
+    if (_children.containsKey(id)) {
+      return _children[id]!.gsPosition;
+    } else {
+      throw _ChildNotFoundException;
+    }
+  }
+
+  // -------- gesture handling --------
+  void handleGesture(ScaleUpdateDetails details) {
+    // handle gestures like pinch to zoom, pan, etc.
+
+    // convention is that if I drag from right to left, dx is negative
+    // for top to bottom, dy is postive ( so usual display conventions and final vector postion - initial vector position )
+    final delta = details.focalPointDelta;
+
+    _gsTopLeftOffset -= delta;
+
+    notifyListeners();
+  }
+
+  List<(Offset, Widget)> widgetsWithScreenPositions() {
+    return _children.values.map((item) {
+      final ssPosition = gsToSs(item.gsPosition, _gsTopLeftOffset);
+      return (ssPosition, item.builder());
+    }).toList();
+  }
+}