support for lazy building widgets within viewport

lib/core/spatial_hashing.dart

@@ -0,0 +1,74 @@
+import 'dart:collection';
+import 'dart:math';
+
+import 'package:flutter/material.dart';
+
+typedef _CellKey = Point<int>;
+
+class PointData<T> {
+  final Point point;
+  final T data;
+
+  PointData(this.point, this.data);
+}
+
+class SpatialHashing<T> {
+  final Size cellSize;
+  final HashMap<Point, T> _pointData = HashMap<Point, T>();
+  final HashMap<_CellKey, List<Point>> _cellMap = HashMap<_CellKey, List<Point>>();
+
+  SpatialHashing({required this.cellSize});
+
+  _CellKey _cellKey(Point point) {
+    int x = (point.x / cellSize.width).floor();
+    int y = (point.y / cellSize.height).floor();
+    return Point(x, y);
+  }
+
+  void add(Point point, T data) {
+    _CellKey cellKey = _cellKey(point);
+    _pointData[point] = data;
+
+    if (!_cellMap.containsKey(cellKey)) {
+      _cellMap[cellKey] = [];
+    }
+    _cellMap[cellKey]!.add(point);
+  }
+
+  void remove(Point point) {
+    final key = _cellKey(point);
+    if (_pointData.containsKey(point)) {
+      _pointData.remove(point); // remove the data
+
+      // cell map must have key then
+      _cellMap[key]!.remove(point); // remove from spatial hash
+      if (_cellMap[key]!.isEmpty) {
+        _cellMap.remove(key);
+      }
+    }
+  }
+
+  List<PointData<T>> getPointsAround(Point point, Offset offset) {
+    _CellKey cellKey = _cellKey(point);
+    List<PointData<T>> results = [];
+
+    // Calculate the range of cells to check
+    int startX = cellKey.x - (offset.dx / cellSize.width).floor();
+    int startY = cellKey.y - (offset.dy / cellSize.height).floor();
+    int endX = cellKey.x + (offset.dx / cellSize.width).ceil();
+    int endY = cellKey.y + (offset.dy / cellSize.height).ceil();
+
+    for (int x = startX; x <= endX; x++) {
+      for (int y = startY; y <= endY; y++) {
+        Point currentCellKey = Point(x, y);
+        if (_cellMap.containsKey(currentCellKey)) {
+          for (Point p in _cellMap[currentCellKey]!) {
+            results.add(PointData(p, _pointData[p] as T));
+          }
+        }
+      }
+    }
+
+    return results;
+  }
+}