couple view and controller + offset working

example/lib/app.dart

@@ -1,4 +1,5 @@
 import 'package:flutter/material.dart';
+import 'package:infinite_lazy_2d_grid/core/controller.dart';
 import 'package:infinite_lazy_2d_grid/infinite_lazy_2d_grid.dart';
 
 class App extends StatefulWidget {
@@ -9,20 +10,21 @@ class App extends StatefulWidget {
 }
 
 class _AppState extends State<App> {
+  final CanvasController controller = CanvasController();
+
+  @override
+  void initState() {
+    super.initState();
+
+    controller.addChild(const Offset(100, 100), () => const InfoContainer(color: Colors.red));
+    controller.addChild(const Offset(200, 200), () => const InfoContainer(color: Colors.green));
+    controller.addChild(const Offset(300, 300), () => const InfoContainer(color: Colors.blue));
+  }
+
   @override
   Widget build(BuildContext context) {
     return Scaffold(
-      body: SizedBox.expand(
+      body: CanvasView(controller: controller, canvasBackground: SingleColorBackround(Colors.white)),
-        child: CanvasView(
-          ssPositions: [Offset(100, 100), Offset(200, 200), Offset(300, 300)],
-          canvasBackground: SingleColorBackround(Colors.white),
-          children: [
-            InfoContainer(color: Colors.red),
-            InfoContainer(color: Colors.green),
-            InfoContainer(color: Colors.blue),
-          ],
-        ),
-      ),
     );
   }
 }

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();
+  }
+}

lib/core/render.dart

@@ -1,5 +1,6 @@
 import 'package:flutter/material.dart';
 import 'package:flutter/rendering.dart';
+import 'package:infinite_lazy_2d_grid/core/controller.dart';
 
 import 'background.dart';
 
@@ -10,19 +11,30 @@ import 'background.dart';
 
 /// An infinite canvas that places all the children at the specified positions.
 class CanvasView extends StatelessWidget {
-  final List<Offset> ssPositions; // screen space positions;
-  final List<Widget> children; // children, in the same order as positions
   final CanvasBackground canvasBackground;
+  final CanvasController controller;
 
-  const CanvasView({required this.ssPositions, required this.children, required this.canvasBackground, super.key})
+  const CanvasView({required this.controller, required this.canvasBackground, super.key});
-    : assert(children.length == ssPositions.length, 'Children and positions must have the same length');
 
   @override
   Widget build(BuildContext context) {
     return Container(
       color: canvasBackground.bgColor,
       child: GestureDetector(
-        child: _CanvasRenderObject(canvasBackground: canvasBackground, ssPositions: ssPositions, children: children),
+        onScaleUpdate: controller.handleGesture,
+        child: ListenableBuilder(
+          listenable: controller,
+          builder: (_, _) {
+            final childrenWithPositions = controller.widgetsWithScreenPositions();
+            final ssPositions = childrenWithPositions.map((e) => e.$1).toList();
+            final children = childrenWithPositions.map((e) => e.$2).toList();
+            return _CanvasRenderObject(
+              canvasBackground: canvasBackground,
+              ssPositions: ssPositions,
+              children: children,
+            );
+          },
+        ),
       ),
     );
   }
@@ -46,7 +58,9 @@ class _CanvasRenderObject extends MultiChildRenderObjectWidget {
 
   @override
   void updateRenderObject(BuildContext context, _CanvasRenderBox renderObject) {
-    // static for now
+    renderObject
+      ..ssPositions = ssPositions
+      ..canvasBackground = canvasBackground;
   }
 }
 
@@ -57,10 +71,12 @@ class _CanvasRenderBox extends RenderBox
     with
         ContainerRenderObjectMixin<RenderBox, _WidgetParentData>,
         RenderBoxContainerDefaultsMixin<RenderBox, _WidgetParentData> {
-  final CanvasBackground canvasBackground;
+  CanvasBackground _canvasBackground;
-  final List<Offset> ssPositions;
+  List<Offset> _ssPositions;
 
-  _CanvasRenderBox({required this.ssPositions, required this.canvasBackground});
+  _CanvasRenderBox({required ssPositions, required canvasBackground})
+    : _ssPositions = ssPositions,
+      _canvasBackground = canvasBackground;
 
   @override
   void setupParentData(RenderBox child) {
@@ -69,6 +85,20 @@ class _CanvasRenderBox extends RenderBox
     }
   }
 
+  set ssPositions(List<Offset> ssPositions) {
+    if (_ssPositions != ssPositions) {
+      _ssPositions = ssPositions;
+      markNeedsPaint();
+    }
+  }
+
+  set canvasBackground(CanvasBackground canvasBackground) {
+    if (_canvasBackground != canvasBackground) {
+      _canvasBackground = canvasBackground;
+      markNeedsPaint();
+    }
+  }
+
   @override
   void performLayout() {
     size = constraints.biggest; // expand as much as possible for the parent
@@ -89,12 +119,12 @@ class _CanvasRenderBox extends RenderBox
     final canvas = context.canvas;
 
     // use the canvas background painter, pass it the canvas and that should handle drawing the background
-    canvasBackground.paint(canvas, size);
+    _canvasBackground.paint(canvas, size);
 
     RenderBox? child = firstChild;
     for (int idx = 0; child != null; idx++) {
       final _WidgetParentData parentData = child.parentData! as _WidgetParentData;
-      context.paintChild(child, ssPositions[idx]);
+      context.paintChild(child, _ssPositions[idx]);
       child = parentData.nextSibling;
     }
   }
@@ -109,11 +139,11 @@ class _CanvasRenderBox extends RenderBox
 
     // Check children in reverse order (last painted = first hit)
     RenderBox? child = lastChild;
-    for (int idx = ssPositions.length - 1; child != null; idx--) {
+    for (int idx = _ssPositions.length - 1; child != null; idx--) {
       final _WidgetParentData parentData = child.parentData! as _WidgetParentData;
 
       // Transform the hit position relative to the child's position
-      final childPosition = ssHitPosition - ssPositions[idx];
+      final childPosition = ssHitPosition - _ssPositions[idx];
 
       if (child.hitTest(BoxHitTestResult.wrap(result), position: childPosition)) {
         return true;

lib/utils/conversions.dart

@@ -0,0 +1,11 @@
+import 'dart:ui';
+
+/// Convert grid space coordinates to screen space coordinates
+Offset gsToSs(Offset gsPosition, Offset gsTopLeft) {
+  return gsPosition - gsTopLeft;
+}
+
+/// Convert screen space coordinates to grid space coordinates
+Offset ssToGs(Offset ssPosition, Offset gsTopLeft) {
+  return ssPosition + gsTopLeft;
+}

lib/utils/offset_extensions.dart

@@ -6,4 +6,8 @@ extension OffsetCartesionOps on Offset {
     final dy = this.dy - other.dy;
     return dx * dx + dy * dy;
   }
+
+  Offset operator -(Offset other) {
+    return Offset(dx - other.dx, dy - other.dy);
+  }
 }