canvas with panning

lib/core/background.dart

@@ -0,0 +1,30 @@
+import 'package:flutter/material.dart';
+
+abstract class CanvasBackground {
+  /// the fill color of background
+  final Color bgColor;
+  const CanvasBackground({this.bgColor = Colors.white});
+
+  /// draw the backround on this context. Implement this to have
+  /// different kinds of backgrounds
+  void paint(Canvas canvas, Offset offset, double scale, Size canvasSize);
+}
+
+class NoBackground extends CanvasBackground {
+  const NoBackground();
+
+  @override
+  void paint(Canvas canvas, Offset offset, double scale, Size canvasSize) {}
+}
+
+class SingleColorBackround extends CanvasBackground {
+  const SingleColorBackround(Color backgroundColor) : super(bgColor: backgroundColor);
+
+  @override
+  void paint(Canvas canvas, Offset offset, double scale, Size canvasSize) {
+    final paint = Paint()
+      ..color = bgColor
+      ..style = PaintingStyle.fill;
+    canvas.drawRect(Rect.fromLTWH(0, 0, canvasSize.width, canvasSize.height), paint);
+  }
+}