Implement nested scroll view.

Author: Yuncong Zhang

Runtime/painting/paint_utilities.cs

@@ -0,0 +1,30 @@
+using Unity.UIWidgets.foundation;
+using Unity.UIWidgets.ui;
+using UnityEngine;
+using Canvas = Unity.UIWidgets.ui.Canvas;
+
+namespace Unity.UIWidgets.painting {
+    public static class PaintingUtilities {
+        public static void paintZigZag(Canvas canvas, Paint paint, Offset start, Offset end, int zigs, float width) {
+            D.assert(MathUtils.isFinite(zigs));
+            D.assert(zigs > 0);
+            canvas.save();
+            canvas.translate(start.dx, start.dy);
+            end = end - start;
+            canvas.rotate(Mathf.Atan2(end.dy, end.dx));
+            float length = end.distance;
+            float spacing = length / (zigs * 2.0f);
+            Path path = new Path();
+            path.moveTo(0.0f, 0.0f);
+            for (int index = 0; index < zigs; index += 1) {
+                float x = (index * 2.0f + 1.0f) * spacing;
+                float y = width * ((index % 2.0f) * 2.0f - 1.0f);
+                path.lineTo(x, y);
+            }
+
+            path.lineTo(length, 0.0f);
+            canvas.drawPath(path, paint);
+            canvas.restore();
+        }
+    }
+}

Runtime/painting/paint_utilities.cs.meta

@@ -69,7 +69,7 @@ protected override float estimateMaxScrollOffset(SliverConstraints constraints,
 
     public class RenderSliverFillRemaining : RenderSliverSingleBoxAdapter {
         public RenderSliverFillRemaining(
-            RenderBox child
+            RenderBox child = null
         ) : base(child: child) {
         }