Create FixedExtentSectionLayout to keep layout data of each section (group) in gallery

mobile/lib/models/gallery/fixed_extent_section_layout.dart

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+import 'package:flutter/material.dart';
+
+// Used to store layout information for a section (group) in a gallery
+
+class FixedExtentSectionLayout {
+  final double tileHeight, mainAxisStride;
+  final int firstIndex, lastIndex, bodyFirstIndex;
+  final double minOffset, maxOffset, bodyMinOffset;
+  final double headerExtent, spacing;
+  final IndexedWidgetBuilder builder;
+
+  const FixedExtentSectionLayout({
+    required this.firstIndex,
+    required this.lastIndex,
+    required this.minOffset,
+    required this.maxOffset,
+    required this.headerExtent,
+    required this.tileHeight,
+    required this.spacing,
+    required this.builder,
+  })  : bodyFirstIndex = firstIndex + 1,
+        bodyMinOffset = minOffset + headerExtent,
+        mainAxisStride = tileHeight + spacing;
+
+  bool hasChild(int index) => firstIndex <= index && index <= lastIndex;
+
+  bool hasChildAtOffset(double scrollOffset) =>
+      minOffset <= scrollOffset && scrollOffset <= maxOffset;
+
+  double indexToLayoutOffset(int index) {
+    index -= bodyFirstIndex;
+    if (index < 0) return minOffset;
+    return bodyMinOffset + index * mainAxisStride;
+  }
+
+  int getMinChildIndexForScrollOffset(double scrollOffset) {
+    scrollOffset -= bodyMinOffset;
+    if (mainAxisStride == 0 || !scrollOffset.isFinite || scrollOffset < 0) {
+      return firstIndex;
+    }
+
+    return bodyFirstIndex + scrollOffset ~/ mainAxisStride;
+  }
+
+  int getMaxChildIndexForScrollOffset(double scrollOffset) {
+    scrollOffset -= bodyMinOffset;
+    if (mainAxisStride == 0 || !scrollOffset.isFinite || scrollOffset < 0) {
+      return firstIndex;
+    }
+    return bodyFirstIndex + (scrollOffset / mainAxisStride).ceil() - 1;
+  }
+}