delegate of SliverPersistentHeader

how_to_use_in_sliver.dart

SliverPersistentHeader(
  delegate: SliverAppBarDelegate((context, _, __, progress) => HistoryPersistentHeaderTile(),
              collapsedHeight: Dimensions.fixedHeaderHeight,
              expandedHeight: Dimensions.fixedHeaderHeight),
   pinned: true),

sliver_app_bar_delegate.dart

import 'dart:math' as math;

import 'package:flutter/widgets.dart';

class SliverAppBarDelegate extends SliverPersistentHeaderDelegate {
  SliverAppBarDelegate(this.builder,
      {@required this.collapsedHeight,
      @required this.expandedHeight,
      this.reverseProgress = false,
      this.limitMinimumToZero = true,
      this.limitMaximumToOne = true});

  final double expandedHeight;
  final double collapsedHeight;
  final bool reverseProgress, limitMinimumToZero, limitMaximumToOne;
  final Widget Function(BuildContext context, double shrinkOffset,
      bool overlapsContent, double progress) builder;

  @override
  double get minExtent => collapsedHeight;

  @override
  double get maxExtent => math.max(expandedHeight, minExtent);

  @override
  Widget build(
      BuildContext context, double shrinkOffset, bool overlapsContent) {
    return builder(
        context,
        shrinkOffset,
        overlapsContent,
        reverseProgress
            ? 1.0 - (shrinkOffset / maxExtent)
            : (shrinkOffset / maxExtent));
  }

  @override
  bool shouldRebuild(SliverAppBarDelegate oldDelegate) {
    return expandedHeight != oldDelegate.expandedHeight ||
        collapsedHeight != oldDelegate.collapsedHeight;
  }
}