Sliver header that can be floating or pinned based on a parameter

sliver_flexible_header.dart

import 'dart:math' as math;

import 'package:flutter/rendering.dart';
import 'package:flutter/widgets.dart';

class SliverFlexibleHeader extends SingleChildRenderObjectWidget {
  const SliverFlexibleHeader({
    Key key,
    @required Widget child,
    this.floating = false, // pinned as default
  }) : super(key: key, child: child);

  final bool floating;

  @override
  _RenderSliverFlexibleHeader createRenderObject(context) =>
      _RenderSliverFlexibleHeader(floating: floating);
}

class _RenderSliverFlexibleHeader extends RenderSliverSingleBoxAdapter {
  _RenderSliverFlexibleHeader({RenderBox child, @required this.floating})
      : super(child: child);

  final bool floating;

  double _lastActualScrollOffset;
  double _effectiveScrollOffset = 0;
  double _childPosition;

  @override
  void performLayout() {
    if (child == null) {
      geometry = SliverGeometry.zero;
      return;
    }
    child.layout(constraints.asBoxConstraints(), parentUsesSize: true);
    final double maxExtent = childExtent;
    final double paintExtent = maxExtent - (_effectiveScrollOffset ?? 0.0);
    final double layoutExtent = maxExtent - constraints.scrollOffset;

    if (_lastActualScrollOffset != null &&
        (constraints.scrollOffset < _lastActualScrollOffset ||
            _effectiveScrollOffset < maxExtent)) {
      double delta = _lastActualScrollOffset - constraints.scrollOffset;
      final bool allowFloatingExpansion =
          constraints.userScrollDirection == ScrollDirection.forward;
      if (allowFloatingExpansion) {
        _effectiveScrollOffset = math.min(_effectiveScrollOffset, maxExtent);
      } else {
        delta = math.min(delta, 0);
      }
      _effectiveScrollOffset =
          (_effectiveScrollOffset - delta).clamp(0.0, constraints.scrollOffset);
    } else {
      _effectiveScrollOffset = constraints.scrollOffset;
    }

    final paintedChildExtent = math.min(
      childExtent,
      constraints.remainingPaintExtent - constraints.overlap,
    );
    geometry = SliverGeometry(
      paintExtent: floating
          ? paintExtent.clamp(0.0, constraints.remainingPaintExtent)
          : paintedChildExtent,
      maxPaintExtent: maxExtent,
      paintOrigin:
          floating ? math.min(constraints.overlap, 0) : constraints.overlap,
      scrollExtent: maxExtent,
      layoutExtent: floating
          ? math.min(
              paintExtent.clamp(0.0, constraints.remainingPaintExtent),
              layoutExtent.clamp(0.0, constraints.remainingPaintExtent),
            )
          : math.max(0.0, paintedChildExtent - constraints.scrollOffset),
      hasVisualOverflow: floating ? true : paintedChildExtent < childExtent,
    );
    _childPosition = math.min(0, paintExtent - childExtent);
    _lastActualScrollOffset = constraints.scrollOffset;
  }

  double get childExtent {
    if (child == null) {
      return 0;
    }
    assert(child.hasSize);
    assert(constraints.axis != null);
    switch (constraints.axis) {
      case Axis.vertical:
        return child.size.height;
      case Axis.horizontal:
        return child.size.width;
    }
    return null;
  }

  @override
  void paint(PaintingContext context, Offset offset) {
    if (child != null && geometry.visible) {
      assert(constraints.axisDirection != null);
      switch (applyGrowthDirectionToAxisDirection(
        constraints.axisDirection,
        constraints.growthDirection,
      )) {
        case AxisDirection.up:
          offset += Offset(
            0,
            geometry.paintExtent - childMainAxisPosition(child) - childExtent,
          );
          break;
        case AxisDirection.down:
          offset += Offset(0, childMainAxisPosition(child));
          break;
        case AxisDirection.left:
          offset += Offset(
            geometry.paintExtent - childMainAxisPosition(child) - childExtent,
            0,
          );
          break;
        case AxisDirection.right:
          offset += Offset(childMainAxisPosition(child), 0);
          break;
      }
      context.paintChild(child, offset);
    }
  }

  @override
  double childMainAxisPosition(RenderBox child) {
    assert(child == this.child);
    return floating ? _childPosition : 0;
  }
}

null safety support

import 'dart:math' as math;

import 'package:flutter/rendering.dart';
import 'package:flutter/widgets.dart';

class SliverFlexibleHeader extends SingleChildRenderObjectWidget {
  const SliverFlexibleHeader({
    Key? key,
    required Widget child,
    this.floating = false, // pinned as default
  }) : super(key: key, child: child);

  final bool floating;

  @override
  _RenderSliverFlexibleHeader createRenderObject(BuildContext context) =>
      _RenderSliverFlexibleHeader(floating: floating);
}

class _RenderSliverFlexibleHeader extends RenderSliverSingleBoxAdapter {
  _RenderSliverFlexibleHeader({RenderBox? child, required this.floating}) : super(child: child);

  final bool floating;

  double? _lastActualScrollOffset;
  double _effectiveScrollOffset = 0;
  double _childPosition = 0;

  @override
  void performLayout() {
    if (child == null) {
      geometry = SliverGeometry.zero;
      return;
    }
    child!.layout(constraints.asBoxConstraints(), parentUsesSize: true);
    final double maxExtent = childExtent;
    final double paintExtent = maxExtent - (_effectiveScrollOffset);
    final double layoutExtent = maxExtent - constraints.scrollOffset;

    if (_lastActualScrollOffset != null &&
        (constraints.scrollOffset < _lastActualScrollOffset! || _effectiveScrollOffset < maxExtent)) {
      double delta = _lastActualScrollOffset! - constraints.scrollOffset;
      final bool allowFloatingExpansion = constraints.userScrollDirection == ScrollDirection.forward;
      if (allowFloatingExpansion) {
        _effectiveScrollOffset = math.min(_effectiveScrollOffset, maxExtent);
      } else {
        delta = math.min(delta, 0);
      }
      _effectiveScrollOffset = (_effectiveScrollOffset - delta).clamp(0.0, constraints.scrollOffset);
    } else {
      _effectiveScrollOffset = constraints.scrollOffset;
    }

    final paintedChildExtent = math.min(
      childExtent,
      constraints.remainingPaintExtent - constraints.overlap,
    );
    geometry = SliverGeometry(
      paintExtent: floating ? paintExtent.clamp(0.0, constraints.remainingPaintExtent) : paintedChildExtent,
      maxPaintExtent: maxExtent,
      paintOrigin: floating ? math.min(constraints.overlap, 0) : constraints.overlap,
      scrollExtent: maxExtent,
      layoutExtent: floating
          ? math.min(
              paintExtent.clamp(0.0, constraints.remainingPaintExtent),
              layoutExtent.clamp(0.0, constraints.remainingPaintExtent),
            )
          : math.max(0.0, paintedChildExtent - constraints.scrollOffset),
      hasVisualOverflow: floating ? true : paintedChildExtent < childExtent,
    );
    _childPosition = math.min(0, paintExtent - childExtent);
    _lastActualScrollOffset = constraints.scrollOffset;
  }

  double get childExtent {
    if (child == null) {
      return 0;
    }
    assert(child!.hasSize);
    switch (constraints.axis) {
      case Axis.vertical:
        return child!.size.height;
      case Axis.horizontal:
        return child!.size.width;
    }
  }

  @override
  void paint(PaintingContext context, Offset offset) {
    if (child != null && geometry?.visible == true) {
      switch (applyGrowthDirectionToAxisDirection(
        constraints.axisDirection,
        constraints.growthDirection,
      )) {
        case AxisDirection.up:
          offset += Offset(
            0,
            geometry!.paintExtent - childMainAxisPosition(child!) - childExtent,
          );
          break;
        case AxisDirection.down:
          offset += Offset(0, childMainAxisPosition(child!));
          break;
        case AxisDirection.left:
          offset += Offset(
            geometry!.paintExtent - childMainAxisPosition(child!) - childExtent,
            0,
          );
          break;
        case AxisDirection.right:
          offset += Offset(childMainAxisPosition(child!), 0);
          break;
      }
      context.paintChild(child!, offset);
    }
  }

  @override
  double childMainAxisPosition(RenderBox child) {
    assert(child == this.child);
    return floating ? _childPosition : 0;
  }
}