SliverAppBar的使用&自定义

SliverAppBar

1. expandedHeight

This does not include the status bar height (which will be automatically included if [primary] is true).

合并的高度，默认是状态栏的高度加AppBar的高度

[primary]:true (不包含状态栏高度，反之包含)

3. backgroundColor

背景颜色(折叠后的背景色也是这个属性)

4. leading

左侧的图标或文字，多为返回箭头

5. automaticallyImplyLeading

没有leading为true的时候，默认返回箭头

没有leading且为false，则显示title

7. title

标题

8. actions

标题右侧的操作

9. primary

是否显示在状态栏的下面,false就会占领状态栏的高度

10. centerTitle

标题是否居中显示

11. pinned

标题栏是否固定

12. elevation

阴影大小

13. forceElevated

是否显示阴影

14. brightness

状态栏主题色

15. titleSpacing

标题间距

16. flexibleSpace(*)

SliverAppBar的背景内容区,通常使用 FlexibleSpaceBar

17. collapsedHeight

折叠之后的高度 默认 toolbarHeight 的高度

18. floating

否会发生下拉立即出现SliverAppBar

19. snap

必须与floating:true联合使用，表示显示SliverAppBar之后，如果没有完全拉伸，是否会完全展开

20. stretch

看这个哇，解释的很到位，图文，medium.com/flutter-com…

FlexibleSpaceBar

1. collapseMode

CollapseMode.pin: 一起滚动，直到消失 CollapseMode.parallax: 视觉渐变，抽屉效果更明显

2. stretchModes

StretchMode.zoomBackground- >背景小部件将展开以填充额外的空间。

StretchMode.blurBackground- >使用[ImageFilter.blur]效果，背景将模糊。

StretchMode.fadeTitle- >随着用户过度滚动，标题将消失。

自定义SliverAppBar，实现appbar title只有在完全折叠的状态显示

1. 关键代码

bool _verticalOffsetZero = false;

AppBarStateChangeController _appBarStateChangeController =
      AppBarStateChangeController();

_appBarStateChangeController.addListener(() {
  var verticalOffsetZero = _appBarStateChangeController.verticalOffsetZero;
  if (_verticalOffsetZero != verticalOffsetZero) {
    logger("verticalOffsetZero >>> $verticalOffsetZero");
    WidgetsBinding.instance?.addPostFrameCallback((_) {
      //需要创建的小组件
      setState(() {});
    });
  }
  _verticalOffsetZero = verticalOffsetZero;
});

MySliverAppBar(
appBarStateChangeController: _appBarStateChangeController,
)

2. MySliverAppBar部分（完全把SliverAppBar copy的出来，进行改动，如果没有我这种需求，还是建议用SliverAppBar）

import 'package:flutter/foundation.dart';
import 'package:flutter/material.dart';
import 'package:flutter/rendering.dart';
import 'package:flutter/services.dart';
import 'dart:math' as math;

class MySliverAppBar extends StatefulWidget {
  /// Creates a material design app bar that can be placed in a [CustomScrollView].
  ///
  /// The arguments [forceElevated], [primary], [floating], [pinned], [snap]
  /// and [automaticallyImplyLeading] must not be null.
  const MySliverAppBar({
    Key? key,
    this.leading,
    this.automaticallyImplyLeading = true,
    this.title,
    this.actions,
    this.flexibleSpace,
    this.bottom,
    this.elevation,
    this.shadowColor,
    this.forceElevated = false,
    this.backgroundColor,
    this.foregroundColor,
    this.brightness,
    this.iconTheme,
    this.actionsIconTheme,
    this.textTheme,
    this.primary = true,
    this.centerTitle,
    this.excludeHeaderSemantics = false,
    this.titleSpacing,
    this.collapsedHeight,
    this.expandedHeight,
    this.floating = false,
    this.pinned = false,
    this.snap = false,
    this.stretch = false,
    this.stretchTriggerOffset = 100.0,
    this.onStretchTrigger,
    this.shape,
    this.toolbarHeight = kToolbarHeight,
    this.leadingWidth,
    this.backwardsCompatibility,
    this.toolbarTextStyle,
    this.titleTextStyle,
    this.systemOverlayStyle,
    this.appBarStateChangeController,
  })  : assert(automaticallyImplyLeading != null),
        assert(forceElevated != null),
        assert(primary != null),
        assert(floating != null),
        assert(pinned != null),
        assert(snap != null),
        assert(stretch != null),
        assert(toolbarHeight != null),
        assert(floating || !snap,
            'The "snap" argument only makes sense for floating app bars.'),
        assert(stretchTriggerOffset > 0.0),
        assert(collapsedHeight == null || collapsedHeight >= toolbarHeight,
            'The "collapsedHeight" argument has to be larger than or equal to [toolbarHeight].'),
        super(key: key);

  /// {@macro flutter.material.appbar.leading}
  ///
  /// This property is used to configure an [AppBar].
  final Widget? leading;

  /// 监听app bar 高度变化，custom熟悉
  final AppBarStateChangeController? appBarStateChangeController;

  /// {@macro flutter.material.appbar.automaticallyImplyLeading}
  ///
  /// This property is used to configure an [AppBar].
  final bool automaticallyImplyLeading;

  /// {@macro flutter.material.appbar.title}
  ///
  /// This property is used to configure an [AppBar].
  final Widget? title;

  /// {@macro flutter.material.appbar.actions}
  ///
  /// This property is used to configure an [AppBar].
  final List<Widget>? actions;

  /// {@macro flutter.material.appbar.flexibleSpace}
  ///
  /// This property is used to configure an [AppBar].
  final Widget? flexibleSpace;

  /// {@macro flutter.material.appbar.bottom}
  ///
  /// This property is used to configure an [AppBar].
  final PreferredSizeWidget? bottom;

  /// {@macro flutter.material.appbar.elevation}
  ///
  /// This property is used to configure an [AppBar].
  final double? elevation;

  /// {@macro flutter.material.appbar.shadowColor}
  ///
  /// This property is used to configure an [AppBar].
  final Color? shadowColor;

  /// Whether to show the shadow appropriate for the [elevation] even if the
  /// content is not scrolled under the [AppBar].
  ///
  /// Defaults to false, meaning that the [elevation] is only applied when the
  /// [AppBar] is being displayed over content that is scrolled under it.
  ///
  /// When set to true, the [elevation] is applied regardless.
  ///
  /// Ignored when [elevation] is zero.
  final bool forceElevated;

  /// {@macro flutter.material.appbar.backgroundColor}
  ///
  /// This property is used to configure an [AppBar].
  final Color? backgroundColor;

  /// {@macro flutter.material.appbar.foregroundColor}
  ///
  /// This property is used to configure an [AppBar].
  final Color? foregroundColor;

  /// {@macro flutter.material.appbar.brightness}
  ///
  /// This property is used to configure an [AppBar].
  final Brightness? brightness;

  /// {@macro flutter.material.appbar.iconTheme}
  ///
  /// This property is used to configure an [AppBar].
  final IconThemeData? iconTheme;

  /// {@macro flutter.material.appbar.actionsIconTheme}
  ///
  /// This property is used to configure an [AppBar].
  final IconThemeData? actionsIconTheme;

  /// {@macro flutter.material.appbar.textTheme}
  ///
  /// This property is used to configure an [AppBar].
  final TextTheme? textTheme;

  /// {@macro flutter.material.appbar.primary}
  ///
  /// This property is used to configure an [AppBar].
  final bool primary;

  /// {@macro flutter.material.appbar.centerTitle}
  ///
  /// This property is used to configure an [AppBar].
  final bool? centerTitle;

  /// {@macro flutter.material.appbar.excludeHeaderSemantics}
  ///
  /// This property is used to configure an [AppBar].
  final bool excludeHeaderSemantics;

  /// {@macro flutter.material.appbar.titleSpacing}
  ///
  /// This property is used to configure an [AppBar].
  final double? titleSpacing;

  /// Defines the height of the app bar when it is collapsed.
  ///
  /// By default, the collapsed height is [toolbarHeight]. If [bottom] widget is
  /// specified, then its height from [PreferredSizeWidget.preferredSize] is
  /// added to the height. If [primary] is true, then the [MediaQuery] top
  /// padding, [EdgeInsets.top] of [MediaQueryData.padding], is added as well.
  ///
  /// If [pinned] and [floating] are true, with [bottom] set, the default
  /// collapsed height is only the height of [PreferredSizeWidget.preferredSize]
  /// with the [MediaQuery] top padding.
  final double? collapsedHeight;

  /// The size of the app bar when it is fully expanded.
  ///
  /// By default, the total height of the toolbar and the bottom widget (if
  /// any). If a [flexibleSpace] widget is specified this height should be big
  /// enough to accommodate whatever that widget contains.
  ///
  /// This does not include the status bar height (which will be automatically
  /// included if [primary] is true).
  final double? expandedHeight;

  /// Whether the app bar should become visible as soon as the user scrolls
  /// towards the app bar.
  ///
  /// Otherwise, the user will need to scroll near the top of the scroll view to
  /// reveal the app bar.
  ///
  /// If [snap] is true then a scroll that exposes the app bar will trigger an
  /// animation that slides the entire app bar into view. Similarly if a scroll
  /// dismisses the app bar, the animation will slide it completely out of view.
  ///
  /// ## Animated Examples
  ///
  /// The following animations show how the app bar changes its scrolling
  /// behavior based on the value of this property.
  ///
  /// * App bar with [floating] set to false:
  /// {@animation 476 400 https://flutter.github.io/assets-for-api-docs/assets/material/app_bar.mp4}
  /// * App bar with [floating] set to true:
  /// {@animation 476 400 https://flutter.github.io/assets-for-api-docs/assets/material/app_bar_floating.mp4}
  ///
  /// See also:
  ///
  /// * [MySliverAppBar] for more animated examples of how this property changes the
  /// behavior of the app bar in combination with [pinned] and [snap].
  final bool floating;

  /// Whether the app bar should remain visible at the start of the scroll view.
  ///
  /// The app bar can still expand and contract as the user scrolls, but it will
  /// remain visible rather than being scrolled out of view.
  ///
  /// ## Animated Examples
  ///
  /// The following animations show how the app bar changes its scrolling
  /// behavior based on the value of this property.
  ///
  /// * App bar with [pinned] set to false:
  /// {@animation 476 400 https://flutter.github.io/assets-for-api-docs/assets/material/app_bar.mp4}
  /// * App bar with [pinned] set to true:
  /// {@animation 476 400 https://flutter.github.io/assets-for-api-docs/assets/material/app_bar_pinned.mp4}
  ///
  /// See also:
  ///
  /// * [MySliverAppBar] for more animated examples of how this property changes the
  /// behavior of the app bar in combination with [floating].
  final bool pinned;

  /// {@macro flutter.material.appbar.shape}
  ///
  /// This property is used to configure an [AppBar].
  final ShapeBorder? shape;

  /// If [snap] and [floating] are true then the floating app bar will "snap"
  /// into view.
  ///
  /// If [snap] is true then a scroll that exposes the floating app bar will
  /// trigger an animation that slides the entire app bar into view. Similarly
  /// if a scroll dismisses the app bar, the animation will slide the app bar
  /// completely out of view. Additionally, setting [snap] to true will fully
  /// expand the floating app bar when the framework tries to reveal the
  /// contents of the app bar by calling [RenderObject.showOnScreen]. For
  /// example, when a [TextField] in the floating app bar gains focus, if [snap]
  /// is true, the framework will always fully expand the floating app bar, in
  /// order to reveal the focused [TextField].
  ///
  /// Snapping only applies when the app bar is floating, not when the app bar
  /// appears at the top of its scroll view.
  ///
  /// ## Animated Examples
  ///
  /// The following animations show how the app bar changes its scrolling
  /// behavior based on the value of this property.
  ///
  /// * App bar with [snap] set to false:
  /// {@animation 476 400 https://flutter.github.io/assets-for-api-docs/assets/material/app_bar_floating.mp4}
  /// * App bar with [snap] set to true:
  /// {@animation 476 400 https://flutter.github.io/assets-for-api-docs/assets/material/app_bar_floating_snap.mp4}
  ///
  /// See also:
  ///
  /// * [MySliverAppBar] for more animated examples of how this property changes the
  /// behavior of the app bar in combination with [pinned] and [floating].
  final bool snap;

  /// Whether the app bar should stretch to fill the over-scroll area.
  ///
  /// The app bar can still expand and contract as the user scrolls, but it will
  /// also stretch when the user over-scrolls.
  final bool stretch;

  /// The offset of overscroll required to activate [onStretchTrigger].
  ///
  /// This defaults to 100.0.
  final double stretchTriggerOffset;

  /// The callback function to be executed when a user over-scrolls to the
  /// offset specified by [stretchTriggerOffset].
  final AsyncCallback? onStretchTrigger;

  /// {@macro flutter.material.appbar.toolbarHeight}
  ///
  /// This property is used to configure an [AppBar].
  final double toolbarHeight;

  /// {@macro flutter.material.appbar.leadingWidth}
  ///
  /// This property is used to configure an [AppBar].
  final double? leadingWidth;

  /// {@macro flutter.material.appbar.backwardsCompatibility}
  ///
  /// This property is used to configure an [AppBar].
  final bool? backwardsCompatibility;

  /// {@macro flutter.material.appbar.toolbarTextStyle}
  ///
  /// This property is used to configure an [AppBar].
  final TextStyle? toolbarTextStyle;

  /// {@macro flutter.material.appbar.titleTextStyle}
  ///
  /// This property is used to configure an [AppBar].
  final TextStyle? titleTextStyle;

  /// {@macro flutter.material.appbar.systemOverlayStyle}
  ///
  /// This property is used to configure an [AppBar].
  final SystemUiOverlayStyle? systemOverlayStyle;

  @override
  _MySliverAppBarState createState() => _MySliverAppBarState();
}

// This class is only Stateful because it owns the TickerProvider used
// by the floating appbar snap animation (via FloatingHeaderSnapConfiguration).
class _MySliverAppBarState extends State<MySliverAppBar>
    with TickerProviderStateMixin {
  FloatingHeaderSnapConfiguration? _snapConfiguration;
  OverScrollHeaderStretchConfiguration? _stretchConfiguration;
  PersistentHeaderShowOnScreenConfiguration? _showOnScreenConfiguration;

  void _updateSnapConfiguration() {
    if (widget.snap && widget.floating) {
      _snapConfiguration = FloatingHeaderSnapConfiguration(
        curve: Curves.easeOut,
        duration: const Duration(milliseconds: 200),
      );
    } else {
      _snapConfiguration = null;
    }

    _showOnScreenConfiguration = widget.floating & widget.snap
        ? const PersistentHeaderShowOnScreenConfiguration(
            minShowOnScreenExtent: double.infinity)
        : null;
  }

  void _updateStretchConfiguration() {
    if (widget.stretch) {
      _stretchConfiguration = OverScrollHeaderStretchConfiguration(
        stretchTriggerOffset: widget.stretchTriggerOffset,
        onStretchTrigger: widget.onStretchTrigger,
      );
    } else {
      _stretchConfiguration = null;
    }
  }

  @override
  void initState() {
    super.initState();
    _updateSnapConfiguration();
    _updateStretchConfiguration();
  }

  @override
  void didUpdateWidget(MySliverAppBar oldWidget) {
    super.didUpdateWidget(oldWidget);
    if (widget.snap != oldWidget.snap || widget.floating != oldWidget.floating)
      _updateSnapConfiguration();
    if (widget.stretch != oldWidget.stretch) _updateStretchConfiguration();
  }

  @override
  Widget build(BuildContext context) {
    assert(!widget.primary || debugCheckHasMediaQuery(context));
    final double bottomHeight = widget.bottom?.preferredSize.height ?? 0.0;
    final double topPadding =
        widget.primary ? MediaQuery.of(context).padding.top : 0.0;
    final double collapsedHeight =
        (widget.pinned && widget.floating && widget.bottom != null)
            ? (widget.collapsedHeight ?? 0.0) + bottomHeight + topPadding
            : (widget.collapsedHeight ?? widget.toolbarHeight) +
                bottomHeight +
                topPadding;
    return MediaQuery.removePadding(
      context: context,
      removeBottom: true,
      child: SliverPersistentHeader(
        floating: widget.floating,
        pinned: widget.pinned,
        delegate: _SliverAppBarDelegate(
          vsync: this,
          leading: widget.leading,
          appBarStateChangeController: widget.appBarStateChangeController,
          automaticallyImplyLeading: widget.automaticallyImplyLeading,
          title: widget.title,
          actions: widget.actions,
          flexibleSpace: widget.flexibleSpace,
          bottom: widget.bottom,
          elevation: widget.elevation,
          shadowColor: widget.shadowColor,
          forceElevated: widget.forceElevated,
          backgroundColor: widget.backgroundColor,
          foregroundColor: widget.foregroundColor,
          brightness: widget.brightness,
          iconTheme: widget.iconTheme,
          actionsIconTheme: widget.actionsIconTheme,
          textTheme: widget.textTheme,
          primary: widget.primary,
          centerTitle: widget.centerTitle,
          excludeHeaderSemantics: widget.excludeHeaderSemantics,
          titleSpacing: widget.titleSpacing,
          expandedHeight: widget.expandedHeight,
          collapsedHeight: collapsedHeight,
          topPadding: topPadding,
          floating: widget.floating,
          pinned: widget.pinned,
          shape: widget.shape,
          snapConfiguration: _snapConfiguration,
          stretchConfiguration: _stretchConfiguration,
          showOnScreenConfiguration: _showOnScreenConfiguration,
          toolbarHeight: widget.toolbarHeight,
          leadingWidth: widget.leadingWidth,
          backwardsCompatibility: widget.backwardsCompatibility,
          toolbarTextStyle: widget.toolbarTextStyle,
          titleTextStyle: widget.titleTextStyle,
          systemOverlayStyle: widget.systemOverlayStyle,
        ),
      ),
    );
  }
}

class _SliverAppBarDelegate extends SliverPersistentHeaderDelegate {
  _SliverAppBarDelegate({
    required this.leading,
    required this.automaticallyImplyLeading,
    required this.title,
    required this.actions,
    required this.flexibleSpace,
    required this.bottom,
    required this.elevation,
    required this.shadowColor,
    required this.forceElevated,
    required this.backgroundColor,
    required this.foregroundColor,
    required this.brightness,
    required this.iconTheme,
    required this.actionsIconTheme,
    required this.textTheme,
    required this.primary,
    required this.centerTitle,
    required this.excludeHeaderSemantics,
    required this.titleSpacing,
    required this.expandedHeight,
    required this.collapsedHeight,
    required this.topPadding,
    required this.floating,
    required this.pinned,
    required this.vsync,
    required this.snapConfiguration,
    required this.stretchConfiguration,
    required this.showOnScreenConfiguration,
    required this.shape,
    required this.toolbarHeight,
    required this.leadingWidth,
    required this.backwardsCompatibility,
    required this.toolbarTextStyle,
    required this.titleTextStyle,
    required this.systemOverlayStyle,
    required this.appBarStateChangeController,
  })  : assert(primary || topPadding == 0.0),
        assert(
          !floating ||
              (snapConfiguration == null &&
                  showOnScreenConfiguration == null) ||
              vsync != null,
          'vsync cannot be null when snapConfiguration or showOnScreenConfiguration is not null, and floating is true',
        ),
        _bottomHeight = bottom?.preferredSize.height ?? 0.0;

  final Widget? leading;
  final AppBarStateChangeController? appBarStateChangeController;
  final bool automaticallyImplyLeading;
  final Widget? title;
  final List<Widget>? actions;
  final Widget? flexibleSpace;
  final PreferredSizeWidget? bottom;
  final double? elevation;
  final Color? shadowColor;
  final bool forceElevated;
  final Color? backgroundColor;
  final Color? foregroundColor;
  final Brightness? brightness;
  final IconThemeData? iconTheme;
  final IconThemeData? actionsIconTheme;
  final TextTheme? textTheme;
  final bool primary;
  final bool? centerTitle;
  final bool excludeHeaderSemantics;
  final double? titleSpacing;
  final double? expandedHeight;
  final double collapsedHeight;
  final double topPadding;
  final bool floating;
  final bool pinned;
  final ShapeBorder? shape;
  final double? toolbarHeight;
  final double? leadingWidth;
  final bool? backwardsCompatibility;
  final TextStyle? toolbarTextStyle;
  final TextStyle? titleTextStyle;
  final SystemUiOverlayStyle? systemOverlayStyle;
  final double _bottomHeight;

  @override
  double get minExtent => collapsedHeight;

  @override
  double get maxExtent => math.max(
      topPadding +
          (expandedHeight ?? (toolbarHeight ?? kToolbarHeight) + _bottomHeight),
      minExtent);

  @override
  final TickerProvider vsync;

  @override
  final FloatingHeaderSnapConfiguration? snapConfiguration;

  @override
  final OverScrollHeaderStretchConfiguration? stretchConfiguration;

  @override
  final PersistentHeaderShowOnScreenConfiguration? showOnScreenConfiguration;

  @override
  Widget build(
      BuildContext context, double shrinkOffset, bool overlapsContent) {
    final double visibleMainHeight = maxExtent - shrinkOffset - topPadding;
    final double extraToolbarHeight = math.max(
        minExtent -
            _bottomHeight -
            topPadding -
            (toolbarHeight ?? kToolbarHeight),
        0.0);
    final double visibleToolbarHeight =
        visibleMainHeight - _bottomHeight - extraToolbarHeight;

    final bool isPinnedWithOpacityFade =
        pinned && floating && bottom != null && extraToolbarHeight == 0.0;
    final double toolbarOpacity = !pinned || isPinnedWithOpacityFade
        ? (visibleToolbarHeight / (toolbarHeight ?? kToolbarHeight))
            .clamp(0.0, 1.0)
        : 1.0;
    appBarStateChangeController?.setValue((math.max(minExtent, maxExtent - shrinkOffset) == minExtent));
    final Widget appBar = FlexibleSpaceBar.createSettings(
      minExtent: minExtent,
      maxExtent: maxExtent,
      currentExtent: math.max(minExtent, maxExtent - shrinkOffset),
      toolbarOpacity: toolbarOpacity,
      child: AppBar(
        leading: leading,
        automaticallyImplyLeading: automaticallyImplyLeading,
        title: title,
        actions: actions,
        flexibleSpace:
            (title == null && flexibleSpace != null && !excludeHeaderSemantics)
                ? Semantics(child: flexibleSpace, header: true)
                : flexibleSpace,
        bottom: bottom,
        elevation: forceElevated ||
                overlapsContent ||
                (pinned && shrinkOffset > maxExtent - minExtent)
            ? elevation
            : 0.0,
        shadowColor: shadowColor,
        backgroundColor: backgroundColor,
        foregroundColor: foregroundColor,
        brightness: brightness,
        iconTheme: iconTheme,
        actionsIconTheme: actionsIconTheme,
        textTheme: textTheme,
        primary: primary,
        centerTitle: centerTitle,
        excludeHeaderSemantics: excludeHeaderSemantics,
        titleSpacing: titleSpacing,
        shape: shape,
        toolbarOpacity: toolbarOpacity,
        bottomOpacity: pinned
            ? 1.0
            : ((visibleMainHeight / _bottomHeight).clamp(0.0, 1.0)),
        toolbarHeight: toolbarHeight,
        leadingWidth: leadingWidth,
        backwardsCompatibility: backwardsCompatibility,
        toolbarTextStyle: toolbarTextStyle,
        titleTextStyle: titleTextStyle,
        systemOverlayStyle: systemOverlayStyle,
      ),
    );
    return floating ? _FloatingAppBar(child: appBar) : appBar;
  }

  @override
  bool shouldRebuild(covariant _SliverAppBarDelegate oldDelegate) {
    return leading != oldDelegate.leading ||
        automaticallyImplyLeading != oldDelegate.automaticallyImplyLeading ||
        title != oldDelegate.title ||
        actions != oldDelegate.actions ||
        flexibleSpace != oldDelegate.flexibleSpace ||
        bottom != oldDelegate.bottom ||
        _bottomHeight != oldDelegate._bottomHeight ||
        elevation != oldDelegate.elevation ||
        shadowColor != oldDelegate.shadowColor ||
        backgroundColor != oldDelegate.backgroundColor ||
        foregroundColor != oldDelegate.foregroundColor ||
        brightness != oldDelegate.brightness ||
        iconTheme != oldDelegate.iconTheme ||
        actionsIconTheme != oldDelegate.actionsIconTheme ||
        textTheme != oldDelegate.textTheme ||
        primary != oldDelegate.primary ||
        centerTitle != oldDelegate.centerTitle ||
        titleSpacing != oldDelegate.titleSpacing ||
        expandedHeight != oldDelegate.expandedHeight ||
        topPadding != oldDelegate.topPadding ||
        pinned != oldDelegate.pinned ||
        floating != oldDelegate.floating ||
        vsync != oldDelegate.vsync ||
        snapConfiguration != oldDelegate.snapConfiguration ||
        stretchConfiguration != oldDelegate.stretchConfiguration ||
        showOnScreenConfiguration != oldDelegate.showOnScreenConfiguration ||
        forceElevated != oldDelegate.forceElevated ||
        toolbarHeight != oldDelegate.toolbarHeight ||
        leadingWidth != oldDelegate.leadingWidth ||
        backwardsCompatibility != oldDelegate.backwardsCompatibility ||
        toolbarTextStyle != oldDelegate.toolbarTextStyle ||
        titleTextStyle != oldDelegate.titleTextStyle ||
        systemOverlayStyle != oldDelegate.systemOverlayStyle;
  }

  @override
  String toString() {
    return '${describeIdentity(this)}(topPadding: ${topPadding.toStringAsFixed(1)}, bottomHeight: ${_bottomHeight.toStringAsFixed(1)}, ...)';
  }
}

class _FloatingAppBar extends StatefulWidget {
  const _FloatingAppBar({Key? key, required this.child}) : super(key: key);

  final Widget child;

  @override
  _FloatingAppBarState createState() => _FloatingAppBarState();
}

// A wrapper for the widget created by _SliverAppBarDelegate that starts and
// stops the floating app bar's snap-into-view or snap-out-of-view animation.
class _FloatingAppBarState extends State<_FloatingAppBar> {
  ScrollPosition? _position;

  @override
  void didChangeDependencies() {
    super.didChangeDependencies();
    if (_position != null)
      _position!.isScrollingNotifier.removeListener(_isScrollingListener);
    _position = Scrollable.of(context)?.position;
    if (_position != null)
      _position!.isScrollingNotifier.addListener(_isScrollingListener);
  }

  @override
  void dispose() {
    if (_position != null)
      _position!.isScrollingNotifier.removeListener(_isScrollingListener);
    super.dispose();
  }

  RenderSliverFloatingPersistentHeader? _headerRenderer() {
    return context
        .findAncestorRenderObjectOfType<RenderSliverFloatingPersistentHeader>();
  }

  void _isScrollingListener() {
    if (_position == null) return;

    // When a scroll stops, then maybe snap the appbar into view.
    // Similarly, when a scroll starts, then maybe stop the snap animation.
    final RenderSliverFloatingPersistentHeader? header = _headerRenderer();
    if (_position!.isScrollingNotifier.value)
      header?.maybeStopSnapAnimation(_position!.userScrollDirection);
    else
      header?.maybeStartSnapAnimation(_position!.userScrollDirection);
  }

  @override
  Widget build(BuildContext context) => widget.child;
}

class AppBarStateChangeController extends ChangeNotifier {
  bool _verticalOffsetZero = false;

  setValue(bool verticalOffset) {
    this._verticalOffsetZero = verticalOffset;
    notifyListeners();
  }

  bool get verticalOffsetZero => _verticalOffsetZero;
}