【 Flutter Unit 解牛篇 】代码折叠展开面板，怎么没有线?

class TolyExpandTile extends StatefulWidget {

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

class _TolyExpandTileState extends State<TolyExpandTile>
    
    with SingleTickerProviderStateMixin {
  var _crossFadeState = CrossFadeState.showFirst;

  bool get isFirst => _crossFadeState == CrossFadeState.showFirst;

  @override
  Widget build(BuildContext context) {
    return Container(
      padding: EdgeInsets.all(20),
      child: Column(
        children: [
          Row(
            children: [
              Expanded(
                child: Container(),
              ),
              GestureDetector(
                  onTap: _togglePanel,
                  child: Padding(
                    padding: const EdgeInsets.all(8.0),
                    child: Icon(Icons.code),
                  ))
            ],
          ),
          _buildPanel()
        ],
      ),
    );
  }

  void _togglePanel() {
    setState(() {
      _crossFadeState =
          !isFirst ? CrossFadeState.showFirst : CrossFadeState.showSecond;
    });
  }

  Widget _buildPanel() => AnimatedCrossFade(
        firstCurve: Curves.easeInCirc,
        secondCurve: Curves.easeInToLinear,
        firstChild: Container(),
        secondChild: Container(
          height: 150,
          color: Colors.blue,
        ),
        duration: Duration(milliseconds: 300),
        crossFadeState: _crossFadeState,
      );
}
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// Copyright 2017 The Chromium Authors. All rights reserved.
// Use of this source code is governed by a BSD-style license that can be
// found in the LICENSE file.

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

const Duration _kExpand = Duration(milliseconds: 200);

class NoBorderExpansionTile extends StatefulWidget {

  const ExpansionTile({
    Key key,
    this.leading,
    @required this.title,
    this.subtitle,
    this.backgroundColor,
    this.onExpansionChanged,
    this.children = const [],
    this.trailing,
    this.initiallyExpanded = false,
  }) : assert(initiallyExpanded != null),
        super(key: key);

  final Widget leading;
  
  final Widget title;

  final Widget subtitle;
  
  final ValueChanged<bool> onExpansionChanged;
  
  final List children;

  final Color backgroundColor;

  final Widget trailing;

  final bool initiallyExpanded;

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

class _ExpansionTileState extends State<ExpansionTile> with SingleTickerProviderStateMixin {
  static final Animatable<double> _easeOutTween = CurveTween(curve: Curves.easeOut);
  static final Animatable<double> _easeInTween = CurveTween(curve: Curves.easeIn);
  static final Animatable<double> _halfTween = Tween<double>(begin: 0.0, end: 0.5);

  final ColorTween _borderColorTween = ColorTween();
  final ColorTween _headerColorTween = ColorTween();
  final ColorTween _iconColorTween = ColorTween();
  final ColorTween _backgroundColorTween = ColorTween();

  AnimationController _controller;
  Animation<double> _iconTurns;
  Animation<double> _heightFactor;
  Animation _borderColor;
  Animation _headerColor;
  Animation _iconColor;
  Animation _backgroundColor;

  bool _isExpanded = false;

  @override
  void initState() {
    super.initState();
    _controller = AnimationController(duration: _kExpand, vsync: this);
    _heightFactor = _controller.drive(_easeInTween);
    _iconTurns = _controller.drive(_halfTween.chain(_easeInTween));
    _borderColor = _controller.drive(_borderColorTween.chain(_easeOutTween));
    _headerColor = _controller.drive(_headerColorTween.chain(_easeInTween));
    _iconColor = _controller.drive(_iconColorTween.chain(_easeInTween));
    _backgroundColor = _controller.drive(_backgroundColorTween.chain(_easeOutTween));

    _isExpanded = PageStorage.of(context)?.readState(context) ?? widget.initiallyExpanded;
    if (_isExpanded)
      _controller.value = 1.0;
  }

  @override
  void dispose() {
    _controller.dispose();
    super.dispose();
  }

  void _handleTap() {
    setState(() {
      _isExpanded = !_isExpanded;
      if (_isExpanded) {
        _controller.forward();
      } else {
        _controller.reverse().then<void>((void value) {
          if (!mounted)
            return;
          setState(() {
            // Rebuild without widget.children.
          });
        });
      }
      PageStorage.of(context)?.writeState(context, _isExpanded);
    });
    if (widget.onExpansionChanged != null)
      widget.onExpansionChanged(_isExpanded);
  }

  Widget _buildChildren(BuildContext context, Widget child) {

    return Container(
      color: _backgroundColor.value ?? Colors.transparent,
      child: Column(
        mainAxisSize: MainAxisSize.min,
        children: [
          ListTileTheme.merge(
            iconColor: _iconColor.value,
            textColor: _headerColor.value,
            child: ListTile(
              onTap: _handleTap,
              leading: widget.leading,
              title: widget.title,
              subtitle: widget.subtitle,
              trailing: widget.trailing ?? RotationTransition(
                turns: _iconTurns,
                child: const Icon(Icons.expand_more),
              ),
            ),
          ),
          ClipRect(
            child: Align(
              heightFactor: _heightFactor.value,
              child: child,
            ),
          ),
        ],
      ),
    );
  }

  @override
  void didChangeDependencies() {
    final ThemeData theme = Theme.of(context);
    _borderColorTween
      ..end = theme.dividerColor;
    _headerColorTween
      ..begin = theme.textTheme.subhead.color
      ..end = theme.accentColor;
    _iconColorTween
      ..begin = theme.unselectedWidgetColor
      ..end = theme.accentColor;
    _backgroundColorTween
      ..end = widget.backgroundColor;
    super.didChangeDependencies();
  }

  @override
  Widget build(BuildContext context) {
    final bool closed = !_isExpanded && _controller.isDismissed;
    return AnimatedBuilder(
      animation: _controller.view,
      builder: _buildChildren,
      child: closed ? null : Column(children: widget.children),
    );
  }
}
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ClipRect(
  child: Align(
    alignment: Alignment.topCenter,
    heightFactor: _heightFactor.value,
    child: child,
  ),
),