flutter_flutter/packages/flutter/lib/src/material/data_table.dart

// Copyright 2016 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 'dart:math' as math;

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

import 'checkbox.dart';
import 'colors.dart';
import 'debug.dart';
import 'dropdown.dart';
import 'icons.dart';
import 'ink_well.dart';
import 'material.dart';
import 'theme.dart';
import 'theme_data.dart';
import 'tooltip.dart';

/// Signature for [DataColumn.onSort] callback.
typedef void DataColumnSortCallback(int columnIndex, bool ascending);

/// Column configuration for a [DataTable].
///
/// One column configuration must be provided for each column to
/// display in the table. The list of [DataColumn] objects is passed
/// as the `columns` argument to the [new DataTable] constructor.
@immutable
class DataColumn {
  /// Creates the configuration for a column of a [DataTable].
  ///
  /// The [label] argument must not be null.
  const DataColumn({
    @required this.label,
    this.tooltip,
    this.numeric: false,
    this.onSort
  }) : assert(label != null);

  /// The column heading.
  ///
  /// Typically, this will be a [Text] widget. It could also be an
  /// [Icon] (typically using size 18), or a [Row] with an icon and
  /// some text.
  ///
  /// The label should not include the sort indicator.
  final Widget label;

  /// The column heading's tooltip.
  ///
  /// This is a longer description of the column heading, for cases
  /// where the heading might have been abbreviated to keep the column
  /// width to a reasonable size.
  final String tooltip;

  /// Whether this column represents numeric data or not.
  ///
  /// The contents of cells of columns containing numeric data are
  /// right-aligned.
  final bool numeric;

  /// Called when the user asks to sort the table using this column.
  ///
  /// If null, the column will not be considered sortable.
  ///
  /// See [DataTable.sortColumnIndex] and [DataTable.sortAscending].
  final DataColumnSortCallback onSort;

  bool get _debugInteractive => onSort != null;
}

/// Row configuration and cell data for a [DataTable].
///
/// One row configuration must be provided for each row to
/// display in the table. The list of [DataRow] objects is passed
/// as the `rows` argument to the [new DataTable] constructor.
///
/// The data for this row of the table is provided in the [cells]
/// property of the [DataRow] object.
@immutable
class DataRow {
  /// Creates the configuration for a row of a [DataTable].
  ///
  /// The [cells] argument must not be null.
  const DataRow({
    this.key,
    this.selected: false,
    this.onSelectChanged,
    @required this.cells
  }) : assert(cells != null);

  /// Creates the configuration for a row of a [DataTable], deriving
  /// the key from a row index.
  ///
  /// The [cells] argument must not be null.
  DataRow.byIndex({
    int index,
    this.selected: false,
    this.onSelectChanged,
    @required this.cells
  }) : assert(cells != null),
       key = new ValueKey<int>(index);

  /// A [Key] that uniquely identifies this row. This is used to
  /// ensure that if a row is added or removed, any stateful widgets
  /// related to this row (e.g. an in-progress checkbox animation)
  /// remain on the right row visually.
  ///
  /// If the table never changes once created, no key is necessary.
  final LocalKey key;

  /// Called when the user selects or unselects a selectable row.
  ///
  /// If this is not null, then the row is selectable. The current
  /// selection state of the row is given by [selected].
  ///
  /// If any row is selectable, then the table's heading row will have
  /// a checkbox that can be checked to select all selectable rows
  /// (and which is checked if all the rows are selected), and each
  /// subsequent row will have a checkbox to toggle just that row.
  ///
  /// A row whose [onSelectChanged] callback is null is ignored for
  /// the purposes of determining the state of the "all" checkbox,
  /// and its checkbox is disabled.
  final ValueChanged<bool> onSelectChanged;

  /// Whether the row is selected.
  ///
  /// If [onSelectChanged] is non-null for any row in the table, then
  /// a checkbox is shown at the start of each row. If the row is
  /// selected (true), the checkbox will be checked and the row will
  /// be highlighted.
  ///
  /// Otherwise, the checkbox, if present, will not be checked.
  final bool selected;

  /// The data for this row.
  ///
  /// There must be exactly as many cells as there are columns in the
  /// table.
  final List<DataCell> cells;

  bool get _debugInteractive => onSelectChanged != null || cells.any((DataCell cell) => cell._debugInteractive);
}

/// The data for a cell of a [DataTable].
///
/// One list of [DataCell] objects must be provided for each [DataRow]
/// in the [DataTable], in the [new DataRow] constructor's `cells`
/// argument.
@immutable
class DataCell {
  /// Creates an object to hold the data for a cell in a [DataTable].
  ///
  /// The first argument is the widget to show for the cell, typically
  /// a [Text] or [DropdownButton] widget; this becomes the [child]
  /// property and must not be null.
  ///
  /// If the cell has no data, then a [Text] widget with placeholder
  /// text should be provided instead, and then the [placeholder]
  /// argument should be set to true.
  const DataCell(this.child, {
    this.placeholder: false,
    this.showEditIcon: false,
    this.onTap,
  }) : assert(child != null);

  /// A cell that has no content and has zero width and height.
  static final DataCell empty = new DataCell(new Container(width: 0.0, height: 0.0));

  /// The data for the row.
  ///
  /// Typically a [Text] widget or a [DropdownButton] widget.
  ///
  /// If the cell has no data, then a [Text] widget with placeholder
  /// text should be provided instead, and [placeholder] should be set
  /// to true.
  final Widget child;

  /// Whether the [child] is actually a placeholder.
  ///
  /// If this is true, the default text style for the cell is changed
  /// to be appropriate for placeholder text.
  final bool placeholder;

  /// Whether to show an edit icon at the end of the cell.
  ///
  /// This does not make the cell actually editable; the caller must
  /// implement editing behavior if desired (initiated from the
  /// [onTap] callback).
  ///
  /// If this is set, [onTap] should also be set, otherwise tapping
  /// the icon will have no effect.
  final bool showEditIcon;

  /// Called if the cell is tapped.
  ///
  /// If non-null, tapping the cell will call this callback. If
  /// null, tapping the cell will attempt to select the row (if
  /// [DataRow.onSelectChanged] is provided).
  final VoidCallback onTap;

  bool get _debugInteractive => onTap != null;
}

/// A material design data table.
///
/// Displaying data in a table is expensive, because to lay out the
/// table all the data must be measured twice, once to negotiate the
/// dimensions to use for each column, and once to actually lay out
/// the table given the results of the negotiation.
///
/// For this reason, if you have a lot of data (say, more than a dozen
/// rows with a dozen columns, though the precise limits depend on the
/// target device), it is suggested that you use a
/// [PaginatedDataTable] which automatically splits the data into
/// multiple pages.
// TODO(ianh): Also suggest [ScrollingDataTable] once we have it.
///
/// See also:
///
///  * [DataColumn], which describes a column in the data table.
///  * [DataRow], which contains the data for a row in the data table.
///  * [DataCell], which contains the data for a single cell in the data table.
///  * [PaginatedDataTable], which shows part of the data in a data table and
///    provides controls for paging through the remainder of the data.
///  * <https://material.google.com/components/data-tables.html>
class DataTable extends StatelessWidget {
  /// Creates a widget describing a data table.
  ///
  /// The [columns] argument must be a list of as many [DataColumn]
  /// objects as the table is to have columns, ignoring the leading
  /// checkbox column if any. The [columns] argument must have a
  /// length greater than zero and cannot be null.
  ///
  /// The [rows] argument must be a list of as many [DataRow] objects
  /// as the table is to have rows, ignoring the leading heading row
  /// that contains the column headings (derived from the [columns]
  /// argument). There may be zero rows, but the rows argument must
  /// not be null.
  ///
  /// Each [DataRow] object in [rows] must have as many [DataCell]
  /// objects in the [DataRow.cells] list as the table has columns.
  ///
  /// If the table is sorted, the column that provides the current
  /// primary key should be specified by index in [sortColumnIndex], 0
  /// meaning the first column in [columns], 1 being the next one, and
  /// so forth.
  ///
  /// The actual sort order can be specified using [sortAscending]; if
  /// the sort order is ascending, this should be true (the default),
  /// otherwise it should be false.
  DataTable({
    Key key,
    @required this.columns,
    this.sortColumnIndex,
    this.sortAscending: true,
    this.onSelectAll,
    @required this.rows
  }) : assert(columns != null),
       assert(columns.isNotEmpty),
       assert(sortColumnIndex == null || (sortColumnIndex >= 0 && sortColumnIndex < columns.length)),
       assert(sortAscending != null),
       assert(rows != null),
       assert(!rows.any((DataRow row) => row.cells.length != columns.length)),
       _onlyTextColumn = _initOnlyTextColumn(columns),
       super(key: key);

  /// The configuration and labels for the columns in the table.
  final List<DataColumn> columns;

  /// The current primary sort key's column.
  ///
  /// If non-null, indicates that the indicated column is the column
  /// by which the data is sorted. The number must correspond to the
  /// index of the relevant column in [columns].
  ///
  /// Setting this will cause the relevant column to have a sort
  /// indicator displayed.
  ///
  /// When this is null, it implies that the table's sort order does
  /// not correspond to any of the columns.
  final int sortColumnIndex;

  /// Whether the column mentioned in [sortColumnIndex], if any, is sorted
  /// in ascending order.
  ///
  /// If true, the order is ascending (meaning the rows with the
  /// smallest values for the current sort column are first in the
  /// table).
  ///
  /// If false, the order is descending (meaning the rows with the
  /// smallest values for the current sort column are last in the
  /// table).
  final bool sortAscending;

  /// Invoked when the user selects or unselects every row, using the
  /// checkbox in the heading row.
  ///
  /// If this is null, then the [DataRow.onSelectChanged] callback of
  /// every row in the table is invoked appropriately instead.
  ///
  /// To control whether a particular row is selectable or not, see
  /// [DataRow.onSelectChanged]. This callback is only relevant if any
  /// row is selectable.
  final ValueSetter<bool> onSelectAll;

  /// The data to show in each row (excluding the row that contains
  /// the column headings). Must be non-null, but may be empty.
  final List<DataRow> rows;

  // Set by the constructor to the index of the only Column that is
  // non-numeric, if there is exactly one, otherwise null.
  final int _onlyTextColumn;
  static int _initOnlyTextColumn(List<DataColumn> columns) {
    int result;
    for (int index = 0; index < columns.length; index += 1) {
      final DataColumn column = columns[index];
      if (!column.numeric) {
        if (result != null)
          return null;
        result = index;
      }
    }
    return result;
  }

  bool get _debugInteractive {
    return columns.any((DataColumn column) => column._debugInteractive)
        || rows.any((DataRow row) => row._debugInteractive);
  }

  static final LocalKey _headingRowKey = new UniqueKey();

  void _handleSelectAll(bool checked) {
    if (onSelectAll != null) {
      onSelectAll(checked);
    } else {
      for (DataRow row in rows) {
        if ((row.onSelectChanged != null) && (row.selected != checked))
          row.onSelectChanged(checked);
      }
    }
  }

  static const double _kHeadingRowHeight = 56.0;
  static const double _kDataRowHeight = 48.0;
  static const double _kTablePadding = 24.0;
  static const double _kColumnSpacing = 56.0;
  static const double _kSortArrowPadding = 2.0;
  static const double _kHeadingFontSize = 12.0;
  static const Duration _kSortArrowAnimationDuration = const Duration(milliseconds: 150);
  static const Color _kGrey100Opacity = const Color(0x0A000000); // Grey 100 as opacity instead of solid color
  static const Color _kGrey300Opacity = const Color(0x1E000000); // Dark theme variant is just a guess.

  Widget _buildCheckbox({
    Color color,
    bool checked,
    VoidCallback onRowTap,
    ValueChanged<bool> onCheckboxChanged
  }) {
    Widget contents = new Padding(
      padding: const EdgeInsets.fromLTRB(_kTablePadding, 0.0, _kTablePadding / 2.0, 0.0),
      child: new Center(
        child: new Checkbox(
          activeColor: color,
          value: checked,
          onChanged: onCheckboxChanged
        )
      )
    );
    if (onRowTap != null) {
      contents = new TableRowInkWell(
        onTap: onRowTap,
        child: contents
      );
    }
    return new TableCell(
      verticalAlignment: TableCellVerticalAlignment.fill,
      child: contents
    );
  }

  Widget _buildHeadingCell({
    BuildContext context,
    EdgeInsets padding,
    Widget label,
    String tooltip,
    bool numeric,
    VoidCallback onSort,
    bool sorted,
    bool ascending
  }) {
    if (onSort != null) {
      final Widget arrow = new _SortArrow(
        visible: sorted,
        down: sorted ? ascending : null,
        duration: _kSortArrowAnimationDuration
      );
      final Widget arrowPadding = const SizedBox(width: _kSortArrowPadding);
      label = new Row(
        children: numeric ? <Widget>[ arrow, arrowPadding, label ]
                          : <Widget>[ label, arrowPadding, arrow ]
      );
    }
    label = new Container(
      padding: padding,
      height: _kHeadingRowHeight,
      alignment: new FractionalOffset(numeric ? 1.0 : 0.0, 0.5), // TODO(ianh): RTL for non-numeric
      child: new AnimatedDefaultTextStyle(
        style: new TextStyle(
          // TODO(ianh): font family should be Roboto; see https://github.com/flutter/flutter/issues/3116
          fontWeight: FontWeight.w500,
          fontSize: _kHeadingFontSize,
          height: _kHeadingRowHeight / _kHeadingFontSize,
          color: (Theme.of(context).brightness == Brightness.light)
            ? ((onSort != null && sorted) ? Colors.black87 : Colors.black54)
            : ((onSort != null && sorted) ? Colors.white : Colors.white70)
        ),
        duration: _kSortArrowAnimationDuration,
        child: label
      )
    );
    if (tooltip != null) {
      label = new Tooltip(
        message: tooltip,
        child: label
      );
    }
    if (onSort != null) {
      label = new InkWell(
        onTap: onSort,
        // TODO(ianh): When we do RTL, we need to use 'end' ordering for the non-numeric case
        child: label
      );
    }
    return label;
  }

  Widget _buildDataCell({
    BuildContext context,
    EdgeInsets padding,
    Widget label,
    bool numeric,
    bool placeholder,
    bool showEditIcon,
    VoidCallback onTap,
    VoidCallback onSelectChanged
  }) {
    final bool isLightTheme = Theme.of(context).brightness == Brightness.light;
    if (showEditIcon) {
      final Widget icon = const Icon(Icons.edit, size: 18.0);
      label = new Expanded(child: label);
      label = new Row(children: numeric ? <Widget>[ icon, label ] : <Widget>[ label, icon ]);
    }
    label = new Container(
      padding: padding,
      height: _kDataRowHeight,
      alignment: new FractionalOffset(numeric ? 1.0 : 0.0, 0.5), // TODO(ianh): RTL for non-numeric
      child: new DefaultTextStyle(
        style: new TextStyle(
          // TODO(ianh): font family should be Roboto; see https://github.com/flutter/flutter/issues/3116
          fontSize: 13.0,
          color: isLightTheme
            ? (placeholder ? Colors.black38 : Colors.black87)
            : (placeholder ? Colors.white30 : Colors.white70)
        ),
        child: IconTheme.merge(
          data: new IconThemeData(
            color: isLightTheme ? Colors.black54 : Colors.white70
          ),
          child: new DropdownButtonHideUnderline(child: label)
        )
      )
    );
    if (onTap != null) {
      label = new InkWell(
        onTap: onTap,
        child: label
      );
    } else if (onSelectChanged != null) {
      label = new TableRowInkWell(
        onTap: onSelectChanged,
        child: label
      );
    }
    return label;
  }

  @override
  Widget build(BuildContext context) {
    assert(!_debugInteractive || debugCheckHasMaterial(context));

    final ThemeData theme = Theme.of(context);
    final BoxDecoration _kSelectedDecoration = new BoxDecoration(
      border: new Border(bottom: new BorderSide(color: theme.dividerColor)),
      // The backgroundColor has to be transparent so you can see the ink on the material
      color: (Theme.of(context).brightness == Brightness.light) ? _kGrey100Opacity : _kGrey300Opacity
    );
    final BoxDecoration _kUnselectedDecoration = new BoxDecoration(
      border: new Border(bottom: new BorderSide(color: theme.dividerColor))
    );

    final bool showCheckboxColumn = rows.any((DataRow row) => row.onSelectChanged != null);
    final bool allChecked = showCheckboxColumn && !rows.any((DataRow row) => row.onSelectChanged != null && !row.selected);

    final List<TableColumnWidth> tableColumns = new List<TableColumnWidth>(columns.length + (showCheckboxColumn ? 1 : 0));
    final List<TableRow> tableRows = new List<TableRow>.generate(
      rows.length + 1, // the +1 is for the header row
      (int index) {
        return new TableRow(
          key: index == 0 ? _headingRowKey : rows[index - 1].key,
          decoration: index > 0 && rows[index - 1].selected ? _kSelectedDecoration
                                                            : _kUnselectedDecoration,
          children: new List<Widget>(tableColumns.length)
        );
      }
    );

    int rowIndex;

    int displayColumnIndex = 0;
    if (showCheckboxColumn) {
      tableColumns[0] = const FixedColumnWidth(_kTablePadding + Checkbox.width + _kTablePadding / 2.0);
      tableRows[0].children[0] = _buildCheckbox(
        color: theme.accentColor,
        checked: allChecked,
        onCheckboxChanged: _handleSelectAll
      );
      rowIndex = 1;
      for (DataRow row in rows) {
        tableRows[rowIndex].children[0] = _buildCheckbox(
          color: theme.accentColor,
          checked: row.selected,
          onRowTap: () => row.onSelectChanged(!row.selected),
          onCheckboxChanged: row.onSelectChanged
        );
        rowIndex += 1;
      }
      displayColumnIndex += 1;
    }

    for (int dataColumnIndex = 0; dataColumnIndex < columns.length; dataColumnIndex += 1) {
      final DataColumn column = columns[dataColumnIndex];
      final EdgeInsets padding = new EdgeInsets.fromLTRB(
        dataColumnIndex == 0 ? showCheckboxColumn ? _kTablePadding / 2.0 : _kTablePadding : _kColumnSpacing / 2.0,
        0.0,
        dataColumnIndex == columns.length - 1 ? _kTablePadding : _kColumnSpacing / 2.0,
        0.0
      );
      if (dataColumnIndex == _onlyTextColumn) {
        tableColumns[displayColumnIndex] = const IntrinsicColumnWidth(flex: 1.0);
      } else {
        tableColumns[displayColumnIndex] = const IntrinsicColumnWidth();
      }
      tableRows[0].children[displayColumnIndex] = _buildHeadingCell(
        context: context,
        padding: padding,
        label: column.label,
        tooltip: column.tooltip,
        numeric: column.numeric,
        onSort: () => column.onSort(dataColumnIndex, sortColumnIndex == dataColumnIndex ? !sortAscending : true),
        sorted: dataColumnIndex == sortColumnIndex,
        ascending: sortAscending
      );
      rowIndex = 1;
      for (DataRow row in rows) {
        final DataCell cell = row.cells[dataColumnIndex];
        tableRows[rowIndex].children[displayColumnIndex] = _buildDataCell(
          context: context,
          padding: padding,
          label: cell.child,
          numeric: column.numeric,
          placeholder: cell.placeholder,
          showEditIcon: cell.showEditIcon,
          onTap: cell.onTap,
          onSelectChanged: () => row.onSelectChanged(!row.selected)
        );
        rowIndex += 1;
      }
      displayColumnIndex += 1;
    }

    return new Table(
      columnWidths: tableColumns.asMap(),
      children: tableRows
    );
  }
}

/// A rectangular area of a Material that responds to touch but clips
/// its ink splashes to the current table row of the nearest table.
///
/// Must have an ancestor [Material] widget in which to cause ink
/// reactions and an ancestor [Table] widget to establish a row.
///
/// The [TableRowInkWell] must be in the same coordinate space (modulo
/// translations) as the [Table]. If it's rotated or scaled or
/// otherwise transformed, it will not be able to describe the
/// rectangle of the row in its own coordinate system as a [Rect], and
/// thus the splash will not occur. (In general, this is easy to
/// achieve: just put the [TableRowInkWell] as the direct child of the
/// [Table], and put the other contents of the cell inside it.)
class TableRowInkWell extends InkResponse {
  /// Creates an ink well for a table row.
  const TableRowInkWell({
    Key key,
    Widget child,
    GestureTapCallback onTap,
    GestureTapCallback onDoubleTap,
    GestureLongPressCallback onLongPress,
    ValueChanged<bool> onHighlightChanged
  }) : super(
    key: key,
    child: child,
    onTap: onTap,
    onDoubleTap: onDoubleTap,
    onLongPress: onLongPress,
    onHighlightChanged: onHighlightChanged,
    containedInkWell: true,
    highlightShape: BoxShape.rectangle
  );

  @override
  RectCallback getRectCallback(RenderBox referenceBox) {
    return () {
      RenderObject cell = referenceBox;
      AbstractNode table = cell.parent;
      final Matrix4 transform = new Matrix4.identity();
      while (table is RenderObject && table is! RenderTable) {
        final RenderTable parentBox = table;
        parentBox.applyPaintTransform(cell, transform);
        assert(table == cell.parent);
        cell = table;
        table = table.parent;
      }
      if (table is RenderTable) {
        final TableCellParentData cellParentData = cell.parentData;
        assert(cellParentData.y != null);
        final Rect rect = table.getRowBox(cellParentData.y);
        // The rect is in the table's coordinate space. We need to change it to the
        // TableRowInkWell's coordinate space.
        table.applyPaintTransform(cell, transform);
        final Offset offset = MatrixUtils.getAsTranslation(transform);
        if (offset != null)
          return rect.shift(-offset);
      }
      return Rect.zero;
    };
  }

  @override
  bool debugCheckContext(BuildContext context) {
    assert(debugCheckHasTable(context));
    return super.debugCheckContext(context);
  }
}

class _SortArrow extends StatefulWidget {
  const _SortArrow({
    Key key,
    this.visible,
    this.down,
    this.duration
  }) : super(key: key);

  final bool visible;

  final bool down;

  final Duration duration;

  @override
  _SortArrowState createState() => new _SortArrowState();
}

class _SortArrowState extends State<_SortArrow> with TickerProviderStateMixin {

  AnimationController _opacityController;
  Animation<double> _opacityAnimation;

  AnimationController _orientationController;
  Animation<double> _orientationAnimation;
  double _orientationOffset = 0.0;

  bool _down;

  @override
  void initState() {
    super.initState();
    _opacityAnimation = new CurvedAnimation(
      parent: _opacityController = new AnimationController(
        duration: widget.duration,
        vsync: this,
      ),
      curve: Curves.fastOutSlowIn
    )
    ..addListener(_rebuild);
    _opacityController.value = widget.visible ? 1.0 : 0.0;
    _orientationAnimation = new Tween<double>(
      begin: 0.0,
      end: math.PI
    ).animate(new CurvedAnimation(
      parent: _orientationController = new AnimationController(
        duration: widget.duration,
        vsync: this,
      ),
      curve: Curves.easeIn
    ))
    ..addListener(_rebuild)
    ..addStatusListener(_resetOrientationAnimation);
    if (widget.visible)
      _orientationOffset = widget.down ? 0.0 : math.PI;
  }

  void _rebuild() {
    setState(() {
      // The animations changed, so we need to rebuild.
    });
  }

  void _resetOrientationAnimation(AnimationStatus status) {
    if (status == AnimationStatus.completed) {
      assert(_orientationAnimation.value == math.PI);
      _orientationOffset += math.PI;
      _orientationController.value = 0.0; // TODO(ianh): This triggers a pointless rebuild.
    }
  }

  @override
  void didUpdateWidget(_SortArrow oldWidget) {
    super.didUpdateWidget(oldWidget);
    bool skipArrow = false;
    final bool newDown = widget.down != null ? widget.down : _down;
    if (oldWidget.visible != widget.visible) {
      if (widget.visible && (_opacityController.status == AnimationStatus.dismissed)) {
        _orientationController.stop();
        _orientationController.value = 0.0;
        _orientationOffset = newDown ? 0.0 : math.PI;
        skipArrow = true;
      }
      if (widget.visible) {
        _opacityController.forward();
      } else {
        _opacityController.reverse();
      }
    }
    if ((_down != newDown) && !skipArrow) {
      if (_orientationController.status == AnimationStatus.dismissed) {
        _orientationController.forward();
      } else {
        _orientationController.reverse();
      }
    }
    _down = newDown;
  }

  @override
  void dispose() {
    _opacityController.dispose();
    _orientationController.dispose();
    super.dispose();
  }

  static const double _kArrowIconBaselineOffset = -1.5;
  static const double _kArrowIconSize = 16.0;

  @override
  Widget build(BuildContext context) {
    return new Opacity(
      opacity: _opacityAnimation.value,
      child: new Transform(
        transform: new Matrix4.rotationZ(_orientationOffset + _orientationAnimation.value)
                             ..setTranslationRaw(0.0, _kArrowIconBaselineOffset, 0.0),
        alignment: FractionalOffset.center,
        child: new Icon(
          Icons.arrow_downward,
          size: _kArrowIconSize,
          color: (Theme.of(context).brightness == Brightness.light) ? Colors.black87 : Colors.white70
        )
      )
    );
  }

}