1. override
Widget build(BuildContext context)

Describes the part of the user interface represented by this widget.

The framework calls this method in a number of different situations:

  • After calling initState.
  • After calling didUpdateConfig.
  • After receiving a call to setState.
  • After a dependency of this State object changes (e.g., an InheritedWidget referenced by the previous build changes).
  • After calling deactivate and then reinserting the State object into the tree at another location.

The framework replaces the subtree below this widget with the widget returned by this method, either by updating the existing subtree or by removing the subtree and inflating a new subtree, depending on whether the widget returned by this method can update the root of the existing subtree, as determined by calling Widget.canUpdate.

Typically implementations return a newly created constellation of widgets that are configured with information from this widget's constructor, the given BuildContext, and the internal state of this State object.

The given BuildContext contains information about the location in the tree at which this widget is being built. For example, the context provides the set of inherited widgets for this location in the tree. The BuildContext argument is always the same as the context property of this State object and will remain the same for the lifetime of this object. The BuildContext argument is provided redundantly here so that this method matches the signature for a WidgetBuilder.

Source 

@override
Widget build(BuildContext context) {
  // TODO(ianh): This whole build function doesn't handle RTL yet.
  ThemeData themeData = Theme.of(context);
  // HEADER
  final List<Widget> headerWidgets = <Widget>[];
  double leftPadding = 24.0;
  if (_selectedRowCount == 0) {
    headerWidgets.add(new Expanded(child: config.header));
    if (config.header is ButtonBar) {
      // We adjust the padding when a button bar is present, because the
      // ButtonBar introduces 2 pixels of outside padding, plus 2 pixels
      // around each button on each side, and the button itself will have 8
      // pixels internally on each side, yet we want the left edge of the
      // inside of the button to line up with the 24.0 left inset.
      // TODO(ianh): Better magic. See https://github.com/flutter/flutter/issues/4460
      leftPadding = 12.0;
    }
  } else if (_selectedRowCount == 1) {
    // TODO(ianh): Real l10n.
    headerWidgets.add(new Expanded(child: new Text('1 item selected')));
  } else {
    headerWidgets.add(new Expanded(child: new Text('$_selectedRowCount items selected')));
  }
  if (config.actions != null) {
    headerWidgets.addAll(
      config.actions.map/*<Widget>*/((Widget widget) {
        return new Padding(
          // 8.0 is the default padding of an icon button
          padding: const EdgeInsets.only(left: 24.0 - 8.0 * 2.0),
          child: widget
        );
      }).toList()
    );
  }

  // FOOTER
  final TextStyle footerTextStyle = themeData.textTheme.caption;
  final List<Widget> footerWidgets = <Widget>[];
  if (config.onRowsPerPageChanged != null) {
    List<Widget> availableRowsPerPage = config.availableRowsPerPage
      .where((int value) => value <= _rowCount)
      .map/*<DropdownMenuItem<int>>*/((int value) {
        return new DropdownMenuItem<int>(
          value: value,
          child: new Text('$value')
        );
      })
      .toList();
    footerWidgets.addAll(<Widget>[
      new Text('Rows per page:'),
      new DropdownButtonHideUnderline(
        child: new DropdownButton<int>(
          items: availableRowsPerPage,
          value: config.rowsPerPage,
          onChanged: config.onRowsPerPageChanged,
          style: footerTextStyle,
          iconSize: 24.0
        )
      ),
    ]);
  }
  footerWidgets.addAll(<Widget>[
    new Container(width: 32.0),
    new Text(
      '${_firstRowIndex + 1}\u2013${_firstRowIndex + config.rowsPerPage} ${ _rowCountApproximate ? "of about" : "of" } $_rowCount'
    ),
    new Container(width: 32.0),
    new IconButton(
      icon: new Icon(Icons.chevron_left),
      padding: EdgeInsets.zero,
      onPressed: _firstRowIndex <= 0 ? null : () {
        pageTo(math.max(_firstRowIndex - config.rowsPerPage, 0));
      }
    ),
    new Container(width: 24.0),
    new IconButton(
      icon: new Icon(Icons.chevron_right),
      padding: EdgeInsets.zero,
      onPressed: (!_rowCountApproximate && (_firstRowIndex + config.rowsPerPage >= _rowCount)) ? null : () {
        pageTo(_firstRowIndex + config.rowsPerPage);
      }
    ),
    new Container(width: 14.0),
  ]);

  // CARD
  return new Card(
    child: new BlockBody(
      children: <Widget>[
        new DefaultTextStyle(
          // These typographic styles aren't quite the regular ones. We pick the closest ones from the regular
          // list and then tweak them appropriately.
          // See https://material.google.com/components/data-tables.html#data-tables-tables-within-cards
          style: _selectedRowCount > 0 ? themeData.textTheme.subhead.copyWith(color: themeData.accentColor)
                                       : themeData.textTheme.title.copyWith(fontWeight: FontWeight.w400),
          child: new IconTheme.merge(
            context: context,
            data: const IconThemeData(
              opacity: 0.54
            ),
            child: new ButtonTheme.bar(
              child: new Container(
                height: 64.0,
                padding: new EdgeInsets.fromLTRB(leftPadding, 0.0, 14.0, 0.0),
                // TODO(ianh): This decoration will prevent ink splashes from being visible.
                // Instead, we should have a widget that prints the decoration on the material.
                // See https://github.com/flutter/flutter/issues/3782
                decoration: _selectedRowCount > 0 ? new BoxDecoration(
                  backgroundColor: themeData.secondaryHeaderColor
                ) : null,
                child: new Row(
                  mainAxisAlignment: MainAxisAlignment.end,
                  children: headerWidgets
                )
              )
            )
          )
        ),
        new ScrollableViewport(
          scrollDirection: Axis.horizontal,
          child: new DataTable(
            key: _tableKey,
            columns: config.columns,
            sortColumnIndex: config.sortColumnIndex,
            sortAscending: config.sortAscending,
            onSelectAll: config.onSelectAll,
            rows: _getRows(_firstRowIndex, config.rowsPerPage)
          )
        ),
        new DefaultTextStyle(
          style: footerTextStyle,
          child: new IconTheme.merge(
            context: context,
            data: const IconThemeData(
              opacity: 0.54
            ),
            child: new Container(
              height: 56.0,
              child: new Row(
                mainAxisAlignment: MainAxisAlignment.end,
                children: footerWidgets
              )
            )
          )
        )
      ]
    )
  );
}