Multi Select GridView in Flutter - by Simon Lightfoot (Drag to select multiple items) Try now here: https://dartpad.dev/experimental/embed-new-flutter.html?id=fbd01e9700bd19af3c71dafa0fbb454b&split=50

multi_select.dart

// MIT License
//
// Copyright (c) 2019 Simon Lightfoot
//
// Permission is hereby granted, free of charge, to any person obtaining a copy
// of this software and associated documentation files (the "Software"), to deal
// in the Software without restriction, including without limitation the rights
// to use, copy, modify, merge, publish, distribute, sublicense, and/or sell
// copies of the Software, and to permit persons to whom the Software is
// furnished to do so, subject to the following conditions:
//
// The above copyright notice and this permission notice shall be included in all
// copies or substantial portions of the Software.
//
// THE SOFTWARE IS PROVIDED "AS IS", WITHOUT WARRANTY OF ANY KIND, EXPRESS OR
// IMPLIED, INCLUDING BUT NOT LIMITED TO THE WARRANTIES OF MERCHANTABILITY,
// FITNESS FOR A PARTICULAR PURPOSE AND NONINFRINGEMENT. IN NO EVENT SHALL THE
// AUTHORS OR COPYRIGHT HOLDERS BE LIABLE FOR ANY CLAIM, DAMAGES OR OTHER
// LIABILITY, WHETHER IN AN ACTION OF CONTRACT, TORT OR OTHERWISE, ARISING FROM,
// OUT OF OR IN CONNECTION WITH THE SOFTWARE OR THE USE OR OTHER DEALINGS IN THE
// SOFTWARE.
//
// Thanks to Hugo Passos.
//
import 'package:flutter/material.dart';
import 'package:flutter/rendering.dart';
import 'package:flutter/widgets.dart';

void main() {
  runApp(
    MaterialApp(
      debugShowCheckedModeBanner: false,
      theme: ThemeData(
        primaryColor: Colors.indigo,
        accentColor: Colors.pinkAccent,
      ),
      home: ExampleScreen(),
    ),
  );
}

class ExampleScreen extends StatefulWidget {
  @override
  _ExampleScreenState createState() => _ExampleScreenState();
}

class _ExampleScreenState extends State<ExampleScreen> {
  final colors = <MaterialColor>[];

  var _selection = MultiChildSelection.empty;

  @override
  void initState() {
    super.initState();
    colors.addAll(Colors.primaries);
    colors.addAll(Colors.primaries);
    colors.addAll(Colors.primaries);
    colors.addAll(Colors.primaries);
  }

  @override
  Widget build(BuildContext context) {
    return Scaffold(
      appBar: MultiSelectAppBar(
        appBar: AppBar(
          title: Text('Multi Select Example'),
        ),
        selection: _selection,
        selectingActions: <Widget>[
          IconButton(
            icon: Icon(Icons.share),
            onPressed: () {},
          )
        ],
      ),
      body: MultiSelectGridView(
        onSelectionChanged: (selection) => setState(() => _selection = selection),
        padding: EdgeInsets.all(8.0),
        itemCount: colors.length,
        itemBuilder: (BuildContext context, int index, bool selected) {
          return ExampleChildItem(
            index: index,
            color: colors[index],
            selected: selected,
          );
        },
        gridDelegate: SliverGridDelegateWithFixedCrossAxisCount(
          crossAxisCount: 3,
          crossAxisSpacing: 8.0,
          mainAxisSpacing: 8.0,
        ),
      ),
    );
  }
}

class ExampleChildItem extends StatefulWidget {
  const ExampleChildItem({
    Key key,
    @required this.index,
    @required this.color,
    @required this.selected,
  }) : super(key: key);

  final int index;
  final MaterialColor color;
  final bool selected;

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

class _ExampleChildItemState extends State<ExampleChildItem> with SingleTickerProviderStateMixin {
  AnimationController _controller;
  Animation<double> _scaleAnimation;

  @override
  void initState() {
    super.initState();
    _controller = AnimationController(
      value: widget.selected ? 1.0 : 0.0,
      duration: kThemeChangeDuration,
      vsync: this,
    );
    _scaleAnimation = Tween<double>(begin: 1.0, end: 0.8).animate(CurvedAnimation(
      parent: _controller,
      curve: Curves.ease,
    ));
  }

  @override
  void didUpdateWidget(ExampleChildItem oldWidget) {
    super.didUpdateWidget(oldWidget);
    if (oldWidget.selected != widget.selected) {
      if (widget.selected) {
        _controller.forward();
      } else {
        _controller.reverse();
      }
    }
  }

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

  @override
  Widget build(BuildContext context) {
    return AnimatedBuilder(
      animation: _scaleAnimation,
      builder: (BuildContext context, Widget child) {
        final color = Color.lerp(widget.color.shade500, widget.color.shade900, _controller.value);
        return Transform.scale(
          scale: _scaleAnimation.value,
          child: DecoratedBox(
            decoration: BoxDecoration(color: color),
            child: child,
          ),
        );
      },
      child: Container(
        alignment: Alignment.center,
        child: Text(
          '${widget.index}',
          style: TextStyle(
            color: Colors.white,
            fontSize: 22.0,
          ),
        ),
      ),
    );
  }
}

class MultiSelectAppBar extends StatelessWidget implements PreferredSizeWidget {
  const MultiSelectAppBar({
    Key key,
    this.appBar,
    this.selection = MultiChildSelection.empty,
    this.selectingActions,
  })  : assert(selection != null),
        super(key: key);

  final AppBar appBar;
  final MultiChildSelection selection;
  final List<Widget> selectingActions;

  @override
  Widget build(BuildContext context) {
    return Container(
      color: appBar.backgroundColor ?? AppBarTheme.of(context).color ?? Theme.of(context).primaryColor,
      child: AnimatedSwitcher(
        duration: kThemeAnimationDuration,
        child: !selection.selecting
            ? appBar
            : AppBar(
                key: Key('multi-select'),
                leading: const BackButton(),
                titleSpacing: 0.0,
                title: Text('${selection.total} items selected…'),
                actions: selectingActions,
              ),
      ),
    );
  }

  @override
  Size get preferredSize => appBar.preferredSize;
}

typedef MultiSelectWidgetBuilder = Widget Function(BuildContext context, int index, bool selected);
typedef MultiSelectSelectionCb = void Function(MultiChildSelection selection);

class MultiSelectGridView extends StatefulWidget {
  const MultiSelectGridView({
    Key key,
    this.onSelectionChanged,
    this.padding,
    @required this.itemCount,
    @required this.itemBuilder,
    @required this.gridDelegate,
    this.scrollPadding = const EdgeInsets.all(48.0),
  }) : super(key: key);

  final MultiSelectSelectionCb onSelectionChanged;
  final EdgeInsetsGeometry padding;
  final int itemCount;
  final MultiSelectWidgetBuilder itemBuilder;
  final SliverGridDelegate gridDelegate;
  final EdgeInsets scrollPadding;

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

class _MultiSelectGridViewState extends State<MultiSelectGridView> {
  final _elements = List<_MultiSelectChildElement>();

  bool _isSelecting = false;
  int _startIndex = -1;
  int _endIndex = -1;

  LocalHistoryEntry _historyEntry;

  @override
  Widget build(BuildContext context) {
    return GestureDetector(
      onTapUp: _onTapUp,
      onLongPressStart: _onLongPressStart,
      onLongPressMoveUpdate: _onLongPressUpdate,
      onLongPressEnd: _onLongPressEnd,
      child: IgnorePointer(
        ignoring: _isSelecting,
        child: GridView.builder(
          padding: widget.padding,
          itemCount: widget.itemCount,
          itemBuilder: (BuildContext context, int index) {
            final start = (_startIndex < _endIndex) ? _startIndex : _endIndex;
            final end = (_startIndex < _endIndex) ? _endIndex : _startIndex;
            final selected = (index >= start && index <= end);
            return _MultiSelectChild(
              index: index,
              child: widget.itemBuilder(context, index, selected),
            );
          },
          gridDelegate: widget.gridDelegate,
        ),
      ),
    );
  }

  void _onTapUp(TapUpDetails details) {
    _setSelection(-1, -1);
    _updateLocalHistory();
  }

  void _onLongPressStart(LongPressStartDetails details) {
    final startIndex = _findMultiSelectChildFromOffset(details.localPosition);
    _setSelection(startIndex, startIndex);
    setState(() => _isSelecting = (startIndex != -1));
  }

  void _onLongPressUpdate(LongPressMoveUpdateDetails details) {
    if (_isSelecting) {
      _updateEndIndex(details.localPosition);
    }
  }

  void _onLongPressEnd(LongPressEndDetails details) {
    _updateEndIndex(details.localPosition);
    setState(() => _isSelecting = false);
  }

  void _updateEndIndex(Offset localPosition) {
    final endIndex = _findMultiSelectChildFromOffset(localPosition);
    if (endIndex != -1) {
      _setSelection(_startIndex, endIndex);
      _updateLocalHistory();
    }
  }

  void _setSelection(int start, int end) {
    setState(() {
      _startIndex = start;
      _endIndex = end;
    });
    if (widget.onSelectionChanged != null) {
      final start = (_startIndex < _endIndex) ? _startIndex : _endIndex;
      final end = (_startIndex < _endIndex) ? _endIndex : _startIndex;
      final total = (start != -1 && end != -1) ? ((end + 1) - start) : 0;
      widget.onSelectionChanged?.call(MultiChildSelection(total, start, end));
    }
  }

  void _updateLocalHistory() {
    final route = ModalRoute.of(context);
    if (route != null) {
      if (_startIndex != -1 && _endIndex != -1) {
        if (_historyEntry == null) {
          _historyEntry = LocalHistoryEntry(
            onRemove: () {
              _setSelection(-1, -1);
              _historyEntry = null;
            },
          );
          route.addLocalHistoryEntry(_historyEntry);
        }
      } else {
        if (_historyEntry != null) {
          route.removeLocalHistoryEntry(_historyEntry);
          _historyEntry = null;
        }
      }
    }
  }

  int _findMultiSelectChildFromOffset(Offset offset) {
    final ancestor = context.findRenderObject();
    for (_MultiSelectChildElement element in List.from(_elements)) {
      if (element.containsOffset(ancestor, offset)) {
        if (widget.scrollPadding != null) {
          element.showOnScreen(widget.scrollPadding);
        }
        return element.widget.index;
      }
    }
    return -1;
  }
}

class _MultiSelectChild extends ProxyWidget {
  const _MultiSelectChild({
    Key key,
    @required this.index,
    @required Widget child,
  }) : super(key: key, child: child);

  final int index;

  @override
  _MultiSelectChildElement createElement() => _MultiSelectChildElement(this);
}

class _MultiSelectChildElement extends ProxyElement {
  _MultiSelectChildElement(_MultiSelectChild widget) : super(widget);

  @override
  _MultiSelectChild get widget => super.widget;

  _MultiSelectGridViewState _ancestorState;

  @override
  void mount(Element parent, newSlot) {
    super.mount(parent, newSlot);
    _ancestorState = ancestorStateOfType(TypeMatcher<_MultiSelectGridViewState>());
    _ancestorState?._elements?.add(this);
  }

  @override
  void unmount() {
    _ancestorState?._elements?.remove(this);
    _ancestorState = null;
    super.unmount();
  }

  bool containsOffset(RenderObject ancestor, Offset offset) {
    final RenderBox box = renderObject;
    final rect = box.localToGlobal(Offset.zero, ancestor: ancestor) & box.size;
    return rect.contains(offset);
  }

  void showOnScreen(EdgeInsets scrollPadding) {
    final RenderBox box = renderObject;
    box.showOnScreen(
      rect: scrollPadding.inflateRect(Offset.zero & box.size),
      duration: const Duration(milliseconds: 150),
    );
  }

  @override
  void notifyClients(ProxyWidget oldWidget) {
    //
  }
}

class MultiChildSelection {
  const MultiChildSelection(this.total, this.start, this.end);

  static const empty = MultiChildSelection(0, -1, -1);

  final int total;
  final int start;
  final int end;

  bool get selecting => total != 0;

  @override
  String toString() => 'MultiChildSelection{$total, $start, $end}';
}