Solved: How to swipe/drag 2 or more buttons in a grid of buttons using flutter

Question

Asked by Karan S on June 01, 2018 (source).

I have made a grid of buttons using flutter but now I want to swipe through 2 or more buttons in a single drag such that all the buttons through which I am dragging gets selected.

I have checked out some questions on the same and I was redirected to use gesture detector but that's not enough. I need certain properties or better a sample code such that I am able to work through it.

an example of the dragable app is http://a5.mzstatic.com/us/r30/Purple60/v4/6f/00/35/6f0035d3-1bab-fcbb-cb13-8ab46cf3c44d/screen696x696.jpeg

Answer

Question answered by Rémi R (source).

You can manually hit test RenderBox and extract a specific RenderObject of your choice.

We could for example add the following renderobject above our buttons:

class Foo extends SingleChildRenderObjectWidget {
  final int index;

  Foo({Widget child, this.index, Key key}) : super(child: child, key: key);

  @override
  RenderObject createRenderObject(BuildContext context) {
    return _Foo()..index = index;
  }

  @override
  void updateRenderObject(BuildContext context, _Foo renderObject) {
    renderObject..index = index;
  }
}

class _Foo extends RenderProxyBox {
  int index;
}

Then use a Listener to extract all _Foo found under the pointer.

Here's a full application using this principle:

enter image description here

import 'package:flutter/gestures.dart';
import 'package:flutter/material.dart';
import 'package:flutter/rendering.dart';

void main() {
  runApp(MyApp());
}

class MyApp extends StatelessWidget {
  @override
  Widget build(BuildContext context) {
    return MaterialApp(
      title: 'Flutter Demo',
      theme: ThemeData(primarySwatch: Colors.blue),
      home: Grid(),
    );
  }
}

class Grid extends StatefulWidget {
  @override
  GridState createState() {
    return new GridState();
  }
}

class GridState extends State<Grid> {
  final Set<int> selectedIndexes = Set<int>();
  final key = GlobalKey();
  final Set<_Foo> _trackTaped = Set<_Foo>();

  _detectTapedItem(PointerEvent event) {
    final RenderBox box = key.currentContext.findRenderObject();
    final result = BoxHitTestResult();
    Offset local = box.globalToLocal(event.position);
    if (box.hitTest(result, position: local)) {
      for (final hit in result.path) {
        /// temporary variable so that the [is] allows access of [index]
        final target = hit.target;
        if (target is _Foo && !_trackTaped.contains(target)) {
          _trackTaped.add(target);
          _selectIndex(target.index);
        }
      }
    }
  }

  _selectIndex(int index) {
    setState(() {
      selectedIndexes.add(index);
    });
  }

  @override
  Widget build(BuildContext context) {
    return Listener(
      onPointerDown: _detectTapedItem,
      onPointerMove: _detectTapedItem,
      onPointerUp: _clearSelection,
      child: GridView.builder(
        key: key,
        itemCount: 6,
        physics: NeverScrollableScrollPhysics(),
        gridDelegate: SliverGridDelegateWithFixedCrossAxisCount(
          crossAxisCount: 3,
          childAspectRatio: 1.0,
          crossAxisSpacing: 5.0,
          mainAxisSpacing: 5.0,
        ),
        itemBuilder: (context, index) {
          return Foo(
            index: index,
            child: Container(
              color: selectedIndexes.contains(index) ? Colors.red : Colors.blue,
            ),
          );
        },
      ),
    );
  }

  void _clearSelection(PointerUpEvent event) {
    _trackTaped.clear();
    setState(() {
      selectedIndexes.clear();
    });
  }
}

class Foo extends SingleChildRenderObjectWidget {
  final int index;

  Foo({Widget child, this.index, Key key}) : super(child: child, key: key);

  @override
  _Foo createRenderObject(BuildContext context) {
    return _Foo()..index = index;
  }

  @override
  void updateRenderObject(BuildContext context, _Foo renderObject) {
    renderObject..index = index;
  }
}

class _Foo extends RenderProxyBox {
  int index;
}
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