//* StaggeredTile.count(結合する列数、結合する行数)

static const List<StaggeredTile> _tiles = [
  //* 上段
  StaggeredTile.count(1, 1),
  StaggeredTile.count(1, 1),
  StaggeredTile.count(1, 1),
  StaggeredTile.count(1, 1),
  //* 下段
  StaggeredTile.count(2, 1),
  StaggeredTile.count(2, 1),
];

static const List<StaggeredTile> _tiles = [
  //* 1行目
  StaggeredTile.count(2, 1),
  StaggeredTile.count(2, 1),
  StaggeredTile.count(2, 1),
  //* 2行目
  StaggeredTile.count(3, 1),
  StaggeredTile.count(3, 1),
  //* 3行目
  StaggeredTile.count(6, 1),
];

StaggeredGridView.countBuilder(
  //* 列数
  crossAxisCount: 6,
  //* アイテム
  itemBuilder: (BuilderContext context, int index) {
    return Text(index);
  },
  //* アイテムに対応したタイル
  staggeredTileBuilder: (int index) {
    return StaggeredTile.count(1, 1)
  }
)

StaggeredGridView.countBuilder({
  Axis scrollDirection: Axis.vertical,
  bool reverse: false,
  ScrollController controller,
  bool primary,
  ScrollPhysics physics,
  bool shrinkWrap: false,
  EdgeInsetsGeometry padding,
  @required int crossAxisCount,
  @required IndexedWidgetBuilder itemBuilder,
  @required IndexedStaggeredTileBuilder staggeredTileBuilder,
  int itemCount,
  double mainAxisSpacing: 0.0,
  double crossAxisSpacing: 0.0,
  bool addAutomaticKeepAlives: true,
  bool addRepaintBoundaries: true,
})

lib/
├── main.dart
└── staggered_grid_view_demo.dart

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

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

class MyApp extends StatelessWidget {
  // This widget is the root of your application.
  @override
  Widget build(BuildContext context) {
    return MaterialApp(
      title: 'Staggered Grid View',
      home: StaggeredGridViewDemo(),
    );
  }
}

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

class StaggeredGridViewDemo extends StatelessWidget {
  @override
  Widget build(BuildContext context) {
    return Scaffold(
      appBar: AppBar(
        title: Text('Staggered Grid View Demo'),
      ),
      body: _StaggeredGridView(),
    );
  }
}

class _StaggeredGridView extends StatelessWidget {
  const _StaggeredGridView({
    Key key,
  }) : super(key: key);

  /*
  * StaggeredTiles
  * 3,2,1列のレイアウトを1グループとして、それをループして表示する場合
  * 引数はdoubleなので、少数も指定できる
  ------------------------------------------*/
  static const List<StaggeredTile> _tiles = [
    StaggeredTile.count(2, 1),
    StaggeredTile.count(2, 1),
    StaggeredTile.count(2, 1),
    StaggeredTile.count(3, 1),
    StaggeredTile.count(3, 1),
    StaggeredTile.count(6, 1),
  ];

  @override
  Widget build(BuildContext context) {
    return Container(
      child: Padding(
        padding: EdgeInsets.all(5.0),
        child: StaggeredGridView.countBuilder(
          //* 最大列数。今回は3列と2列が共存しているため最小公倍数の6を設定する
          crossAxisCount: 6,

          //* アイテム数
          itemCount: 24,

          //* アイテムビルダー
          itemBuilder: (BuildContext context, int index) {
            return _ItemWidget(index: index);
          },

          //* タイルビルダー。同じindexのアイテムに対して、どのタイルを反映させるか。
          staggeredTileBuilder: (int index) {
            //* indexを_tilesの長さで割ったときのあまりが、該当するタイルのインデックスとなる。
            int _tileIndex = index % _tiles.length;
            return _tiles[_tileIndex];
          },

          //* スペーサー
          mainAxisSpacing: 5.0,
          crossAxisSpacing: 5.0,
        ),
      ),
    );
  }
}

class _ItemWidget extends StatelessWidget {
  final int index;
  _ItemWidget({Key key, @required this.index})
      : assert(index != null),
        super(key: key);

  @override
  Widget build(BuildContext context) {
    return Container(
      padding: EdgeInsets.all(5.0),
      color: Colors.black12,
      child: Text('Title $index'),
    );
  }
}