flutter/examples/api/lib/widgets/scroll_view/grid_view.0.dart

// Copyright 2014 The Flutter 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/material.dart';
import 'package:flutter/rendering.dart';

void main() => runApp(const GridViewExampleApp());

class GridViewExampleApp extends StatelessWidget {
  const GridViewExampleApp({super.key});

  @override
  Widget build(BuildContext context) {
    return MaterialApp(
      home: Padding(
        padding: const EdgeInsets.all(20.0),
        child: Card(
          elevation: 8.0,
          child: GridView.builder(
            padding: const EdgeInsets.all(12.0),
            gridDelegate: CustomGridDelegate(dimension: 240.0),
            // Try uncommenting some of these properties to see the effect on the grid:
            // itemCount: 20, // The default is that the number of grid tiles is infinite.
            // scrollDirection: Axis.horizontal, // The default is vertical.
            // reverse: true, // The default is false, going down (or left to right).
            itemBuilder: (BuildContext context, int index) {
              final math.Random random = math.Random(index);
              return GridTile(
                header: GridTileBar(
                  title: Text('$index', style: const TextStyle(color: Colors.black)),
                ),
                child: Container(
                  margin: const EdgeInsets.all(12.0),
                  decoration: ShapeDecoration(
                    shape: RoundedRectangleBorder(
                      borderRadius: BorderRadius.circular(12.0),
                    ),
                    gradient: const RadialGradient(
                      colors: <Color>[ Color(0x0F88EEFF), Color(0x2F0099BB) ],
                    ),
                  ),
                  child: FlutterLogo(
                    style: FlutterLogoStyle.values[random.nextInt(FlutterLogoStyle.values.length)],
                  ),
                ),
              );
            },
          ),
        ),
      ),
    );
  }
}

class CustomGridDelegate extends SliverGridDelegate {
  CustomGridDelegate({ required this.dimension });

  // This is the desired height of each row (and width of each square).
  // When there is not enough room, we shrink this to the width of the scroll view.
  final double dimension;

  // The layout is two rows of squares, then one very wide cell, repeat.

  @override
  SliverGridLayout getLayout(SliverConstraints constraints) {
    // Determine how many squares we can fit per row.
    int count = constraints.crossAxisExtent ~/ dimension;
    if (count < 1) {
      count = 1; // Always fit at least one regardless.
    }
    final double squareDimension = constraints.crossAxisExtent / count;
    return CustomGridLayout(
      crossAxisCount: count,
      fullRowPeriod: 3, // Number of rows per block (one of which is the full row).
      dimension: squareDimension,
    );
  }

  @override
  bool shouldRelayout(CustomGridDelegate oldDelegate) {
    return dimension != oldDelegate.dimension;
  }
}

class CustomGridLayout extends SliverGridLayout {
  const CustomGridLayout({
    required this.crossAxisCount,
    required this.dimension,
    required this.fullRowPeriod,
  }) : assert(crossAxisCount > 0),
       assert(fullRowPeriod > 1),
       loopLength = crossAxisCount * (fullRowPeriod - 1) + 1,
       loopHeight = fullRowPeriod * dimension;

  final int crossAxisCount;
  final double dimension;
  final int fullRowPeriod;

  // Computed values.
  final int loopLength;
  final double loopHeight;

  @override
  double computeMaxScrollOffset(int childCount) {
    // This returns the scroll offset of the end side of the childCount'th child.
    // In the case of this example, this method is not used, since the grid is
    // infinite. However, if one set an itemCount on the GridView above, this
    // function would be used to determine how far to allow the user to scroll.
    if (childCount == 0 || dimension == 0) {
      return 0;
    }
    return (childCount ~/ loopLength) * loopHeight
         + ((childCount % loopLength) ~/ crossAxisCount) * dimension;
  }

  @override
  SliverGridGeometry getGeometryForChildIndex(int index) {
    // This returns the position of the index'th tile.
    //
    // The SliverGridGeometry object returned from this method has four
    // properties. For a grid that scrolls down, as in this example, the four
    // properties are equivalent to x,y,width,height. However, since the
    // GridView is direction agnostic, the names used for SliverGridGeometry are
    // also direction-agnostic.
    //
    // Try changing the scrollDirection and reverse properties on the GridView
    // to see how this algorithm works in any direction (and why, therefore, the
    // names are direction-agnostic).
    final int loop = index ~/ loopLength;
    final int loopIndex = index % loopLength;
    if (loopIndex == loopLength - 1) {
      // Full width case.
      return SliverGridGeometry(
        scrollOffset: (loop + 1) * loopHeight - dimension, // "y"
        crossAxisOffset: 0, // "x"
        mainAxisExtent: dimension, // "height"
        crossAxisExtent: crossAxisCount * dimension, // "width"
      );
    }
    // Square case.
    final int rowIndex = loopIndex ~/ crossAxisCount;
    final int columnIndex = loopIndex % crossAxisCount;
    return SliverGridGeometry(
      scrollOffset: (loop * loopHeight) + (rowIndex * dimension), // "y"
      crossAxisOffset: columnIndex * dimension, // "x"
      mainAxisExtent: dimension, // "height"
      crossAxisExtent: dimension, // "width"
    );
  }

  @override
  int getMinChildIndexForScrollOffset(double scrollOffset) {
    // This returns the first index that is visible for a given scrollOffset.
    //
    // The GridView only asks for the geometry of children that are visible
    // between the scroll offset passed to getMinChildIndexForScrollOffset and
    // the scroll offset passed to getMaxChildIndexForScrollOffset.
    //
    // It is the responsibility of the SliverGridLayout to ensure that
    // getGeometryForChildIndex is consistent with getMinChildIndexForScrollOffset
    // and getMaxChildIndexForScrollOffset.
    //
    // Not every child between the minimum child index and the maximum child
    // index need be visible (some may have scroll offsets that are outside the
    // view; this happens commonly when the grid view places tiles out of
    // order). However, doing this means the grid view is less efficient, as it
    // will do work for children that are not visible. It is preferred that the
    // children are returned in the order that they are laid out.
    final int rows = scrollOffset ~/ dimension;
    final int loops = rows ~/ fullRowPeriod;
    final int extra = rows % fullRowPeriod;
    return loops * loopLength + extra * crossAxisCount;
  }

  @override
  int getMaxChildIndexForScrollOffset(double scrollOffset) {
    // (See commentary above.)
    final int rows = scrollOffset ~/ dimension;
    final int loops = rows ~/ fullRowPeriod;
    final int extra = rows % fullRowPeriod;
    final int count = loops * loopLength + extra * crossAxisCount;
    if (extra == fullRowPeriod - 1) {
      return count;
    }
    return count + crossAxisCount - 1;
  }
}