VariableSizedGridView for Flutter (Masonry style)

variable_sized_grid_view.dart

import 'dart:math' as math;

import 'package:flutter/foundation.dart';
import 'package:flutter/rendering.dart';
import 'package:flutter/widgets.dart';

/// Signature for a function that creates a [TileSize] for a given index.
typedef TileSize IndexedTileSizeBuilder(int index);

/// Creates grid layouts with a fixed number of spans in the cross axis.
/// Each tile can occupy from 1 to this number of spans.
///
/// For example, if the grid is vertical, this delegate will create a layout
/// with a fixed number of vertical spans.
///
/// This delegate creates grids with variable sized but equally spaced tiles.
class SliverGridDelegateWithFixedCrossAxisSpans extends SliverGridDelegate {
  /// The number of spans in the cross axis.
  final int crossAxisSpans;

  /// The number of logical pixels between each child along the main axis.
  final double mainAxisSpacing;

  /// The number of logical pixels between each span along the cross axis.
  final double crossAxisSpacing;

  final double mainAxisRatio;

  /// The function used to get the size of a tile from its index.
  final IndexedTileSizeBuilder tileSizeBuilder;

  /// Creates a delegate that makes grid layouts with variable sized tiles.
  ///
  /// All of the arguments must not be null. The [crossAxisSpans],
  /// [mainAxisSpacing], and [crossAxisSpacing] arguments must not be negative.
  const SliverGridDelegateWithFixedCrossAxisSpans(
      {@required this.crossAxisSpans,
      this.mainAxisSpacing: 0.0,
      this.crossAxisSpacing: 0.0,
      @required this.tileSizeBuilder,
      this.mainAxisRatio})
      : assert(crossAxisSpans != null && crossAxisSpans > 0),
        assert(mainAxisSpacing != null && mainAxisSpacing >= 0),
        assert(crossAxisSpacing != null && crossAxisSpacing >= 0),
        assert(tileSizeBuilder != null);

  @override
  SliverGridLayout getLayout(SliverConstraints constraints) {
    assert(_debugAssertIsValid());

    final double usableCrossAxisExtent =
        constraints.crossAxisExtent - crossAxisSpacing * (crossAxisSpans - 1);
    final double spanCrossAxisExtent = usableCrossAxisExtent / crossAxisSpans;

    return new SliverGridVariableSizedTileLayout(
      crossAxisSpans: crossAxisSpans,
      spanCrossAxisExtent: spanCrossAxisExtent,
      crossAxisSpacing: crossAxisSpacing,
      mainAxisSpacing: mainAxisSpacing,
      tileSizeBuilder: tileSizeBuilder,
      mainAxisRatio: mainAxisRatio,
    );
  }

  @override
  bool shouldRelayout(SliverGridDelegateWithFixedCrossAxisSpans oldDelegate) {
    return oldDelegate.crossAxisSpans != crossAxisSpans ||
        oldDelegate.mainAxisSpacing != mainAxisSpacing ||
        oldDelegate.crossAxisSpacing != crossAxisSpacing;
  }

  bool _debugAssertIsValid() {
    assert(crossAxisSpans > 0);
    assert(mainAxisSpacing >= 0.0);
    assert(crossAxisSpacing >= 0.0);
    return true;
  }
}

/// A [SliverGridLayout] that uses variable sized but equally spaced tiles.
///
/// This layout is used by [SliverGridDelegateWithVariableSizedTiles].
class SliverGridVariableSizedTileLayout extends SliverGridLayout {
  /// The number of spans in the cross axis.
  final int crossAxisSpans;

  /// The number of pixels from the leading edge of one span to the trailing
  /// edge of the same span in the cross axis.
  final double spanCrossAxisExtent;

  /// The number of logical pixels between each span along the cross axis.
  final double crossAxisSpacing;

  /// The number of logical pixels between each child along the main axis.
  final double mainAxisSpacing;

  /// The function used to get the size of a tile from its index.
  final IndexedTileSizeBuilder tileSizeBuilder;

  final double mainAxisRatio;

  final double _spanCrossAxisStride;

  final double _spanMainAxisStride;

  final Map<int, _TileGeometry> _indexToGeometry;

  final List<double> _offsets;

  /// Creates a layout that uses variable sized but equally spaced tiles.
  ///
  /// All of the arguments must not be null and must not be negative. The
  /// [crossAxisSpans] argument must be greater than zero.
  SliverGridVariableSizedTileLayout(
      {@required this.crossAxisSpans,
      @required this.spanCrossAxisExtent,
      @required this.crossAxisSpacing,
      @required this.mainAxisSpacing,
      @required this.tileSizeBuilder,
      this.mainAxisRatio})
      : assert(crossAxisSpans != null && crossAxisSpans > 0),
        assert(spanCrossAxisExtent != null && spanCrossAxisExtent >= 0),
        assert(crossAxisSpacing != null && crossAxisSpacing >= 0),
        assert(mainAxisSpacing != null && mainAxisSpacing >= 0),
        assert(tileSizeBuilder != null),
        _spanCrossAxisStride = spanCrossAxisExtent + crossAxisSpacing,
        _spanMainAxisStride = spanCrossAxisExtent + mainAxisSpacing,
        _indexToGeometry = new Map(),
        _offsets = new List.generate(crossAxisSpans, (i) => 0.0);

  @override
  double estimateMaxScrollOffset(int childCount) {
    // We cannot now just with child count.
    if (childCount == null) return null;
    _getTileGeometry(childCount - 1);
    var maxScrollOffset = _offsets.reduce(math.max);
    return maxScrollOffset;
  }

  @override
  SliverGridGeometry getGeometryForChildIndex(int index) {
    return _getTileGeometry(index).geometry;
  }

  double getMainAxisExtent(double extent) {
    if (mainAxisRatio == null) {
      return extent;
    } else {
      return _spanMainAxisStride * mainAxisRatio * extent - mainAxisSpacing;
    }
  }

  @override
  int getMaxChildIndexForScrollOffset(double scrollOffset) {
    int index = 0;
    List<double> offsets = new List.generate(crossAxisSpans, (i) => 0.0);

    while (true) {
      var tileGeometry = _getTileGeometry(index);
      if (tileGeometry == null) return index - 1;
      for (var i = 0; i < tileGeometry.tileSize.span; i++) {
        offsets[i + tileGeometry.spanIndex] =
            tileGeometry.geometry.trailingScrollOffset;
      }
      if (!offsets.any((i) => i <= scrollOffset)) return index;
      ++index;
    }
  }

  @override
  int getMinChildIndexForScrollOffset(double scrollOffset) {
    int index = 0;
    while (true) {
      var geometry = _getTileGeometry(index)?.geometry;
      if (geometry == null) return index - 1;
      if (geometry.trailingScrollOffset >= scrollOffset) return index;
      ++index;
    }
  }

  /// Finds the first available block with at least the specified [size].
  _Block _findFirstAvailableBlockWithSize(int size) {
    return _findFirstAvailableBlockWithSizeAndOffsets(
        size, new List.from(_offsets));
  }

  /// Finds the first available block with at least the specified [size].
  _Block _findFirstAvailableBlockWithSizeAndOffsets(
      int size, List<double> offsets) {
    _Block block = _findFirstAvailableBlock(offsets);
    if (block.size < size) {
      // Not enough space for the specified size.
      // We have to fill this block in try again.
      for (var i = 0; i < block.size; ++i) {
        offsets[i + block.index] = block.maxOffset;
      }
      return _findFirstAvailableBlockWithSizeAndOffsets(size, offsets);
    } else {
      return block;
    }
  }

  _TileGeometry _getTileGeometry(int index) {
    _TileGeometry tileGeometry = _indexToGeometry[index];

    if (tileGeometry == null) {
      // Populates the tiles geometries from the last computed index to this
      // index.
      for (var i = _indexToGeometry.length; i < index; ++i) {
        var x = _getTileGeometry(i);
        if (x == null) return null;
      }

      var tileSize = _normalizeTileSize(tileSizeBuilder(index));
      if (tileSize == null) return null;

      var block = _findFirstAvailableBlockWithSize(tileSize.span);

      var scrollOffset = block.minOffset;
      var crossAxisOffset = block.index * _spanCrossAxisStride;

      var geometry = new SliverGridGeometry(
          scrollOffset: scrollOffset,
          crossAxisOffset: crossAxisOffset,
          mainAxisExtent: tileSize.extent,
          crossAxisExtent:
              _spanCrossAxisStride * tileSize.span - crossAxisSpacing);
      tileGeometry = new _TileGeometry(tileSize, geometry, block.index);
      _indexToGeometry[index] = tileGeometry;
      var stride = tileSize.extent + mainAxisSpacing;
      for (var i = 0; i < tileSize.span; i++) {
        _offsets[i + block.index] = block.minOffset + stride;
      }
    }
    return tileGeometry;
  }

  TileSize _normalizeTileSize(TileSize tileSize) {
    if (tileSize == null) return null;
    return new TileSize(tileSize.span.clamp(0, crossAxisSpans),
        getMainAxisExtent(tileSize.extent));
  }

  /// Finds the first available block for the specified [offsets] list.
  static _Block _findFirstAvailableBlock(List<double> offsets) {
    int index = 0;
    double minBlockOffset = double.INFINITY;
    double maxBlockOffset = double.INFINITY;
    int size = 1;
    bool contiguous = false;

    for (var i = index; i < offsets.length; ++i) {
      var offset = offsets[i];
      if (offset < minBlockOffset) {
        index = i;
        maxBlockOffset = minBlockOffset;
        minBlockOffset = offset;
        size = 1;
        contiguous = true;
      } else if (offset == minBlockOffset && contiguous) {
        size++;
      } else if (offset < maxBlockOffset && offset > minBlockOffset) {
        contiguous = false;
        maxBlockOffset = offset;
      } else {
        contiguous = false;
      }
    }

    return new _Block(index, size, minBlockOffset, maxBlockOffset);
  }
}

/// The size of a tile that can be used by a [VariableSizedTileGridView].
class TileSize {
  /// The length of this tile in the cross axis.
  final int span;

  /// The number of pixels from the leading edge to the trailing edge of this tile
  /// in the main axis.
  final double extent;

  /// Creates a tile with a span and an extent.
  ///
  /// The arguments must be positive.
  const TileSize(this.span, this.extent)
      : assert(span != null && span >= 0),
        assert(extent != null && extent >= 0);
}

/// A scrollable 2D array of widgets that can have variable sizes.
class VariableSizedTileGridView extends GridView {
  /// Creates a scrollable 2D array of widgets with variable sizes.
  ///
  /// The [crossAxisSpans] is the number of spans in the cross axis.
  /// Typically for a vertical direction, this is the number of columns of
  /// your grid view. Each widget can have a span from 1 to [crossAxisSpans].
  /// It must be strictly positive.
  ///
  /// The [tileSizeBuilder] gives the tile size for the widget at the
  /// specified index.
  ///
  /// The [itemBuilder] has the same meaning as in [new GridView.builder].
  ///
  /// The [mainAxisSpacing] and [crossAxisSpacing] are the number of pixels
  /// between your widgets. These arguments are 0.0 by default and must be
  /// positives.
  ///
  /// The [mainAxisRatio] is the ratio between the computed cross axis extent
  /// for one span and the main axis extent of your widgets.
  /// For example, by setting a [mainAxisRatio] to 1, the widget for a tile
  /// size of (1, 1.0) will be a square.
  /// If you do not set the [mainAxisRatio] or set it to null, the extent of
  /// the tile size will be the main axis extent of your widget.
  VariableSizedTileGridView.builder({
    Key key,
    Axis scrollDirection: Axis.vertical,
    ScrollController controller,
    bool primary,
    ScrollPhysics physics,
    bool shrinkWrap: false,
    EdgeInsetsGeometry padding,
    double mainAxisSpacing: 0.0,
    double crossAxisSpacing: 0.0,
    @required int crossAxisSpans,
    @required IndexedTileSizeBuilder tileSizeBuilder,
    @required IndexedWidgetBuilder itemBuilder,
    double mainAxisRatio,
    int itemCount,
    bool addAutomaticKeepAlives: true,
    bool addRepaintBoundaries: true,
  })
      : assert(crossAxisSpans > 0),
        assert(tileSizeBuilder != null),
        assert(itemBuilder != null),
        assert(mainAxisSpacing != null && mainAxisSpacing >= 0.0),
        assert(crossAxisSpacing != null && crossAxisSpacing >= 0.0),
        super.custom(
            key: key,
            scrollDirection: scrollDirection,
            reverse: false,
            controller: controller,
            primary: primary,
            physics: physics,
            shrinkWrap: shrinkWrap,
            padding: padding,
            gridDelegate: new SliverGridDelegateWithFixedCrossAxisSpans(
              crossAxisSpans: crossAxisSpans,
              crossAxisSpacing: crossAxisSpacing,
              mainAxisSpacing: mainAxisSpacing,
              tileSizeBuilder: tileSizeBuilder,
              mainAxisRatio: mainAxisRatio,
            ),
            childrenDelegate: new SliverChildBuilderDelegate(
              itemBuilder,
              childCount: itemCount,
              addAutomaticKeepAlives: addAutomaticKeepAlives,
              addRepaintBoundaries: addRepaintBoundaries,
            ));
}

class _Block {
  final int index;
  final int size;
  final double minOffset;
  final double maxOffset;

  const _Block(this.index, this.size, this.minOffset, this.maxOffset);
}

class _TileGeometry {
  final TileSize tileSize;
  final SliverGridGeometry geometry;
  final int spanIndex;

  const _TileGeometry(this.tileSize, this.geometry, this.spanIndex);
}

variable_sized_grid_view_usage.dart

import 'package:flutter/material.dart';
import 'package:flutter_app_test_2/sliver_grid.dart';

void main() {
  runApp(new App());
}

class App extends StatelessWidget {

  final List<TileSize> tileSizes = [
    new TileSize(1, 1.0),
    new TileSize(1, 1.0),
    new TileSize(1, 2.0),
    new TileSize(1, 1.0),
    new TileSize(2, 1.0),
    new TileSize(1, 2.0),
    new TileSize(1, 1.0),
    new TileSize(2, 2.0),
    new TileSize(2, 1.0),
    new TileSize(1, 1.0),
    new TileSize(2, 2.0),
    new TileSize(2, 1.0),
    new TileSize(1, 2.0),
    new TileSize(2, 4.0),
    new TileSize(3, 1.0),
    new TileSize(4, 2.0),
    new TileSize(1, 1.0),
    new TileSize(3, 2.0),
    new TileSize(2, 1.0),
  ];

  // This widget is the root of your application.
  @override
  Widget build(BuildContext context) {
    return new MaterialApp(
        title: 'Flutter Demo',
        theme: new ThemeData(
          // This is the theme of your application.
          //
          // Try running your application with "flutter run". You'll see the
          // application has a blue toolbar. Then, without quitting the app, try
          // changing the primarySwatch below to Colors.green and then invoke
          // "hot reload" (press "r" in the console where you ran "flutter run",
          // or press Run > Flutter Hot Reload in IntelliJ). Notice that the
          // counter didn't reset back to zero; the application is not restarted.
          primarySwatch: Colors.blue,
        ),
        home: new Container(
            decoration: new BoxDecoration(
                color: Colors.white, shape: BoxShape.rectangle),
            child: new VariableSizedTileGridView.builder(
                padding: new EdgeInsets.all(10.0),
                itemCount: tileSizes.length,
                mainAxisSpacing: 10.0,
                crossAxisSpacing: 10.0,
                mainAxisRatio: 1.0,
                crossAxisSpans: 4,
                tileSizeBuilder: _tileSizeBuilder,
                itemBuilder: _itemBuilder)));
  }

  Widget _itemBuilder(BuildContext context, int index) {
    if (index >= tileSizes.length) return null;
    var color = index.isEven ? Colors.green : Colors.blue;
    return new Container(
        decoration:
            new BoxDecoration(color: color, shape: BoxShape.rectangle),
        child: new Center(
          child: new CircleAvatar(
            backgroundColor: Colors.white,
            child: new Text("$index"),
          ),
        ));
  }

  TileSize _tileSizeBuilder(int index) {
    if (index >= tileSizes.length) return null;
    return tileSizes[index];
  }
}