sliver_grid.dart 27 KB
Newer Older
Ian Hickson's avatar
Ian Hickson committed
1
// Copyright 2014 The Flutter Authors. All rights reserved.
2 3 4 5 6
// 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;

xster's avatar
xster committed
7
import 'package:flutter/foundation.dart';
8 9 10 11 12 13 14 15 16 17

import 'box.dart';
import 'object.dart';
import 'sliver.dart';
import 'sliver_multi_box_adaptor.dart';

/// Describes the placement of a child in a [RenderSliverGrid].
///
/// See also:
///
18 19 20
///  * [SliverGridLayout], which represents the geometry of all the tiles in a
///    grid.
///  * [SliverGridLayout.getGeometryForChildIndex], which returns this object
21 22 23
///    to describe the child's placement.
///  * [RenderSliverGrid], which uses this class during its
///    [RenderSliverGrid.performLayout] method.
24
@immutable
25 26 27
class SliverGridGeometry {
  /// Creates an object that describes the placement of a child in a [RenderSliverGrid].
  const SliverGridGeometry({
28 29 30 31
    required this.scrollOffset,
    required this.crossAxisOffset,
    required this.mainAxisExtent,
    required this.crossAxisExtent,
32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57
  });

  /// The scroll offset of the leading edge of the child relative to the leading
  /// edge of the parent.
  final double scrollOffset;

  /// The offset of the child in the non-scrolling axis.
  ///
  /// If the scroll axis is vertical, this offset is from the left-most edge of
  /// the parent to the left-most edge of the child. If the scroll axis is
  /// horizontal, this offset is from the top-most edge of the parent to the
  /// top-most edge of the child.
  final double crossAxisOffset;

  /// The extent of the child in the scrolling axis.
  ///
  /// If the scroll axis is vertical, this extent is the child's height. If the
  /// scroll axis is horizontal, this extent is the child's width.
  final double mainAxisExtent;

  /// The extent of the child in the non-scrolling axis.
  ///
  /// If the scroll axis is vertical, this extent is the child's width. If the
  /// scroll axis is horizontal, this extent is the child's height.
  final double crossAxisExtent;

Adam Barth's avatar
Adam Barth committed
58 59 60 61
  /// The scroll offset of the trailing edge of the child relative to the
  /// leading edge of the parent.
  double get trailingScrollOffset => scrollOffset + mainAxisExtent;

62 63 64 65 66 67 68 69 70
  /// Returns a tight [BoxConstraints] that forces the child to have the
  /// required size.
  BoxConstraints getBoxConstraints(SliverConstraints constraints) {
    return constraints.asBoxConstraints(
      minExtent: mainAxisExtent,
      maxExtent: mainAxisExtent,
      crossAxisExtent: crossAxisExtent,
    );
  }
Adam Barth's avatar
Adam Barth committed
71 72 73

  @override
  String toString() {
74 75 76 77 78 79 80
    final List<String> properties = <String>[
      'scrollOffset: $scrollOffset',
      'crossAxisOffset: $crossAxisOffset',
      'mainAxisExtent: $mainAxisExtent',
      'crossAxisExtent: $crossAxisExtent',
    ];
    return 'SliverGridGeometry(${properties.join(', ')})';
Adam Barth's avatar
Adam Barth committed
81
  }
82 83
}

84 85 86 87 88 89 90 91 92 93 94 95 96 97 98 99 100
/// The size and position of all the tiles in a [RenderSliverGrid].
///
/// Rather that providing a grid with a [SliverGridLayout] directly, you instead
/// provide the grid a [SliverGridDelegate], which can compute a
/// [SliverGridLayout] given the current [SliverConstraints].
///
/// The tiles can be placed arbitrarily, but it is more efficient to place tiles
/// in roughly in order by scroll offset because grids reify a contiguous
/// sequence of children.
///
/// See also:
///
///  * [SliverGridRegularTileLayout], which represents a layout that uses
///    equally sized and spaced tiles.
///  * [SliverGridGeometry], which represents the size and position of a single
///    tile in a grid.
///  * [SliverGridDelegate.getLayout], which returns this object to describe the
101
///    delegate's layout.
102 103
///  * [RenderSliverGrid], which uses this class during its
///    [RenderSliverGrid.performLayout] method.
104
@immutable
105 106 107 108 109
abstract class SliverGridLayout {
  /// Abstract const constructor. This constructor enables subclasses to provide
  /// const constructors so that they can be used in const expressions.
  const SliverGridLayout();

110
  /// The minimum child index that intersects with (or is after) this scroll offset.
111 112
  int getMinChildIndexForScrollOffset(double scrollOffset);

113
  /// The maximum child index that intersects with (or is before) this scroll offset.
114 115 116 117 118
  int getMaxChildIndexForScrollOffset(double scrollOffset);

  /// The size and position of the child with the given index.
  SliverGridGeometry getGeometryForChildIndex(int index);

119 120 121 122 123
  /// The scroll extent needed to fully display all the tiles if there are
  /// `childCount` children in total.
  ///
  /// The child count will never be null.
  double computeMaxScrollOffset(int childCount);
124 125 126 127 128 129 130 131 132 133 134 135 136 137 138
}

/// A [SliverGridLayout] that uses equally sized and spaced tiles.
///
/// Rather that providing a grid with a [SliverGridLayout] directly, you instead
/// provide the grid a [SliverGridDelegate], which can compute a
/// [SliverGridLayout] given the current [SliverConstraints].
///
/// This layout is used by [SliverGridDelegateWithFixedCrossAxisCount] and
/// [SliverGridDelegateWithMaxCrossAxisExtent].
///
/// See also:
///
///  * [SliverGridDelegateWithFixedCrossAxisCount], which uses this layout.
///  * [SliverGridDelegateWithMaxCrossAxisExtent], which uses this layout.
139
///  * [SliverGridLayout], which represents an arbitrary tile layout.
140 141 142
///  * [SliverGridGeometry], which represents the size and position of a single
///    tile in a grid.
///  * [SliverGridDelegate.getLayout], which returns this object to describe the
143
///    delegate's layout.
144 145 146 147 148 149 150
///  * [RenderSliverGrid], which uses this class during its
///    [RenderSliverGrid.performLayout] method.
class SliverGridRegularTileLayout extends SliverGridLayout {
  /// Creates a layout that uses equally sized and spaced tiles.
  ///
  /// All of the arguments must not be null and must not be negative. The
  /// `crossAxisCount` argument must be greater than zero.
151
  const SliverGridRegularTileLayout({
152 153 154 155 156 157
    required this.crossAxisCount,
    required this.mainAxisStride,
    required this.crossAxisStride,
    required this.childMainAxisExtent,
    required this.childCrossAxisExtent,
    required this.reverseCrossAxis,
158 159 160 161 162
  }) : assert(crossAxisCount > 0),
       assert(mainAxisStride >= 0),
       assert(crossAxisStride >= 0),
       assert(childMainAxisExtent >= 0),
       assert(childCrossAxisExtent >= 0);
163 164 165 166 167 168 169 170 171 172 173 174 175 176 177 178 179 180 181

  /// The number of children in the cross axis.
  final int crossAxisCount;

  /// The number of pixels from the leading edge of one tile to the leading edge
  /// of the next tile in the main axis.
  final double mainAxisStride;

  /// The number of pixels from the leading edge of one tile to the leading edge
  /// of the next tile in the cross axis.
  final double crossAxisStride;

  /// The number of pixels from the leading edge of one tile to the trailing
  /// edge of the same tile in the main axis.
  final double childMainAxisExtent;

  /// The number of pixels from the leading edge of one tile to the trailing
  /// edge of the same tile in the cross axis.
  final double childCrossAxisExtent;
182

183 184 185 186 187 188 189 190 191 192 193
  /// Whether the children should be placed in the opposite order of increasing
  /// coordinates in the cross axis.
  ///
  /// For example, if the cross axis is horizontal, the children are placed from
  /// left to right when [reverseCrossAxis] is false and from right to left when
  /// [reverseCrossAxis] is true.
  ///
  /// Typically set to the return value of [axisDirectionIsReversed] applied to
  /// the [SliverConstraints.crossAxisDirection].
  final bool reverseCrossAxis;

194
  @override
195
  int getMinChildIndexForScrollOffset(double scrollOffset) {
196
    return mainAxisStride > precisionErrorTolerance ? crossAxisCount * (scrollOffset ~/ mainAxisStride) : 0;
197 198 199 200
  }

  @override
  int getMaxChildIndexForScrollOffset(double scrollOffset) {
201 202 203 204 205
    if (mainAxisStride > 0.0) {
      final int mainAxisCount = (scrollOffset / mainAxisStride).ceil();
      return math.max(0, crossAxisCount * mainAxisCount - 1);
    }
    return 0;
206 207
  }

208
  double _getOffsetFromStartInCrossAxis(double crossAxisStart) {
209
    if (reverseCrossAxis) {
210
      return crossAxisCount * crossAxisStride - crossAxisStart - childCrossAxisExtent - (crossAxisStride - childCrossAxisExtent);
211
    }
212 213 214
    return crossAxisStart;
  }

215 216
  @override
  SliverGridGeometry getGeometryForChildIndex(int index) {
217
    final double crossAxisStart = (index % crossAxisCount) * crossAxisStride;
218
    return SliverGridGeometry(
219
      scrollOffset: (index ~/ crossAxisCount) * mainAxisStride,
220
      crossAxisOffset: _getOffsetFromStartInCrossAxis(crossAxisStart),
221 222 223 224 225 226
      mainAxisExtent: childMainAxisExtent,
      crossAxisExtent: childCrossAxisExtent,
    );
  }

  @override
227
  double computeMaxScrollOffset(int childCount) {
228
    final int mainAxisCount = ((childCount - 1) ~/ crossAxisCount) + 1;
229 230 231
    final double mainAxisSpacing = mainAxisStride - childMainAxisExtent;
    return mainAxisStride * mainAxisCount - mainAxisSpacing;
  }
232 233
}

234 235 236 237 238 239 240 241 242 243 244 245 246 247 248 249 250 251
/// Controls the layout of tiles in a grid.
///
/// Given the current constraints on the grid, a [SliverGridDelegate] computes
/// the layout for the tiles in the grid. The tiles can be placed arbitrarily,
/// but it is more efficient to place tiles in roughly in order by scroll offset
/// because grids reify a contiguous sequence of children.
///
/// See also:
///
///  * [SliverGridDelegateWithFixedCrossAxisCount], which creates a layout with
///    a fixed number of tiles in the cross axis.
///  * [SliverGridDelegateWithMaxCrossAxisExtent], which creates a layout with
///    tiles that have a maximum cross-axis extent.
///  * [GridView], which uses this delegate to control the layout of its tiles.
///  * [SliverGrid], which uses this delegate to control the layout of its
///    tiles.
///  * [RenderSliverGrid], which uses this delegate to control the layout of its
///    tiles.
252 253 254 255 256
abstract class SliverGridDelegate {
  /// Abstract const constructor. This constructor enables subclasses to provide
  /// const constructors so that they can be used in const expressions.
  const SliverGridDelegate();

257 258
  /// Returns information about the size and position of the tiles in the grid.
  SliverGridLayout getLayout(SliverConstraints constraints);
259

260 261 262 263 264 265
  /// Override this method to return true when the children need to be
  /// laid out.
  ///
  /// This should compare the fields of the current delegate and the given
  /// `oldDelegate` and return true if the fields are such that the layout would
  /// be different.
266
  bool shouldRelayout(covariant SliverGridDelegate oldDelegate);
267 268
}

269 270 271 272 273 274 275 276
/// Creates grid layouts with a fixed number of tiles in the cross axis.
///
/// For example, if the grid is vertical, this delegate will create a layout
/// with a fixed number of columns. If the grid is horizontal, this delegate
/// will create a layout with a fixed number of rows.
///
/// This delegate creates grids with equally sized and spaced tiles.
///
277
/// {@tool dartpad}
278 279 280 281
/// Here is an example using the [childAspectRatio] property. On a device with a
/// screen width of 800.0, it creates a GridView with each tile with a width of
/// 200.0 and a height of 100.0.
///
282
/// ** See code in examples/api/lib/rendering/sliver_grid/sliver_grid_delegate_with_fixed_cross_axis_count.0.dart **
283 284
/// {@end-tool}
///
285
/// {@tool dartpad}
286 287 288 289
/// Here is an example using the [mainAxisExtent] property. On a device with a
/// screen width of 800.0, it creates a GridView with each tile with a width of
/// 200.0 and a height of 150.0.
///
290
/// ** See code in examples/api/lib/rendering/sliver_grid/sliver_grid_delegate_with_fixed_cross_axis_count.1.dart **
291 292
/// {@end-tool}
///
293 294 295 296 297 298 299 300 301 302 303
/// See also:
///
///  * [SliverGridDelegateWithMaxCrossAxisExtent], which creates a layout with
///    tiles that have a maximum cross-axis extent.
///  * [SliverGridDelegate], which creates arbitrary layouts.
///  * [GridView], which can use this delegate to control the layout of its
///    tiles.
///  * [SliverGrid], which can use this delegate to control the layout of its
///    tiles.
///  * [RenderSliverGrid], which can use this delegate to control the layout of
///    its tiles.
304
class SliverGridDelegateWithFixedCrossAxisCount extends SliverGridDelegate {
305 306 307
  /// Creates a delegate that makes grid layouts with a fixed number of tiles in
  /// the cross axis.
  ///
308 309 310 311
  /// All of the arguments except [mainAxisExtent] must not be null.
  /// The `mainAxisSpacing`, `mainAxisExtent` and `crossAxisSpacing` arguments
  /// must not be negative. The `crossAxisCount` and `childAspectRatio`
  /// arguments must be greater than zero.
312
  const SliverGridDelegateWithFixedCrossAxisCount({
313
    required this.crossAxisCount,
314 315 316
    this.mainAxisSpacing = 0.0,
    this.crossAxisSpacing = 0.0,
    this.childAspectRatio = 1.0,
317
    this.mainAxisExtent,
318 319 320 321
  }) : assert(crossAxisCount > 0),
       assert(mainAxisSpacing >= 0),
       assert(crossAxisSpacing >= 0),
       assert(childAspectRatio > 0);
322 323 324 325 326 327 328 329 330 331 332 333 334

  /// The number of children in the cross axis.
  final int crossAxisCount;

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

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

  /// The ratio of the cross-axis to the main-axis extent of each child.
  final double childAspectRatio;

335 336 337 338 339 340
  /// The extent of each tile in the main axis. If provided it would define the
  /// logical pixels taken by each tile in the main-axis.
  ///
  /// If null, [childAspectRatio] is used instead.
  final double? mainAxisExtent;

341 342 343 344 345 346 347 348 349
  bool _debugAssertIsValid() {
    assert(crossAxisCount > 0);
    assert(mainAxisSpacing >= 0.0);
    assert(crossAxisSpacing >= 0.0);
    assert(childAspectRatio > 0.0);
    return true;
  }

  @override
350
  SliverGridLayout getLayout(SliverConstraints constraints) {
351
    assert(_debugAssertIsValid());
352 353 354 355
    final double usableCrossAxisExtent = math.max(
      0.0,
      constraints.crossAxisExtent - crossAxisSpacing * (crossAxisCount - 1),
    );
356
    final double childCrossAxisExtent = usableCrossAxisExtent / crossAxisCount;
357
    final double childMainAxisExtent = mainAxisExtent ?? childCrossAxisExtent / childAspectRatio;
358
    return SliverGridRegularTileLayout(
359 360 361 362 363
      crossAxisCount: crossAxisCount,
      mainAxisStride: childMainAxisExtent + mainAxisSpacing,
      crossAxisStride: childCrossAxisExtent + crossAxisSpacing,
      childMainAxisExtent: childMainAxisExtent,
      childCrossAxisExtent: childCrossAxisExtent,
364
      reverseCrossAxis: axisDirectionIsReversed(constraints.crossAxisDirection),
365 366 367 368 369 370 371 372
    );
  }

  @override
  bool shouldRelayout(SliverGridDelegateWithFixedCrossAxisCount oldDelegate) {
    return oldDelegate.crossAxisCount != crossAxisCount
        || oldDelegate.mainAxisSpacing != mainAxisSpacing
        || oldDelegate.crossAxisSpacing != crossAxisSpacing
373 374
        || oldDelegate.childAspectRatio != childAspectRatio
        || oldDelegate.mainAxisExtent != mainAxisExtent;
375 376 377
  }
}

378
/// Creates grid layouts with tiles that each have a maximum cross-axis extent.
379 380 381
///
/// This delegate will select a cross-axis extent for the tiles that is as
/// large as possible subject to the following conditions:
382
///
383 384
///  - The extent evenly divides the cross-axis extent of the grid.
///  - The extent is at most [maxCrossAxisExtent].
385
///
386 387 388 389 390 391 392 393 394 395 396 397 398 399 400 401 402
/// For example, if the grid is vertical, the grid is 500.0 pixels wide, and
/// [maxCrossAxisExtent] is 150.0, this delegate will create a grid with 4
/// columns that are 125.0 pixels wide.
///
/// This delegate creates grids with equally sized and spaced tiles.
///
/// See also:
///
///  * [SliverGridDelegateWithFixedCrossAxisCount], which creates a layout with
///    a fixed number of tiles in the cross axis.
///  * [SliverGridDelegate], which creates arbitrary layouts.
///  * [GridView], which can use this delegate to control the layout of its
///    tiles.
///  * [SliverGrid], which can use this delegate to control the layout of its
///    tiles.
///  * [RenderSliverGrid], which can use this delegate to control the layout of
///    its tiles.
403
class SliverGridDelegateWithMaxCrossAxisExtent extends SliverGridDelegate {
404 405
  /// Creates a delegate that makes grid layouts with tiles that have a maximum
  /// cross-axis extent.
406
  ///
407 408 409
  /// All of the arguments except [mainAxisExtent] must not be null.
  /// The [maxCrossAxisExtent], [mainAxisExtent], [mainAxisSpacing],
  /// and [crossAxisSpacing] arguments must not be negative.
410
  /// The [childAspectRatio] argument must be greater than zero.
411
  const SliverGridDelegateWithMaxCrossAxisExtent({
412
    required this.maxCrossAxisExtent,
413 414 415
    this.mainAxisSpacing = 0.0,
    this.crossAxisSpacing = 0.0,
    this.childAspectRatio = 1.0,
416
    this.mainAxisExtent,
417 418 419 420
  }) : assert(maxCrossAxisExtent > 0),
       assert(mainAxisSpacing >= 0),
       assert(crossAxisSpacing >= 0),
       assert(childAspectRatio > 0);
421

422 423 424 425 426 427 428 429 430 431 432
  /// The maximum extent of tiles in the cross axis.
  ///
  /// This delegate will select a cross-axis extent for the tiles that is as
  /// large as possible subject to the following conditions:
  ///
  ///  - The extent evenly divides the cross-axis extent of the grid.
  ///  - The extent is at most [maxCrossAxisExtent].
  ///
  /// For example, if the grid is vertical, the grid is 500.0 pixels wide, and
  /// [maxCrossAxisExtent] is 150.0, this delegate will create a grid with 4
  /// columns that are 125.0 pixels wide.
433 434 435 436 437 438 439 440 441 442 443
  final double maxCrossAxisExtent;

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

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

  /// The ratio of the cross-axis to the main-axis extent of each child.
  final double childAspectRatio;

444 445 446 447 448 449
  /// The extent of each tile in the main axis. If provided it would define the
  /// logical pixels taken by each tile in the main-axis.
  ///
  /// If null, [childAspectRatio] is used instead.
  final double? mainAxisExtent;

450 451
  bool _debugAssertIsValid(double crossAxisExtent) {
    assert(crossAxisExtent > 0.0);
452 453 454 455 456 457 458 459
    assert(maxCrossAxisExtent > 0.0);
    assert(mainAxisSpacing >= 0.0);
    assert(crossAxisSpacing >= 0.0);
    assert(childAspectRatio > 0.0);
    return true;
  }

  @override
460
  SliverGridLayout getLayout(SliverConstraints constraints) {
461
    assert(_debugAssertIsValid(constraints.crossAxisExtent));
462
    int crossAxisCount = (constraints.crossAxisExtent / (maxCrossAxisExtent + crossAxisSpacing)).ceil();
463 464 465
    // Ensure a minimum count of 1, can be zero and result in an infinite extent
    // below when the window size is 0.
    crossAxisCount = math.max(1, crossAxisCount);
466 467 468 469
    final double usableCrossAxisExtent = math.max(
      0.0,
      constraints.crossAxisExtent - crossAxisSpacing * (crossAxisCount - 1),
    );
470
    final double childCrossAxisExtent = usableCrossAxisExtent / crossAxisCount;
471
    final double childMainAxisExtent = mainAxisExtent ?? childCrossAxisExtent / childAspectRatio;
472
    return SliverGridRegularTileLayout(
473 474 475 476 477
      crossAxisCount: crossAxisCount,
      mainAxisStride: childMainAxisExtent + mainAxisSpacing,
      crossAxisStride: childCrossAxisExtent + crossAxisSpacing,
      childMainAxisExtent: childMainAxisExtent,
      childCrossAxisExtent: childCrossAxisExtent,
478
      reverseCrossAxis: axisDirectionIsReversed(constraints.crossAxisDirection),
479 480 481 482 483 484 485 486
    );
  }

  @override
  bool shouldRelayout(SliverGridDelegateWithMaxCrossAxisExtent oldDelegate) {
    return oldDelegate.maxCrossAxisExtent != maxCrossAxisExtent
        || oldDelegate.mainAxisSpacing != mainAxisSpacing
        || oldDelegate.crossAxisSpacing != crossAxisSpacing
487 488
        || oldDelegate.childAspectRatio != childAspectRatio
        || oldDelegate.mainAxisExtent != mainAxisExtent;
489 490 491
  }
}

492
/// Parent data structure used by [RenderSliverGrid].
493
class SliverGridParentData extends SliverMultiBoxAdaptorParentData {
494 495 496 497 498 499
  /// The offset of the child in the non-scrolling axis.
  ///
  /// If the scroll axis is vertical, this offset is from the left-most edge of
  /// the parent to the left-most edge of the child. If the scroll axis is
  /// horizontal, this offset is from the top-most edge of the parent to the
  /// top-most edge of the child.
500
  double? crossAxisOffset;
501 502 503 504 505

  @override
  String toString() => 'crossAxisOffset=$crossAxisOffset; ${super.toString()}';
}

506
/// A sliver that places multiple box children in a two dimensional arrangement.
507 508 509 510 511 512 513 514 515 516 517
///
/// [RenderSliverGrid] places its children in arbitrary positions determined by
/// [gridDelegate]. Each child is forced to have the size specified by the
/// [gridDelegate].
///
/// See also:
///
///  * [RenderSliverList], which places its children in a linear
///    array.
///  * [RenderSliverFixedExtentList], which places its children in a linear
///    array with a fixed extent in the main axis.
518
class RenderSliverGrid extends RenderSliverMultiBoxAdaptor {
519 520 521 522
  /// Creates a sliver that contains multiple box children that whose size and
  /// position are determined by a delegate.
  ///
  /// The [childManager] and [gridDelegate] arguments must not be null.
523
  RenderSliverGrid({
524
    required super.childManager,
525
    required SliverGridDelegate gridDelegate,
526
  }) : _gridDelegate = gridDelegate;
527 528 529

  @override
  void setupParentData(RenderObject child) {
530
    if (child.parentData is! SliverGridParentData) {
531
      child.parentData = SliverGridParentData();
532
    }
533 534
  }

535
  /// The delegate that controls the size and position of the children.
536 537
  SliverGridDelegate get gridDelegate => _gridDelegate;
  SliverGridDelegate _gridDelegate;
538
  set gridDelegate(SliverGridDelegate value) {
539
    if (_gridDelegate == value) {
540
      return;
541
    }
542
    if (value.runtimeType != _gridDelegate.runtimeType ||
543
        value.shouldRelayout(_gridDelegate)) {
544
      markNeedsLayout();
545
    }
546
    _gridDelegate = value;
547 548 549 550
  }

  @override
  double childCrossAxisPosition(RenderBox child) {
551
    final SliverGridParentData childParentData = child.parentData! as SliverGridParentData;
552
    return childParentData.crossAxisOffset!;
553 554 555 556
  }

  @override
  void performLayout() {
557
    final SliverConstraints constraints = this.constraints;
558
    childManager.didStartLayout();
559 560
    childManager.setDidUnderflow(false);

561
    final double scrollOffset = constraints.scrollOffset + constraints.cacheOrigin;
562
    assert(scrollOffset >= 0.0);
563 564 565
    final double remainingExtent = constraints.remainingCacheExtent;
    assert(remainingExtent >= 0.0);
    final double targetEndScrollOffset = scrollOffset + remainingExtent;
566

567 568 569
    final SliverGridLayout layout = _gridDelegate.getLayout(constraints);

    final int firstIndex = layout.getMinChildIndexForScrollOffset(scrollOffset);
570
    final int? targetLastIndex = targetEndScrollOffset.isFinite ?
571
      layout.getMaxChildIndexForScrollOffset(targetEndScrollOffset) : null;
572 573

    if (firstChild != null) {
574 575
      final int oldFirstIndex = indexOf(firstChild!);
      final int oldLastIndex = indexOf(lastChild!);
576
      final int leadingGarbage = (firstIndex - oldFirstIndex).clamp(0, childCount); // ignore_clamp_double_lint
577 578
      final int trailingGarbage = targetLastIndex == null
        ? 0
579
        : (oldLastIndex - targetLastIndex).clamp(0, childCount); // ignore_clamp_double_lint
580 581 582
      collectGarbage(leadingGarbage, trailingGarbage);
    } else {
      collectGarbage(0, 0);
583 584
    }

585
    final SliverGridGeometry firstChildGridGeometry = layout.getGeometryForChildIndex(firstIndex);
586 587

    if (firstChild == null) {
588 589 590
      if (!addInitialChild(index: firstIndex, layoutOffset: firstChildGridGeometry.scrollOffset)) {
        // There are either no children, or we are past the end of all our children.
        final double max = layout.computeMaxScrollOffset(childManager.childCount);
591
        geometry = SliverGeometry(
592 593 594
          scrollExtent: max,
          maxPaintExtent: max,
        );
595
        childManager.didFinishLayout();
596 597 598 599
        return;
      }
    }

600 601
    final double leadingScrollOffset = firstChildGridGeometry.scrollOffset;
    double trailingScrollOffset = firstChildGridGeometry.trailingScrollOffset;
602
    RenderBox? trailingChildWithLayout;
603

604
    for (int index = indexOf(firstChild!) - 1; index >= firstIndex; --index) {
605
      final SliverGridGeometry gridGeometry = layout.getGeometryForChildIndex(index);
606
      final RenderBox child = insertAndLayoutLeadingChild(
Ian Hickson's avatar
Ian Hickson committed
607
        gridGeometry.getBoxConstraints(constraints),
608
      )!;
609
      final SliverGridParentData childParentData = child.parentData! as SliverGridParentData;
610
      childParentData.layoutOffset = gridGeometry.scrollOffset;
611 612 613
      childParentData.crossAxisOffset = gridGeometry.crossAxisOffset;
      assert(childParentData.index == index);
      trailingChildWithLayout ??= child;
Adam Barth's avatar
Adam Barth committed
614
      trailingScrollOffset = math.max(trailingScrollOffset, gridGeometry.trailingScrollOffset);
615 616 617
    }

    if (trailingChildWithLayout == null) {
618
      firstChild!.layout(firstChildGridGeometry.getBoxConstraints(constraints));
619
      final SliverGridParentData childParentData = firstChild!.parentData! as SliverGridParentData;
620
      childParentData.layoutOffset = firstChildGridGeometry.scrollOffset;
621 622 623 624
      childParentData.crossAxisOffset = firstChildGridGeometry.crossAxisOffset;
      trailingChildWithLayout = firstChild;
    }

625
    for (int index = indexOf(trailingChildWithLayout!) + 1; targetLastIndex == null || index <= targetLastIndex; ++index) {
626 627
      final SliverGridGeometry gridGeometry = layout.getGeometryForChildIndex(index);
      final BoxConstraints childConstraints = gridGeometry.getBoxConstraints(constraints);
628
      RenderBox? child = childAfter(trailingChildWithLayout!);
629
      if (child == null || indexOf(child) != index) {
630 631 632 633 634 635 636 637 638
        child = insertAndLayoutChild(childConstraints, after: trailingChildWithLayout);
        if (child == null) {
          // We have run out of children.
          break;
        }
      } else {
        child.layout(childConstraints);
      }
      trailingChildWithLayout = child;
639
      final SliverGridParentData childParentData = child.parentData! as SliverGridParentData;
640
      childParentData.layoutOffset = gridGeometry.scrollOffset;
641 642
      childParentData.crossAxisOffset = gridGeometry.crossAxisOffset;
      assert(childParentData.index == index);
Adam Barth's avatar
Adam Barth committed
643
      trailingScrollOffset = math.max(trailingScrollOffset, gridGeometry.trailingScrollOffset);
644 645
    }

646
    final int lastIndex = indexOf(lastChild!);
647 648

    assert(debugAssertChildListIsNonEmptyAndContiguous());
649
    assert(indexOf(firstChild!) == firstIndex);
650
    assert(targetLastIndex == null || lastIndex <= targetLastIndex);
651 652 653 654 655 656 657 658

    final double estimatedTotalExtent = childManager.estimateMaxScrollOffset(
      constraints,
      firstIndex: firstIndex,
      lastIndex: lastIndex,
      leadingScrollOffset: leadingScrollOffset,
      trailingScrollOffset: trailingScrollOffset,
    );
Adam Barth's avatar
Adam Barth committed
659
    final double paintExtent = calculatePaintOffset(
660
      constraints,
661
      from: math.min(constraints.scrollOffset, leadingScrollOffset),
662 663
      to: trailingScrollOffset,
    );
664 665 666 667 668
    final double cacheExtent = calculateCacheOffset(
      constraints,
      from: leadingScrollOffset,
      to: trailingScrollOffset,
    );
669

670
    geometry = SliverGeometry(
671
      scrollExtent: estimatedTotalExtent,
Adam Barth's avatar
Adam Barth committed
672
      paintExtent: paintExtent,
673
      maxPaintExtent: estimatedTotalExtent,
674
      cacheExtent: cacheExtent,
675
      hasVisualOverflow: estimatedTotalExtent > paintExtent || constraints.scrollOffset > 0.0 || constraints.overlap != 0.0,
676 677
    );

678 679
    // We may have started the layout while scrolled to the end, which
    // would not expose a new child.
680
    if (estimatedTotalExtent == trailingScrollOffset) {
681
      childManager.setDidUnderflow(true);
682
    }
683
    childManager.didFinishLayout();
684 685
  }
}