layout method

@override void layout(BoxConstraints constraints)

Implementation

@override
void layout(BoxConstraints constraints) {
  super.layout(constraints);

  final maxWidth =
      width ??
      (constraints.hasBoundedWidth ? constraints.maxWidth.round() : 0);
  final boundedHeight = constraints.hasBoundedHeight
      ? constraints.maxHeight.round()
      : null;
  final viewportHeight = height ?? boundedHeight;
  final separatorBreaks = _separatorBreaks(separator);
  final itemCount = _itemCount;
  if (variableHeight) {
    if (_lastMaxWidth != maxWidth) {
      assert(
        _lastMaxWidth == null || (_lastMaxWidth! - maxWidth).abs() <= 100,
        'layout: maxWidth jumped from $_lastMaxWidth to $maxWidth',
      );
      final widthDiff = _lastMaxWidth != null
          ? (_lastMaxWidth! - maxWidth).abs()
          : 0;
      _traceScroll(
        'virtual_list.clear_measurements '
        'zone=$zoneId reason=_lastMaxWidth($_lastMaxWidth)!=maxWidth($maxWidth)'
        ' diff=$widthDiff',
      );
      _lastMaxWidth = maxWidth;
      if (widthDiff > 4) {
        _clearMeasurements();
      }
    }
    final baseEstimate = math
        .max(1, estimatedItemExtent ?? itemExtent)
        .toInt();
    final estimate = _resolveAdaptiveEstimate(baseEstimate);
    _syncCache(itemCount, separatorBreaks, estimate);
    _preMeasureItems(
      itemCount: itemCount,
      maxWidth: maxWidth,
      estimate: estimate,
      separatorBreaks: separatorBreaks,
    );
    final contentHeight = _estimatedContentHeight();
    final layoutHeight = viewportHeight ?? contentHeight;
    _setMetrics(
      viewportExtent: layoutHeight.toInt(),
      contentExtent: contentHeight.toInt(),
    );
    size = constraints.constrain(
      Size(maxWidth.toDouble(), layoutHeight.toDouble()),
    );
    return;
  }

  final itemHeight = math.max(1, itemExtent).toInt();
  final contentHeight = itemCount == 0
      ? 0
      : itemCount * itemHeight + (itemCount - 1) * separatorBreaks;
  final layoutHeight = viewportHeight ?? contentHeight;

  _setMetrics(
    viewportExtent: layoutHeight.toInt(),
    contentExtent: contentHeight.toInt(),
  );

  size = constraints.constrain(
    Size(maxWidth.toDouble(), layoutHeight.toDouble()),
  );
}