performLayout method

  1. @override
void performLayout ()
override

Do the work of computing the layout for this render object.

Do not call this function directly: call layout instead. This function is called by layout when there is actually work to be done by this render object during layout. The layout constraints provided by your parent are available via the constraints getter.

If sizedByParent is true, then this function should not actually change the dimensions of this render object. Instead, that work should be done by performResize. If sizedByParent is false, then this function should both change the dimensions of this render object and instruct its children to layout.

In implementing this function, you must call layout on each of your children, passing true for parentUsesSize if your layout information is dependent on your child's layout information. Passing true for parentUsesSize ensures that this render object will undergo layout if the child undergoes layout. Otherwise, the child can change its layout information without informing this render object.

Implementation 

@override
void performLayout() {
  childManager.didStartLayout();
  childManager.setDidUnderflow(false);

  final double scrollOffset = constraints.scrollOffset + constraints.cacheOrigin;
  assert(scrollOffset >= 0.0);
  final double remainingExtent = constraints.remainingCacheExtent;
  assert(remainingExtent >= 0.0);
  final double targetEndScrollOffset = scrollOffset + remainingExtent;

  final SliverGridLayout layout = _gridDelegate.getLayout(constraints);

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

  if (firstChild != null) {
    final int oldFirstIndex = indexOf(firstChild);
    final int oldLastIndex = indexOf(lastChild);
    final int leadingGarbage = (firstIndex - oldFirstIndex).clamp(0, childCount);
    final int trailingGarbage = targetLastIndex == null ? 0 : (oldLastIndex - targetLastIndex).clamp(0, childCount);
    collectGarbage(leadingGarbage, trailingGarbage);
  } else {
    collectGarbage(0, 0);
  }

  final SliverGridGeometry firstChildGridGeometry = layout.getGeometryForChildIndex(firstIndex);
  final double leadingScrollOffset = firstChildGridGeometry.scrollOffset;
  double trailingScrollOffset = firstChildGridGeometry.trailingScrollOffset;

  if (firstChild == null) {
    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);
      geometry = SliverGeometry(
        scrollExtent: max,
        maxPaintExtent: max,
      );
      childManager.didFinishLayout();
      return;
    }
  }

  RenderBox trailingChildWithLayout;

  for (int index = indexOf(firstChild) - 1; index >= firstIndex; --index) {
    final SliverGridGeometry gridGeometry = layout.getGeometryForChildIndex(index);
    final RenderBox child = insertAndLayoutLeadingChild(
      gridGeometry.getBoxConstraints(constraints),
    );
    final SliverGridParentData childParentData = child.parentData;
    childParentData.layoutOffset = gridGeometry.scrollOffset;
    childParentData.crossAxisOffset = gridGeometry.crossAxisOffset;
    assert(childParentData.index == index);
    trailingChildWithLayout ??= child;
    trailingScrollOffset = math.max(trailingScrollOffset, gridGeometry.trailingScrollOffset);
  }

  if (trailingChildWithLayout == null) {
    firstChild.layout(firstChildGridGeometry.getBoxConstraints(constraints));
    final SliverGridParentData childParentData = firstChild.parentData;
    childParentData.layoutOffset = firstChildGridGeometry.scrollOffset;
    childParentData.crossAxisOffset = firstChildGridGeometry.crossAxisOffset;
    trailingChildWithLayout = firstChild;
  }

  for (int index = indexOf(trailingChildWithLayout) + 1; targetLastIndex == null || index <= targetLastIndex; ++index) {
    final SliverGridGeometry gridGeometry = layout.getGeometryForChildIndex(index);
    final BoxConstraints childConstraints = gridGeometry.getBoxConstraints(constraints);
    RenderBox child = childAfter(trailingChildWithLayout);
    if (child == null) {
      child = insertAndLayoutChild(childConstraints, after: trailingChildWithLayout);
      if (child == null) {
        // We have run out of children.
        break;
      }
    } else {
      child.layout(childConstraints);
    }
    trailingChildWithLayout = child;
    assert(child != null);
    final SliverGridParentData childParentData = child.parentData;
    childParentData.layoutOffset = gridGeometry.scrollOffset;
    childParentData.crossAxisOffset = gridGeometry.crossAxisOffset;
    assert(childParentData.index == index);
    trailingScrollOffset = math.max(trailingScrollOffset, gridGeometry.trailingScrollOffset);
  }

  final int lastIndex = indexOf(lastChild);

  assert(childScrollOffset(firstChild) <= scrollOffset);
  assert(debugAssertChildListIsNonEmptyAndContiguous());
  assert(indexOf(firstChild) == firstIndex);
  assert(targetLastIndex == null || lastIndex <= targetLastIndex);

  final double estimatedTotalExtent = childManager.estimateMaxScrollOffset(
    constraints,
    firstIndex: firstIndex,
    lastIndex: lastIndex,
    leadingScrollOffset: leadingScrollOffset,
    trailingScrollOffset: trailingScrollOffset,
  );

  final double paintExtent = calculatePaintOffset(
    constraints,
    from: leadingScrollOffset,
    to: trailingScrollOffset,
  );
  final double cacheExtent = calculateCacheOffset(
    constraints,
    from: leadingScrollOffset,
    to: trailingScrollOffset,
  );

  geometry = SliverGeometry(
    scrollExtent: estimatedTotalExtent,
    paintExtent: paintExtent,
    maxPaintExtent: estimatedTotalExtent,
    cacheExtent: cacheExtent,
    // Conservative to avoid complexity.
    hasVisualOverflow: true,
  );

  // We may have started the layout while scrolled to the end, which
  // would not expose a new child.
  if (estimatedTotalExtent == trailingScrollOffset)
    childManager.setDidUnderflow(true);
  childManager.didFinishLayout();
}