layoutChild method

Size layoutChild (Object childId, BoxConstraints constraints)

Ask the child to update its layout within the limits specified by the constraints parameter. The child's size is returned.

Call this from your performLayout function to lay out each child. Every child must be laid out using this function exactly once each time the performLayout function is called.

Implementation 

Size layoutChild(Object childId, BoxConstraints constraints) {
  final RenderBox child = _idToChild[childId];
  assert(() {
    if (child == null) {
      throw FlutterError(
        'The $this custom multichild layout delegate tried to lay out a non-existent child.\n'
        'There is no child with the id "$childId".'
      );
    }
    if (!_debugChildrenNeedingLayout.remove(child)) {
      throw FlutterError(
        'The $this custom multichild layout delegate tried to lay out the child with id "$childId" more than once.\n'
        'Each child must be laid out exactly once.'
      );
    }
    try {
      assert(constraints.debugAssertIsValid(isAppliedConstraint: true));
    } on AssertionError catch (exception) {
      throw FlutterError(
        'The $this custom multichild layout delegate provided invalid box constraints for the child with id "$childId".\n'
        '$exception\n'
        'The minimum width and height must be greater than or equal to zero.\n'
        'The maximum width must be greater than or equal to the minimum width.\n'
        'The maximum height must be greater than or equal to the minimum height.'
      );
    }
    return true;
  }());
  child.layout(constraints, parentUsesSize: true);
  return child.size;
}