insertBefore method

@override void insertBefore(Node node, Node? before)

Implementation

@override
void insertBefore(Node node, Node? before) {
  // Can't add document
  if (node is Document) {
    throw ArgumentError.value(node, 'node');
  }

  // Can't add node into itself
  if (identical(node, this)) {
    throw ArgumentError.value(node, 'node');
  }

  // If 'before' is non-null, it must be a child
  if (before != null && !identical(before._parent, this)) {
    throw ArgumentError.value(before, 'before');
  }

  // Mark as dirty
  _markDirty();

  // Remove from old parent
  if (node.parentNode != null) {
    node.remove();
  }

  // Validate state
  if (node.parentNode != null ||
      node.previousNode != null && node.nextNode != null) {
    throw StateError('Node is not detached.');
  }

  node._parent = this;
  if (before == null) {
    final oldLastChild = _lastChild;
    if (oldLastChild == null) {
      _firstChild = node;
    } else {
      oldLastChild._nextNode = node;
    }
    node._previousNode = oldLastChild;
    _lastChild = node;
  } else {
    final previous = before._previousNode;
    if (previous == null) {
      // Set first sibling
      _firstChild = node;
    } else {
      previous._nextNode = node;
      node._previousNode = previous;
    }
    node._nextNode = before;
    before._previousNode = node;
  }

  // Validate state
  if (_firstChild == null || _lastChild == null || node.parentNode == null) {
    throw StateError('Node is not attached.');
  }
}