merge method

Selection merge(Selection other)

Returns a new selection merging this selection with the specified other selection or transition.

The returned selection has the same number of groups and the same parents as this selection. Any missing (null) elements in this selection are filled with the corresponding element, if present (not null), from the specified selection. (If the other selection has additional groups or parents, they are ignored.)

This method is used internally by selection.joind to merge the enter and update selections after binding data. You can also merge explicitly, although note that since merging is based on element index, you should use operations that preserve index, such as selection.select instead of selection.filter. For example:

final odd = selection
    .select((Element thisArg, JSAny? d, int i, List<Element?> nodes) {
  return i.isOdd ? thisArg : null;
}.u21);
final even = selection
    .select((Element thisArg, JSAny? d, int i, List<Element?> nodes) {
  return i.isOdd ? null : thisArg;
}.u21);
final merged = odd.merge(even);

See selection.dataBind for more.

This method is not intended for concatenating arbitrary selections, however: if both this selection and the specified other selection have (non-null) elements at the same index, this selection’s element is returned in the merge and the other selection’s element is ignored.

Implementation

Selection merge(Selection other) {
  var selection = other.selection(),
      groups0 = _groups,
      m0 = groups0.length,
      merges = List<List<Element?>>.filled(m0, []),
      j = 0;
  for (var groups1 = selection._groups, m1 = groups1.length, m = min(m0, m1);
      j < m;
      ++j) {
    Element? node;
    for (var group0 = groups0[j],
            group1 = groups1[j],
            n = group0.length,
            merge = merges[j] = List<Element?>.filled(n, null),
            i = 0;
        i < n;
        ++i) {
      if ((node = group0[i] ?? group1[i]) != null) {
        merge[i] = node;
      }
    }
  }

  for (; j < m0; ++j) {
    merges[j] = groups0[j];
  }

  return Selection._(merges, _parents);
}